NOT ALL IS LOST
Even though they may be locked up by chemical action, all forms of phosphorus are not lost forever. True, certain combinations with iron or aluminum are so highly resistant to change that they can never be dissolved by any chemical that would be safe to use on soils in which plants are growing. Other phosphorus compounds, however, such as fluorapatite and hydroxyapatite, do become available by weathering, by bacterial action and by exposure to soil acids and alkaline solutions. For example, raising the pH of an acid soil from 5.5 to a reading of 6.4 increased the availability of phosphorus to ten times the original level. In another case, reducing the pH of a higher alkaline soil (from 8.3 to 6.9) resulted in a 500 per cent increase in phosphorus availability.
PHOSPHORUS DOES NOT MOVE
One difficulty experienced in supplying phosphorus to plants is its lack of mobility in soil-a result of its low solubility. Phosphorus may become slightly soluble and move somewhat in soil water, but even when this happens, it will hardly have time to move far before
it is fixed in less soluble form. For this reason, if plants are to obtain enough phosphorus, their roots must grow out to meet it. Phosphorus is the mountain, roots are Mahomet.
Because phosphorus moves so little once it is in contact with soil, placement is highly important. The usual practice of scattering superphosphate on top of the soil is of little value, at least to the current growth. Because it is so stable, the phosphorus will still be in place, unused, when soil is prepared the following year.
Adding superphosphate to topsoil just before spading or tilling does have the virtue of getting some of it down into the ground. However, any of the material that remains above the area in which roots grow is of no use to them.
For maximum use, the best placement for phosphorus fertilizers is in the soil, worked down deeply before planting. It should be as close to the root zone as possible. For shallow-rooted plants such as petunias, lettuce or alyssum, this means within 3 to 4 inches of the surface, but for deep-rooted woody plants, such as trees and shrubs, it might mean working in superphosphate to a depth of 3 to 4 feet.
IN THE LAWN
Soils for lawns present a special problem. Mistakes in feeding
vegetables and annuals can be corrected a year later, but it is not
easy to roll up an established sod to incorporate superphosphate.
Since it does not move downward, phosphorus in liberal amounts
should be used in original lawn soil preparation, with hopes that
this will become slowly available through the years