Manure and Compost

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 Regulations under Fundamentals of Soil Health and Soil Fertility).

Nitrogen in manures and other waste products: The N content of manures is highly variable. Differences are due to the species of animal, the animal's age and diet, the moisture content of the manure, handling and storage, and the amount of bedding in the manure. The N fertilizer equivalent of a manure varies not only with the total N content of the manure, but also with the timing and method of manure application. The values in Table 7 are based on analyses of Vermont manures as well as published data from other states. If specific manure analysis data is not available, growers should estimate N credits using these or other book values. The time elapsed between spreading and incorporation of manure is also important. About half of the N in dairy manure and three quarters of the N in poultry manure is in the form of ammonium (NH4) which easily turn to ammonia gas (NH3) and be volatilized (lost to the air). The longer that manure is left on the soil surface, and not incorporated, the greater NH3 volatilization losses become (Table 7a). Broadcast application of slurry manure without incorporation should be avoided at all times because this method increases air contact and allows time for all ammonia to be lost. Research has shown that in reduced or no-till fields where manure must be surface applied without incorporation, ammonia can be best conserved if applied during cold temperatures, low wind speeds and especially to a growing cover. A growing cover also reduces manure run-off and leaching losses.  NOTE: Manure often contains human pathogens. Serious illness has occurred from eating produce where fresh manure was applied without an adequate waiting period (see Food Safety).

Previous manure applications: Up to 50% of the total N in cow manure is available to crops in the year of application. Between 5% and 10% of the total N applied is released the year after the manure is added. Smaller amounts are furnished in subsequent years. The quantity of N released the year after a single application of 20 tons per acre of cow manure is small (about 15 lb N per acre). However, in cases where manure has been applied at high rates (30 to 40 tons per acre) for several years, the N furnished from previous manure increases substantially. The buildup of a soil's capacity to supply N resulting from previous applications of manure has important consequences for efficient N management, including: 1) The amount of fertilizer N needed for the crop decreases annually; and 2) If all the crop's N needs are being supplied by manure, the amount of manure needed decreases yearly.

WIth poultry manure (as compared with manure from cattle) a higher percentage of the total N in the manure is converted to plant-available forms in the year of application. Consequently, there is relatively less carry-over of N to crops in succeeding years. This does not mean, however, that there is never any carry-over of N from poultry manure applications. If excessive rates of poultry manure (or commercial N fertilizers) are used, high levels of residual inorganic N, including nitrate (NO3), may accumulate in soil. High levels of soil nitrate in the fall, winter and spring have the potential to pollute groundwater and coastal seawater.

Table 7: Nitrogen Credits from Manure Applied Before Planting

Type of manure Dry Matter Total N NH4-N Organic N P2O5 K2O
   

------------------------- lbs/1,000 gallons  -----------------

Dairy, liquid <5% 12-16 4.9 7.3 4.8 15.1
Dairy, slurry 5%-10% 22.3 7.6 14.7 8.9 22.0
    ---------------------------  lbs/ton  -----------------------
Dairy, semi-solid 10%-20% 8.5 1.8 6.7 4.1 6.1
Dairy, solid >20% 5-12 1.4 10.9 8.1 10.0
Beef (paved lot) 29% 14 5 9 9 13
Swine (hoop barn) 40% 26 6 20 15 18
Sheep 25% 23 n/a n/a 8 20
Poultry, layer 41% 16-37 18 19 55 32
Poultry, broiler 69% 75 15 60 27 33
Horse 20% 12 n/a n/a 5 9

Adapted from Nutrient Recommendations for Field Crops in Vermont (2018). Dairy manure values are from Vermont samples analyzed by University of Maine, 2012-2016, others are adapted from University of Nebraska-Lincoln NebGuide G 1335 and Penn State Agronomy Guide (2016). Values do not include bedded pack. Manures vary greatly, so obtaining a manure analysis is always best practice. n/a = data not available.

Table 7a: Availability of ammonium nitrogen from spring or summer applied manure (% fertilizer N equivalent)

 

Cattle1

Thin (<5% DM)

cattle

MEdium (5%-10% DM)

cattle

Semi-SOlid (>10% DM)

Cattle

Solid (>20% DM)

poultry

Solid (>20% DM)

Time to incorporation by tillage or rain -------------- % NH4 - N available to crop -----------
Immediate 95 95 90 95 95
< 8 hrs 80 70 60 80 90
1 day 70 55 40 60 85
2 days 65 50 30 45 80
3-4 days 65 45 23 35 70
5-7 days 60 40 25 25 60
>7 days, or not incorporated 60 40 20 10 50
1 Dairy cattle or other livestock.  Adapted from Nutrient Recommendations for Field Crops in Vermont (2018).

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 most of the easily decomposed components of the material have been broken down and biological activity has slowed. At this time, the pile returns to ambient temperature, and it does not reheat when mixed or turned. 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.

Application of unfinished compost could affect plant growth adversely because the compost-making microbes may compete with the crop for nitrogen. 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 are also often high in ammonium, which can change to ammonia gas and 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 be 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 manure or municipal yard wastes to compost on-site. Others purchase compost from the increasing number of commercial composters. 

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. The 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 ratio 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. The amount of organic amendments that can be added without building up excessive phosphorus depends primarily on: 1) the existing soil test P level of the field; and 2) the P2O5 content of the amendment. Table 9 shows the effect of both soil test P categories and the P2O5 concentration of an organic amendment on the suggested maximum amount of material to apply. If these rates of amendments are applied every year, analyze the soil for extractable P annually to ensure that soil test P has not risen to excessive levels. Large additions when the soil test P level is Optimum could increase the soil test P to Above Optimum levels, which would preclude the application of more compost until the soil test P level fell back into the Optimum range.

Table 9. Maximum Compost or Organic Amendment Application and total P2O5 per Soil Test Category and P2O5 Concentration1

  Soil test phosphorus (P) Category

Compost/organic amendment P2O5 content

Very Low/Low Optimum Above Optimum
% P2O5 (dry wt.) P2O5 (lb/acre) Compost (tons/acre) P2O5 (lb/acre) Compost (tons/acre)  
Low (0.1-0.5%)1 330 120 82 30 No application
Medium (0.5-1.5%) 330 30 55 5 No application
High (1.5-3.0%) 330 15 No application No application

1 Assumes moisture content of the compost or organic amendment of 45%.

2 Average rates used to calculate amounts of P2O5 applied for various rates of compost applications.

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 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.