Sustainable Agriculture Research and Education - Publications - Building Soils for Better Crops- Chapter 9 - Animal Manures for Increasing Organic Matter and Supplying Nutrients

The quickest way to rebuild a poor soil is to practice dairy farming, growing forage crops,
buying . . . grain rich in protein, handling the manure properly,
and returning it to the soil promptly.
J. L. Hills, C. H. Jones, and C. Cutler, 1908

Once cheap fertilizers became widely available after World War II, many farmers, extension agents, and scientists looked down their noses at manure. People thought more about how to get rid of manure than how to put it to good use. In fact, some scientists tried to find out the absolute maximum amount of manure that could be applied to an acre without reducing crop yields. Some farmers who didn't want to spread manure actually piled it next to a stream and hoped that next spring's flood waters would wash it away. We now know that manure, like money, is better spread around than concentrated in a few places. The economic contribution of farm manures can be considerable. The value of the nutrients in manure from a 70-cow dairy farm may exceed $7,000 per year; manure from a 50-sow farrow-to-finish operation is worth about $4,000; and manure from a 20,000-bird broiler operation is worth about $3,000. The other benefits to soil organic matter build-up, such as enhanced soil structure and better diversity and activity of soil organisms, may double the value of the manure. If you're not getting the full fertility benefit from manures on your farm, you may be wasting money.

Animal manures can have very different properties, depending on the animal species, feed, bedding, and manure-storage practices. The amounts of nutrients in the manure that become available to crops also depend on what time of year the manure is applied and how quickly it is worked into the soil. In addition, the influence of manure on soil organic matter and plant growth is influenced by soil type. In other words, it's impossible to give blanket manure application recommendations. They need to be tailored for every situation.

We'll deal mainly with dairy cow manure, because there's more information about its use on cropland. We'll also offer general information about the characteristics and uses of some other animal manures.

Solid versus Liquid
The type of barn on the farmstead frequently determines how manure is handled on a dairy farm. Dairy-cow manure containing a fair amount of bedding, usually around 13 to 20 percent dry matter, is spread as a solid. This is most common on farms where cows are kept in individual stanchions. Liquid manure-handling systems are common where animals are kept in a "free stall" barn with little bedding. Liquid manure is usually in the range of from 2 to 10 percent dry matter (90 percent or more water). Manures with characteristics between solid and liquid are usually referred to as semi-solid or slurry, depending on the method of handling.

Composting manures is becoming an increasingly popular option for farmers. By composting manure you help stabilize nutrients, have a smaller amount of material to spread, and have a more pleasant material to spread (and if neighbors have complained about manure odors, that might be a big plus). Although it's easier to compost manure that has been handled as a solid, some farmers are separating the solids from liquid manure and then irrigating with the liquid and composting the solids. For a more detailed discussion of composting, see chapter 12.

Storage of Manure
Researchers have been investigating how best to store manure to reduce the problems that come with year-round manure spreading. Storage allows the farmer to apply manure when it's best for the crop and during appropriate weather conditions. This reduces nutrient loss from the manure caused by water runoff from the field. However, significant losses of nutrients from stored manure also may occur. One study found that, during the year, dairy manure stored in uncovered piles lost 3 percent of the solids, 10 percent of the nitrogen, 3 percent of the phosphorus, and 20 percent of the potassium. Covered piles or well-contained liquid systems, which tend to form a crust on the surface, do a better job of conserving the nutrients and solids than unprotected piles. Poultry manure, with its high amount of ammonium, may lose 50 percent of its nitrogen during storage as ammonia gas volatilizes, unless precautions are taken to conserve nitrogen.

A high percentage of the nutrients in feeds passes right through animals and ends up in their manure. Over 70 percent of the nitrogen, 60 percent of the phosphorus, and 80 percent of the potassium in feeds may be available in manures for use on cropland. In addition to the nitrogen, phosphorus, and potassium contributions given in table 9.1, manures also contain significant amounts of other nutrients, such as calcium, magnesium, and sulfur. In regions where the micronutrient zinc tends to be deficient, there is rarely any deficiency on soils receiving regular manure applications.

The values given in table 9.1 must be viewed with some caution, because the characteristics of manures from even the same type of animal may vary considerably from one farm to another. Differences in feeds, mineral supplements, bedding materials, and storage systems make manure analysis quite variable. Yet, as long as feeding, bedding, and storage practices remain unchanged on a given farm, manure characteristics will be similar from year to year.

The major difference among all the manures is that poultry manure is significantly higher in nitrogen and phosphorus than the other manure types. This is partially due to the difference in feeds given poultry versus other farm animals. The relatively high percentage of dry matter in poultry manure is also partially responsible for the higher analyses of certain nutrients, when expressed on a wet ton basis.

It is possible to take the guesswork out of estimating manure characteristics; most soil-testing laboratories will now analyze manure. Manure analysis should become a routine part of the soil fertility management program on animal-based farms.

Effects on Organic Matter
When considering the influence of any residue or organic material on soil organic matter, the key question is the amount of solids returned to the soil. Equal amounts of different types of manures will have different effects on soil organic matter levels. Dairy and beef manure contain undigested parts of forages, as well as bedding. They, therefore, have a high amount of complex substances, such as lignin, that do not decompose readily in soils. Using this type of manure results in a much greater long-term influence on soil organic matter than does a poultry manure without bedding. More solids are commonly applied to soil with solid manure-handling systems than with liquid systems, because greater amounts of bedding are usually included.

When conventional tillage is used to grow a crop (such as corn silage), where the entire above-ground portion is harvested, research indicates that an annual application of 20 to 30 tons of the solid type of dairy manure per acre is needed to maintain soil organic matter (table 9.2). As discussed above, a nitrogen-demanding crop, such as corn, may be able to use all of the nitrogen in 20 to 30 tons of manure. If more residues are returned to the soil by just harvesting grain, lower rates of manure application will be sufficient to maintain or build up soil organic matter.

Application of manures causes many soil changes biological, chemical, and physical. A few of these types of changes are indicated in table 9.2, which contains the results of a long-term experiment in Vermont with continuous corn silage on a clay soil. Manure counteracted many of the negative effects of a monoculture cropping system in which few residues are returned to the soil. Soil receiving 20 tons of dairy manure (wet weight, including bedding) maintained organic matter and CEC levels and close to the original pH (although acid-forming nitrogen fertilizers also were used). Manures, such as dairy and poultry, have liming effects and actually counteract acidification.

High rates of manure addition caused a build-up of both phosphorus and potassium to high levels. Soil in plots receiving manures were better aggregated and less dense and, therefore, had greater amounts of pore space than fields receiving no manure.

An example of how manure addition might balance annual loss is given in figure 9.1. One large Holstein "cow year" worth of manure is about 20 tons. Although 20 tons of anything is a lot, when considering dairy manure, it translates into a much smaller amount of solids. If the approximately 5,200 pounds of solid material in the 20 tons is applied over the surface of one acre and mixed with the 2 million pounds of soil present to a 6-inch depth, it would raise the soil organic matter by about 0.3 percent. However, much of the manure will decompose during the year, so the net effect on soil organic matter will be even less. Let's assume that 75 percent of the solid matter decomposes during the first year and the carbon ends up as atmospheric CO₂. At the beginning of the following year, only 25 percent of the original 5,200 pounds, or 1,300 pounds of organic matter is added to the soil. The net effect is an increase in soil organic matter of 0.065 percent (the calculation is [1,300/2,000,000] x 100). Although this does not seem like much added organic matter, if a soil had 2.17 percent organic matter and 3 percent of this was decomposed annually during cropping, then the loss would be 0.065 percent per year and the manure addition would just balance this loss.

Figure 9.1 Example of dairy manure addition just balancing soil organic matter losses.

Manures, like other organic residues that decompose easily and rapidly release nutrients, are usually applied to soils in quantities judged to supply sufficient nitrogen for the crop being grown in the current year. It might be better for building and maintaining soil organic matter to apply manure at higher rates, but doing so may cause undesirable nitrate accumulation in leafy crops and excess nitrate leaching to groundwater. High nitrate levels in leafy-vegetable crops are undesirable in terms of human health, and the leaves of many plants seem more attractive to insects. In addition, salt damage to crop plants can occur from high manure application rates, especially when there is insufficient leaching by rainfall or irrigation. Very high amounts of added manures, over a period of years, also lead to high soil phosphorus levels (table 9.2). It is a waste of money and resources to add unneeded nutrients to the soil, nutrients which will only be lost by leaching or runoff, instead of contributing to crop nutrition.

Application Rates A common per-acre rate of dairy-manure application is 10 to 30 tons fresh weight of solid, or 4,000 to 11,000 gallons of liquid manure. These rates will supply approximately 50 to 150 pounds of available nitrogen (not total) per acre. If you are growing crops that don't need that much nitrogen, such as small grains, 10 to 15 tons of solid manure should supply sufficient nitrogen per acre. For a crop that needs a lot of nitrogen, such as corn, 20 to 30 tons per acre may be necessary to supply its nitrogen needs. Low rates of about 10 tons per acre are also suggested for each of the multiple applications used on a grass hay crop. In total, grass hay crops need at least as much total nitrogen applied as does a corn crop. There has been some discussion about applying manures to legumes. This practice has been discouraged because the legume uses the nitrogen from the manure, and much less nitrogen is fixed from the atmosphere. However, the practice makes sense on animal farms where there is excess nitrogen.

For the most nitrogen benefit to crops, manures should be incorporated into the soil immediately after spreading on the surface. About half of the total nitrogen in dairy manure comes from the ammonium (NH₄⁺) in urine. This ammonium represents almost all of the readily available nitrogen present in dairy manure. As materials containing urea or ammonium dry on the soil surface, the ammonium is converted to ammonia gas (NH₃) and lost to the atmosphere. If dairy manure stays on the soil surface, about 25 percent of the nitrogen is lost after one day and 45 percent is lost after four days but that 45 percent of the total represents around 70 percent of the readily available nitrogen! This problem is significantly lessened if about ½ inch of rainfall occurs shortly after manure application, leaching ammonium from manure into the soil. Leaving manure on the soil surface is also a problem because runoff waters may carry significant amounts of nutrients from the field. When this happens, crops don't benefit as much from the manure application and surface waters become polluted. Some liquid manures those with low solids contents penetrate the soil more deeply. When applied at normal rates, these manures will not be as prone to lose ammonia by surface drying.

Other nutrients contained in manures, in addition to nitrogen, make important contributions to soil fertility. The availability of phosphorus and potassium in manures should be similar to that in commercial fertilizers. (However, some recommendation systems assume that only around 50 percent of the phosphorus and 90 percent of the potassium is available.) The phosphorus and potassium contributions of 20 tons of dairy manure is approximately equivalent to about 30 to 50 lbs. of phosphate and 180 to 200 lbs. of potash from fertilizers. The sulfur content as well as trace elements in manure, such as the zinc previously mentioned, also add to the fertility value of this resource.

Because one-half of the nitrogen and almost all of the phosphorus is in the solids, much of these nutrients remain in sediments at the bottom when a liquid system is emptied without properly agitating the manure. On the other hand, almost all of the potassium will be applied with the liquid portion, even if it's applied without the solids. A manure system that allows significant amounts of surface water penetration and then drainage, such as a manure stack of well-bedded dairy or beef cow manure, may lose a lot of potassium. The 20 percent leaching loss of potassium from stacked dairy manure mentioned above occurred because potassium was mostly found in the liquid portion of the manure.

Timing of Applications
Manures are best applied to annual crops, such as corn, small grains, and vegetables, in one dose just before soil tillage (unless a high amount of bedding is used, which might tie-up nitrogen for a while see discussion of C:N in chapter 8). This allows for rapid incorporation by plow, chisel, harrow, or disk. Even with reduced tillage systems, application close to planting time is best, because the possibility of loss by runoff and erosion is reduced. It also is possible to inject liquid manures either just before the growing season starts or as a sidedress to row crops. Fall manure applications on annual row crops, such as corn, may result in considerable nitrogen loss, even if manure is incorporated. Losses of nitrogen from fall-applied manure in humid climates may be around 50 percent resulting from leaching and denitrification before nitrogen is available to next year's crop.

Without any added nitrogen, perennial grass hay crops are constantly nitrogen deficient. Application of a moderate rate of manure about 50 lbs. worth of available nitrogen in early spring and following each harvest is the best way to apply manure. However, wet soils in early spring may not allow manure application without causing significant compaction.

Although the best use of manure is to apply it near the time when the crop needs the nutrients, sometimes insufficient storage capacity causes farmers to apply it at other times. In the fall, manure can be applied to grasslands that don't flood or to tilled fields that will either be fall plowed or planted to a winter cover crop. Although legal in most states, it is not a good practice to apply manures when the ground is frozen or covered with snow. The nutrient losses that can occur with runoff from winter-applied manure are both an economic loss to the farm as well as an environmental hazard. Winter spreading should be done only on an emergency basis. However, new research on frost tillage has shown that there are windows of opportunity for incorporating winter-applied manure during periods when the soil has a shallow frozen layer, 2 to 4 inches thick (see chapter 15). Farmers may use this time window to inject manure during the winter.

Excessive manure applications may cause plant-growth problems. It is especially important not to apply excess poultry manure, because the high soluble-salt content can harm plants.

Plant growth is sometimes retarded when high rates of fresh manure are applied to soil immediately before planting. This problem usually doesn't occur if the fresh manure decomposes for a few weeks in the soil and can be avoided by using a solid manure that has been stored for a year or more. Injection of liquid manure sometimes causes problems when used on poorly drained soils in wet years. The extra water applied and the extra use of oxygen by microorganisms may mean less aeration for plant roots.

Ever hear of E. Coli 0157:H7?

A bacteria strain known as E. Coli (an abbreviation that is pronounced e-COLE-eye) 0157: H7 has caused numerous outbreaks of severe illness in people who ate contaminated meat. It also has caused one known outbreak when water used to wash lettuce was contaminated with animal manure.

This particular bacteria is a resident of cows' digestive systems. It does no harm to the cow, but probably because of the customary practice of feeding low levels of antibiotics when raising cattle it is resistant to a number of commonly used antibiotics. This problem only reinforces the common sense approach to manure use. When using manure that has not been thoroughly composted to grow crops for direct human consumption especially leafy crops like lettuce that grow low to the ground and root crops such as carrots and potatoes special care should be taken. Before planting your crop, avoid problems by planning a three-month period between incorporation and harvest. For short season crops, this means that the manure should be incorporated long before planting. Although there has never been a confirmed instance of contamination of vegetables by E. Coli 0157: H7 or other disease organisms from manure incorporated into the soil as a fertility amendment, being cautious and erring on the side of safety is well justified.

When manures are applied regularly to a field to provide enough nitrogen for a crop like corn, phosphorus and potassium may build up to levels way in excess of crop needs (see table 9.2). Erosion of phosphorus-rich topsoils contributes sediments and phosphorus to streams and lakes, polluting surface waters. When very high phosphorus build-up occurs from the continual application of manure applied at rates to satisfy crop nitrogen needs, it may be wise to switch the application to other fields or to use strict soil-conservation practices to trap sediments before they enter a stream. Including rotation crops, such as alfalfa that do not need manure allows a "draw-down" of phosphorus that accumulates from manure application to grains. (However, this may mean finding another location to apply manure. For a more detailed discussion of nitrogen and phosphorus management, see chapter 17.)

Farms that purchase much of their animal feed may have too much manure to safely use on their own land. Although they don't usually realize it, they are importing large quantities of nutrients in the feed that remain on the farm as manures. If they apply all these nutrients on a small area of soil, nitrogen and phosphorus pollution of groundwater and surface water will occur. It is a good idea to make arrangements with neighbors for use of the excess manure. Another option, if local outlets are available, is to compost the manure (see chapter 14) and sell the product to vegetable farmers, garden centers, landscapers, and directly to home gardeners.

Sources
Elliott, L. F., and F. J. Stevenson, eds. 1977. Soils for Management of Organic Wastes and Wastewaters. Soil Science Society of America.

Madison, WI. Madison, F., K. Kelling, J. Peterson, T. Daniel, G. Jackson, and L. Massie. 1986. Guidelines for Applying Manure to Pasture and Cropland in Wisconsin. Agricultural Bulletin A3392. Madison, WI.

Magdoff, F. R., and J. F. Amadon. 1980. Yield trends and soil chemical changes resulting from N and manure application to continuous corn. Agronomy Journal 72:161164. See this reference for dairy manure needed to maintain or increase organic matter and soil chemical changes under continuous cropping for silage corn.

Magdoff, F. R., J. F. Amadon, S. P. Goldberg, and G. D. Wells. 1977. Runoff from a Low-cost Manure Storage Facility. Transactions of the American Society of Agricultural Engineers 20:658660, 665. This is the reference for the nutrient loss that can occur from uncovered manure stacks.

Magdoff, F. R. and R. J. Villamil, Jr. 1977. The Potential of Champlain Valley Clay Soils for Waste Disposal. Proceedings of the Lake Champlain Environmental Conference, Chazy, NY. July 15, 1976.

Maryland State Soil Conservation Committee. Undated. Manure Management HandbookA Producer's Guide. College Park, MD.

Ontario Ministry of Agriculture and Food. 1994. Livestock and Poultry Waste Management. Best Management Practices Series. Available from the Ontario Federation of Agriculture, Toronto, Ontario (Canada).

Ontario Ministry of Agriculture and Food. 1997. Nutrient Management. Best Management Practices Series. Available from the Ontario Federation of Agriculture, Toronto, Ontario (Canada).

Soil Conservation Society of America. 1976. Land Application of Waste Materials. Soil Conservation Society of America. Ankeny, IA.

van Es, H.M., A.T. DeGaetano, and D.S. Wilks. 1998. Space-time upscaling of plot-based research information: frost tillage. Nutrient Cycling in Agroecosystems 50:8590.