ORGANIC NITROGEN COMPOUNDS
During the warm-weather times of the year, when soil bacteria and fungi are working at top speed, ammonia and nitrate nitrogen are present in the soil in considerable amounts. Nevertheless, most of the nitrogen in soils exists as organic compounds. Although a plant's root system cannot "eat" them, these compounds still are valuable sources of nutrition (as discussed in Chapter Five). Organic compounds are mentioned briefly here because they often give a false picture of fertility when the soil is tested, particularly when the analysis is turned over to the beginner without explanation.
In a fertile, humus-rich soil, availability of nitrogen will vary from season to season or even from day to day. In early spring, none may seem to be present because all the free nitrogen has been taken up by soil organisms. With practically no bacterial or fungal action going on in the cold soil, nitrogen is not released. Weekly tests as the soil warms up will show a gradually increasing nitrogen supply, with a high point late in June (in the region north of the Ohio River). This levels off soon thereafter and gradually subsides until July, when a sharp deficiency of nitrogen may be registered (a partially false reading). If summer rains are abundant (preventing drought-death of soil organisms) this leveling-off in July may not take place. When cooler weather comes in fall, nitrogen will again accumulate as plants use less and less of it. Yet the surplus will again gradually be blotted up by bacteria, fungi, actinomyces and protozoa, until a nitrogen "deficiency" is again registered.
Farther south, where soil organisms can attack organic matter over a much longer period, it is soon used up, so that its end product -humus-has little chance to accumulate. In tropical countries, where decay is continuous, humus formation is a negligible factor in soil fertility.
PHOSPHORUS: AN ELUSIVE ELEMENT
Although our knowledge of how plants actually use phosphorus is still elemental, we are much better off than we were just before World War II. Production of radioactive forms of phosphorus in atomic piles has made possible a study of its movement through plant tissues, unlocking many secrets of a decade or more ago.
Why is phosphorus a difficult element to maintain in soil in a form that plants can use? It is extremely quick to react with-and be locked up by-other chemicals. In fact, experts estimate that less than 1 per cent of the total phosphorus reserves of a given soil are ever used.