Many people think of manure nutrient value mainly in terms of nitrogen, and it’s true that manure can be a great source of economical nitrogen. From the information in the last chapter, you can see that manure also contains substantial amounts of the other macronutrients (phosphorus and potassium), the minor nutrients (calcium, magnesium, and sulfur), and the micronutrients that crops need to grow. These nutrients are all essential to getting good yields of high-quality crops and pasture, even though the amounts needed of each of them vary.
Before you apply manure, test your soil to see what nutrients your fields need and which fields could benefit the most from manure. Manure provides all the nutrients that plants need, but some soils need more added fertility to perform well than others do.
Soil samples can be tested through a university or state lab where you live, or there are many reputable privately owned labs. For the purpose of figuring out where to spread manure, the basic type of test that measures phosphorus, potassium, organic matter, and pH (acidity or alkalinity) is sufficient. Even so, tests that measure the minor and micronutrients are very useful. On some farms where most fields have adequate levels of phosphorus and potassium, it might be micronutrient levels that determine the best places to apply manure.
Manure is a very good source of nitrogen, but it also contains substantial quantities of phosphorus, potassium, and other nutrients. Because there are other ways of supplying nitrogen to crops (like plowed-down forage legume crops or green manure crops), I’d encourage you to consider the total nutrient needs of your various fields. You may find that the manure you have available is more valuable for fields that need more phosphorus and potassium than on fields that will grow crops that need a lot of nitrogen.
In chapter 1, I discussed the nutrient content of manure from various species of animals. Those numbers are based on average values in manure from many farms. The nutrient content of manure on your farm may be close to these values or substantially different. Average manure nutrient values are often good enough for less intensive cropping situations and for home gardens, but if you raise high-value crops or if you want to get the most value from your manure, you should also test the manure. If you have a fairly consistent feed and bedding system and your storage system doesn’t change, the numbers you get from this kind of testing should be fairly similar from year to year. Many soil testing labs can also do nutrient testing on manure, and they often include both the total nutrient content and an estimate of how much of the major nutrients will be available to crops the first year after you apply it.
Whether you are testing soil or manure, it’s essential to follow a good sampling procedure. There will be variations in the nutrient levels of your soil as you move across a field, as well as in the nutrient content of manure from one point in the storage system to the next. The goal in any such program is to collect representative samples that will reflect the average conditions of the whole unit. With this in mind, take lots of individual samples that you can mix together to form a good composite sample, and don’t collect samples from any places that don’t seem representative.
There are good publications on both soil and manure sampling, and you should familiarize yourself with these before you collect samples to send in to the lab for testing. Your university Extension service is an excellent place to look for references on these testing options and will provide suggestions that are appropriate for your region.
For both soil and manure tests, as well as fertilizer recommendations, it’s important to understand how the results are expressed.
Nitrogen (N) is usually expressed in its elemental equivalent when it comes to fertilizer materials; in other words, how much pure nitrogen is in the sample if it could be separated out from the rest of the material.
Phosphorus (P) in fertilizer is usually represented by its oxide equivalent (an old convention dating back to earlier days of chemistry). The chemical formula for its oxide is P2O5, usually referred to as phosphate.
Potassium (K) in fertilizer is likewise represented by its oxide equivalent, K2O, usually referred to as potash.
Soil tests may or may not show nitrogen levels, because nitrogen is difficult to measure in soils and can be present in many different forms. In fact, most of the time when we have ordinary soil samples analyzed for nitrogen, the results aren’t very useful. This is because nitrogen can be present in so many forms in soils, and labs often measure only nitrate. There are valid ways to measure soil nitrate to provide useful information, but you must follow a very specific protocol to get meaningful results. Newer tests such as the Solvita biological respiration test and the Haney test give better results, and these can be performed on ordinary soil samples.
Soil tests usually give the analysis of phosphorus and potassium listed in their elemental form, because agronomists are used to interpreting soil test results according to numbers expressed that way. But if they include recommendations for applying phosphorus and potassium fertilizer, those are usually given in the oxide form.
To further complicate the matter, the elemental concentrations in soil samples can be given as either parts per million or pounds per acre! Don’t worry, though — the conversion between these is pretty straightforward: an acre of completely dry average soil to a depth of about 6 inches is assumed to weigh 2 million pounds, so to convert parts per million to pounds per acre, just multiply by two. To convert pounds per acre to parts per million, divide by two. Easy!
When fertilizer is sold, the analysis is always listed with a set of three numbers according to this formula: Percent nitrogen as its elemental basis (N)-percent phosphate (P2O5)-percent potash (K2O). For example, a fertilizer with an analysis of 9-23-30 would translate as 9 percent elemental nitrogen, 23 percent phosphate (P2O5), and 30 percent potash (K2O).
With manure, the nutrient value is usually expressed as pounds of nutrients per ton of manure (for solid manure) or per 1,000 gallons (for liquid). Although manure contains all the nutrients necessary for plants to grow, labs don’t usually test it for anything beyond nitrogen, phosphorus, and potassium (although they occasionally test for sulfur). Manure tests are designed to measure how much fertilizer value the manure has with respect to these major nutrients. Manure test results may report the content of the major nutrients in their elemental form, as their oxide form (for phosphorus and potassium), or both.
When you’re calculating how much manure to apply in order to meet the recommendations on your soil test report, it’s usually easiest to use the oxide basis to compare the nutrient value of the manure to the amount of fertilizer it would take to supply the same amount of nutrients. Note: Some of the nutrient value will be available during the first year after you apply the manure, and some will be available the following year(s), and labs often give you estimates of both the first-year nutrient value and total nutrients. Manure is a gift that keeps on giving.
Below are the nitrogen (N), phosphorus (P2O5), and potassium (K2O) fertilizer credits per ton of manure from different livestock types and storage systems. The nitrogen credit compares availability at different times of spreading.
* The first number refers to the first-year nitrogen credit, and the number in parentheses refers to the second-year credit.
Source: “Nutrient Management Fast Facts,” University of Wisconsin Nutrient and Pest Management (NPM) Program (see Resources). The actual publication also gives figures for liquid manure and for turkey manure (which is very similar to chicken manure).
This table shows two points very nicely:
Testing is the best way to know the nutrient value of the manure you’re working with, but if you don’t have that information you can get some idea of the average value of manure from many good sources. (They vary somewhat, but they’re usually fairly close to each other.) One thing to keep in mind is that there’s a portion of the manure we apply that plants can use quickly (in the first growing season after we apply it), while some decomposes more slowly and is available later.
It’s important to understand the nutrient requirements of the crop you intend to raise where you spread manure.
Crops such as corn and potatoes generally need higher nutrient levels (especially for nitrogen) than other crops.
Legumes such as alfalfa, red clover, and birdsfoot trefoil don’t need the nitrogen that manure can provide at all if the seed has been properly inoculated and the soil pH is adequate. They rely on the bacteria living in their roots to biologically “fix” nitrogen (see Nitrogen Fixation). (When you inoculate the seed you introduce those bacteria into the soil as you plant, and you don’t usually need to re-inoculate each year.) Even if they have the bacteria to fix nitrogen, these legumes still benefit from the phosphorus, potassium, and other nutrients in the manure if the soil is deficient in these.
Many leafy vegetables are susceptible to quality problems if they get too much nitrogen, so be careful about using manure on those produce crops.
Whatever crop you’re growing, you can get excellent information on the nutrients they need from the university Extension office in your area. Many soil test reports will also provide recommendations for specific crops.
After you have your soil test results back, you’ll be able to see which fields will benefit most from the manure you have available. If all your fields test the same for nutrient levels, maybe you’ll just elect to divide the amount of manure you have across all the fields equally — there’s nothing wrong with that, as long as applying manure won’t cause any of the nutrient levels to become overloaded. It’s more typical that some fields need more fertility help than others, and manure is often used as a fertilizer to amend nutrient deficiencies in those fields or pastures.
A wheelbarrow and a pitchfork work fine for small-scale applications.
When we focus on using manure for its nitrogen value, we lose track of how good it is at providing other nutrients, especially phosphorus and potassium. Green manure crops and old hay fields can provide most of the nitrogen that many farmers need to grow good crops, but sometimes that’s not enough. For organic crop farmers who don’t have enough old hay fields to plow under to provide the nitrogen they need, manure is usually the cheapest form of supplemental nitrogen, even if they have to buy it. With this awareness, farmers often apply manure first to fields where high nitrogen-demand crops such as corn will be grown. Depending on the amount of nitrogen available from the previous crop and the overall nutrient levels in all the fields, that may or may not be the best place to use the manure.
Corn is often grown as part of a rotation that includes forage crops with a legume component of some sort (alfalfa, clover, or trefoil). Even a poor stand of legume hay or pasture can provide most or all of the nitrogen the corn crop will need. As you consider where to spread manure, look beyond its nitrogen content and capitalize on its phosphorus and potassium value. In most cases where corn follows hay, a very light application of manure will be enough for a good corn crop, and the rest of your manure supply can go to the fields that have the lowest phosphorus and potassium levels.
This is the least expensive way to provide P and K to fields that test low in these. For organic farmers, phosphorus in manure is also the most available form of that nutrient that we can apply.
To use manure as a fertilizer, check your soil test results and the manure analysis to determine how much to apply. Soil tests often provide fertilizer recommendations, so simply consider manure in terms of the equivalent amount of fertilizer it contains.
Let’s use potassium as an example. A common potassium fertilizer, potassium chloride, has a potash (K2O) equivalent of 60 percent, and it has no appreciable nitrogen or phosphorus content, so its fertilizer analysis is listed as 0-0-60. Suppose our soil test recommendations call for applying 150 pounds per acre (lb/a) of K2O. To determine how much 0-0-60 we should apply per acre:
The same method works for phosphorus, using the phosphate numbers from the soil test recommendations.
In a similar way we can determine how much manure to apply to meet the soil test recommendations. The “textbook” approach to figuring out how much manure to apply is just a matter of knowing how many nutrients are in the manure you have and how many nutrients you need to apply to the field. Then it should be simple to select the right application rate and put it on the land, right?
Well, maybe it's not quite that simple. The problem is that the proportions of the various nutrients in manure are fixed, and the proportions we need to apply to our fields vary from place to place. Soil tests almost never show that the nutrients you need to apply are in the same proportions that are in the manure, so you have to decide how best to employ this resource. Usually we pick one or two of the most pressing needs and choose a rate of manure that will meet (or come close to meeting) those needs. This often means that some other nutrients will be either under- or over-applied.
Here’s an example. A soil test reports that a field is slightly low in phosphorus and very low in potassium. The recommendation from the lab calls for applying 50 lb/a of phosphate and 300 lb/a of potash. Suppose we have horse manure on hand. According to the manure nutrient tables above, average horse manure will supply 5 lb of P2O5 and 6 lb of K2O per ton. To meet the phosphorus recommendation, we could apply 10 ton/a of the horse manure. But that amount of manure would provide much less potassium than the soil test recommended.
Small ground-driven spreaders can be pulled behind small tractors, ATVs, or horses.
If you apply the manure at the 10 ton/a rate, you could just apply another form of potassium (potassium chloride or potassium sulfate) in addition to the manure to make up the difference. Or you can increase the rate of manure to 50 tons per acre to meet the potassium recommendation.
A large liquid manure spreader with injectors.
If you do that, however, you’ll apply a lot more phosphorus than recommended. Fifty tons of manure per acre is a very high application rate. If the field doesn’t already have excessively high levels of phosphorus, this might be acceptable. If the level in the field is already high, applying this much manure could drive the phosphorus up to where it could cause environmental or agronomic problems.
There are advantages and drawbacks to either way of using the manure, depending on the background fertility and whether you have extra manure you can apply or extra money you can spend on fertilizer.
Sometimes there isn’t enough manure available to completely meet the nutrient needs of the land you’re working with. Here are a couple of principles to keep in mind.
The fields with the lowest fertility will respond best to any nutrients you’re able to add. If you have a limited supply of manure, you should apply manure to the least fertile fields first.
The first unit of fertilizer you apply to a low-testing soil will give you the biggest response. For instance, imagine a situation where a soil test indicates that you should apply 100 lb/a of phosphate. You should get a profitable response from applying the entire 100 lb/a, but if you aren’t able to do that, the first 50 lb/a that you apply will give you a greater response than the second 50 lb/a increment will. The yield response you get from manure applications works this same way — the first increment you apply gives a disproportionately high response compared to the successive increments.
If the soil nutrient levels are low, whatever you do apply will help. If all the fields have roughly the same need for extra nutrients, then put the same rate of manure on all those fields, even if it means not putting as much on each field as you’d like.
A typical rear-discharge solid manure spreader behind a tractor.
Here are some miscellaneous additional thoughts about applying manure.
Composted manure. If the manure you’re going to use has been composted (or even just well rotted from being aged a long time) it won’t behave exactly the same as fresh manure. Aside from nitrogen, most of the nutrients present in the original material are preserved during composting, but the volume is reduced substantially. This means that most of the nutrients in composted manure are more concentrated than in the fresh material. Accordingly, we can use much lower rates of compost to improve fertility than if we were using fresh manure. Some of the nitrogen content of the original manure will be lost during the composting process, so this element won’t be concentrated to the same degree that the others are.
Forage-crop nutrient removal by mechanical harvest. Each ton of forage dry matter you harvest from a field removes 12 to 15 pounds of P2O5 and 50 to 60 pounds of K2O. Depending on the type of soil you have and its starting fertility, you may have to replace these nutrients in order to keep the land productive. Where the starting fertility is low, you may need to supply even more than these amounts of nutrients as you harvest forage in order to get the land to its optimal productivity. On the other hand, if you start with excessively high levels of nutrients, you can also use forage harvests to bring these nutrient levels down to the more optimum ranges.
Grazing management. When livestock graze, they don’t just eat — they also poop and pee. Since most of the nutrients animals eat pass through their bodies, with good grazing management we can recycle most of what the animals are eating from the pasture. This means controlled grazing (such as rotational grazing or management-intensive grazing), not just having a pasture area where livestock can roam around at will for as long as they want.
If you follow excellent grazing practices, up to 60 percent of the nutrients that dairy cattle harvest can be returned to the pasture (for beef, it could be 75 percent). If you have ruminant livestock and graze them, I encourage you to develop your grazing system according to the principles of rotational grazing. For an excellent introduction to the principles of rotational grazing, see “Pastures for Profit,” published by the University of Wisconsin-Extension (see Resources).
The season you apply manure affects how it performs as a fertilizer, which influences the yield you get, and it may also affect the quality of the crop. The best time to apply manure to a certain area depends on the soil in that area and the crop you intend to grow. Here are some general rules of thumb.
If the crop is an existing perennial forage stand (a hay field or pasture), I recommend applying the manure in the late summer or fall. During that time of year, perennial plants are designed to take up nutrients and energy and store them in their roots and crowns (the base of the stem, located at the surface of the soil) so they can live through the winter and send up new shoots in the spring. Applying manure at this time of the year allows these plants to take up whatever portion of the nutrients is in the soluble form.
This not only strengthens the plants, it conserves the nutrients by holding them so they won’t leach away during the winter. This is especially important with nitrogen and potassium, the two nutrients that are most likely to be present in soluble form when the manure goes on the field.
Composted manure is loose and friable and has a nice earthy odor.
Why fall instead of spring? In the spring, the plant is bringing nutrients up out of the roots and crowns and putting it into the new growth, so the flow of nutrients is the opposite of what happens in fall. Nitrogen and potassium that are applied in the spring get taken up readily by the plants and much of that goes right into the herbage of the first crop of forage. This means that the fertilizer value of those nutrients behaves more like a “flash in the pan,” and there isn’t a lot of residual benefit for the successive cuttings or grazings of the forage.
Not only that, but if the forage is for dairy cattle, too much potassium early in the year can cause excessive levels of that nutrient in the feed for the cows, which can lead to metabolic disorders like grass tetany (hypomagnesemia) or milk fever. Nutrients applied to perennial crops in the fall strengthen the vigor of the entire plant, and they can be used by the plant during more of the following season.
If you decide to spread manure on a pasture to bump up the fertility, be aware that it will be quite a while before the livestock graze those pastures again (at least if the manure is from the same species). All livestock come designed with a self-protection feature that warns them against grazing too close to their own feces, because doing so would likely expose them to a fresh batch of internal parasites (worms), shed as eggs through the animal’s manure. (Urine doesn’t contain worm eggs and presents nowhere near the refusal problem that dung patches do.) Therefore, when we spread fresh manure on a pasture, we effectively coat the entire grazing area with material that warns away the animals we’re trying to feed.
There are two ways to get around this problem. One is to spread the manure in the fall when the stock are done grazing (by the time spring rolls around, there is unlikely to be as much refusal). The other is to spread composted (or even very well-rotted) manure. This material doesn’t smell like manure either to us or to the livestock, and it won’t cause feed refusal. Because the nutrients in compost or well-rotted manure are in a very stable form (and any internal parasites would have died by then), it’s okay to spread this material whenever it’s convenient.
If you use cover crops in your cropping system (a very good practice!), apply manure just before you plant the crop to conserve nutrients and beef up the benefits of the cover crops. This is true for whatever season you plant them.
Many farmers like to apply manure in the fall of the year before they plant corn, soybeans, or spring-seeded small grains. It’s a way to cut down on some of the work that needs to be done in the spring and often helps control perennial weeds that have established in the preceding crop. This can be a good practice in some cases, but it’s best to do it in conjunction with a cover crop or an existing forage stand if at all possible, so that you won't lose nutrients to leaching over the winter.
Never apply manure to bare ground in the fall on fields with sandy soil, since many of the soluble nutrients will most likely leach away over winter. On sandy ground, it’s almost always better to apply the manure in the season you’ll plant, so that the growing plants can use the nutrients as they become available.
If you grow vegetable crops, incorporate manure into the ground at least 90 days before harvesting any crop. If the crop you are growing has direct contact with the soil surface (potatoes, onion, radishes, etc.) then you must wait at least 120 days between application and harvest. This time restriction doesn’t apply if the manure is composted in accordance with the NOP rules. Aged manure that has not gone through the documented composting process, even if it’s as old as the hills, is still considered to be raw manure, and you need to follow the time interval rules.
Knowing how much manure we want to spread is important, but you must also know the amount you are actually applying, so as to adjust it as needed. If you use a spreader, it’s a good idea to calibrate it so that you know what your application rate is.
Calibrating a spreader boils down to measuring the weight or volume of manure in one spreader load and then calculating the area that one load covers. If we divide the weight (usually expressed in tons) or volume (usually expressed in gallons) by the area of land (usually expressed in acres) we have our application rate. Knowing the nutrient value of the manure we spread allows us to calculate how many pounds of the major nutrients we’re applying.
Many soil and water land conservation departments or Extension offices offer help with calibrating manure spreaders. If that isn’t an option or you prefer to do this yourself, it’s not hard. There are a number of good references available that give detailed information on calibrating manure spreaders. One recommended resource is the University of Vermont (see Resources).
There are several ways to calibrate manure application systems.
The first method involves doing a simple calculation considering the volume (or weight) of the full spreader and the amount of land covered by that load. You’ll determine the amount of weight your spreader can hold in tons, and divide that by the amount of land in acres covered by one full load.
Step 1: Determine manure quantity. If you have liquid manure, the specifications for your spreader will give you an accurate figure for the volume of a full spreader load. With solid manure, the most accurate way to know how much weight you apply with each load is to weigh the full spreader. University Extension offices or land and water conservation departments in your area may have portable scales you can use to do this. Otherwise it will involve a couple of trips across a load scale — one with the spreader empty and another with it full. These types of scales are common at feed mills and trucking companies.
Another method for solid manure is to estimate the volume of the load you have, and then use average manure density values to estimate the weight of a load. According to the University of Vermont, solid cattle manure varies from 55 to 62 lb/cubic foot, depending on moisture and the type of bedding used. Not only does the density of solid manure vary a lot, but it’s also hard to estimate just how much volume is in a load because of the many different ways people load spreaders. This method is a lot better than just guessing, but it would be easy to make a substantial miscalculation this way.
Step 2: Determine land covered. Next, measure the amount of land you cover with one load. For example, let’s say we find that the spreader covers a swath 20 feet wide and we can drive 435 feet to empty the load. This covers 8,700 square feet (20 feet times 435 feet). We need to convert this to the equivalent land area expressed as acres. There are 43,560 square feet in one acre. The land area we covered is 8,700 square feet, so if we divide this by 43,560 square feet per acre, we get about 0.2 acre.
Step 3: Calculate rate. Finally, divide the weight of the manure in a full load by the area covered. For instance, if we know that our spreader holds 6 tons of manure with each load and we cover 0.2 acres, our application rate is 6 tons divided by 0.2 acres, or 30 tons per acre (t/a). That’s a fairly heavy application rate. In most cases it’s probably not a good idea to spread heavier than that and 20 t/a would be even better for a single application. (With poultry manure, you should use much lower application rates than you would with larger livestock because the nutrient content is much higher. I usually suggest applying no more than around 2 t/a of poultry manure.)
If you can’t weigh the spreader and you aren’t confident of the volume of the spreader, there’s another simple way to calibrate. This method involves using three (or more) tarps (or uniform-sized pieces of heavy plastic sheeting) that you put in the path you’re spreading. With this method, you collect the manure that lands on each of the sheets and actually weigh it. Suppose the sheets you use are 6 feet by 6 feet. That’s 36 square feet per sheet. Now let’s say the average weight of the manure we caught on the sheets is 20 lb. To find the equivalent application rate in tons per acre, we can use this formula:
Plugging in the numbers from the example, this is (20 lb/36 square feet × 43,560 square feet/acre) divided by 2,000 lb/t = 12 t/a (rounded to the nearest ton).
The reason for using three sheets is that most manure spreaders don’t do a very good job of macerating the manure as it comes out, and there can be a lot of variability in what actually lands on the ground from one place to another. You can also use these three tarps to see how the application rate varies from one side of the spreading path to the other. A lot of beater-type spreaders put more manure directly behind than they do on the sides of the travel path. To check this, place one tarp in the center of the path and one on each side of the spreading pattern. If you see a much heavier application rate in the center of the spread, you may choose to overlap your spreads a bit to compensate.
If you find that your spreading rate is higher or lower than you’d prefer, change the rate by one of three methods: adjust your spreader, adjust your speed, or adjust your load. You may or may not be able to easily adjust the rate that you’re spreading. Some spreaders allow you to change settings to adjust the rate somewhat. If the terrain is forgiving, you may be able to adjust your travel speed. Driving slower (by driving in a lower gear) will increase your application rate, and driving faster while you spread will decrease your rate. With a box-type (horizontal beater) spreader and solid manure, adjusting the depth of the manure in the load may give you some flexibility in the rate you apply. Loading the spreader to a lower height will result in a lower application rate if you can’t increase the ground speed enough, but it will also mean more trips across the field.
This discussion has focused on solid manure systems, because they are more common on small farms. If you use a liquid manure system, calibrating your spreader is even easier, because the manufacturer’s information for your spreader will tell you its capacity in gallons. Unlike a box-type solid manure spreader, when a liquid spreader is full, it’s full — there’s no guesswork involved. Calibration becomes a simple matter of dividing the number of gallons in one full load by the area of land you cover as you spread it. Simply multiply the width of the spread by the length in feet, and divide this number of square feet by 43,560 to determine the acreage. Then divide the weight of the manure by the acreage to determine the application rate.
If you’re a gardener or a small-scale produce grower, you may be interested in spreading manure by hand. The same principles apply for small plots: determine the amount of manure you’d like to apply, and then distribute it uniformly over the appropriate amount of land area. Depending on the lab you use for soil testing and the needs of your crop, you may get recommendations based on pounds of nutrients per 100 square feet or per 1,000 square feet. Weigh the container you use to haul the manure, such as a 5-gallon bucket, and determine how many of those buckets you need to apply over the desired area. Keep in mind that this isn’t as precise as brain surgery — it’s perfectly fine to round numbers off to make things easier.
Here’s an example: You have a garden that’s 35 feet wide and 70 feet long. That’s 2,450 square feet. The soil test report recommends applying various nutrients, but phosphorus seems to be the most limiting factor. The report calls for applying 4.5 lb of P2O5 per 1,000 square feet.
Weigh plastic pails full of manure to determine application rates for small areas.
The way you apply manure has a lot to do with how much benefit your land and crops get from it. Manure can either be spread on top of the ground (top-dressed) and left for weather and the soil biology to deal with, or it can be incorporated into the soil by cultivation.
You can spread composted manure with a feed bag right in the rows of your garden.
Whatever system you use, you should be putting the manure where it does the most good. You should also apply it at rates that are appropriate for what the soil needs with respect to the crops you’ll grow on that piece of ground. An application system that can distribute the manure uniformly across the area being spread is always the best.
Top-dressing manure is often the quickest, easiest, and least expensive way to apply it. Top-dressed manure can act like a mulch, shading the ground from the hot sun and helping to conserve moisture. There are also some drawbacks to doing this. Some of the nitrogen in uncomposted top-dressed manure will be lost as ammonia volatizes, and less soluble nutrients in manure on top of the ground will take a long time to become available to the crop. Manure on the surface of the soil can be washed away during intense rainfall or rapid snowmelt. During dry weather, some of the manure on the soil surface may not continue to break down until more rainfall wets it again.
Incorporating manure means putting it underground either by tilling it in after spreading it or by injecting it (for liquid manure systems). This requires more equipment, and it usually is only practical where the ground is going to be worked up to prepare for the next crop. Incorporating manure cuts down on odor and conserves more of the nitrogen in the manure, because any ammonia gas that would otherwise escape to the atmosphere is held in the soil.
If you use ordinary tillage implements to incorporate manure, it’s usually better to work the manure into the ground with a disc or harrow rather than using a moldboard plow. Organic matter like manure, or even a heavy sod that is only plowed under, often gets buried at the base of the plow layer where there is little oxygen. In this condition, the manure will basically be entombed in an anaerobic state. Without enough oxygen, the manure doesn’t decompose naturally as it would if air were available. Anaerobic decomposition proceeds very slowly and can actually produce gases that are toxic to the desirable organisms in the soil.