ORGANIC Materials May Hurt Crops If Applied Undecayed

Most soils which are constantly under cultivation have a tendency to become low in organic matter or humus content unless farm practices are planned so as to make provision for the regular return of crop residues, stable manure, green crops, or other forms of organic materials, The necessity for maintenance of the humus supply in the soil is well understood, but the best method of doing this is not so obvious. However, many recent, studies have given us the information needed for making practical recommendations.

Whenever readily decomposable organic materials are incorporated with the soil a rapid multiplication of bacteria and fungi occurs. These microorganisms are the agents of decay primarily responsible for the conversion of plant proteins into ammonia and nitrates, and the sugars, starch cellulose, and other carbohydrate materials into carbon dioxide. The more resistant portions of the plant substances remain to form a part of the soil humus. The growth requirements of the organisms which produce these changes are similar to those of higher plants, except that they obtain their energy from the residues of higher plants rather than directly from sunlight. They require adequate moisture, mineral matter, usually good aeration, and available forms of nitrogen.

It is this last requirement that needs most consideration. If a green-plant material is added to the soil it usually contains more nitrogen than needed by the decay organisms. There occurs a marked formation of ammonia, a considerable portion of which is later converted into nitrates. The bacteria and fungi assimilate a portion of the available nitrogen, but there remains a surplus for the crop. On the other hand, if straw, corn stalks, fresh stable manure, or other materials which are rather easily decomposed but which contain a very low percentage of nitrogen, are applied, they may not furnish enough nitrogen to meet the needs of the microorganisms. Unless the soil is especially high in nitrogen there is likely to be a temporary deficiency and the crop may suffer partial nitrogen starvation, often evidenced by a rather pale, yellowish-green color, and a stunted growth. Later, a return to normal may take place, but usually not in time to avoid crop injury if the planting was done at the time of the application of the organic materials.

The procedure to be followed in applying materials which are likely to produce crop injuries must vary with the existing conditions. The plant substances may be allowed to decay partially before application, preferably in a compost heap, they may be used as bedding for livestock, or they may be applied several weeks before planting.  If application must be made immediately before planting, the quantity used should be very limited and the crop grown should be one that thrives on decaying organic matter. The efficient utilization of crop residues may require modifications of farm practices, but such changes usually prove profitable. Not only is the soil organic matter content maintained, but the energy supplied is partly utilized by bacteria which grow independent of higher plants and fix atmospheric nitrogen.