Figure 4.1
In this soil profile, a clay or caliche layer impedes water drainage and root growth. Deep-ripping the soil can sometimes break up the impermeable layers and improve drainage.
Early organic horticulturists focused heavily on soil health, declaring that healthy soils were essential for healthy crops. Although organic advocates have continued to use that mantra for more than a century, the concept was widely ridiculed or minimized by many commercial fruit growers and agricultural scientists. By adding enough fertilizers and using herbicides, orchardists were able to produce abundant crops with little regard to the soil, although root and collar diseases and replant disorders were common. We are finding, however, that orchard soils are far more complex than we once thought and do, indeed, play a key role in tree health and productivity.
Soils are critically important in organic systems because they impact not only tree health and productivity; they also affect beneficial organisms that are keystones in our pest and disease control programs. The soil is, quite literally, the foundation of your orchard. If you don’t get the soil right, not much else you do will matter. All organic systems emphasize healthy, biologically active soils that:
To prepare or upgrade your orchard, you will need to develop healthy soils. This is accomplished by identifying and correcting any drainage problems, setting soil standards, establishing the right pH and salinity levels for your soil, adding the appropriate amendments to your soil, and controlling weeds and pests. In chapter 9, we will go into detail about how to maintain healthy soils after your trees are established.
The following sections provide guidance on preparing a new orchard site for planting or upgrading an established orchard site. The steps are:
Regardless of the crop, fruit trees perform best on deep, well-drained soils. Poor soil drainage creates many diseases and physiological disorders in fruit crops. Unfortunately, not everyone has ideal soils, and extra work can be needed to prepare a planting site. Correcting drainage problems may involve grading down high spots and filling in low spots, installing drainage tiles, and installing drainage ditches and culverts. These steps are best done before staking out planting blocks and roads. It will be enough to have identified your general orchard outline at this point.
Telltale signs of poor drainage are standing water; very dark soils (sometimes with a foul or swamp odor); low-lying areas with sparse vegetation; or the presence of rushes, sedges, reeds, cattails, and other wetland plants. As mentioned in chapter 2, some wetlands are regulated by federal, state, or provincial laws and you may be limited on what you do with such lands. If in doubt as to the status of your land, check with your regulatory agency before making any modifications to wetlands.
For commercial orchards or a large home orchard, take the time to dig several pits or trenches about 6 feet deep. Look for hardpans, other impermeable layers, and dark bluish or greenish streaks and layers in the soil (see chapter 2). Map out areas of the orchard where drainage appears to be poor. They will require remedial treatment before planting and possibly special management after planting. Poorly drained spots may never be suitable for fruit trees and may best be used for non-tree crops or staging equipment.
To improve poor soil drainage, you can employ the following strategies:
Adding sand or organic materials. In chapter 2, we discussed how soil texture or type affects drainage. Unfortunately, there is little you can do to change soil texture in any but the smallest plantings. Hauling in enough sand or clay to modify the soil on a large scale is generally prohibitively expensive. What is more, adding sand or organic matter to heavy, low-lying soils seldom improves the drainage because the water still has no place to drain to. For example, say you have a slight depression where water stands for a day or more after rain. Filling the depression with well-drained soil will not improve the drainage because the water will still perch on top of the heavier soil at the bottom. This practice simply creates a pot without a hole in the bottom and can actually make a poorly drained soil even worse.
If you choose to amend your soil with sand and/or organic matter, spread the amendments across the entire planting area, including alleys, and till them into the soil. Then lay out your tree rows and plant the trees. Do not simply dig a trench along a tree row and fill it with amended soil or add amendments to the planting holes. Doing so creates a boundary layer between the amended and native soils that impedes root growth and the movement of water. Again, you have created a pot without a hole in the bottom.
Sand and organic amendments can effectively lighten soils and improve drainage when combined with raised beds, and such practices can be effective for small orchards.
Identifying and breaking up hardpans. Hardpans are impermeable layers of soil that prevent water from draining through the soil. This limits the movement of oxygen into the root zone, reduces micro- and macrobiological activity, increases root and collar diseases, and makes certain nutrients less available to plants. Hardpans also limit how deeply roots grow and can create weakly anchored trees that experience further root damage as they sway excessively in the wind.
Hardpans can be detected by digging test pits in your planting site (see chapter 2). Puddles that remain more than a few hours after a heavy rain or snow melt are also indications of hardpans. In many cases, impermeable layers result from poor soil management practices, such as excessive tilling (see box on page 81) or compaction from equipment and livestock.
Hardpans also occur naturally due to clay layers (sometimes called lenses) or mineral deposits (caliche). The mineral deposits are usually some form of carbonate (lime) and occur most often in arid regions. Frequent, shallow irrigation that draws soil minerals to the surface and deposits them when the water evaporates can also create drainage problems. Less frequent and deeper irrigation will help flush excess mineral salts out of the root zone.
In this soil profile, a clay or caliche layer impedes water drainage and root growth. Deep-ripping the soil can sometimes break up the impermeable layers and improve drainage.
If a hardpan is not too deep, you can often break it up using a long, narrow chisel plow or shank in a process called “deep ripping.” Shallow tiller- or plow-pans can usually be broken up by ripping using a medium-sized tractor. Deeper and harder pans usually require a large tractor or bulldozer, often pulling two to several deep-ripping shanks. Ripping is more effective with mechanical or mineral hardpans than with clay layers because the clay particles tend to settle back into their original positions when the soils are wet.
For new or replanted commercial orchards, growers commonly cross-rip the fields before planting, first deep-ripping in one direction and then ripping at right angles to the first plowed rows. Cross ripping before planting can significantly reduce problems due to compacted soils and hardpans. A small investment now often reaps large benefits later.
Rototillers are especially likely to create hardpans for two reasons:
Mechanical tillers with vertical tines that stir the soil, rather like an eggbeater, are less likely to create a tiller pan than tillers with traditional flat, revolving blades. Disk and plow blades are less likely to damage soil structure than tillers, but they can create mechanical compaction when used excessively. While rototillers, disks, and plows are useful farm implements, use them carefully and sparingly.
Creating ditches, installing drain tiles, and grading. These practices can all be effective in improving drainage, provided you have a lower area to drain the water into.
Simply creating drainage ditches through your orchard may help improve soil drainage. Keep the ditches on more-or-less level ground to reduce problems with erosion. Install and cover culverts where equipment and people need to cross over the ditches. Due to safety concerns, open ditches are best not used in orchards where young children may be present.
Drain tiles or perforated pipe can be buried in the orchard to accumulate excess soil water and carry it off site or to a storage pond. The great advantage tiles and perforated pipes have is that they are entirely underground, require little or no maintenance, and do not interfere with orchard operations or movement of people and equipment. Be sure that the tiles and pipes are surrounded with weed barrier fabric to allow water to enter the pipes but prevent tree roots from entering into and clogging them.
Orchard sites can be graded to provide level or at least uniform sloping planting areas. Grading can also be used to remove high spots and fill in low-lying areas. Be especially cautious with grading, however, and ensure that you do not reduce portions of your orchard to infertile subsoil by scraping away all of the topsoil.
Grading fields, establishing drainage ditches, and installing drain tiles often require specialized equipment and expertise. For commercial orchards, these activities are usually best left to professionals. Correcting drainage problems is much more difficult once the trees are planted, so be sure to get this process right from the start.
Using raised beds. Raised beds are simply mounds of soil that lift crop plants above the surrounding soil and provide increased rooting depth. In gardens, raised beds are often used and quite typically are enclosed on the sides with stone, brick, wood, or composite materials. In orchards, raised beds are most often simply flattened mounds of soil. These beds will be formed after the orchard is laid out, usually just before planting the trees. This section is included here so that you can consider whether raised beds will meet your needs later or if you also need to take other remedial action to correct drainage problems before laying out the planting blocks.
A) A raised bed along the center of the fruit crop row can be used in home and commercial orchards.
B) Raised planting areas work well for fruit crops grown in a landscape setting.
Planting on raised beds is rapidly becoming a standard practice in commercial berry production in many parts of the world. For very little investment, you can ensure excellent soil drainage. Saskatoons and bush cherries and plums, for example, can be grown in raised beds that are 10 to 12 inches high and 3 to 4 feet wide. Even for the larger tree fruits, more fruit specialists are now recommending the use of raised beds. Because fruit trees have larger root systems than berries, the beds must be somewhat wider. While the use of raised beds in commercial tree fruit orchards is still largely novel and untested, it appears likely that the practice would work best with relatively small trees on low-vigor rootstocks.
Raised beds are useful on poorly drained home orchard sites where you can create a berm or raised ridge to plant your trees on. Berms are generally higher and wider than the raised beds used in orchards and gardens. Likewise, raised landscape beds perhaps 10 feet or more in diameter can be used to improve soil drainage. If your native soil has high amounts of clay and silt, combine it with sand and organic materials to create an amended soil that will improve drainage and biological activity in the berms or raised beds. Figure 4.2 shows two raised bed designs.
Excessive drainage usually occurs on soils high in sand and/or gravel. They have little water-holding capacity, and without irrigation, plant growth is sparse. On a small scale, you can improve the water-holding capacity of droughty soil by amending it with clay or silt soils, peat soils, compost, wood chips, bark, or similar materials. On a large scale, adding large amounts of off-site soils or organic amendments is usually prohibitively expensive.
In all cases, tilling under one to several green manure crops before planting your trees will start the process of building organic matter. After the trees are established, annual alley crops, such as barley, can be tilled into the soil. If you use permanent alley cover crops, blow the clippings into the tree rows during mowing. Increasing soil organic matter concentrations, however, is a slow process, and you may need 5 to 10 years of careful management (discussed in chapter 9) to achieve significant increases. If you plan to establish an orchard on a droughty site, ensure that you have ample irrigation water and install an effective irrigation system before you plant any trees.
With a few improvements, some orchard owners that I consulted with turned a poorly drained site into a productive berry orchard. This family-run farm was located on level to rolling pasture land with a low-lying strip next to the adjacent county road. I examined the site during late winter when areas of standing water from rain and melting snow were clearly visible in some of the low-lying areas of the field.
To correct the problem, the owners devised a series of ditches between the planting blocks, providing drainage into an inlet from a nearby lake. They used grading to level the fields and to fill in some low-lying areas in the main body of the orchard, and they installed drain tiles leading into the ditches. The strip along the county road could not be improved enough to grow fruit on without great expense, so it was kept as a buffer strip for a future roadside stand and parking area. The result was that most of the 15-acre property was converted from rather marginal pasture and forage fields into manageable orchard land.
Before you stake out planting blocks and other parts of your orchard, take the time to carefully examine the site and identify problem areas, as we discussed in chapters 2 and 3. Of particular importance, identify areas that are poorly or excessively drained, rocky outcrops unsuitable for fruit trees, and other portions of your site that require special management. Then adjust your orchard design, if necessary, to take these areas into account.
Once you have corrected drainage problems, lay out your orchard blocks and roads according to the design you created in chapter 3 and modified after a close inspection of the site. For this process, you simply need stakes, brightly colored flagging tape, and a measuring tape. Bamboo plant stakes from the garden center and 1- or 2-inch-square tomato stakes 3 to 4 feet long work well.
You can purchase marking flags on plastic or metal stakes from Internet retailers. You will find that these are much easier to carry and insert into the ground than are wooden stakes. You might find it convenient to make several metal stakes from 1⁄4- or 5⁄16-inch diameter wire rod to hold the end of your measuring tape in place as you lay out and measure reference lines. These stout, metal stakes are different from the marking stakes that you leave in place to identify corners and reference lines. The rod should slip easily through the end grommet on your measuring tape. Bend the rod to form an L with the main shank about 24 inches long and the handle about 6 inches long, or weld a cross piece onto the main shank to form a T-shaped stake. Tie flagging tape to the tops of the rods to make them easy to see. Fiberglass measuring tapes 200 to 300 feet long are inexpensive and ideal for both measuring lines and creating accurate right angles.
Metal rods 1⁄4" to 5⁄16" in diameter are useful in holding measuring tapes when staking out planting blocks and tree rows. Brightly colored flagging tape makes the rods easy to see.
Mark buffer strips and roads. Fences and roads provide good reference lines for the edges of your orchard. If roads or fences are not available, stretch string or baling twine along the property line. Simply mark the width of the buffer strip, perhaps 30 feet, by placing a flagged stake at a right angle to your reference line. Place stakes every 50 to 100 feet around the entire perimeter of where your trees will be planted. If using a windbreak, be sure to allow room for it outside the buffer strip.
The buffer strips also serve as roads and headlands, so before going any further, test to make sure they are wide enough for your equipment. Try pulling your largest equipment from the orchard block into the headlands, turning around, and returning into the orchard block as if you were entering the next alleyway or every other alleyway. If you cannot easily make the turn, increase the width of the headlands. Cramped headlands waste time and cause damage to trees and equipment.
Create right angles for planting blocks. If your design includes right angles for the corners of the orchard blocks, you will need a reference line along the orchard block, a starting corner, three stout stakes, and a measuring tape 200 to 300 feet long. The idea is to create a triangle that has side lengths in a ratio of 3:4:5. This is easier to do than it sounds.
Place stake (A) at the corner of the planting block and on the reference line. Fasten the end of a 200-foot-long tape measure and the 120-foot mark of the tape to stake (A), forming a loop. Place stake (B) on the reference line and 30 feet from (A). Bring the tape to the outside of stake (B) and walk in the direction of (C) holding onto the 80-foot mark on the tape. When both legs of the tape from stakes (A) and (B) are tight, you have reached (C) and formed a 90-degree angle. Place stake (C) on your new reference line.
Place one stake (A) on the reference line where you want the corner and slip the end grommet of the measuring tape over the metal stake (see figure 4.3). Wrap the 120-foot mark on the tape around stake (A) and fasten the tape in place so that it cannot slip. Stake (A) forms the first corner of the triangle. Measure 30 feet along the reference line that you laid out earlier and place a second stake (B) at this point. Run the measuring tape on the outside of the stake. This is the second corner of the triangle. Holding onto the 80-foot mark on the measuring tape, walk at approximately right angles to your reference line from stake (A), stretching the tape until both segments from stakes (A) and (B) are tight. Place a stake at (C) to mark the line. You have now formed a triangle at a right angle to your reference line. The stakes serve as the corners and the measuring tape shows the sides of the triangle.
For greater accuracy, use a 300-foot-long tape and make the sides of the triangle 60, 80, and 100 feet long. With a 300-foot tape, place the end grommet and the 240-foot mark on the measuring tape at stake (A). Place stake (B) 60 feet along the reference line and run the measuring tape on the outside of the stake. Holding onto the 160-foot mark on the measuring tape, walk in the general direction of (C), as described above. When both segments of the tape from stakes (A) and (B) are tight, place stake (C).
Extend your right angle line for the width of the block by stretching your measuring tape or twine from the corner stake (A) and just touching the right angle corner stake (C). Repeat the procedure until all of the planting blocks and roads are marked.
Later, you will need to lay out the tree planting rows (see chapter 7). For now, it is enough to have the planting areas, buffer strips, roads, and headlands clearly marked.
All too often, organic growers apply soil conditioners and fertilizers blindly in the hope that something good will happen. Unless you know what you are starting with, however, adding materials to the soil is a gamble, at best, and can actually create problems in an orchard. Make the investment to have your soil tested before planting a new orchard or changing management practices in an established orchard. That small investment will reap large benefits.
Test your soil. If you are not already sure what your soil type is, have your soils tested for particle size density (texture or soil type). Also test for pH, liming requirement, organic matter concentrations, cation exchange capacity (CEC), nutrient concentrations, and salinity (see chapter 2 for specific tests that are needed). Remember that soils can change dramatically over very short distances. If necessary, divide your orchard into separate management areas based on topography and soils. The topography of your site often provides clues as to where different soils might be. Given a site with a ridge, level area, and low-lying area, you might consider testing the soils in each of those areas. Patches of different types of vegetation on your site may indicate corresponding areas of different soils.
Even in a small home orchard where the site is not uniform, establishing management zones is important. For example, an organic home orchard I visited covered less than an acre, but their planting sites ranged from a low-lying creek side with rather gravelly soils to a level area with silt loam to a low-lying, poorly drained meadow on silty-clay soil. Each of the areas required its own preparation, planting, and management strategies. If you have two or more different management zones in your orchard, have separate soil samples tested for each of the zones.
Strategize for amendments. Consider how you will apply amendments in your fruit planting. In berry and grape culture, growers often amend only the soil within the crop rows. For crops with small root systems, this practice works well, but for crops like fruit trees that develop extensive root systems that spread far from the trunks, it is not effective. Apple tree roots can spread over a diameter two to three times as far as a tree is tall and extend downward until they hit a permanent water table or impervious layer of subsoil. For that reason, orchardists usually amend all of the soil in their planting blocks, rather than only those soils within the tree rows. In a home orchard with a few trees, estimate the final height of the trees and draw circles around the planting spots and extending out 1 to 1.5 times that height. Apply amendments, as described below, throughout the entire circles. Figure 4.5 shows the above- and below-ground structure of a fruit tree.
Fruit tree roots can spread up to three times the width of the canopy and downward until they are stopped by a water table, impervious soil layer, or other unfavorable soil environments.
Unless you know what the soil pH, nutrient concentrations, and organic matter should be, even the best analytical results will mean little. Now is the time to begin developing standards. Soil standards are simply minimum benchmarks or ranges in pH, soil nutrient concentrations, and organic matter. We have similar standards for foliar nutrient concentrations, as we will discuss in chapter 8. These standards help you decide which amendments to apply and help you monitor changes in your orchard over time. The goal is to create and maintain a stable, pH-appropriate, nutrient-rich, biologically active soil.
A soil’s pH affects the availability of mineral nutrients to plants, so it’s important to get it right. Remember that a pH of 7.0 is neutral. Values above that are alkaline (basic) and those below 7.0 are acidic. A laboratory will provide the most accurate analysis, although there are several types of kits you can purchase to measure pH yourself. A handheld pH meter purchased from an agricultural or laboratory supply company will provide accurate results when used properly and can be a good investment for growers who wish to test soil pH frequently. You can also purchase one of several “color kits” for measuring soil pH and nutrient concentrations. Some are extremely accurate (and expensive!) while others provide poor accuracy. For serious fruit growers, it is best to send a sample to the laboratory or purchase a good-quality pH meter. Simple pH “meters” with metal probes are readily available in garden centers, but these often provide very poor accuracy and should be avoided.
As we discussed in chapter 2, aside from blueberries and a few other acid-loving crops, temperate zone fruit crops grow and produce best on soils that are slightly to moderately acidic. As the soil becomes more strongly acidic (lower pH), the macronutrients nitrogen, phosphorus, potassium, magnesium, calcium, and sulfur become less available to plants, as does the micronutrient molybdenum.
As soils become alkaline, the micronutrients boron, cobalt, copper, iron, manganese, and zinc become less available. At pH values above roughly 7.5, iron chlorosis often develops in apples, peaches, cherries, and other fruit tree crops. This disorder is characterized by yellow to white leaf blades with dark green veins. Unless corrected, affected trees become weakened and stunted with poor fruit production. Under severe conditions, trees may eventually die. Similar micronutrient deficiency disorders are sometimes seen with a lack of available boron, manganese, and zinc, even when the nutrients are relatively abundant in the soil. Figure 4.6 shows how soil pH affects nutrient availability to plants.
Most tree fruits tolerate pH values between 5.5 and 7.5. For a productive, healthy, long-lived orchard, a pH of 6.0 to 7.0 should produce excellent results. Bear in mind that the soil pH fluctuates during the year due to wetting and drying, microbial activity, and chemicals added to and removed from the soil by plants. In western Oregon orchards, for example, the soil pH typically fluctuates up to 0.5 pH unit and can fluctuate twice that amount on sandy soils. There, the highest soil pH is usually recorded in late winter. Soil drying, increased microbial and plant activity, and fertilizer applications cause the pH to drop during the growing season, reaching an annual low in late summer or early fall.
Soil pH affects the availability of mineral nutrients to plants. In this chart, the wider the bar, the more available that nutrient is to plants. As the bars become narrower, nutrient availability decreases. For pome and stone fruit crops, both macro- and micronutrients are readily available at pH 6–7.
For organic production, some form of limestone or dolomite is normally used to adjust soil pH.
Limestone. It is quite easy to raise pH on acidic soils by using limestone (calcium carbonate). If your soil test shows both low pH and low magnesium, apply dolomitic limestone (also known as dolomite), which is primarily calcium-magnesium carbonate. Do not use dolomitic liming products if your soil calcium is low (see table 4.6) unless soil tests show soil magnesium concentrations are also low. This is particularly important for apples. Magnesium can interfere with the uptake and utilization of calcium in fruit trees, increasing problems with bitter pit disorder in stored apples.
Wood ashes. Strongly alkaline, wood ashes can be used to raise soil pH while also supplying some nutrients, as shown in table 8.4. They are allowed in organic production, although they can be problematic. Use only ashes from untreated and unpainted wood. Ashes created from treated woods, colored paper, plastic, and other materials are prohibited under organic certification guidelines. Never use coal ash. Apply ashes sparingly because excessive amounts can cause pH and nutrient imbalances. Wood ash is highly alkaline and reacts very quickly in the soil to raise the soil pH.
Unfortunately, the liming equivalent of wood ashes varies greatly depending on the source and handling of the ashes. In general, wood ashes have a calcium carbonate equivalent (CCE) of 25 to 60 percent of calcium carbonate, and you need to add two to four times as much wood ash as limestone, by weight, to create the same change in soil pH.
Do not apply wood ashes to soils that are already at or above the desired pH, and do not add more than 20 pounds per 1,000 square feet per year. At this rate, you would be adding the equivalent of about 220 to 520 pounds of 100 percent calcium carbonate per acre per year.
Sugar beet lime. Also called spent lime, sugar beet lime can be used for raising the soil pH in some organic orchards (check with your certifying agency). This material is a by-product of sugar beet processing in which a suspension of finely ground calcium hydroxide (slaked lime or hydrated lime) is mixed with carbon dioxide and the sugar extracted from sugar beets. The lime does not come from the sugar beets themselves or any other plant material.
Sugar beet lime can be problematic. Its high water content increases transportation costs and makes uniform application difficult. The uneven application of lime in your orchard can create serious production problems. Unless you have a particular reason for using sugar beet lime, traditional liming materials will probably provide better results.
Hydrated lime is allowed for the preparation of Bordeaux fungicide and can be used in organic livestock production but is not allowed as a soil amendment.
Quicklime (burnt lime) is more potent than limestone. It is very reactive and is difficult and hazardous to handle, however, as well as damaging to plants. The U.S. National Organic Program makes no mention of quicklime, and it is best avoided.
Adding the liming material. The amount of liming material you should add depends on your soil’s pH, texture, and buffering capacity. The pH value alone tells you only whether your soil is acidic or alkaline — it will not tell you how much liming material to add. Your soil analyses should include a “liming requirement” test, which will tell you how much standard liming material to add to raise soil pH by a given amount. You will then need to calculate how much material to apply, based on that material’s CCE. Pure calcium carbonate has a CCE of 100 percent. Hydrated lime has a CCE of 135 percent. Most commonly available liming materials have CCE values less than 100 percent.
If the material you plan to use has a CCE of 50 percent, you would need to add twice as much as you would for pure calcium carbonate. In reality, determining how much material to apply is very easy once you receive your liming requirement results and know the CCE of your liming material. Your soil test report should tell you how much 100 percent CCE material to add.
Allow time for the liming materials to react in the soil and for the soil pH to stabilize. If you are making substantial changes to the soil pH, apply liming materials at least one full year before planting your trees. If the soil pH in an established orchard is 5.0 or below, raise it gradually by adding liming materials over 2 or 3 years to avoid damaging the trees and to allow the soil pH to stabilize. Be sure to monitor the soil pH at least once yearly before liming so as not to add too much liming material.
While raising soil pH is relatively easy and inexpensive, lowering soil pH can be both difficult and expensive, particularly on a large scale. In order to significantly lower pH in an organic orchard soil, you usually need to add elemental sulfur.
Lowering soil pH is more technically difficult than raising pH with liming materials. Liming materials undergo simple chemical reactions with the soil particles. Elemental sulfur, however, must be metabolized by soil bacteria and converted into the acidic SO4- ion, which then reacts with basic (alkaline) compounds in the soil. Although simple in theory, sulfur oxidation depends on which types and how many soil microorganisms are present, the soil type and moisture level, the carbon nutrient sources for the microbes, soil temperature, and more.
Note also that sulfates, such as calcium sulfate and magnesium sulfate, are salts formed by the reaction of SO4- ions and do not alter soil pH. This is because the SO4- has already reacted and is no longer acidic. In conventional farming, ammonium sulfate is sometimes used to lower soil pH. In these cases, it is the ammonium, not the sulfate, that reacts to acidify the soil. Ammonium sulfate is sometimes recommended for acidifying soils, but it is not allowed in organic production. Aluminum sulfate is sometimes recommended for acidifying soils, but it is not mentioned in the NOP. Although some aluminum sulfate products are marketed as “organic,” nearly all aluminum sulfate used commercially is manufactured synthetically. The naturally occurring form is rare and typically found in burning coal mining waste dumps and volcanoes. The improper use of aluminum sulfate can cause aluminum toxicity in sensitive plants. It should not be used in an organic orchard.
Elemental sulfur is a yellow solid and is usually sold as fine powders and small pellets. It is readily available at many garden centers and through farm supply stores. The fine powder is generally used as a fungicide to control diseases. Pellets are more commonly used to amend soil. Costs vary according to the form of the sulfur and the amount purchased. While various organic materials, such as pine needles and cottonseed meal, are somewhat acidifying, it is better to use organic acidifying amendments to help maintain a desirable pH once you have it within your desired range.
The approximate amount of elemental sulfur, in pounds, to add per acre to acidify the soil pH to 6.5.
Adapted from Cornell University and Oregon State University guidelines. Do not add more than 3,000 pounds of sulfur per acre per year.
Adding the sulfur. The best approach to lowering soil pH is to move slowly and conservatively. Deciding how much sulfur to add is often difficult because of the factors mentioned above and the strong influence of soil type and original soil pH. Table 4.1 provides estimates of how much elemental sulfur you will need to lower the pH of alkaline soils to between pH 6 and 7. The more finely ground the sulfur, the more quickly it will react in the soil to lower pH.
Avoid adding excessive sulfur and acidifying the soil too much. In no case should you add more than 3,000 pounds of elemental sulfur per acre at one time, regardless of the amounts listed in this or any other table. If you need to add more than 3,000 pounds of sulfur per acre, you should do so in stages over a period of several years. Allow time for the sulfur to be metabolized and for the soil pH to stabilize, processes that can take 2 years or more from the time you apply the sulfur. The acidification process can be hard on plants when making dramatic changes, and major adjustments to the soil pH are best done before planting your orchard crops.
For new orchards, apply the sulfur uniformly across your planting block at least a year before planting your trees and mix it thoroughly into the top 6 to 8 inches of soil. Remember that the soil must be moist, but not saturated, in order for the microorganisms to metabolize the sulfur. Six months after application, measure the soil pH from several locations across your planting block. Follow up with additional pH tests 12 months after applying the sulfur to see how the process is progressing. It is best to wait for 2 years from the first application before applying more sulfur.
For established orchards, broadcast the sulfur throughout the orchard and cultivate it into the soil no more than 2 inches deep to avoid damaging the tree roots. If the pH is 7.5 to 8.0, apply half of the needed sulfur one year and half the next year. If the pH is above 8.0, apply one-third of the needed sulfur one year and one-third during each of the two following years. Test the soil pH each year, and if more sulfur is needed, adjust the amount accordingly.
As we discussed in chapter 2, soil salinity, or soluble salts, refers to electrolytes, such as sodium chloride, that dissolve in the soil water. When soil salinity levels become excessive, plants take up less water and grow roots more slowly. The tips of leaves often die and become brown, sometimes referred to as “burning” or “scorching.” Shoots grow more slowly, and plants produce fewer and poorer-quality fruits. At still higher salt concentrations, most plants die. Tree fruits tolerate somewhat higher salinity than do most berry crops and should perform reasonably well with salinity levels of 2 to 4 millisiemens per centimeter (mS•cm-1), although lower levels are generally preferred (see page 33 for more information on salinity measures). If your orchard includes blueberries, raspberries, or strawberries, keep your soil salinity levels at 2 mS•cm-1 or less.
Provided that you have a sufficient amount of low-salt irrigation water available, you can sometimes reclaim an orchard site that has excessively salty soils by applying large amounts of water to leach the salts from the soil. You will need a sprinkler irrigation system that provides 100 percent coverage of the orchard blocks. Reclaiming a site this way works well when the excess salts accumulate because of applying too much fertilizer or manure, or if frequent, shallow irrigation concentrated the salts near the soil surface. Depending on calcium, sulfur, and magnesium levels (discussed below), you can help displace some of the unwanted salts from the soil particles by applying gypsum to the soil before leaching.
Unfortunately, some saline soils in arid regions of North America are associated with irrigation water that is also high in salts and, therefore, unsuitable for leaching purposes. On sites with extreme soil salinity, producing orchard crops can be difficult or impossible.
Soil tests provide a starting point for amending your orchard soils before planting trees (see tables 4.2 through 4.5 for suggested pre-plant soil concentrations and application rates for selected nutrients).
Our goal at this stage is to make sure ample supplies of nutrients will be available to the fruit trees, particularly those nutrients that move slowly in the soil. Once your trees are planted, it can be difficult to amend the soil with certain nutrients. On low-pH (strongly acidic) soils rich in aluminum and iron, for example, phosphorus is very immobile, and applying it after the trees are planted is rather ineffective. When phosphorus is applied to the surface of such soils, it tends to become tied up near the soil surface and remains unavailable to the plant roots. Calcium, potassium, and sometimes boron can also be relatively immobile in soil and are best applied before you plant your orchard. Most other amendments can be applied to the soil up to the time of planting the trees.
Bear in mind that soil tests do not necessarily show the actual amount of nutrients in the soil. Instead, they show the approximate amounts that should be available to plants, taking the soil pH and chemical forms of the nutrients into account. Nitrogen availability is shown as equivalent amounts of ammonium (NH4+) and nitrate (NO3-) nitrogen. Phosphorus, being chemically reactive, is found in many different chemical compounds in the soil and is defined as the equivalent of P2O5 (phosphorus pentoxide), regardless of the chemical forms actually present. Potassium is reported as equivalents of K2O (potassium oxide or potash).
Rates are in P2O5, based on soil tests. Soil test P can be determined by several procedures, three of which are shown here: sodium acetate (NaOAc), Bray I method, or by sodium bicarbonate (NaHCO3). Sodium bicarbonate should not be used on soils with pH values less than 6.2. Use the column indicated by your soil test report.
P × 2.29 = P2O5, or P2O5 × 0.44 = P, where P refers to the actual amount of pure phosphorus.
Adapted from University of Idaho guidelines.
Rates are in K2O, based on soil tests using sodium acetate as the extractant.
K × 1.20 = K2O or K2O × 0.83 = K, where K refers to the actual amount of pure potassium.
Application rates are in actual S, based on soil tests.
Adapted from University of Idaho guidelines
Broadcast boron-containing fertilizers throughout the planting site or orchard. Never band them in rows. Boron-containing sprays can be applied to the fruit crop foliage in established orchards. Boron becomes toxic to plants at very low concentrations. Do not exceed the rates in table 4.5. Rates are in actual B, based on soil tests.
Adapted from University of Idaho guidelines
Nitrogen is essential to plants and is the nutrient most often deficient in orchards. In conventional orchards, nitrogen is easy to add in the form of rapidly available industrial fertilizers, and it is seldom added before planting trees unless soil concentrations are very low. The situation for organic orchardists is less clear because many organic nitrogen fertilizers become available to plants relatively slowly. In some organic orchard trials, the establishment and growth of trees in organic plots treated with organic nitrogen fertilizers was very poor when compared with adjacent conventional treatments. The problem has likely been due to low amounts of available soil nitrogen and the inability of growers to rapidly increase available nitrogen to the newly planted trees using slow-release, organic materials.
Compounding the problem is that the nitrogen cycle in the soil is very complex, and nitrogen is constantly shifting from one chemical form to another as it moves through micro- and macroorganisms and plants. Rather than simply looking at nitrogen (NH4+ and NO3-) concentrations in a soil test report, a better guide to available nitrogen for a planned orchard is to look at the soil test results for organic matter. As organic matter decomposes in the soil, it adds nitrogen that is available to the plants.
Each 1 percent of soil organic matter adds up to about 20 pounds of plant-available nitrogen per year. Soil temperatures and moisture affect the actual amount of nitrogen released, which will be less in cold, wet soils. In reasonably well-drained soils, organic matter concentrations of 5 percent or more should provide sufficient available nitrogen for planting tree fruit crops. Additional nitrogen may be needed later, as the trees mature and begin bearing fruit.
Physical signs of low soil nitrogen are stunted and yellowish crop plants and weeds on the planting site. If crop and weed growth are lush and vigorous, and leaf colors are normally green, nitrogen deficiency should not be a problem for your newly planted trees.
Composts, manures, alfalfa pellets, alfalfa meal, soybean meal, feather meal, fish emulsions and meal, and cottonseed meal provide moderate to slow nitrogen release into the soil. These products, their mineral contents, and their uses in organic orchards are described in chapter 8. If you need a quick fix of nitrogen just before or at the time you plant your trees, nitrogen from dried blood (blood meal) (12 percent N) is available almost immediately. Blood meal must be used with care as it can damage plants when too much is applied.
Sodium nitrate (also known as Chilean nitrate) (16 percent N) provides nitrogen rapidly, but it is discouraged in organic crop production because of its high sodium content (26 percent Na), which can damage sensitive plants and contaminate groundwater. Certified organic growers should check with their certifying organization before using sodium nitrate, as some organizations limit sodium nitrate to no more than 20 percent of the total nitrogen applied, and some certifying organizations prohibit its use altogether.
Nitrogen-fixing green manure crops can help get your trees off to a good start. Plow them under the year before planting. Clovers, beans, and peas are typically used to fix nitrogen as green manure crops. Alfalfa establishes more slowly than these crops and, depending on your location, is best left in place for two or more growing seasons before tilling it into the soil. Inoculate your legume seeds with a Rhizobium preparation to ensure that your crops fix nitrogen. Check the inoculant label carefully to be sure that it is organically certified and not produced using genetically modified organisms.
Depending on the materials that went into its making, compost is also a good source of nitrogen and can be a cornerstone of nutrition programs in small orchards. Composting has the added benefit of recycling discarded fruit, prunings, and other waste products from the orchard.
Apply nitrogen fertilizers directly to the soil. Although some people advocate “foliar feeding” by spraying fertilizers onto the leaves of crop plants, nitrogen and other macronutrients are generally not taken up by foliage in amounts large enough to benefit the plants. Foliar feeding can be used effectively for some micronutrients.
If the level of organic matter in your soil is below about 4 to 5 percent and nitrogen concentrations are also low, as shown in a soil test, you may want to begin incorporating slow-release forms of nitrogen before planting your trees. Your soil test should recommend suggested amounts of nitrogen to add. If you are using slow-release forms of nitrogen, such as compost, add the amendment at least by the summer or fall before planting your trees. For fertilizers that release nitrogen more rapidly, such as blood meal, apply one-half of the needed amount 1 to 4 months before planting the orchard. Apply the other half shortly after planting your trees.
Be careful not to apply too much nitrogen before or after planting. In young trees, excessive nitrogen can cause too much shoot growth at the expense of root growth. In established orchards, excessive nitrogen causes poor fruit set; soft, poorly colored fruit; and excessive amounts of lush, pest- and disease-prone foliage and shoots that require much labor to manage. Remember that organic soil systems move more slowly than those in conventional orchards. If you have incorporated a green manure crop or applied composts or manures, the amount of nitrogen available to the plants will increase for several years after each application. Unlike a conventional orchard where soil nutrient concentrations can be changed quickly, we want to change organic soils gradually, nudging them into the desired condition and not overshooting the mark.
Phosphorus is rather a puzzle when it comes to orchard crops. Although it is required by plants and soils can be deficient in available phosphorus, we seldom see shoot growth or fruit yield increases following phosphorus fertilization of fruit trees.
Rock phosphate, also called hard rock phosphate, is mined from ancient ocean deposits and contains only 1 to 2 percent available phosphate in a form that is available very slowly to most plants, particularly in alkaline soils. It typically becomes available over a period of years to centuries. From economic and horticultural perspectives, this is an inefficient and expensive form of phosphorus.
Colloidal phosphate, also known as soft rock phosphate, consists of clay particles surrounded by naturally derived phosphorus. Although it contains around 20 percent phosphorus, only about 2 to 3 percent is available phosphate. Its phosphorus is also available to plants slowly, but it is more readily available than the phosphorus from hard rock phosphate.
Bonemeal is a rich source of phosphorus that is readily available to plants. Use steamed bonemeal to reduce the risk of pathogens that can infect humans. Other organic sources of phosphorus are available, although most have low concentrations of phosphorus.
Wood ashes can be added to supply phosphorus if potassium is low and the soil pH is below 6.5 (see page 92).
Dried blood, fish meal, shrimp waste, and oyster shell products also provide phosphorus, but usually at low rates of available phosphate. The downside of commercial organic phosphorus fertilizers is their cost, which can be prohibitive for large orchards.
Another effective use of colloidal phosphate and rock phosphate is to include them with animal manures during the composting process. Acids in the manures dissolve the available phosphate, which then helps to stabilize the nitrogen in the manures.
Phosphorus is relatively immobile in soils under most conditions and is best applied before planting your trees. Unfortunately, phosphorus can react with other minerals in the soil and become unavailable to plants. On moderately to strongly acidic soils, phosphorus combines with aluminum and iron. Under alkaline conditions, phosphorus reacts with calcium to become unavailable. For instance, when applied directly to the orchard floor, rock phosphates are most available and effective on acidic soils and remain generally unavailable on neutral to alkaline soils. A key way to effectively manage phosphorus is to keep your soil pH between 6.0 and 7.0 and, ideally, 6.5 to 7.0.
Base your phosphorus applications on “available phosphate” or P2O5. Many materials contain high concentrations of phosphorus, but they are in forms that plants cannot use. The given amount of phosphorus in all fertilizers refers to the amount of available phosphate that is released from fertilizer materials in a weakly acidic solution. This solution mimics chemicals that are released by roots to make certain mineral nutrients more available and assist in their uptake.
Because of its tendency to become immobilized in the soil, you may find it beneficial to band phosphorus materials along your tree rows, rather than broadcasting them throughout the orchard. Do not band materials that contain boron, nitrogen, or potassium.
To make phosphorus available more quickly to your fruit trees, add the phosphate materials to the soil 1 to 2 years before planting your trees and then grow one or more green manure crops of buckwheat. Cornell University reports that buckwheat is very effective at using these sources of phosphorus. When the buckwheat is tilled into the soil, the phosphorus in its tissues becomes available to other plants. Be sure to till under the buckwheat before it sets seeds because it can become a weed problem.
Potassium is abundant in many North American soils and often does not need to be added before planting an orchard. Check your soil analysis results to see if potassium is deficient before adding this nutrient to your planting site.
Granite dust contains about 5 percent potassium, but little of it is available for plant growth, and this material has little value for organic fruit growers. If used, the potassium is most rapidly available on acidic soils. Granite dust can contain silica, known to cause lung cancer. If you choose to apply granite dust, use respiratory protection during all handling and application activities.
Greensand is a naturally occurring sandstone found worldwide, often in association with other marine deposits of chalk and clay. It contains glauconite, a greenish-colored iron potassium silicate compound. Greensand typically contains about 6 percent K2O and 1 to 2 percent P2O5. Both are very slowly available to plants, and greensand is best applied to an orchard during preplant preparation. Greensand is more effective on sandy soils than on heavier-textured soils.
Black mica, also known as biotite, refers to a class of micas, rather than to a specific chemical formula. These micas contain variable amounts of potassium, usually several percent, in a form that is quite available to plants in biologically active soils. If you need to add potassium and have ready access to inexpensive sources of biotite, it can be an effective soil amendment. Unfortunately, biotite fertilizers can be hard to find. Also, at least one European study found that one source of biotite included barium and strontium that could be taken up by horticultural crops in sufficient concentrations to possibly be toxic to humans.
More effective and cost-effective sources of potassium include the following: Sulfate of potash-magnesia (sul-po-mag or langbeinite) is a rich source of potassium that also supplies sulfur and magnesium. Wood ashes are rich in potassium and, when used cautiously (see page 92), can be beneficial, especially on acidic soils. Do not use wood ashes if the soil pH is 6.5 or above.
Potassium sulfate is a good source of rapidly available potassium, but ensure that your source meets organic certification standards. Some organic certifying organizations prohibit it altogether. One form of potassium sulfate fertilizer is created industrially using sulfuric acid and is not suitable for organic growers. Natural potassium sulfate is mined or extracted through evaporation of water from saline lakes.
Potassium chloride contains 60 percent potash that is rapidly available to plants. It also contains a high amount of chloride, so it’s not a good idea to use it frequently or in large doses. The U.S. National Organic Program allows the use of potassium chloride only if it is “derived from a mined source and applied in a manner that minimizes chloride accumulation in the soil.” Before applying potassium chloride, check with your organic certifying organization to ensure you can use this material.
Potassium is relatively immobile on heavy-textured soils and, in such cases, is best broadcast throughout the orchard before planting your trees. On sandy soils, potassium can be highly mobile and is best broadcast shortly before or at the time of planting. On loams and heavier soils, apply the potassium 1 or 2 years before planting fruit trees. Base your potassium applications on “available potash” or K2O. As with phosphorus, many materials contain high concentrations of potassium, but not necessarily in forms that are available to plants.
Be cautious not to add too much potassium, as it can interfere with calcium uptake and metabolism in fruit trees. The problem is especially serious with apples, where excessive amounts of potassium may increase bitter pit problems in stored fruit.
Sulfur is seldom deficient in soils to the extent that it is necessary to apply sulfur amendments before planting. Deficiencies are most likely to occur on sandy soils that are low in organic matter, which allows the sulfur to leach from the root zone. If your soil test shows less than 10 ppm sulfur and your soil pH is 6.5 or below, add about 175 pounds of gypsum per acre any time before planting. Repeat annually, as necessary, until soil tests show 10 ppm or more sulfur. If your soil pH is above 7.0, add elemental sulfur a year before planting your trees. Table 4.4 shows the amount of elemental sulfur needed, depending on soil pH and soil type.
Calcium is seldom deficient in soils, particularly in arid regions. It is most likely to be deficient in sandy, acidic soils. The amount of calcium available to crops, however, depends not only on the amount of calcium present in the soil, but also on the cation exchange capacity of the soil (CEC), which is reported in milliequivalents/100 grams of soil. CEC refers to the ability of negatively charged clay particles and humus in the soil to attract and hold positively charged cations, including calcium, which has two positive charges.
The higher the CEC, the greater the soil’s ability to bind to cations. Likewise, soils with low CEC values have less binding ability. This is important because nearly all plant mineral nutrients are positively charged, so as the CEC increases, so does the soil’s ability to bind to and store these nutrients. Nitrogen, being negatively charged, does not bind to the negatively charged soil particles. This is one of the reasons that nitrogen is easily lost from the soil and why it is the nutrient most likely to be deficient in soils.
Soils with low CEC values quickly saturate with cations and have high nutrient ratings, even when soil tests show low concentrations of those nutrients. This apparent contradiction arises because low-CEC soils have relatively few spaces for the nutrient cations to fill. On a low-CEC soil, all of the sites may be filled with calcium, leading to a high calcium saturation rating, even though the actual amount of calcium present in the soil is low. Soils with high CEC values require larger concentrations of calcium to bind to their available sites, and calcium ratings can be low, even when abundant calcium is present in the soil. Table 4.6 gives calcium ratings based on CEC and calcium soil test results.
If your soil’s calcium rating is low and the pH is 6.5 or above, add gypsum as a soil amendment up to the time of planting (see the sulfur section above). Consult with your soil testing lab for recommended amounts of gypsum to add.
If the soil pH is less than 6.5 and soil magnesium levels are adequate, add limestone, preferably several months or more before planting your trees. If the pH is less than 6.5 and magnesium is also deficient, add dolomitic limestone. Remember that pH is an important factor in calcium management. Keep your soil between pH 6.0 and 7.0 and, ideally, around 6.5. Use the amounts of limestone or dolomite needed to adjust your soil pH to 6.5.
Soil calcium ratings as determined by soil calcium concentrations and cation exchange capacity (CEC). To determine your soil’s calcium rating (low, medium, or high), first locate the soil CEC value in the top row, as shown in your soil test. Then drop down that column to find your soil calcium concentration, also shown on your soil test. Read off your calcium rating from the left side of the chart. For example, a soil with a CEC value of 15 meq and a calcium concentration of 4,500 ppm would have a calcium rating of “medium.”
Adapted from Soil Test Interpretations and Recommendation Guide: Commercial Fruits, Vegetables and Turf, 1999, University of Missouri.
Magnesium deficiency is occasionally a problem in orchards and is known to cause leaf chlorosis in cherries and peaches. Deficiency problems are most likely to occur when soil pH is too low or soil calcium concentrations are too high. If your soil test shows less than 100 ppm magnesium, you should amend the soil with magnesium before planting your trees.
Epsom salts (magnesium sulfate) are an easy-to-use amendment. If sulfur and potassium are also low, sul-po-mag can be applied to add all three nutrients. Your soil test should give you suggested application rates for magnesium. Use the magnesium percentages in table 8.4 to determine how much of a particular fertilizer to add.
If both calcium and magnesium concentrations in the soil are low, dolomitic limestone can supply both nutrients while raising the soil pH. Use the amount of liming material recommended in your soil analyses. Do not apply dolomitic limestone if the pH is 6.5 or above, or when soil calcium concentrations are above 5,000 ppm.
Boron can be difficult for orchardists to work with because the trees need the nutrient only in very small quantities. Unlike most other nutrients, however, the adequate range of boron concentration within the tissues is very small, and boron becomes toxic at extremely low concentrations. If your soil test shows less than 0.5 ppm of boron, add 1 to 2 pounds of actual boron per acre.
Broadcast the boron amendment throughout the entire planting block. Never apply boron-containing materials to the soil in bands. According to the Organic Materials Research Institute, boric acid, hydrated forms of sodium tetraborate, sodium borate derivatives, disodium octaborate and its hydrated forms, and hydrated forms of colemanite may be used to supply boron but should be applied to the soil or plants only when you have a documented deficiency, as shown in soil or foliar analyses. The OMRI website lists recommended boron-containing amendments.
Organic amendments can improve water-holding capacity on droughty soils, improve tilth, and help you get a start on building concentrations of soil organic matter.
The best materials to add are those that have already been composted, for several reasons: Provided you have done the composting correctly, the process generates enough heat to kill many weed seeds, pathogens, and pests, while still leaving beneficial microorganisms. Raw organic materials can be sources of diseases, pests, and weeds. Properly composted materials are also more chemically stable and predictable than fresh materials. If you use animal manures in your compost, the process allows excess salts to leach out that might otherwise damage your crop.
And very importantly, good compost has a carbon-nitrogen balance that is suitable for plant growth — usually around 10 parts of carbon for each part of nitrogen. Green plant materials, such as green manure crops, are typically 12:1 to 17:1. Cattle and horse manures have ratios around 18:1. These materials can be added directly to soils with no fear of creating nitrogen deficiencies in your crops.
Materials that are rich in cellulose, such as straw (80:1) or sawdust (400:1), have high carbon-to-nitrogen ratios. When these materials are incorporated directly into the soil, microorganisms begin decomposing the carbon compounds. The microorganisms, however, require large amounts of nitrogen as they form structural and enzymatic proteins. Since the woody material has little nitrogen, the microorganisms take the nitrogen from the soil itself, which can create temporary deficits in available nitrogen. Once the microorganisms have completed decomposing the organic material, they die and release the nitrogen back into the soil. Until that nitrogen is released, however, your crops can be deprived of adequate nitrogen and suffer stunting or even death.
In general, increasing soil organic matter concentrations by adding off-farm amendments is too expensive in terms of labor and transportation costs for any but small orchards. In some areas, community waste recycling programs supply mulch from tree trimmings and yard wastes. While the recycling is commendable and the materials are generally available in large quantities at low cost, beware! Such mulches have been found to be contaminated with pesticides from roadside and home herbicide and pesticide applications. Determining whether a municipal source of mulch or compost is pesticide-free may be impossible.
If you choose to incorporate uncomposted woody materials into your soil, add nitrogen at the same time. For every cubic yard of bark, hardwood sawdust, or hardwood wood chips, add 4 to 5 pounds of actual nitrogen the first year and half that amount the following year. For straw and softwood sawdust or chips, apply 2 to 2.5 pounds of nitrogen the first year and half that amount the following year. In this case, if you were adding one cubic yard of hardwood sawdust and wanted to provide the nitrogen in the form of dry, composted cattle manure (approximately 0.5 percent nitrogen), you would need to add about 800 pounds of manure. Leaving uncomposted woody materials on the soil surface as mulches usually does not deplete soil nitrogen, and supplemental nitrogen often does not need to be applied.
Adding uncomposted woody materials as a soil amendment can require large quantities of the amendment and nitrogen fertilizer. Time is another factor. If you incorporate uncomposted woody materials into the soil, it is best to wait at least one full year (preferably two or more) before planting trees, even when you apply extra nitrogen. From these standpoints, amending soil with sawdust and similar materials is seldom feasible except for the smallest orchards.
An effective way to add organic matter to the soils of orchards large and small is to grow one or more green manure crops and cultivate them into the soil before planting fruit trees. Although they require time to produce, green manure crops cost very little in terms of money and labor compared with purchasing, hauling, and incorporating sawdust or similar materials. Some green manure crops add nitrogen to the soil, and none require extra nitrogen in order to decompose.
Depending on where you live, many good green manure crops are suitable for preparing an orchard site. Look for crops that grow quickly, form dense stands that quickly cover the soil surface, produce large amounts of organic matter, out-compete most weeds, are easy to grow, have few pest and disease problems, and are easy to incorporate into the soil. The goals are to shade or crowd out weeds while producing large amounts of organic matter that will break down in the soil without causing nitrogen depletion.
Buckwheat, barley, clover, vetch, oats, beans, peas, and rapeseed are popular green manure crops. Avoid sod-forming grasses (hard to kill and incorporate) and row crops that leave bare areas of soil (poor weed competitors). In milder climates, you can often grow two or three green manure crops in a single season.
Green manure crops have other important advantages over other forms of organic amendments: Some crops serve as natural soil fumigants and can be valuable in reducing pest and disease problems. Particularly valuable are Brassica varieties (canola, mustard, or rapeseed) that have been selected for high concentrations of biofumigant compounds called glucosinolates. When glucosinolates come into contact with a group of plant enzymes called myrosinase during mowing or cultivating, the glucosinolates break down to form isothiocyanates, nitriles, epithionitriles, and thiocyanates. Isothiocyanates are the most common and abundant by-product and are considered highly toxic to nematodes and valuable as general biocides. The isothiocyanates are relatively volatile and remain in the soil for a few days to a few weeks.
Not all Brassica varieties are equally effective biofumigants. Canola varieties for human consumption have had most of the glucosinolates bred out of them. Two biofumigant varieties rich in glucosinolates, however, were developed at the University of Idaho: ‘Humus’ rapeseed and ‘Ida Gold’ mustard. If you are using open-pollinated seed, rather than named varieties, brown mustard appears to be a somewhat more effective biofumigant than white mustard or rapeseed.
Mustard, canola, and rapeseed crops produce seeds quickly and in enormous quantities. Be sure to till the green manure crops into the soil before they set mature seeds. Shredding the crop with a flail mower immediately before rototilling it into the soil maximizes its biofumigant value.
Instead of growing the Brassica plants, you can use cold-pressed meal from crushed seed. The biofumigant properties of the seed vary greatly, however, depending on the type and variety of the mustard, canola, or rapeseed used. While progress is being made in seed meal biofumigation, results often leave much to be desired, and it is best suited for small orchard applications.
Mycorrhizae are soil fungi that form beneficial relationships with plants. The name literally means “fungus root,” and these organisms are extremely valuable in helping plants take up moisture and nutrients by effectively extending the plants’ root systems. These organisms can also, in some cases, help protect plants against pathogens and parasitic nematodes by competing with other microorganisms for space on plant roots and by forming a physical barrier to nematodes. There are many different mycorrhizal species. Some are specific to particular plant species or types, while others are generalists. Some mycorrhizae are adapted to acid soils, others to neutral or alkaline soils.
In a healthy soil, native mycorrhizae should already be present and do not need to be added as a soil amendment. When native mycorrhizae have been killed or damaged by fumigation, overuse of inorganic fertilizers, or other causes, it can be beneficial to inoculate an orchard soil at the time of planting. For example, you might want to place mycorrhizae in bands in your tree rows immediately before planting if you have grown biofumigant crops. Blends of several mycorrhizal species are generally more effective than applying a single species. You might also choose to dip the roots of bare root trees in a slurry of mycorrhizae at the time of planting. Be sure that the product you choose has been approved for use in organic production. Chapter 7 goes into detail on different mycorrhizal species and application methods for these products at the time of planting.
Adding algae and other microbes to the soil is seldom beneficial to organic fruit production. Despite producers’ claims that their microbes fix nitrogen or somehow make nutrients more available to crops, the reality is that few of these products create significant benefits under the conditions that we want to maintain in our orchards. Nitrogen-fixing bacteria are very important in rice production, for example, but few of us would want to grow fruit trees in the wet, oxygen-poor environments needed by these microorganisms. An exception would be Rhizobium radiobacter strains that help protect roots against crown gall when used as root dips at the time of planting.
Soil amendments can be beneficial when used for specific purposes, such as to correct a nutrient deficiency. They are not, however, a substitute for proper site selection, preparation, and maintenance. Far more important are correcting soil drainage problems, adjusting soil pH, developing and maintaining reasonable organic matter concentrations, utilizing green manure and cover crops, and creating moist, well-drained, biologically active soils. Until you have accomplished those goals, soil amendments are unlikely to be of great benefit.
Although some organic farmers and gardeners are convinced of the value of various “soil-building” amendments, the reality is that many of these products do little to improve an organic orchard. These products are typically very expensive for the small benefits that they provide.
Besides being ineffective, some products contain prohibited substances that can cost you your organic certification. Before applying any amendments, certified organic growers should be sure that those amendments meet their certifying agency’s guidelines. For commercial products, purchase only those that you know, by brand name, are approved for organic production. If they are not specifically approved by OMRI or another recognized organic certification organization, avoid them.
There are a few exceptions, including the phosphorus and potassium amendments we discussed earlier. While rock dusts and humate products have little to offer orchardists, zeolites can be beneficial in cropping systems. According to the U.S. Geological Survey (USGS), “Zeolites are porous minerals with high cation-exchange capacity that can help control the release of plant nutrients in agricultural systems. Zeolites can free soluble plant nutrients already in soil, and may improve soil fertility and water retention. Because zeolites are common, these unique minerals could be useful on a large-scale in agriculture.” Zeolites are most useful on soils with low CEC and act rather like sponges, holding and releasing plant mineral nutrients. In chapter 8, we will go into more detail on other plant nutrient sources and programs to use after you have planted your crops.
At this point, we have corrected soil drainage problems, laid out planting blocks and other operating areas of the orchard, adjusted the soil pH, and amended the soil (according to soil tests!) to ensure adequate nutrition for our orchard crops. The next step is to address pre-plant weed control. Eliminating the most serious weeds or bringing the populations under control is a key step in establishing a healthy, productive orchard.
Until the late 1990s, we generally referred to managing vegetation on the orchard floor as weed control or weed management. Prior to the 1950s, trees were planted far apart in orchards to allow cross-cultivation and keep orchard floors bare of vegetation. As a result, the soil of these orchards was often heavily eroded and compacted, and it was difficult to access trees in wet weather. As preemergent and selective herbicides became available beginning in the 1950s, many orchardists began maintaining grass in alleyways and eliminating all grasses and broadleaf weeds in the tree rows. This remains the standard practice in conventional orchards today. Other benefits are reduced erosion and soil compaction, better access during wet weather, more soil organic matter, and a larger habitat for beneficial organisms.
As commercial organic orchards increased in size and number, it became more important to find cost-effective weed control methods. By studying the problem and testing different weed control strategies, we began to discover advantages in maintaining rather diverse vegetation on orchard floors. While it is relatively easy to manage alleyways in both organic and conventional systems, it is much more difficult and expensive for commercial organic growers to manage vegetation within tree rows.
Planting fruit trees into dense vegetation of any kind generally causes poor tree survival rates, and those trees that live are often stunted and unproductive. For that reason, it is important to eliminate, to the greatest degree possible, competitive plant species before planting your trees. Serious orchard weeds vary across the country, but they typically include aggressive perennial species such as quack grass, Bermuda grass, johnsongrass, nutsedge, bindweed, and Canada thistles.
For many organic growers, bringing weeds under control before planting is the most difficult step in developing an orchard and requires varied cultural practices. Serious infestations of perennial weeds are hardest to control. Unfortunately, organic herbicides, including soaps, essential oils, and vinegar, are largely ineffective in controlling established perennial weeds. The same limitation applies to thermal weed control methods, such as flaming, infrared, and infrared plus steam. Only plant tissues directly contacted by the herbicides or heat are killed, and perennial weeds quickly resprout from underground organs. Thermal weeding and organic herbicides can be helpful in controlling annual weeds and young seedlings of perennial weeds.
This method is more effective for annual weeds than perennial weeds. With aggressive perennial weeds, particularly those that spread by rhizomes, one or a few passes of a tiller or disk are more likely to increase the weed problem than to reduce it.
If your soil is deep enough and the weeds are confined to the top 4 inches of soil, try deep plowing with an inverting plow before adding the soil amendments we discussed earlier in this chapter. The goal is to bury the seeds and rhizomes so deeply that they cannot resprout. A typical moldboard plow may not bury the weeds and seeds deeply enough. California fruit specialists recommend using a Kverneland rollover plow for this operation. When farming on shallow soils, be careful not to turn your productive topsoil under and leave unproductive subsoil on the surface.
Repeatedly tilling or disking bare fallow ground for one or two growing seasons can also help destroy rhizomes, roots, and tubers from which perennial weeds resprout. Shallow disking or harrowing is less likely to create a hardpan or damage soil structure than is rototilling. Cultivate the field as early in spring as you can without causing the soil to compact. As soon as the field begins to green up, cultivate it again to kill the young sprouts and seedlings and gradually deplete the food reserves in the underground tissues. To reduce the adverse effects of leaving the soil bare for long periods of time, you can plant a biofumigant green manure crop in early spring and till it in before the seeds mature. Keep the soil bare using cultivation during the summer, and replant the biofumigant crop in fall as a late green manure crop. Repeat this process for 1 or 2 years.
If you choose to keep your orchard land bare using repeated cultivation to achieve weed control, follow up with 1 or 2 years of biofumigant and green manure crops. Use crops that form dense stands that shade out and out-compete weed seedlings. In warmer areas with prolonged growing seasons, you should be able to establish and till in at least two green manure crops in a single growing season. Be sure to till these crops under before they set seeds and become weeds themselves. Avoid using sod-forming crops, such as turf grasses.
University of California orchard specialists recommend solarization as one means of reducing weed populations in the crop rows before planting trees. To solarize the soil, stretch one, or preferably two, layers of clear, 2 to 4 mil plastic film over bare, moist soil and cover the edges of the plastic with soil. Make sure that the soil is smooth, level, and free from clods and surface debris. The plastic should be as close to the soil as possible. Under ideal conditions, the top 2 inches of soil will be heated to around 140°F (60°C) and soil 18 inches deep will be as hot as 102°F (39°C). These temperatures are high enough to kill many seeds and pathogens. Nematodes in the top several inches of soil may also be killed. The films may need to be left in place an entire summer.
Solarization works well in parts of California and other areas with hot summers and clear weather, but in cooler or cloudier areas, it has proven to be ineffective. In cooler climates, the plastic films serve as greenhouses, producing lush weed growth under the plastic. Solarization also creates a disposal problem for the plastic film, which is usually derived from a nonrenewable resource. Except for very small-scale growers in hot climates, solarization is not recommended.
At the risk of offending some readers, there is another strategy for bringing serious weed problems under control before establishing orchard trees. For sites heavily infested with hard-to-control perennial weeds, some orchardists elect to use synthetic, translocatable herbicides to kill the weeds during their transition to an organic orchard. While I do not promote this practice for organic fruit growers, it is an option for reclaiming a badly maintained site and converting it to a productive organic orchard.
Glyphosate, also known as Roundup and sold under a variety of brand names, is often the herbicide of choice because it moves from the leaves and stems of weeds into the roots and rhizomes, killing the weeds and preventing resprouting. Translocatable herbicides are often used as part of a season-long bare ground fallowing, followed by biofumigant and green manure crops, as we discussed above. The herbicides can be broadcast-sprayed throughout an entire field or spot-sprayed on weed patches.
Most organic certification programs require a wait of at least 3 years after the last application of a prohibited substance before that site can be certified organic. When used early in the site preparation process, herbicides provide a way to rapidly and economically control weeds, and organic certification is possible by the time the trees bear their first or second crops.
At the same time you begin your preplant vegetation management program, also begin controlling gophers, voles, and mice. We will discuss rodent control in more detail in chapter 11. For now, remove debris from the site, including piles of wood, boards, and anything these pests can hide under. Use repeated, shallow cultivation to destroy burrows and nesting sites. Keep vegetation around your planting site mowed very short to reduce cover. Especially for gophers, it is generally more effective to trap the animals than to poison them, and it poses little risk to pets and other off-target animals.
If you have birds of prey (raptors) that feed on small mammals in your area, install perches for them around the planting blocks. Owls and hawks are quite effective in hunting rodents.
Planting the biofumigant green manure crops we discussed earlier can help reduce nematode pest populations. Removing host plants from hedgerows and other non-crop areas on your site will help reduce pest and disease problems after planting your trees. Chapters 10 and 11 describe what plants serve as hosts of pests and diseases and what to remove from your property.
Congratulations! At this point, you are well on your way. By investing the time and effort to select a good location or evaluate your present site, create a great orchard design, and prepare or upgrade a planting site, you have given yourself excellent prospects for a healthy and productive organic orchard. Our next step will be what many growers consider one of the most enjoyable parts of fruit growing — selecting crops and varieties.