We once thought that phosphorus was used by plants only as they approached maturity and was not essential to young growth. Old garden books are full of recommendations for "hardening soft growth" with phosphorus (and potash). You may still see references such as "apply phosphorus to tomatoes toward maturity to 'firm up the fruit.'" Many special dahlia and potato fertilizers were low in nitrogen but high in phosphorus because phosphorus was thought to bring about earlier maturity. Actually, young growth in particular needs phosphorus. It is so essential to such growth that if it is not present in sufficient quantities for all parts of the plant, it will be withdrawn from older leaves and translocated to more active growing tips and young foliage. For this reason, a phosphorus deficiency is among the first things that plant nutritionists suspect when a plant's lower foliage is poor but younger leaves seem normal.
Phosphorus is a major ingredient in the nuclei of cells, and is present as well in cytoplasm surrounding each nucleus. We know that phosphorus has something to do with transfer of inheritance factors from one generation to the next. Exactly how it works, we do not know.
OUT OF BALANCE
For all its importance, phosphorus is removed by plants in amazingly small amounts compared with amounts applied to soil in order to supply it. A crop of corn may remove less than 25 pounds of phosphorus per acre. Yet to supply that amount, between 200
and 300 pounds of superphosphate may have to be applied. Hay may remove only 2 to 3 pounds of actual phosphorus from an application of 50 to 100 pounds of superphosphate.
Because of its tendency to lock up, phosphorus accumulates when high-phosphate fertilizers are used regularly. I have seen analyses of lawn soils from the Chicago area, where such fertilizers had been applied for several years without letup, in which the phosphorus content was so high that the soil itself could have been used as a low-grade source of that element I
An old-time phosphate fertilizer is bone meal. It is still used by many, but is a poor value because of its low solubility. It is often said that one application of bone meal will last in soil for 15 years- which is presented as an argument in its favor. (The pros and cons of bone meal are discussed under fertilizers in Chapter Five.) I know of an instance of bone meal remaining virtually unchanged in the soil for half a century.