Using Manures, Composts, and Legumes

Nutrients in commercial fertilizers are highly soluble, so nutrient availability is quite predictable and nutrients are quickly available to plants. Organic fertilizers, however, vary widely in how and how quickly they make nutrients available for crops. Nutrient availability depends on the source, whether it be manure, compost, or a cover crop used as green manure. In general, these organic fertilizers mineralize and release nutrients, such as N and sulfur, at a very slow rate.

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 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.

Compost - Adding organic matter to your soil: making your own compost

Buy or make a simple composting bin (four stakes and some chicken wire) to contain your compost. Try to choose a site on grass or soil so that earthworms can move in and out of the compost heap. Start off by adding a layer of coarse material e.g. (twigs and branches) and build up the heap by adding layers of garden and kitchen wastes.

Woody stems should be cut up into small pieces. Weeds that can re-root from small pieces must be left out to die for several days before adding to the heap. Grass cuttings should be mixed with coarser material to allow air circulation and to stop them from turning into a wet mass.

Vegetable waste from the kitchen is good to add too. But cooked food and meats should be avoided because they may attract vermin. Old woollen and cotton clothes, newspaper and cardboard cut into pieces will compost if soaked and mixed in. Wood ash from fires and even waste from the vacuum cleaner can all be composted in little quantities.

To help decomposition, moisten the heap. If you want to speed things up, intersperse with layers of manure from a local farm. Try not to let the compost heap dry out completely or it will not rot down. Keep heat in and rain out by putting on a lid of newspaper, plastic sheeting or carpet.

Chemicals to accelerate decomposition are not essential but they do add bacteria and enzymes that improve composting. It is the action of these micro-organisms that generates the heat which is so beneficial in the composting process, especially for killing weed seeds. When the heap begins to shrink in size and starts to cool, take out the contents, shake up any compressed matter and return them to continue the composting process. When the compost is brown and crumbly you can add it to your soil as needed.

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 very important to make sure compost is finished, before adding it to the soil. In practical terms, this means that compost is ready or finished when it looks, feels and smells like rich, dark earth rather than rotting vegetables. In other words, it should be dark brown, small particle size, crumbly and smell like earth.

Figure 6- Manure Compost
Source: http://www.agrinord.it/gallerydetails/usi-compost-5

Immature 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, 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.

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. Composts made primarily from manures supply phosphorus over the growing season at 70 to 100% of the availability of triple superphosphate fertilizer.

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.

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. An efficient way to evaluate the effect of compost on the fertility of a soil is to obtain a soil test after applying compost. It is advised to wait 6 to 8 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 s.

Using Legumes as Nitrogen Sources

Increasingly, organic growers are using legume cover crops as green manures in rotations to meet the N needs of cash crops. Legume cover crops fix significant amounts of N for use by subsequent crops. Through a symbiotic association with the legumes, rhizobia bacteria convert atmospheric N2 into an organic form that the legume uses for growth. The accumulation of N via cover crops depends on the length of the growing season, climate, and soil conditions. Sometimes a legume that is grown as a green manure crop can supply enough biomass N to meet the entire N requirement of the next crop. This depends on the climate, species of legume, soil conditions, and the length of time the legume is allowed to grow before it is killed.

Sown shortly after harvest of a cash crop, winter and summer legume covers serve as trap crops for leftover nutrients that might otherwise be lost from the cropping system. These trap crops prevent excess N and inorganic phosphorous from leaching into ground and surface water. Legume residues contain phosphorous, potassium, and other nutrients that are recycled in relatively available forms for subsequent crop use. Where soil P and K sufficiency index values are high and soil pH is appropriate, legume cover crops can provide nitrogen for subsequent crops without contributing to problematic increases in soil P, K, and trace metal concentrations. Removing legume or other trap crop biomass from the field provides a means of reducing soil concentrations of these and other nutrients.

Environmental and Regulatory Considerations

Applications of manure, compost, and other organic amendments should be limited on fields where significant environmental hazards or concerns are present, for example on highly erodible land (HEL). Uniform application of organic materials on highly erodible land is often physically difficult. Surface-applied materials on HEL are subject to runoff. Nutrient rates should be based on realistic yield expectations (RYE) for the crop and on plant-available nitrogen or phosphorous, as described previously in this publication. For amendments with significant nitrogen content, applications should not be made to HEL fields more than 30 days before planting. Complying with this last recommendation can complicate manure management for certified organic growers. Manure cannot be applied within 120 days of harvesting a crop that will come into contact with soil or soil particles. If a leaf lettuce crop (fertilized with manure) requires 45 days from planting to harvest, manure would have to be applied at least 75 days before planting. This is in obvious conflict with the recommendation not to apply manure more than 30 days before planting.