Parameters for Compost Use

Quantified Parameters:

In order to assure successful compost use and overall satisfaction, it is suggested that quantitative data be provided to compost users by producers and marketers. The following eight quantitative parameters have been recommended to assist with this suggestion.

Definitions

pH
pH is the numerical measure of the acidity (or alkalinity), or hydrogen ion activity of the soil. The pH scale ranges from 0 to 14, with a pH of 7 indicating neutrality. Most compost has a pH of between 6 and 8. Since specified plant species can persist and flourish when grown within a specific pH range, and based on typical compost application rates, it is understood that the addition of compost can affect the pH of growing media. Therefore, to estimate the effect, which in turn will affect maintenance practices or system management, pH is a necessary parameter to be aware of. pH is adjusted through the use of such materials as lime (alkaline) and sulfur (acidic). If liming agents are used in the production of your compost product, this information should also be provided to end users. Although the addition of lime in the composting process may not dramatically effect the compost's pH, it will have a pronounced effect on calcium availability. pH adjustments of limed composts are much more difficult, due to its higher buffering capacity, and therefore, may not be appropriate for specific applications.

Soluble Salts (Salinity)
Soluble salts concentration is the concentration of soluble ions in a solution, which is measured by the ability to carry an electric current. Many nutrients are supplied to plants in salt form, while some soluble salts are more detrimental to plants than others. Most plant species have a salinity tolerance rating and maximum tolerable quantities are known. Excess soluble salts can cause phytotoxicity to plants. Soluble salts are measured in mmhos/cm or dS/m. Compost may contribute to, or dilute, the cumulative soluble salts content of growing media or soil. Manure compost tends to be low in soluble salts, where sludge (biosolids) and yard debris composts are more variable. Reduction in soluble salts content can be achieved through thorough watering (leaching). However, management practices for leaching will be dependent on the salinity of the irrigation water.

Nutrient Content (N-P-K)
Nitrogen (N), Phosphorous (P), and Potassium (K) are the three nutrients used by plants in the greatest quantities (macronutrients), and therefore, are the nutrients most often applied through fertilization. These three nutrients are measured and expressed in a dry weight basis in the form of a percentage (%). The content of phosphorous should be expressed as P2O, and potassium as K20, which are the plant available forms. The total nitrogen content should be expressed and the amount of water soluble (N03 and NH4) and insoluble nitrogen forms should be known. The content of these nutrients, as well as magnesium and calcium, should be known to allow end users to make correct decisions regarding supplemental nutrition and pH adjustment. Providing data relative to the content of other nutrients can also be helpful, and may be necessary for specific applications. Although large quantities of nutrients are not typically found in compost, in comparison to most fertilizer products, compost is usually applied at greater rates and, therefore, can have a significant cumulative effect.

Water Holding Capacity
Water holding capacity is the ability of a given volume of compost to hold water under one atmosphere of pressure. Water holding capacity measures the potential benefit of reducing the required frequency of irrigation, as well as gross water requirements. The water holding capacity should be known to allow end users to monitor, or estimate, the compost's effect on their crop watering regime and growing media. Water holding capacity is measured as a percent of dry weight.

Bulk Density
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 (i.e., 1 inch application rate). Bulk density is also used to determine the volume of compost which may be transported on a given occasion, taking into account that most vehicles have a specific maximum gross weight which may not be legally surpassed. A product's bulk density may also affect other handling issues. Bulk density is typically measured in grains per cubic centimeter, then converted to pounds per cubic yard.

Moisture Content
Moisture content is the measure of the amount of water in a compost product, expressed as a percent of total solids. The moisture content of compost affects its bulk density, and, therefore, may affect transportation. Moisture content is also relevant because it affects product handling. Compost which is dry can be dusty and irritating to work with, while compost which is wet can become heavy and clumpy, making its application more difficult and delivery more expensive.

Organic Matter Content
Organic matter content is the measure of carbon?based materials in compost. Organic matter content is typically expressed as a percentage of dry weight. To derive more accurate measure of organic matter content, materials that are combustible, but also considered to be man-made inerts, should not be considered organic matter in the expressed figure. Being aware of a product's organic matter content is necessary for determining compost application rates on specific applications, such as soil incorporation and topsoil production. In these applications, standard agricultural soil test kits are often used to determine the recommended application rate of compost. However, these application rates are specified as the quantity of organic matter needed per acre. Typically, rates are not specified for specific types of organic matter. Therefore, the compost product's organic matter content must be known to convert the application to usable form.

Particle Size
The specificity in which compost particle size is measured should be based on the product's end use or your customer's specific needs. For most applications, merely specifying the product's maximum particle size or the screen size through which it passes, is sufficient. However, for specific applications, such as potting/nursery media component, a full particle size distribution may be required. A compost's particle size distribution may be required. A compost's particle size distribution will effect the porosity of the media to which it is added. Porosity is related to the ability of a potting/nursery mix to resist water logging, and low media oxygen levels. Particle size distribution measures the amount of compost meeting a specific particle size range, by using a series of sieves (screens) to capture compost particles of specific size. Particle size distribution figures are expressed as the percent of materials retained per sieve size. A compost product's particle size may also determine its usability in specific applications. For example, a compost product with a maximum particle size of 1/2 inch or greater may not be acceptable as a turf topdressing, whereas a product with a maximum particle size of 3/8 inch or less would be acceptable.