Compost and Manure Use and Nutrient Management

Animal Manure

Animal manure is an excellent source of nutrients and organic matter. Many of the nutrients, especially nitrogen, are readily available from fresh livestock manure.  Nutrient content varies by animal species, their diets and the form of their manure. About half of the nitrogen in fresh dairy manure and 75% of the nitrogen in poultry manure is in the form of ammonia. Ammonia is subject to loss through volatilization if not incorporated immediately after spreading. In the soil, ammonia is converted to nitrate and is available for plant use or is directly absorbed as ammonium. However, nitrate is subject to leaching and large applications (more than 40 tons per acre dry or 20,000 gallons per acre liquid) should generally be avoided. There are times when readily available nitrogen is needed, but many people prefer to compost manure before field application (see below). This stabilizes the nitrogen. Manure can be mixed with other materials for composting. Manure samples can be analyzed by the Universities of Maine and Vermont Laboratories. Manure carries pathogens that are of concern to human health. Uncomposted manure should not be applied within 90 or 120 days of harvesting vegetable crops, depending on whether or not the edible part of the crop has contact with the soil. Manure applications rates are now regulated in many New England States (see Nutrient Management section for more details).

Compost

Composting livestock manure and other organic matter stabilizes the nutrients by partially decomposing the materials.  Nutrients from finished compost are more slowly released than from fresh livestock manure.  Compost is considered mature (i.e., finished) when the energy and nutrient containing materials have been combined into a stable organic mass.  At this time the pile returns to ambient temperature, and it does not reheat on mixing even though it is moist, not overly wet, and well aerated. The composting process results in a dark-brown material in which the initial constituents are no longer recognizable and further degradation is not noticeable. The length of time needed to achieve finished compost will vary with many factors and can take anywhere from a couple of weeks to over a year.

It is important to make sure compost is finished before adding it to the soil as unfinished compost may contain plant pathogens or be a weed seed bank among other problems. Application of unfinished, carbonaceous compost could affect plant growth adversely because the compost may compete for nitrogen with plant roots as the breakdown to maturity continues in the soil. Applying compost at least one week before transplanting or seeding a crop will allow a margin of safety in case the compost is immature. Immature composts made from nitrogen-rich feedstock also are often high in ammonium, which can be toxic to plant growth. High ammonium concentrations are not typically a problem if the compost is field applied, but if compost will be used in a greenhouse mix, it is important that it is low in ammonium.  

Vegetable growers can make compost on the farm although most don’t have enough raw materials to satisfy their needs. Some bring in additional materials such as municipal yard wastes to compost on site. Others purchase compost from the increasing number of commercial composters. If organic materials are imported onto a farm, it is recommended to test the soil at least every two years and obtain a test of the compost being applied.

Compost as a nutrient source. Finished compost is a dilute fertilizer, typically having an analysis of about (1-1-1 N-P2O5-K2O), but the analysis can vary greatly depending on the types of materials used to make the compost and how they were composted. Composts should be analyzed for their available N, total N, P2O5, and K2O content before application to agriculture fields.

Carbon to Nitrogen Ratio. Recommended C: N ratio for finished compost is 15-18:1.   The C: N ratio plays a crucial role in the availability of nitrogen in any organic material added to the soil. If the C: N is much above 30:1 microorganisms will immobilize (i.e., consume and make unavailable for plant uptake) soil nitrogen.  This soil nitrogen will remain unavailable until the carbonaceous material is consumed by the bacteria.

Table 8: Typical Carbon-to-Nitrogen Ratios

Material

C:N Ratio

Legume hay

15-19:1

Non-legume hay

24-41:1

Corn stalks

42:1

Oat straw

70:1

Rye straw

82:1

Cow manure

18:1

Finished compost

17-20:1

Agricultural soils

8-14:1

Hardwood sawdust

500:1

 

Nitrogen. The majority of the nitrogen in finished compost (usually over 90%) has been incorporated into organic compounds that are resistant to decomposition. Rough estimates are that only 10% to 30% of the nitrogen in these organic compounds will become available in the first season following application. Some of the remaining nitrogen will become available in subsequent years and at much slower rates than in the first year. Repeated annual applications of compost at high rates above 400 pounds of nitrogen per acre can result in excessive amounts of nitrate in the soil.

Phosphorus. There is not much research information published about the availability of phosphorus from compost. The few papers published show that composts made primarily from manures supply phosphorus over the growing season at 70 to 100% of the availability of triple superphosphate fertilizer. See discussion above in the “Building Soil Organic Matter” section for more information about soil organic matter and phosphorus management.  

Potassium. Potassium in finished compost is much more available for plant uptake than nitrogen because potassium is not incorporated into organic matter. However, some of the potassium can be leached from the compost because it is water soluble. In one study, potassium levels were reduced by 25% when finished compost was left uncovered in the open over a winter.

Soluble Salts. In general, soluble salts are not a concern from additions of composts to field soil. However, soluble salts can be a serious problem when using compost in greenhouse mixes. Incorporation of 40 tons/acre of compost in the top 6” of field soil would be a ratio of 50 parts soil to one part of compost. Compost used in the preparation of greenhouse media will make up a much greater percentage of the whole mix and therefore will have a greater influence on all aspects of fertility, including soluble salts. It is important to have composts tested for salt levels. Electrical conductivity (EC) is a measure of salt level, and compost used in greenhouse mixes should have EC < 1 mmhos/cm.

Compost and pH. The pH of finished compost is usually slightly alkaline. In general, composts will not raise soil pH to undesirably alkaline levels because of the low total alkalinity of composts. However, caution should be taken if the compost has been “stabilized” with the addition of lime (thus increasing the total alkalinity) or with heavy applications to certain crops such as potatoes, for which the soil pH should be about 5.2. Heavy applications can cause increases in soil pH that might last for a growing season.

Heavy Metals and Trace Elements. The danger of heavy metals in some composts has received much attention. At one time, some heavy metals in some composts were high enough to be toxic to plants (copper, nickel, zinc) or of concern to human health (cadmium). There have been documented cases where elements such as boron have been raised to toxic levels with repeated applications of compost. These composts with high metals or boron were made from materials with high concentrations of these elements. Governmental regulations control the materials that may be used in composts for applications to farmland. None of these toxicity problems are likely to occur with compost that has been made from farm manures or crop residues or with the commercially available composts of today.

Herbicide Residues in Compost. There are broadleaf herbicides registered for use on turfgrass, pastures, and hay crops that retain activity in the manure of animals that have fed upon them, as well as through the composting process of crop residues from areas treated with these herbicides. There have been many cases where vegetable growers have unknowingly purchased organic amendments such as manure and composts that are contaminated with herbicides and have damaged vegetable crops. If you purchase organic amendments, you should be aware of this possibility and get assurance that herbicides are not present in the manures and composts that you purchase. 

Have Compost Analyzed. No compost should be applied to field soil or used in greenhouse mixes without testing for nutrient content. If the compost will be used in greenhouse mixes, it should also be tested for maturity. Some soil test labs (see list of labs in the Soil Testing section) will test compost. Check to be sure the lab analyzes compost before submitting samples, and make sure to have it tested as a compost sample, not as field soil.

Take Soil Test After Applying Compost. A good way to evaluate the effect of compost on the fertility of a soil is to obtain a soil test after applying compost. It is best to wait six to eight weeks after application before testing the soil to allow the compost and soil to equilibrate. The soil test can measure available plant nutrients, soil pH, organic matter, and heavy metal content of the soil.