Composts

Nontraditional Soil Amendments

COMPOSTS

Composts technically should not be called by-product materials. Instead, they are intentionally produced from waste or by-product organic materials. Composting is nothing more than managing the natural process of decay to greatly accelerate the biological decomposition of organic materials. Properly produced composts are stable and will not undergo further rapid decomposition in the soil. Thus, the greatest benefit of composts for agricultural soils is the organic matter they add. Increasing soil organic matter content improves soil tilth, increases water infiltration and water storage, and increases cation exchange capacity. Composts contain low levels of plant nutrients and would equate to a very low-analysis fertilizer. Most of the nitrogen in composts is in an organic form, and because of the stable nature of the organic matter in compost, it is converted to plant-available mineral nitrogen forms (ammonium and nitrate) at very slow rates. For most composts, only about 10 percent of the organic nitrogen will become available during the year following application.

One of the most critical characteristics of compost is its carbon to nitrogen (C: N) ratio. Fully mature and stable compost will have a C:N ratio between 30:1 and 10:1. If the C:N ratio is above 30:1, soil application of the compost could result in immobilization of soil N and a temporary unavailability of nitrogen to crops. High-quality compost also will be low in soluble salts and trace elements, will not contain recognizable feedstock (leaves, pieces of wood), and will be free of inert contaminants such as plastic, glass, and metal. High-quality compost also has a high commercial value and is generally sold in the landscaping and horticultural markets at prices that preclude its economical use for agronomic crop production. High-quality manure compost may be economically produced and used on-farm. Lower-quality composts may be economically feasible for field crop production, but farmers need to be careful about using them. All composts should be analyzed for stability, nutrient content, C: N ratio, soluble salts, trace elements, and inerts. Farmers should obtain several such analyses of the compost produced from a given source over a period of time. Penn State’s Agricultural Analytical Services Lab offers two compost testing options. Test 1 includes total and ammonium nitrogen, total phosphorus, total potassium, percent solids, carbon, carbon: nitrogen ratio, soluble salts, and pH. Test 2 includes all of the analyses in Test 1 plus total calcium, magnesium, sulfur, copper, zinc, manganese, sodium, iron, and aluminum. Similar testing programs are available from commercial labs.