Table 8.2
Suggested Nutrients in Pome Fruits
Adapted from Leaf Analysis for Fruit Trees. Rutgers NJAES Cooperative Extension.
Nutrient management is critically important for all orchards and is an area on which many organic fruit growers concentrate. In the early days of conventional agriculture, when an abundance of cheap nitrogen fertilizers became available, this chapter would have been titled “Fertilization.” Fields long managed by organic methods or virgin lands were often already rich in macro- and micronutrients, and little was required to produce a fruit crop except to dump commercially available, nitrogen-rich fertilizers into the field. These concentrated forms of plant nutrients eliminated the need to transport and apply large quantities of fertilizers or to grow green manure crops, and they fit perfectly into the concept of modern horticulture at that time.
As our knowledge of soil environments and plant-soil interactions has grown, the concept of fertilization has also grown. We now recognize the soil as far more than an inert sponge that holds up the trees and provides a place for the roots. Soil ecosystems are incredibly complicated and diverse, and the health of those ecosystems greatly influences the health of the orchard crops. Soil ecosystems are also dynamic, changing with the seasons and from one year to the next. Every change you make in the orchard and in your management practices affects the soil and the macro- and microorganisms that dwell there, in turn affecting your fruit trees and bushes.
Unlike fertilization, which often has a quick or short-term effect, nutrient management requires a long-term approach, particularly in an organic system. Preplant applications of colloidal phosphorus, for example, will provide phosphorus to the fruit crops for decades. The green manure crop that you till in this season will supply nitrogen and micronutrients to your crops for years. Our goal is to guide the nutrient status of the soil so that the crop plants always have adequate, but not excessive, readily available supplies of all required nutrients. In some cases, quick fixes will still be required, even in the best organic orchard. That should be the exception, however, not the rule.
At the very beginning, a caution is needed: Having been a professional fruit specialist for 30 years, I have found nutrient deficiencies in market and home orchards to be quite rare and problems associated with excessive fertilization quite common. The old adage that “if a little is good, more is better” does not apply to fruit growing. Quite the contrary is true! The abundant, lush foliage created by excessive applications of nitrogen is generally accompanied by many problems that are described below. Adequate but not excessive is the rule.
A second caution: As the size of an organic orchard grows, so does the difficulty and cost of supplying adequate nitrogen in the form of fertilizers. Organic fertilizers typically have low concentrations of nitrogen, compared with industrial fertilizers, and are required in much greater quantities. Composts, manures, alfalfa pellets, alfalfa meal, soybean meal, feather meal, fish emulsions and meal, and cottonseed meal are all fine organic sources of nitrogen. They tend, however, to be expensive to purchase and ship in the large amounts needed for a big commercial orchard. Along with pest and disease management, supplying adequate nitrogen is one of the greatest challenges in operating a large organic orchard. As in the days before industrial fertilizers, fruit growers near livestock operations or other sources of inexpensive, nitrogen-rich materials have a distinct advantage over their more distant counterparts.
Newly established organic orchards and orchards transitioning from conventional to organic systems are most likely to suffer nutrient deficiencies. The problem is compounded by weed competition, which can greatly impair the survival and growth of young trees. These facts emphasize the need for careful and thorough preplant soil testing, applications of soil amendments, corrections of soil pH, and weed management.
As we will discuss in chapter 9, new variations on old cultural practices allow us to supply much of the needed nitrogen on-site using nitrogen -fixing cover crops. While we still have much to learn in selecting and managing cover crops, orchard floor management has great potential to reduce the need for off-site nitrogen, while enhancing and maintaining soil structure and biological activity.
Plants take chemicals called “essential elements” from their environments to create living tissues. We refer to them as essential because plants must have them in order to complete their life cycles. Sixteen elements are generally considered necessary for normal plant growth and development. Among them are carbon, hydrogen, and oxygen, which come from water and air and combine to form cellulose, starches, sugars, and other carbohydrates that make up the bulk of plant tissues. The remaining 13 essential elements (mineral nutrients) come from the soil and are divided into “macronutrients” and “micronutrients.” Although both macro- and micronutrients are equally important, plants need larger amounts of macronutrients, which are normally measured as percentages, by weight, of plant tissues. Micronutrients are needed in tiny amounts, ranging from about 5 to 400 parts of micronutrients per million parts of plant tissues (ppm). Tables 8.2 and 8.3 list macro- and micronutrients, along with their recommended ranges for leaf tissues in orchard crops.
Nitrogen. The nutrient most often deficient in orchards is nitrogen, for three reasons: First, nitrogen is needed in greater amounts than any other mineral nutrient. Second, nitrogen is not derived from the mineral portion of the soil, coming instead from the atmosphere by way of nitrogen-fixing plants and microorganisms. Third, nitrogen is negatively charged, as are soil and organic matter particles. Rather than binding to these particles like Ca++ or K+, nitrogen is repelled and washes from the root zone or volatizes into the atmosphere.
Although nitrogen is essential and needed in large amounts, too much nitrogen forces excessive, lush growth that is highly susceptible to damage from pests and diseases and creates dense canopies that reduce air movement, further increasing disease problems. Excess nitrogen also delays fruit crop bearing; reduces flower bud and fruit formation; results in large, soft, poorly colored fruits; and creates excessive, erect shoots, thereby increasing pruning requirements and complicating training.
In orchards that are transitioning to organic and in newly planted organic orchards, nitrogen deficiency can be severe and often limits tree survival and health. Proper site preparation, combined with careful orchard floor management and supplemental nitrogen fertilizers, should prevent most nitrogen deficiencies from developing.
Research has demonstrated that organic apple orchards with low, although acceptable, amounts of nitrogen have better crops and healthier trees than those orchards with high, although still acceptable, amounts of nitrogen. The amount of nitrogen needed depends on the crop and variety. In apples, for example, ‘Stayman’, ‘Turley’, ‘McIntosh’, ‘Jerseyred’, ‘Gravenstein’, ‘Starr’, ‘Summer Pippin’, and ‘Britemac’ require less nitrogen than ‘Red Delicious’, ‘Rome Beauty’, ‘Golden Delicious’, and ‘Jonathan’.
Potassium. Potassium is abundant in many soils across North America. Deficiencies can arise in orchard production because some fruits are relatively rich in potassium and large amounts of the nutrient are transported off-site in the harvested fruits. Potassium, although often considered rather immobile in the soil, can leach out of the root zone in poorly managed orchards. Remember: When applying potassium fertilizers, base your rates on the K2O analysis for your fertilizer (see page 28).
Other nutrients. While other macronutrients can be deficient in orchard soils, careful preplant soil testing and site preparation should prevent significant problems from developing. Boron, iron, and zinc are micronutrients that are occasionally deficient in orchards. In many cases, the nutrients are actually available in adequate amounts in the soil, but their availability to plants is limited by pH values that are too high or low, excessive amounts of other nutrients, and soils that are wet and cold. Depending on soil conditions, boron can leach from the root zone. These elements are easily added to the orchard by applying compost, boric acid, chelated foliar sprays, and other materials. Tissue testing, as discussed below, is the most effective method for tracking macro- and micronutrient status in orchard crops.
It bears repeating: In planning and carrying out your site preparation and soil nutrition program, avoid the temptation to add excessive amounts of nutrients to the soil. Adding too much calcium or phosphorus, for example, can trigger the onset of iron chlorosis and apparent deficiencies of copper and zinc, even though these nutrients are plentiful in the soil. Excessive potassium can trigger or exacerbate apparent calcium and magnesium deficiencies by interfering with plant uptake of these nutrients. Strive to develop and maintain balanced soil and plant nutrition profiles.
You need to know the nutrient status of your crop to make effective decisions about fertilization and orchard floor management. You can find recommendations to apply 60 to 90 pounds of nitrogen per acre per year, but such recommendations do not take into account your orchard’s soil types, precipitation and irrigation, climate, training systems, fruit varieties, and management practices. In short, they are not particularly useful, especially for a commercial orchard. Several methods can help you judge the nutrient status of your crops.
Regular scouting is a critical part of maintaining an orchard. While scouting for pests and diseases, take the time to observe the growth and appearance of trunks, foliage, and fruit. Our goals are to maintain moderate vegetative growth and sustainable yields of high-quality fruit. Excessive nutrients can cause excessive vegetative growth or toxicity to your crop. Inadequate nutrition results in spindly trees and poor fruit crops.
To judge nutritional status, you will need to know what a healthy tree or bush should look like, which comes through experience. Appearance varies from one crop to another and even from one variety to another, and plant responses to poor pruning and other cultural practices can mimic nutritional disorders. To assist your learning, keep a journal of your observations. Over a period of a few years, you will discover patterns of annual growth and development and correlate these with fruit yields and quality. You will learn how your orchard system responds to fertilization, pruning and training, and orchard floor management. Keep your observations separate for each crop and each variety. To help get you started, here are some general guidelines:
Leader growth. A tree or bush with adequate nutrition will produce average leader growth. In a young apple tree, for example, you should see the leader grow 18 to 30 inches annually. As trees mature, the leader’s elongation slows. In all crops, there should be enough lateral branches to fill the canopy. Short leaders and few or short laterals indicate problems. Most newly developed lateral branches should form at wide-enough angles to the trunks to avoid bark inclusions.
Shoot growth. Excessively vigorous trees develop too many shoots and shoots that are too long and upright. The result is a dense, columnar tree that requires much effort to prune out undesirable branches and spread branches that you want to keep. In a vicious cycle, heavy pruning stimulates more vigorous, vertical growth.
Fruit color and size. Overly vigorous, upright fruit trees are slow to come into bearing and typically bear light crops of large, soft, poorly colored fruit. There are far greater pest and disease pressures on an excessively vigorous tree than on a healthy tree exhibiting moderate growth. In such a case, your approach might be to stop applying nitrogen, bend branches down to form 45- to 90-degree angles with the trunks, and prune as little as possible, but enough to keep the canopy open and allow light exposure to the trunk and interior branches.
Leaf color. Normally, leaves on most fruit trees should be green, exceptions being ornamental varieties selected for purple or other exotic leaf colors. Leaves on nitrogen-deficient trees are yellowish or pale green and may be smaller than normal. Nitrogen is mobile in plants and deficiencies often show up first in older leaves. Very dark green leaves usually indicate too much nitrogen. Sulfur deficiency also causes leaf yellowing, but it is much less common than nitrogen deficiency.
Potassium-deficient leaves show browning (scorching) around the leaf margins, with the leaf margins on stone fruits often curling upward. Magnesium-deficient leaves show yellowing between the veins of older leaves, forming a herringbone pattern on the leaves. Iron chlorosis shows up as yellow to white areas between dark green veins and is often associated with stunted shoot growth. Iron chlorosis is more often due to high soil pH and/or excessive soil moisture.
Leaf size. Leaves should be of normal size for that particular variety. Very large leaves might suggest too much nitrogen and/or shading caused by a canopy that is too dense. Very small, elongated leaves, especially when they are clustered at the tips of branches (rosetting) often indicate zinc or copper deficiencies, although glyphosate herbicide drift can cause the same symptoms.
Typical symptoms associated with nutrient deficiencies and excesses are described in tables 8.1a and 8.1b.
Symptoms. Appear on older leaves first. Symptoms include small, light green to yellowish leaves, possibly with dead tissues at the leaf tips. New shoots are stunted and contain little vigor. Low fruit set is common.
Treatment. Apply blood meal or other rapidly available nitrogen in two or three split applications during May through July.
Symptoms. Visual symptoms of P deficiency are uncommon in tree fruits. Leaves may be reduced in size and bluish-green in color, possibly with purplish veins and undersides of the leaves. Symptoms appear first on older leaves. Flower bud formation can be reduced, and older leaves may drop early.
Treatment. Apply 250 lb/A (9 oz/100 square feet) of bonemeal.
Symptoms. Potassium deficiency in tree fruits typically appears as necrotic (dead) leaf margins and brownish-yellow spots on the leaf blades, starting with older leaves. Leaves may take on a bronzed appearance. Spurs and new shoots may exhibit weak growth and early defoliation. Fruits may be small, poorly colored, and lacking in acidity. Potassium-deficient trees fail to properly develop cold hardiness.
Treatment. Apply 400 lb/A (15 oz/100 square feet) of sul-po-mag or 160 lb/A (6 oz/100 square feet) of potassium sulfate. Apply when symptoms develop. Potassium should normally be applied during early spring or late fall.
Symptoms. Calcium deficiencies seldom appear in leaves, although root and shoot growth can be reduced and leaf tips scorched. Symptoms are more often seen as fruit disorders, such as bitter pit, cork spot, and internal breakdown of apples.
Treatment. Keep soil pH correctly adjusted. Add 1,000 lb/A (2.3 pounds per 100 square feet) of gypsum in spring or whenever symptoms develop. Foliar calcium sprays during fruit development may help reduce bitter pit and cork spot in apples.
Symptoms. Yellowish-brown necrotic areas appear, generally first between the veins on older leaves. Spurs and new shoots may be thin, weak, and brittle. Fruiting spurs may fail to develop (blind wood). Leaves and fruits may drop prematurely.
Treatment. Apply 500 lb/A (18 oz/100 square feet) of magnesium sulfate (Epsom salts) or potassium magnesium sulfate (sul-po-mag) when symptoms develop or in early spring. Apply to the soil, not to the foliage.
Symptoms. The most characteristic of B deficiency symptoms include delayed bud break, poor flower development, and poor fruit set. Shoot terminals and spurs may abort, and leaves are often small, narrow, and elongated. Apple fruits may develop corky spots within the flesh and wrinkled skins.
Treatment. Apply a soluble boron-containing product as a foliar spray during the growing season or to the soil in early spring. CAUTION — boron becomes toxic to plants at very low concentrations. Never band boron fertilizers within crop rows. Use soil and foliar tests to determine the need for boron. Do not apply more than 1–2 pounds of actual boron per acre. Follow label directions when applying foliar spray materials.
Symptoms. Copper is not mobile in plants, and deficiency symptoms usually appear on younger leaves first. The leaves are stunted or misshapen, and the margins may be irregular. Yellowish or white mottling may appear between the leaf veins. Fruits may be small, poorly colored, and of poor quality.
Treatment. Seldom a problem in organic orchards where copper fungicides are used to control diseases.
Symptoms. Iron deficiency is among the easiest nutritional disorders to spot. Leaf blades turn yellow to white but the veins remain dark green. Symptoms appear first on younger leaves and are generally worse in early spring when soils are cold and wet. Necrotic areas can develop along the leaf margins and between the veins. New shoots are stunted and may die back. Flowering and fruiting are decreased.
Treatment. Foliar sprays of approved iron-containing products provide quick but temporary relief. Follow label directions. Iron chlorosis is seldom due to Fe being deficient in the soil, and is more often due to high soil pH, wet soils, cold soils, and/or excess Ca and P. It is often caused by overirrigation or poor drainage, especially on alkaline soils. Ensure that irrigation water is evenly distributed throughout the field. Drain or do not plant on wet sites. Use sulfur to lower soil pH to 6.5 on alkaline sites.
Symptoms. Manganese deficiency resembles iron chlorosis somewhat but tends to appear as dull, chlorotic herringbone patterns on the older leaves. Young terminal leaves remain green. Shoot tips may die back, and flowering and fruit set are reduced.
Treatment. Apply a foliar spray of an approved Mn-containing product as soon as leaves are well developed. Follow label directions. High soil pH can create manganese deficiency. Use sulfur to lower soil pH to 6.5 on alkaline sites.
Symptoms. Zinc deficiency produces stunted shoots. Leaves are small and narrow with a striped, irregular chlorotic pattern between green veins. Rosettes of leaves form at the tips of shoots, with bare wood below the rosettes in a condition called “little leaf disease.” Fruit set, size, and quality are reduced.
Treatment. Apply an approved Zn-containing product as a foliar spray beginning in early spring or as soon as symptoms develop. Follow label directions.
Adapted, in part, from Cornell University tree fruit guidelines and Garcia, E. Orchard Nutrition. University of Vermont Extension.
Symptoms. The numbers and growth of new shoots are excessive, and growth tends to be unusually upright. Leaves are dark green, and the foliage is lush and succulent. Fruit set is poor, and the fruits are often large, soft, and poorly colored. Susceptibility to pests and diseases (particularly fire blight) increases greatly. Growth fails to harden off in the fall and winter injury to the branches and trunk increases.
Treatment. Reduce N fertilization and the use of N-fixing alley and in-row cover crops.
Symptoms. Excessive amounts of P can trigger apparent Cu, Fe, and Zn deficiency symptoms.
Treatment. Keep soil pH adjusted correctly. Avoid excessive P applications.
Symptoms. Excess K can trigger or exacerbate apparent Ca and Mg deficiency symptoms.
Treatment. Avoid excessive K applications.
Symptoms. Excess C does not produce visual symptoms but can trigger phosphorus deficiency.
Treatment. Avoid excessive Ca applications.
Symptoms. Excess Mg can hinder Ca uptake and make Ca deficiency symptoms more prevalent when soil Ca supplies are low.
Treatment. Avoid use of dolomitic limestone or other Mg-containing materials when soil Mg concentrations are high.
Symptoms. Symptoms of excess B are similar to deficiency symptoms. Leaves may show yellowing (chlorosis) along the midrib, and shoots may defoliate early, beginning at the tips. Fruits may crack and drop early.
Treatment. Avoid excessive applications of boron-containing materials.
Symptoms. Excess copper may damage or kill roots and can trigger symptoms of other micronutrient imbalances.
Treatment. Use copper-containing fungicides and bactericides carefully and according to label directions to avoid excessive accumulation of copper in the soil under the trees.
Symptoms. None
Symptoms. Symptoms of excess manganese are only seen on ‘Red Delicious’ apple in the form of measles-like pimpling of the bark.
Treatment. Avoid excessive applications of manganese-containing materials.
Symptoms. Excess Zn can trigger symptoms of apparent Cu deficiency.
Treatment. Avoid excessive applications of zinc-containing materials.
Adapted, in part, from Cornell University tree fruit guidelines and Garcia, E. Orchard Nutrition. University of Vermont Extension.
As we discussed in earlier chapters, testing the soil is a very important part of preparing an orchard for planting. Once the trees or bushes are established, soil tests for nutrient concentrations become much less useful, for several reasons: Trees and fruit bushes have spreading root systems that gather nutrients from large volumes of soil. These plants are also very efficient at concentrating mineral nutrients in their tissues at much higher concentrations than are found in the soil. And unlike annual crops that grow and die in a single season and release the nutrients back into the soil, woody plants recycle and store nutrients from year to year. For these reasons, soil nutrient tests on established orchards often correlate poorly with fruit crop performance.
It is important to test your soil’s pH levels throughout the life of the orchard. Remember that soil pH has a strong impact on nutrient availability, as shown in figure 4.6. An excellent goal is to maintain the pH between 6.0 and 7.0 and as near 6.5 as possible, recognizing that soil pH varies throughout the year. Soil pH also varies across the orchard. Research has shown that soil pH in the planting row often differs significantly from the pH within the alley, just a few feet away.
Plan on testing the soil within crop rows every 2 to 3 years. While the time of sampling is not critical, you should sample at the same time each year because soil pH varies with the seasons. I normally sample the soil in mid-spring, when plant growth is most rapid and the need for plant nutrients is the greatest. See box on page 260 for how to collect a sample.
When gathering samples, an easy approach is to walk diagonally from one corner of the orchard or planting block to the opposite corner, sampling as you cross tree rows. Repeat the process on the other diagonal to obtain a representative sample for that block. Mix the samples inside the plastic pail and take out enough to fill the sample bag provided by your testing laboratory. If you are concerned about the pH effects on alley crops, you may want to repeat the procedure along the centers of the alleys.
Many universities with agriculture programs offer soil testing, and there are many good private laboratories in the United States and Canada. Choose a laboratory as close as possible to your site. Soil testing procedures vary greatly from one region to another, based on different soil types. Always use a laboratory that is familiar with your soils and stick with that lab. A common complaint from fruit growers is that testing results from different laboratories are not the same, even for the same sample. Use consistent sampling procedures and the same testing facility. Cooperative Extension offices and Ministry of Agriculture offices can usually provide recommendations for a reliable testing laboratory in your area.
Collect samples within tree rows throughout the orchard. In large orchards, sample each planting block separately. For bush fruits and high-density tree plantings, collect samples midway between plants.
By far the most reliable method of determining the nutrient status of perennial fruit crops is to analyze the leaves and petioles for mineral nutrient concentrations, a process called tissue testing or foliar analysis. As mentioned above, the results of soil analyses of established orchards are not good indicators of tree health and productivity. Visual observations of growth, leaf color, and fruit characteristics are valuable and necessary, but they do not reveal problems until damage has already occurred and fruit quality and yields have been reduced. Tissue testing, on the other hand, tells you precisely how much of each essential mineral nutrient is present in the plant.
While foliar analysis is seldom needed for home orchards, it is standard for commercial plantings. Foliar analyses help guide your nutrition and orchard floor management programs and provide early warnings of nutrient deficiencies and toxicities. Knowing, with accuracy, your crop’s nutrient status takes the guesswork out of fertilization programs, saving time and money on unneeded fertilizer applications and helping ensure healthy plants and high-quality fruit.
In commercial orchards, foliar analyses are usually conducted yearly. Nutrient concentrations fluctuate greatly during a growing season, and it is best to sample after shoot and leaf growth have ceased but before the trees begin senescing (leaves changing color or dropping). At that time, the nutrient concentrations in the leaves are fairly stable and allow comparison from one year to another. In USDA Plant Hardiness Zones 4 to 6, the last week of July through the first week of August is the most popular time for sampling. In warmer or cooler climates, you may need to adjust the timing for leaf collection. Other fruit growers, state and provincial fruit specialists, and your analytical laboratory can give you guidance for your particular region.
Here are some general guidelines for collecting leaves to be sent away for tissue testing.
Sample collecting procedures. Collecting leaves for testing is easy and straightforward. As with soil tests, you are looking for average values across many trees, if possible. Sample and test each variety separately because different fruit varieties of a single crop often show differences in foliar nutrient concentrations. Do not sample soon after a cover spray of macro- or micronutrients or materials such as kelp or fish emulsion.
Collect only from typical trees. If most trees in a planting appear healthy and one or a few isolated trees do not, it is unlikely that the problem is being caused by nutrient deficiencies or excesses and is more likely to be pests, diseases, or localized irrigation or soil problems. All of the trees sampled should be the same age.
Sample from the mid-height of the trees or bushes, typically about 5 to 7 feet above the ground for mature fruit trees and 3 to 6 feet for young trees. Collect samples from the middle portion of shoots, taking no more than about two leaves per shoot, and from several shoots per tree. The shoots should be of average vigor for the trees. Do not collect spur leaves or leaves from excessively vigorous or weak shoots, such as those near pruning cuts.
Shoots should be well exposed to sunlight. If collecting more than 60 to 70 days after petal fall, collect the first and second fully mature leaves below the shoot tip. Collect only clean leaves that are typical of those on the tree. Avoid those damaged by pests, diseases, weather, or mechanical injury, and do not collect leaves showing unusual symptoms, unless they are typical of the trees being sampled. Collect both the leaf blades and petioles by grasping the petioles (leaf stems) firmly and pulling them downward and away from the shoot.
Repeat the leaf collecting process across the orchard or planting block, much as you did for soil samples, collecting roughly 60 to 100 leaves in total for each variety. If possible, collect from at least 5 to 10 trees and do not collect all of your samples from a few trees planted closely together. Repeat for different varieties and crops, if necessary.
Place the leaves inside a paper lunch sack and allow them to air dry at room temperature until they are brittle. Transfer the leaves to the sample bag provided by the analytical laboratory and send the samples to the laboratory.
Growers often ask whether they should wash the leaves before drying them to remove dust or other contaminants. In general, most analytical laboratories prefer that you do not wash the leaves. If the leaves are dusty, covered with kaolin clay (Surround), or have recently received a cover spray, you may want to wash them. The following guidelines are courtesy of Cornell University’s Agro-One Soils Laboratory.
Wash the leaf samples while they are still fresh and before they wilt. If a large number of samples are involved, they may be stored overnight in a refrigerator or ice chest to keep them from drying out.
Clean one sample at a time, taking care not to mix different samples together. Use distilled water, available at most drug stores, for washing and rinsing the samples. Change the water if it becomes dirty or after 8 to 10 samples (whichever occurs first).
Analyzing test results. Interpreting foliar analyses is generally straightforward. Much research has been conducted on commercial orchard crops, and tables showing recommended foliar nutrient concentrations are available. Most tables list ranges of concentrations, from deficient to adequate to excessive. In general, you want to be in the adequate range. Because organic orchard systems perform best with less nitrogen than in conventional orchards, you will want the nitrogen values to fall toward the lower end of the adequate range. Tables 8.2 and 8.3 provide suggested nutrient values. Because published standards are not available for mayhaw, saskatoon, quince, medlar, and bush cherries, the suggested values are extrapolated from similar crops. Consider these values as starting points, and adjust, as necessary, for your crops, varieties, and cultural practices.
Adapted from Leaf Analysis for Fruit Trees. Rutgers NJAES Cooperative Extension.
Adapted from Leaf Analysis for Fruit Trees. Rutgers Rutgers NJAES Cooperative Extension.
In developing organic certification programs for the United States, Canada, and other countries, organic growers, fruit specialists, legislators, and others worked together to identify materials suitable for sustainable organic crop production. The following list of fertilizers, growth promoters, and soil amendments is approved for use in organic crop production by the U.S. National Organic Program. Similar lists have been developed for Canadian certified organic growers. Even if you are not interested in organic certification, the approved lists provide excellent guidance for developing effective and environmentally friendly orchard management practices.
Some approved materials have certain restrictions regarding their use in organic crop production, and some organic certifying organizations follow standards more restrictive than those detailed in the following list. If certification is important to you, become thoroughly familiar with your certifying organization’s rules. Always carefully read labels and any other documentation. Be sure that the materials you purchase meet organic certification standards.
Certification standards allow for the use of many naturally occurring and a few synthetic materials for crop production. In the following section, we will cover some common soil amendments and fertilizers used in organic fruit production. Because organic certification standards vary between Canada and the United States and some U.S. certifying organizations enforce stricter rules than the NOP, be sure you know your certifying organization’s rules if you are to be certified. Table 8.4 provides information on the nutritional values of selected organic fertilizers.
Algae. Algae are incredibly diverse, from giant kelps along ocean coastlines to one-celled organisms floating in a pond. Perhaps the most diverse are the blue-green algae (also called cyanobacteria) that are found almost everywhere life exists, as free-floating blooms, strands, and sheets in lakes and rivers to the symbiotes living inside lichens on desert rocks. For growers, the primary benefits of algae are as sources of plant nutrients.
One problem with using algae is that most algae has little dry matter and is primarily water, making harvesting and processing very labor intensive. You can harvest sheets, strands, and balls of algae from a farm pond and dump the materials around your fruit trees. You may also choose to wait until the pond has dried and scrape off the surface crust and algal remains to apply as a soil amendment. For a very small orchard, this approach might yield some benefits, depending on the soil conditions. You will add very little organic matter, and the primary benefit will be the addition of micronutrients. Any microorganisms you add will be adapted to watery environments — certainly not what you want in an orchard soil.
Probably most useful to organic orchardists are kelps (brown algae), when used in small quantities and as commercially available products. If you live close to kelp forests along a coastline, you might economically obtain large enough quantities to use as a source of nitrogen and other macronutrients. However, the energy and labor required to harvest the material and process it into a form suitable for orchard applications would make kelp a very expensive option for most growers. When using large quantities of kelp, be aware that it contains large quantities of sodium that can damage orchard soils by displacing calcium, phosphorus, and other plant nutrients from the soil and organic matter particles. Commercially available kelp meals and extracts are available and can serve as good sources of micronutrients, depending on how the kelp was processed. Some blue-green algae products may also be available. The greatest value of these kelp and blue-green algae products will probably be as foliar sprays to provide micronutrients.
vs = very slow; ms = moderately slow; s = slow; m = moderate; r = rapid; mr = moderately rapid; vr = very rapid
To convert between % P and % P2O5: % P × 2.29 = % P2O5, % P2O5 × 0.44 = % P
To convert between % K and % K2O: % K × 1.2 = % K2O, % K2O × 0.83 = % K
Animal manures. Historically, animal manures have been a valuable source of organic matter and macro- and micronutrients, and they remain so today. Economics and governmental rules, however, have changed how we obtain and use manures. Large, integrated orchard and livestock operations are quite rare today. Even 60 years ago, operators of such enterprises often found they were making money on the fruit but just breaking even on the livestock and were essentially working very hard for manure. Larger growers usually find it most effective to specialize on their orchards and let someone else produce the livestock and manure. Separating crop and livestock operations creates transportation problems, however, and raises the cost of manure products unless the livestock operations are very close. Integrated market and home orchards and livestock production are still common.
The nutrient content of fresh manures varies according to the kind of animal and what the animal has been eating. As manures dry and age, leach, or are composted, nutrient concentrations are reduced, but so are the salt concentrations and the chances of burning plants. Fresh manures also contain large amounts of water, making them difficult to handle and expensive to transport. They can damage plants by providing too much nitrogen too quickly, and the high concentrations of salts in fresh fertilizers can build up in soils.
Raw (fresh or dried) manures can contain human pathogens, and NOP organic certification regulations do not allow raw animal manures to be applied to tree orchards within 90 days of harvest, or small fruit orchards within 120 days of harvest. The NOP states that, for crops intended for human consumption, raw manures must be incorporated into the soils at least 90 or 120 days, respectively, before harvest.
Both bat and bird forms of guano are allowed under the U.S. National Organic Program. Use these materials as you would other manures. Nitrogen contents range from about 5 percent for bat guano to 13 percent for bird products. Guanos also contain 8 to 12 percent P2O5 and about 2 percent K2O. These nutrients are available to plants at moderate rates and can be applied as late as the spring for the coming growing season.
Composting solves many of the problems with manures, providing a stable, relatively light (if bulky) material from which excessive salts have been leached out. Composting various mineral products, such as colloidal phosphate, with animal manures can make the mineral nutrients more available to plants. Proper composting also helps destroy potential human pathogens, such as E. coli bacteria. Manures can be contaminated with pesticides used to control pests in and around barns, corrals, and feedlots. When obtaining composts or materials for making compost, ensure that they are pesticide-free. Note that the U.S. National Organic Program prohibits the use of ash from burned manures.
If you are meeting certification standards, be aware that the NOP has specific requirements for composting manure so that it can be applied to an orchard within the 90 or 120 days-to-harvest window mentioned above. Composted plant and animal materials must be produced though a process that establishes an initial C:N ratio of between 25:1 and 40:1, respectively. The temperature must be maintained between 131 and 170°F (55–77°C) for 3 days using an in-vessel or static aerated pile system, or you must maintain the temperature between 131 and 170°F for 15 days if you are using a windrow composting system. During the 15-day period, you must turn the materials a minimum of five times.
Biodynamic preparations. Biodynamic agriculture was one of the cornerstones of today’s organic agriculture, beginning in 1924 with Rudolf Steiner in Germany and quickly spreading to the UK, Australia, and North America. Biodynamic farming emphasizes an integrated, holistic approach, where the soil and all organisms on a farm are managed as a single, integrated organism. Many of the practices we discuss in this book fit well into a biodynamic approach.
The term “biodynamic” is trademarked by Demeter-International, which represents an association of biodynamic farmers. Some biodynamic farmers bring religious, occult, and astrological considerations into farming, which is beyond the scope and intent of this book. Scientific studies have shown that biodynamic farming operations have better soil health and structure than similar conventional farms and are more energy efficient. Biodynamic farming has not produced results that are significantly different from similar organic farms. Whether to follow strict biodynamic practices or the organic practices we cover here is a personal choice. Both approaches are very similar in most respects and produce similar results.
Blood meal. Blood meal is a by-product of slaughterhouse operations and is one of the few relatively high-concentration, rapidly available sources of nitrogen approved for organic farmers. Blood meal also supplies iron and some other micronutrients. It is most useful when establishing an orchard or transitioning from conventional to organic, when soil organic matter is low and soil nitrogen is not yet available in the concentrations needed by the orchard crops. As the orchard matures and soil organic matter and slow-release sources of soil nitrogen build, blood meal becomes less necessary.
Due to its relatively low cost, rapid availability, and ease of application, blood meal is also useful in established orchards where a quick fix of nitrogen is needed. Follow recommended rates carefully, as you can damage plants with excessive rates of blood meal.
While they provide nitrogen and organic matter, these products are usually expensive sources of both and are best used in very small orchards. While advertised as a natural preemergence herbicide, research trials on corn gluten generally show it to be ineffective as an herbicide for fruit crops.
Bonemeal. Like blood meal, bonemeal is a by-product of slaughterhouse operations. It is a rich source of calcium (23 percent) and P2O5 (12 to 14 percent) and contains trace amounts of micronutrients. Depending on its source and preparation, it may contain 1 to 4 percent nitrogen.
Bonemeal is particularly valuable when establishing an orchard. Use steamed bonemeal preparations to reduce the likelihood of human pathogens and increase the availability of phosphorus. Because it contains very little nitrogen or potassium, excess amounts of bonemeal will not “burn” plants, although excess amounts can create nutrient imbalances in young orchard crops, as we discussed earlier.
Boron products. Boron deficiency can cause poor flowering, fruit set, fruit development, and shoot development. Boron, however, becomes toxic to plants at very low concentrations. Boron products can be applied to the soil or as foliar sprays when tissue analyses show boron deficiencies. Use only organic-approved boron products and do not apply more than 1 to 2 pounds of actual boron per acre. Follow label directions very carefully. See page 107 for allowable sources of boron.
Chelates. Chelated micronutrient sprays may be used when soil and/or plant tissue analyses show nutrient deficiencies. Chelates are used to protect nutrient molecules from becoming chemically bound up in the soil and unavailable to plants. Some chelates may also facilitate the uptake of nutrients by plants, which normally release chelates from their roots to make soil nutrients more available. Amino acids, lignosulphate, citric acid, malic acid, tartaric acid, and other diacid and triacid chelates are acceptable chelates for organic orchards.
Chelated products may be applied to the soil, but they are most useful in orchards as foliar sprays to correct micronutrient deficiencies. Use only products approved for organic use.
Cocoa bean hulls. Cocoa bean hulls are the shells of cocoa beans and are removed from the beans during roasting. The hulls are generally applied as mulches and slowly decompose. They contain about 2 to 3 percent nitrogen and small amounts of phosphorus and potassium. While the mulch is weed-free, organic growers need to ensure and document that the products they use have been tested for pesticide residues and found to be clean. Cocoa bean hull mulch is best used for home orchards, as its cost is prohibitive for larger, commercial operations.
Compost. Compost is an outstanding amendment for orchards, providing both organic matter and nutrients. Because it is derived, in large part, from plant materials, compost normally has most or all of the nutrients plants need and in the correct proportions.
The quality of compost and its nutrient content depend on what goes into it and how it is prepared. It can be made strictly from plant materials or include animal manures and/or amendments such as colloidal phosphate. Properly managed compost has temperatures high enough to kill most weed seeds and pathogenic microorganisms. Many beneficial microorganisms tolerate the composting. Storey Publishing carries several guides for composting. The On-Farm Composting Handbook, available online from the Natural Resource, Agriculture, and Engineering Service (see Resources), is an excellent guide for large farm-scale composting.
When purchasing or making compost, take great care to ensure that none of the components have pesticide residues that can damage your crops or cost you certification. Several years ago, farmers and nursery growers in Washington State obtained compost from a municipal source. This compost contained high concentrations of the herbicide clopyralid, once used to control dandelions and other broadleaf weeds in lawns. Clopyralid is not destroyed during the composting process, and many crops were damaged or destroyed by the residues remaining in the compost. Clopyralid is, reportedly, still registered for use in some forage and grain crops.
When making your own compost (highly recommended), do not include diseased or pest-infested plant materials or weed seeds. Burn these materials and add the ashes to your compost, if you wish. Be very careful not to include materials that may contain pesticide residues. Grass clippings, leaves, wood chips, and other plant debris from municipal sources should generally not be used to produce compost for organic orchards unless you are absolutely sure they are free of pesticides.
Cottonseed meal. Cottonseed meal is the dry matter left after cotton has been ginned and the oil extracted from the seeds. This high-protein product is often used for animal feed in the southern and western United States and is used as an organic fertilizer. It provides 6 to 9 percent of slow- to moderately rapid–release nitrogen and lesser amounts of phosphorus and potassium. If you have access to a cost-effective source of this product, it can be a good orchard amendment. Ensure that any product you use has been tested for pesticide residues and you have documentation that it is free from such contamination.
Dolomite. Dolomite (also known as dolomitic limestone) is primarily mined from sedimentary deposits and consists largely of calcium magnesium carbonate (CaMg(CO3)2). Its primary use in horticulture is to raise soil pH levels while supplying magnesium. Because this is a natural product, the percentages and chemical compositions of calcium and magnesium vary, as does its calcium carbonate equivalent value (liming ability). As we discussed in chapter 4, do not apply dolomite unless soil magnesium concentrations and pH are both low. Excessive amounts of magnesium can interfere with calcium uptake and increase bitter pit problems in apples.
Enzymes. Various organic products containing enzymes, often combined with supposedly beneficial microbes and plant nutrients, are marketed as fertilizers and soil amendments. According to the U.S. National Organic Program, enzymes are acceptable if derived microbiologically from natural materials and not fortified with synthetic plant nutrients. In actual practice, it is extremely difficult to understand how applying enzymes to the soil would benefit orchard crops.
Enzymes are chemically complex biochemicals that carry out or regulate very specific chemical reactions. Microbes, soil macroorganisms, plants, and all other living things produce the enzymes they need for their life processes. While adding enzymes to the soil or spraying them on your fruit crops is not likely to cause any damage, you will earn much greater returns on your investments of time and money by focusing on creating the conditions needed for a biologically rich and active soil containing adequate amounts of plant nutrients. We have covered many such practices, such as correcting soil drainage problems, adjusting soil pH, incorporating green manure crops, applying compost, and adding optimal amounts of plant nutrients.
Epsom salts or magnesium sulfate. Epsom salts are a rich source of magnesium and are best applied before planting orchard crops. They are much more rapidly available than dolomite, however. According to the U.S. National Organic Program, they may be used when you can document a soil magnesium deficiency. Base your application rates on soil or tissue analyses. Do not apply to apples when soil calcium concentrations are also low. Kieserite is a hydrated form of magnesium sulfate and used in the production of Epsom salts.
Feather meal. This is a by-product of poultry slaughterhouses and contains 7 to 12 percent nitrogen, depending on how it is processed. The nitrogen is released slowly over 4 months or more. Nitrogen from feather meal generally costs about 10 to 15 percent more than that from the more rapidly available nitrogen in blood meal. If you can obtain feather meal inexpensively, it can be an effective slow-release fertilizer. Be sure to obtain a product that is free from pesticides.
Fish emulsions. Emulsions contain only about half of the nitrogen, phosphorus, and potassium found in fish meal, but the nutrients are available much more rapidly. According to the U.S. National Organic Program, forms that are fortified with urea or other synthetic plant nutrients are prohibited. Phosphoric acid can be used as a stabilizer, but the resulting product may not exceed 1 percent, by weight, of P203.
While some people recommend applying fish emulsion to the foliage in order to correct nitrogen deficiencies, the practice is not effective. Nitrogen and the other macronutrients generally cannot be taken up through the foliage in large enough quantities to significantly improve a plant’s nutritional status. Studies have shown that, in cases where foliar feeding increased the macronutrient concentrations in orchard trees, the fertilizer solution dripping from the leaves onto the soil provided the benefits. For nitrogen, phosphorus, and potassium deficiencies, apply fish emulsions to the soil. Fish emulsion can be applied as a foliar spray, however, to supply micronutrients; these can be taken up in significant quantities through the leaves.
Fish meal. Fish meal is a rich source of nitrogen (8 to 9 percent) and also provides phosphorus and potassium. Depending on the preparation, fish meal can also provide micronutrients. Nitrogen from fish meal is available rather slowly and is best used as part of a soil-building program, not as a quick fix for a nitrogen deficiency.
Fruit pomaces. Grape, apple, and other fruit pomaces (spelled incorrectly as “pomades” in the U.S. National Organic Program) are the pulpy materials left after the juice has been extracted from the fruits. Pomaces may be nearly dry or wet, depending on the press. They can be applied directly to the soil, although the relatively large amounts of water make handling and applying the materials somewhat difficult and transportation over great distances expensive. The fruit odor and sugar will also attract flies and other insects, possibly including fruit pests. Where facilities and equipment are available, composting pomaces along with other organic materials can be an effective practice to recycle orchard wastes and create nutrient-rich compost. In an orchard operation where cider or juice is extracted, an excellent practice is to apply the pomace waste to the orchard floor, preferably after composting.
Granite dust. This is sometimes used as a source of potassium (see page 103).
Green manure crops. These crops are not grown to be harvested for human consumption but are incorporated into the soil as part of a soil-building program. As we discussed in earlier chapters, growing and incorporating green manure crops before planting an orchard is an excellent strategy for adding organic matter and nitrogen to the soil. Used properly, green manure crops are also important in reducing weed problems. Green manure crops are at the heart of organic annual vegetable and grain production. In vegetable or grain systems, once a cash crop is harvested, you can plant and turn in a green manure crop before replanting the cash crop. The use of green manure crops in established, long-lived perennial crops is more challenging because you are not removing and replanting your cash crop frequently. In some orchard systems, green manure crops can be used as annual alley crops or as in-row companion plantings. We will discuss these strategies in chapter 9.
Greensand. This is a source of potassium (see page 104).
Gypsum. Also known as calcium sulfate, gypsum adds both calcium and sulfur to the soil without changing the soil pH (see page 105 for more information). When calcium is needed and the soil is already at pH 6.5 or greater, gypsum is an excellent source of calcium. Likewise, when sulfur is needed and the soil is pH 6.5 or lower, gypsum is the material of choice.
Gypsum is often touted as a “soil conditioner.” That claim is true, but only in very specific cases. Gypsum is used to reclaim saline-sodic soils that are found most often in the arid regions of western North America. These soils have both high pH values and high sodium contents. They are very compacted and drain poorly, often forming what appear to be oily patches, earning them the common name of “slick soils.” High concentrations of sodium in such soils displace calcium and other nutrients, destroying soil aggregates and creating dense, virtually lifeless soils. When gypsum is combined with tillage and water to flush the sodium soils out of the root zone, the calcium in gypsum occupies the negatively charged sites on soil particles, flocculating (fluffing up) the soil and forcing the sodium out. Aside from its effectiveness on saline-sodic soils, however, gypsum’s “soil conditioner” properties are highly questionable. Although it is a valuable tool for orchardists, be careful not to give gypsum more credit than it deserves.
In conventional cropping systems, borated gypsum is often used to supply boron. This material typically contains about 1 percent boron and is applied at a rate of about 175 pounds per acre, broadcast across the entire field. There are naturally occurring sources of boron-containing gypsum, but not many. If you choose to apply borated gypsum, ensure that it is approved for organic use and ensure that the boron concentration is not high enough to damage your crops. Limit boron applications to no more than 1 to 2 pounds of actual boron per acre and only when soil or tissue analyses document a boron deficiency. Never band boron-containing fertilizers into crop rows.
Hoof and horn meal. Containing 12 to 14 percent nitrogen and 2 percent K2O, hoof and horn meal are suitable sources of slow-release nitrogen for use as soil builders. Like blood meal, they are by-products of the slaughterhouse industry.
Humates and humic acids. Humic materials are derived from plant materials, such as lignin, and have weathered to the point that they are very resistant to further degradation. They are important components of the soil because they help bind the soil particles together, creating aggregations and structure.
The term “humic acid” refers to a large group of related compounds. Although they can be created synthetically, humic materials are generally mined from lignite coal or an oxidized clay-like form of lignite called leonardite. The latter is particularly rich in humic acids. While humic acids are certainly critical components in a biologically active soil, the benefits of adding them to the soil are less clear. You would need to add vast amounts to increase soil organic matter even slightly. Mined humates are chemically different from those found in the soil. From a fruit grower’s perspective, commercial humate products appear to have little to offer.
Kelp meal and extracts. See algae entry, page 267.
K-mag, sul-po-mag, or sulfate of potassium magnesia. This naturally occurring material is mined for use as a fertilizer. It is moderately to rapidly available to plants and contains 23 percent sulfur, 22 percent K2O, and 11 percent magnesium. This is a valuable product when you are faced with potassium, magnesium, and sulfur deficiencies. It has no effect on soil pH. Make your applications based on recommendations from a soil analysis.
Limestone. Limestone is a naturally occurring material comprising mostly various concentrations of calcium carbonate. A typical limestone contains about 40 percent calcium, which is slowly available to plants. Its greatest value in orchards is its ability to raise the soil pH. Dolomitic limestone, or dolomite, is a magnesium-containing form of limestone.
Leather meal is a by-product of the tanning industry. Although a relatively rich source of nitrogen (10 percent), it is often contaminated with chromium and cannot be used in organic orchards.
Micronutrients. Boron, copper, iron, manganese, molybdenum, and zinc are occasionally deficient in orchards. While sulfates of these nutrients are available, your best results will probably come from applying foliar sprays of chelated products (see page 107), described above. Apply them only when there is a documented deficiency of the nutrient in the soil or tissues.
Mushroom compost. Mushroom compost is usually comprised mostly of sawdust, rice hulls, straw, or similar woody organic materials that have been used to grow mushrooms for human consumption or medicinal purposes. It typically contains less than 1 percent each of nitrogen, phosphorus, and potassium and is most valuable as a source of organic material for mulching. Test the material for pesticides and document that it is pesticide-free before applying it to your soil.
Peat moss. Peat moss is extremely valuable for containerized plant production, but it is normally prohibitively expensive for all but a small home orchard. Its value as an amendment is to increase the soil’s water-holding capacity and is most useful on sandy soils. It is also acidic and somewhat useful in temporarily lowering soil pH. Ensure that any peat moss you use has not been amended with synthetic fertilizers or wetting agents (surfactants).
Potassium sulfate. Also called potash of sulfate, potassium sulfate is a valuable fertilizer in conventional farming systems. Unfortunately, it is usually produced in factories and naturally occurring forms that can be mined are rare. As described in chapter 4, potassium sulfate can also be derived from evaporating saline lake water. Not all sources of potassium sulfate are suitable for organic production and not all certifying organizations allow its use. Sul-po-mag is a good alternative product. If you use potassium sulfate, ensure that it is allowed under your certification guidelines.
Rock phosphates and colloidal phosphates. See page 102.
Shells. Ground oyster, clam, lobster, and crab shells are similar to bonemeal in some ways. They are rich in calcium but rather low in phosphorus. A typical crab shell–based fertilizer might contain 2 to 3 percent nitrogen, 3 percent P2O5, 0.5 percent K2O, and 23 percent calcium. Nutrients from shell products become available slowly. Shell products are probably best used as slow-release forms of calcium.
Sodium nitrate. Also known as Chilean nitrate, this is a nitrogen-rich, rapidly available fertilizer. Because of its high sodium content, most organic certification programs discourage or prohibit its use. Some certification programs allow Chilean nitrate to be used, but only enough to provide 20 percent of the total amount of nitrogen applied. This material is best used sparingly during the early stages of establishing an orchard or as an occasional supplement when a quick fix of nitrogen is needed. Ensure your organic certification organization allows its use before applying it to your crops.
Sugar beet lime. In processing sugar beets, limestone is heated to produce calcium oxide and carbon dioxide, which are then mixed with the beet juice to absorb impurities. During the process, the calcium oxide and carbon dioxide recombine to form calcium carbonate, which must be disposed of. Sugar beet lime can be useful as a soil amendment to raise pH, but the material often contains large amounts of water, making it difficult and expensive to transport and apply in an orchard. Obtaining uniform applications of the wet product is also difficult, and uneven applications can create pH and nutritional problems for your orchard crops. Be sure you have an accurate measurement of the calcium equivalent of sugar beet lime before applying it to your soil.
If you are in an area where sugar beet lime is readily available, it can be a good choice as a soil amendment. Ensure that any product you use has been tested to ensure it is pesticide-free, and check with your certification organization to ensure it is allowed for your operation.
Sulfur. We discussed the use of elemental sulfur in chapter 4 as a means of lowering the pH in alkaline soils. Elemental sulfur can also be used to add sulfur to the soil when pH is high and soil sulfur concentrations are low. Elemental sulfur is available as pellets, prills, and powders. The smaller the product particles are (the finer it is ground), the more rapidly the sulfur reacts in the soil to lower pH and the more rapidly available it is to plants. If the soil pH is already in the desired range but sulfur concentrations are low, add sulfur in the form of gypsum or sul-po-mag.
Wood ashes. Wood ashes are allowed in organic production but can be problematic. They contain 1 to 2 percent P2O5 and 3 to 7 percent K2O, both of which are rapidly available to plants. Wood ash is highly alkaline and reacts very quickly in the soil to raise the soil pH. Do not apply ashes to soils that are already at or above the desired pH range and do not add more than 20 pounds of wood ash per 1,000 square feet per year. Use only ashes from untreated and unpainted wood. They must not be created using colored paper, plastic, or other prohibited substances. Excessive applications of ash can cause pH and nutrient imbalances. Never use coal ash.
Worm castings. Worm castings are literally the manure formed by worms. This organically rich material contains variable amounts of nutrients, based on the worms’ diets. Few commercially available worm casting fertilizers provide a fertilizer analysis, making it impossible to determine application rates. At best, worm castings would be a very expensive fertilizer and source of organic matter for any but the smallest orchards. A better strategy is to create a biologically active soil where your own worms produce the castings in the soil where they are needed by your orchard crops.
Soil tests, tissue analyses, and rule-of-thumb guides generally include estimates of how much nitrogen, phosphorus, and other nutrients are needed. Recommendations are usually given in pounds per acre or pounds per 1,000 square feet in the United States or kilograms per hectare or kilograms per 100 square meters in Canada. The question then becomes how much fertilizer is needed to provide those amounts of nutrients. Commercial fertilizers, organic and nonorganic, are analyzed for the concentrations of nutrients in them, and the fertilizer analyses are printed on the containers. At least three numbers are shown, representing nitrogen, phosphorus, and potassium, in that order. Other macro- and micronutrients, if present, may be listed in a table.
Ammonium sulfate, a popular conventional fertilizer, has an analysis of 21-0-0, indicating 21 percent N and no P2O5 or K2O. A typical conventional garden fertilizer might be labeled as 16-16-16, indicating that it has at least 16 percent each of N, P2O5, and K2O. Most fertilizers contain at least trace amounts of micronutrients. Manufacturers are not required to list these nutrients unless they advertise that fertilizer as a source of those particular nutrients. If they do list micronutrients, the claim is usually that the fertilizer contains “at least” the amounts listed on the label.
Unfortunately, some manufacturers of commercial organic fertilizers choose not to include the analyses and sell the products as “soil amendments” or “soil conditioners” without specifying exactly what is in them. When buying such products, you have no clear idea of the benefits, if any, that you are receiving. You are far better off using materials that are clearly and accurately labeled. Treat testimonials in advertisements with a huge grain of salt. If a material is intended to be used as a source of plant nutrients and no fertilizer analysis is printed on the label, there is usually a very good reason why it is missing.
There are, however, some good commercial organic fertilizers, including some naturally occurring materials, that do not come with fertilizer analyses. These products include such things as feather meal, greensand, and soybean meal. Table 8.4 provides approximate concentrations of plant nutrients for many common organic fertilizers. Like conventional fertilizer labels, phosphorus values are given as P2O5 and potassium as K2O equivalents.
For an example, say that a foliar analysis of your young apple trees showed low levels of nitrogen and the laboratory recommended applying 60 pounds of nitrogen per acre (fruit crops often need about 60 to 90 pounds of nitrogen per acre [68 to 103 kg/ha] per year). We will further assume that this is a newly established orchard and that we do not yet have a nitrogen-fixing alley crop in place, nor much decomposing organic matter in the soil. All nitrogen must come from the applied fertilizer.
Because the need for nitrogen is acute and our soil’s organic matter is not yet releasing much nitrogen, we will use a fairly rapid-release fertilizer in the form of blood meal. Blood meal contains about 12 percent nitrogen. If you were to buy a commercial blood meal fertilizer, it might have a label something like 12-2-0.6, indicating 12 percent N, 2 percent P2O5, and 0.6 percent K2O.
Amount of fertilizer per acre. To calculate how much fertilizer to apply, divide the amount of nitrogen needed by the percentage of nitrogen in the fertilizer. The same procedure is used to calculate the amount of phosphorus, potassium, or other nutrient to add. In this case:
60 pounds of N per acre ÷ 0.12 = 500 pounds of blood meal fertilizer per acre
67 kilograms per hectare ÷ 0.12 = 558 kilograms of blood meal fertilizer per hectare
In this example, we decide to split the fertilization into two applications in order to reduce the amount of nutrients that are lost from the soil by leaching and volatilization. We will make the first application as the buds begin swelling and the second 1 month later. It is common practice to split the application of rapidly available fertilizer materials, such as blood meal. For materials that are available to plants at moderate or slow rates, the entire amount of fertilizer is usually applied at once, allowing plenty of time for it to become available to the crop.
Amount of fertilizer for less than one acre. To calculate the amount of fertilizer needed for orchards smaller than one acre, you can easily convert the rate per acre to the rate per 1,000 square feet or the rate per tree.
One acre contains 43,560 square feet. To calculate the amount of nitrogen per 1,000 square feet (an area 10 feet wide by 100 feet long), divide the recommended rate per acre by 43.56.
500 ÷ 43.56 = 11.5 pounds of blood meal per 1,000 square feet (11.5 lbs per 1,000 ft2)
To calculate a per-tree rate, divide the number of trees per acre, based on your planting density. Say, for example, that your apple trees are 6 feet apart in rows 12 feet apart.
6 feet in rows × 12 feet between rows = 72 square feet per tree
43,560 square feet per acre ÷ 72 square feet per tree = 605 trees per acre
500 pounds of blood meal per acre ÷ 605 trees per acre = 0.82 pounds of blood meal per tree
For small orchards, you may need small amounts of fertilizer and it may be more convenient to convert pounds to ounces (dry) of fertilizer.
0.82 pounds of blood meal × 16 ounces per pound = 13.2 ounces of blood meal per tree
Amount of fertilizer for metric areas. Exactly the same procedures apply when calculating kilograms of fertilizer per hectare or 100 square meters.
Soil and tissue test recommendations in Canada are likely to be given in kilograms of nutrient per hectare. If you are converting between U.S. and metric units, remember that kg/ha = 1.12 × lbs/acre. Say that your analyses show potassium and magnesium to be deficient and you plan to correct the problem using sul-po-mag. From table 8.4, you see that sul-po-mag contains about 22 percent K2O and 11 percent magnesium. The laboratory’s recommendation is to apply 60 kg of K2O and 30 kg of Mg per hectare.
60 kg of K2O ÷ 0.22 = 273 kg of sul-po-mag per hectare
30 kg Mg ÷ 0.11 = 273 kg of sul-po-mag per hectare
One hectare = 10,000 square meters. To calculate the amount of sul-po-mag needed per 100 square meters:
273 kg per 10,000 square meters ÷ 100 = 2.73 kg of sul-po-mag per 100 square meters
If your tree density is 1,400 trees per hectare:
273 kg of sul-po-mag per hectare ÷ 1,400 trees = 0.195 kg = 195 g of sul-po-mag per tree
Amount of fertilizer in mature orchards. As our organic orchard matures and we develop organically rich soils and nitrogen-fixing alley and/or companion crops, determining how much nitrogen fertilizer to apply becomes much more complicated. We need to account for the nitrogen that is fixed in the alley and companion crops and available to our fruit crops. We also need to account for the estimated amount of nitrogen that becomes available to plants each year as the soil organic matter decomposes.
The most effective strategies are to 1) carefully observe and document tree growth and productivity each year, and 2) use annual foliar tissue analyses and compare them with your observations. Be consistent each year in where you sample and from which varieties. Keep the samples for different crops and varieties separate. Keep accurate records of alley crops, companion crops, and types and amounts of compost, fertilizers, mulches, and other materials added to the orchard. At the end of each growing season, try to determine how these practices affected your crops.
In the long term, we want to greatly reduce or eliminate the need for off-farm nitrogen fertilizers. Our focus will be on building abundant supplies of soil organic matter and developing an orchard floor management system that helps provide or conserve nitrogen and make it available to our fruit trees.
Because we are covering many different crops, ranging from a few trees to large commercial plantings, it is hard to make specific recommendations on how to apply fertilizers. It is most important to make accurate applications. For large orchards, dry, granular fertilizers and soil amendments are often broadcast using cone spreaders mounted on the backs of tractors. Some fertilizer spreader designs allow the product to be applied in narrow bands along the tree rows. Smaller cone spreaders are available that work with garden tractors and ATVs (all-terrain vehicles). For smaller orchards, handheld spreaders (belly grinders) can be used to broadcast fertilizers across the orchard floor. For a few trees, hand spreading fertilizer from a small pail works well.
Whether you are using a large commercial fertilizer spreader or are sprinkling fertilizer out of a pail, be sure you know how much material you are applying in a given area. Mark off an area on bare ground or on a parking lot, measuring it so that you know exactly how much area it covers. Carefully weigh the fertilizer and apply it across the area, then measure the amount of fertilizer you have left and calculate how much you applied to that known area. Keep practicing until you know very closely how much fertilizer you are applying to a given area in your orchard.
In an orchard, fertilizers can be broadcast across the entire orchard floor or banded within the tree rows. While both methods have advantages and disadvantages, the fact is that both work. I prefer to broadcast fertilizers across the orchard, knowing that the tree roots spread far beyond the rows and into and beyond the alleys. This strategy is particularly effective in high-density plantings with narrow alleys. Other fruit growers enjoy good success concentrating the fertilizers within tree rows. This strategy is particularly effective when the trees are planted farther apart within and between rows.
To a large degree, the orchard floor management system that you choose will determine your fertilization practices. In areas with abundant precipitation or ample overhead irrigation, you may choose to plant permanent alley crops. In drier areas, these permanent alley crops might consist of grasses that go dormant during the summer to reduce competition with the trees for moisture. Annual crops, such as cereal grains, can be planted as temporary alley crops. In arid regions where water for alley crops is unavailable or prohibitively expensive, you might maintain bare ground between the trees. With annual or perennial alley crops, you need to address their nutritional needs, as well as those of the fruit crops. In such cases, broadcasting fertilizers can be a good strategy. In a bare ground situation, fertilizers might best be kept within the drip lines or planting rows of the trees.
In summary, ensure that your trees are well fed — but not too well fed. Our goals are to establish and maintain healthy trees that produce moderate growth and sustainable harvests of high-quality fruit.