In organic gardening, all nutrients are supplied through the process of mineralization. As organic matter is decomposed by the microorganisms that digest the cellulose and hemi-cellulose, minerals contained within the cells of the animal or plant tissues are released into the soil. After the microorganisms have digested all digestible cells, they die. Since their bodies consist mostly of proteins, the proteins are broken down by enzymes, releasing more nutrients, mostly nitrogen (N), into the soil.
The rate of mineralization is dependent on temperatures in the soil.
Under laboratory conditions, mineralization rates are measured at room temperature, 72 degrees. Moist soil samples are held in temperature-controlled containers for several days, then the amount of available nitrogen in the soil is measured. This process is repeated until the figures are stable. Mineralization rates are faster at temperatures above room temperature and significantly slower at temperatures below room temperature. At 72 degrees, the mineralization of compost is between eight and 10 percent. Mineralization of organic matter stops when soil temperatures approach the freezing point.
The rate of mineralization has a major effect on plant growth.
Because soils are cooler in the early spring, the rate of growth is often reduced for early spring crops such as peas, cabbage, broccoli, cauliflower, lettuce and spinach. Cooler soils mean fewer nutrients becoming available. This problem can be minimized by selecting south-facing slopes for early spring and late fall crops. Planting the crops on ridges is another method of encouraging early warming of soils. A soil raised above a natural grade warms faster than a soil that is level on grade. Covering the area to be planted with a sheet of clear polyethylene several weeks before planting, followed by ridging and covering the ridges with black plastic mulch, is labor intensive but will stimulate early mineralization. Soils warm very rapidly under clear plastic due to the greenhouse effect. However, anticipate early growth of spring weeds, requiring light cultivation or spraying with horticultural vinegar. Ridging and mulching with black plastic will also provide weed control.
Apply no more than four cubic yards of compost or animal manure per 1,000 square feet in any one year. Five percent is one year’s limit for organic matter added to the soil. Excessive applications of either can stimulate excessive vegetative growth and weak spindly plants. With the mineralization rate eight to 10 percent, 90 to 92 percent of the minerals remain in the soil’s organic matter. So repeated applications of compost and organic matter should be based on soil test results.
If existing soils contain less than three percent organic matter, an initial application of four cubic yards of compost or animal manure the first year followed by repeated applications at two cubic yards in successive years (or on alternate years for sandy soils) can be adequate. In silt or clay loam soils, these levels may be excessive, requiring greater dependency on soil test results.
Initially, compost or animal manure should be incorporated to a depth of six to eight inches, deeper if possible. Because organic matter reduces the bulk density of soils, deep incorporation promotes deep rooting, making crops more tolerant to drought. As deep incorporation of organic matter promotes deep rooting, the roots that penetrate this region will continue to maintain the organic matter concentration in that region.
Repeated applications of compost or animal manure should be incorporated only in the upper three inches of soil. This results in concentrating the nutrients in the region where seed germination occurs and where roots of new transplants initiate growth. Leaching will move nutrients deeper into the soil as the growing season progresses.