SPECIES OF PLANTS differ markedly in their capacity to absorb soil and fertilizer phosphorus. At Beltsville, Md., phosphorus from phosphate rock was found to be most available to buckwheat. Legumes (alfalfa, crotalaria, and Ladino clover) extracted more phosphorus from phosphate rock than grasses (orchardgrass, bromegrass, perennial ryegrass, millet, and oats).

One explanation for these differences in uptake is that plants vary in their capacity to absorb cations, such as calcium, from the soil solution. A lowering of the calcium content of the soil solution increases the solubility of phosphorus. A plant with a high capacity to adsorb calcium on its root surfaces, such as buckwheat and the legumes, reduces the calcium concentration in the soil solution more than does a plant with a low capacity, such as oats. Therefore the buckwheat absorbs more -phosphorus than oats because of the higher solubility of the phosphorus in the soil solution.

Another explanation is that plant roots release carbon dioxide to the soil solution. Carbon dioxide lowers the pH and increases the solubility of calcium phosphate. More carbon dioxide is given off by roots of a high exchange capacity than by roots with a low exchange capacity. This mechanism could also explain the relative differences between buckwheat and oats. It would be least effective on iron phosphate and aluminum phosphate or acid soils generally and most effective on calcium phosphates.

Crops with a low capacity to absorb the relatively insoluble calcium phosphates should benefit if they follow a crop with a high capacity. Planting pasture mixtures that combine these two types of crops may make it possible for the species with a low capacity to absorb these phosphates.

M. Drake and J. E. Steckel, of the Massachusetts Agricultural Experiment Station, reported that oats grown in association with red clover gained 32 percent in yield and 62 percent in total phosphorus uptake compared to oats alone.

PLACING THE FERTILIZER in bands reduces soil contact and increases the phosphorus concentration in a small area. The proportion of the added phosphorus fixed in the more unavailable forms is greater when the fertilizer is broadcast and mixed with the whole mass of acid soils.

Band placement of phosphorus fertilizer greatly increases the availability of the phosphorus in acid soils. In alkaline or calcareous soils, placement and thorough mixing with the soil lead to about the same results, except possibly with potatoes.

The time phosphate fertilizer is applied is more important on acid soils than on calcareous soils. The major change in solubility of concentrated superphosphate occurs within 2 or 3 days after mixing.

Water-soluble phosphorus diffuses away from a fertilizer particle through the soil a distance of about 1 inch; then its movement becomes very slow. Within this small zone around the fertilizer particle, water-soluble phosphorus in the soil increases. Any mixing from cultivation or plowing, however, will expose the phosphate-rich soil around the fertilizer particle to soil capable of adsorbing more phosphorus, with the result that the phosphorus will transfer to other soil surfaces. The greater the adsorption capacity for phosphorus, the more significant the reduction in solubility will be.

Farmers with acid soils usually get best results by applying phosphorus at planting time, but -farmers with alkaline or calcareous soils may satisfactorily apply the phosphorus several months before planting.

The availability of soil phosphorus is also affected by the available moisture supply. J. L. Haddock observed at the Utah Agricultural Experiment Station that conditions of soil moisture were as important in making phosphorus available to sugar beets as either phosphorus applications or placement.

Drying tends to make phosphorus less available. That may be due to a drop in amount of phosphorus in solution or to other factors associated with root activity and the production of carbon dioxide. The effects of soil moisture on phosphorus availability emphasize the importance of maintaining high rates of water infiltration and of reducing evaporation losses on non-irrigated soils subject to drought.

Less than 20 percent of the applied phosphorus is utilized by the crop the year of application. When a farmer follows a practice of applying phosphate as inorganic fertilizer or manures to his soil each year, or even every 2 or 3 years, the amount of phosphorus the crop removes is usually less than the amount of phosphorus added.

Residual phosphate accumulates in the soil, with the result that the plant-available phosphorus level increases.

In several areas intensively farmed or devoted to large cash crops, phosphate fertilization has been so high that the soils no longer respond very much to applications of phosphate. This phosphate fertilizer probably could be utilized more efficiently on soils on which chemical tests show low values for extractable phosphorus.

If the level of available phosphorus is low, as indicated by a soil test for extractable phosphorus or by plant response to applied phosphorus, a large application is needed to bring the available phosphorus level within a suitable range.

In high-fixing acid soils, the residual or carryover effects of phosphorus for a second crop usually are small, unless the soil has been fertilized for many years. At the start of a phosphate fertilization program, therefore, the rate may have to be high, and annual applications may be necessary. As the residual phosphorus gradually accumulates in the soil, the rate and frequency of application may be reduced.