On lawns, maintaining proper pH will be of tremendous help. Fortunately, superphosphate is not harmful to roots and can be used liberally to build up a reserve for the future.
ORGANIC MATTER TO CONSERVE PHOSPHORUS
One of the roles of organic matter is conserving phosphorus. Organic matter keeps up soil moisture. A characteristic of phosphorus is that when it exists as small crystals, it is much more soluble than when larger crystals form. In moist soil, smaller crystals are formed. These go into solution more readily.
Too, as organic matter decays it produces certain organic acids such as tartaric, isocitric, and so on. These combine rapidly with any free iron and aluminum to form metal-organic ions which do not combine readily with phosphorus. By using up free iron and aluminum, these acids prevent formation of less-soluble compounds of phosphorus.
In addition, organic matter itself contains considerable phosphorus, the amount depending upon the origin of the organic compound. As cells of decomposition bacteria and fungi die, they release their phosphorus for use by higher plants.
The amount of organic phosphorus available varies from time to time. If liberal amounts of nitrogen are present, soil organisms may increase so rapidly that instead of releasing nutrients they will use up all surplus food and cause a temporary shortage. Fortunately, the life span of these organisms is short, so that plant roots will not have to wait long before the food elements are again available.
PHOSPHORUS IN EARLY GROWTH
Because phosphorus is needed by young plant growth, it should be applied early in the season. At least half the total annual phosphorus consumption by annuals and perennials will be absorbed before these plants have made one-fifth their annual growth. In case of grasses, early uptake may be even higher: perhaps 80 to 90 per cent of their annual consumption will be taken up during the first few weeks of growth.
This need for phosphorus early in the growth cycle poses a problem. Phosphorus should be supplied just before it is needed, but not too far ahead of need. For grasses and perennials, this means in early spring. Bedding plants, however, and tender vegetables such as tomatoes, eggplant and peppers usually are not set outdoors until early June. Superphosphate worked under in April would already be combining in less soluble compounds by June, and so would not be of maximum value to these late-set plants.
Here the so-called transplanting solutions serve a useful purpose. These are chemicals to be dissolved in water and applied to seedlings as they are transplanted. These solutions are low in nitrogen but high in phosphorus. They are completely soluble and are taken
up by seedlings and transplants before phosphorus fixing can take place.