Compost Management on Organic Farms
Properties of Compost
The quality of the compost can vary widely because of the diverse natural of the raw materials used, degree of decomposition, moisture content, nutrient content, salt content, acidity/alkalinity, contaminants (organic and non-organic materials or heavy metals), and the composting method employed. Quality control during compost production should ensure adequate chemical and physical properties, as well as an adequate degree of stability and maturity. Successful use of compost relies on evaluating the soil to be amended followed by an evaluation of the compost and its properties. Some of the more important properties of mature and stable compost include: nutrient concentration, pH, EC, C:N ratio, moisture content, organic matter content, bulk density, and heavy metals.
Nutrient Concentration of Compost
Although the nutrient content of compost is low compared to synthetic fertilizer products, compost is usually applied at greater rates and therefore nutrient contribution can be significant. The most commonly required nutrients are nitrogen, phosphorus, and potassium. Composts are often analyzed for total and available nutrients.
The nitrogen content of composts will vary according to the source material and how it was composted. In general, nitrogen concentration is lower than that in animal manures but tends to be more uniform. Inorganic nitrogen, sometimes called “plant-available nitrogen,” this is composed of soluble ions (ammonium and nitrate), and is reported as ammonium-N (NH4+) and nitrate-N (NO3?). Ammonium-N and nitrate-N are released from decomposition of organic nitrogen.
Similar to nitrogen, much of the phosphorus in finished compost is not readily available for plant uptake since it is incorporated in organic matter.
Potassium in finished compost is much more available for plant uptake than nitrogen and phosphorus since potassium is not incorporated into organic matter.
Compost typically possesses a pH between 5.0 and 8.5. Liming agents are sometimes used in the production of compost.
Soluble salt concentration is the concentration of soluble ions in solution. Electrical conductivity is usually reported in units of mmhos/cm, mS/cm, or dS/m of a saturated extract of either soil or compost. Soluble salt levels in compost can vary considerably, depending of feed stock and processing.
The moisture content of compost should be between 30 to 40 percent wet weight basis. If the moisture content of the compost is too low, then the product may cause dust issues when being transported or spread.
Organic Matter Content
Organic matter is the measure of carbon based materials in the compost. High quality compost will usually have a minimum of 50 percent organic content based on dry weight.
The carbon to nitrogen ratio is a parameter used to determine if a compost is nitrogen stable. Composts that are derived primarily from wood by-products have high carbon to nitrogen ratios unless additional nitrogen is added during the composting process. Biosolids and manures generally have low carbon to nitrogen ratios since these materials are nitrogen rich.
Bulk Density is the weight per unit volume of compost. Bulk density is used to convert compost application rates from tonnage to cubic yards. In a field application, cubic yards per acre would subsequently be extrapolated to express an application rate represented as a depth in inches (e.g., 1 inch application rate).
Heavy Metals in Compost
Heavy metals are trace elements whose concentration are regulated due to the potential for toxicity to humans, animals, or plants. The quantity of these elements are measured on a dry weight basis and expressed in parts per million (ppm) or milligrams per kilogram (mg/kg).
Compost Stability and Maturity
Stability and maturity are key elements of compost quality and help to determine its fitness for purpose (suitability for a specific use).
Compost stability is a measure of how stable the organic substances are in the compost. More stable composts have less microbial activity (of the type associated with decomposition) and have less easily degradable organic matter. A product becomes more stable as it moves through the various composting phases.
Maturity is achieved through allowing the composting process to continue through a lower temperature maturation phase prior to application. Poorly composted products can contain animal and plant pathogens, weeds, excessive levels of ammonium nitrogen (can be toxic) and can cause temporary nutrient draw down (nitrogen and phosphorus).
Other Undesirable Substances in Compost
Some manure is contaminated with hormones, antibiotics, pesticides, disease organisms, and other undesirable substances. Many of the organic compounds, pathogens, protozoa, or viruses can be eliminated through high-temperature aerobic composting. Caution is advised, however, as some disease-causing agents, e.g. Salmonella and E. coli bacteria, may survive the composting process.
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