Soil Phosphorus and Fertility

Sterling R. Olsen and Maurice Fried.

Phosphorus is present in all living tissue. It is particularly concentrated in the younger parts of the plant and in the flowers and the seed.

It is necessary for such life processes as photosynthesis, the synthesis and breakdown of carbohydrates, and the transfer of energy within the plant.

It is a major part of the nucleus of the cell and is present in the cytoplasm, where it is involved in the organization of cells and the transfer of hereditary characteristics.

Growth is arrested when the supply of phosphorus in the soil is too low, and phosphorus from the older tissues moves to the younger tissues. Usually, therefore, signs of too little phosphorus appear first in the lower leaves, which are the older ones. The symptoms may be a lack of chlorophyll, a deepening of the green color, or a red color in the leaves. Usually also the roots are stunted and poorly branched. A deficiency of phosphorus may delay maturity of the plant.

Different species of plants and different parts of a plant vary in their content and requirement of phosphorus. The cobs in a 75-bushel corn crop contain about 0.1 pound of phosphorus, but the stover may contain 5 pounds and the grain about 12 pounds. The roots of the corn plant contain less phosphorus than the tops. The plants that produce the 75 bushels may take up 20 pounds of phosphorus. Although 35 bushels of wheat may contain only half as much phosphorus as the 75-bushel corn crop, the required level of readily available phosphorus in the soil is probably greater for wheat than for corn.

The amount of phosphorus removed by a harvested crop depends on its total yield, its phosphorus content, and on how much of the plant is actually removed. If only the grain is removed, as in sweet corn, the soil loses less phosphorus than when the whole corn plant silage corn, as an example is harvested. Grass harvested for seed may remove less phosphorus than grass harvested as hay.

The phosphorus content of a particular species tends to be low when the available supply is low, but the amount in a plant may be increased by using phosphate fertilizer. The increase usually is small, but if it is needed it can improve the plant and its nutritive value.

Since phosphorus is an essential nutrient of plants, the total amount of phosphorus in cultivated and virgin soils is one of our important natural resources. It ranges from less than 100 to 4,000 pounds an acre and averages about 1,000 pounds.

The coastal areas in the South Atlantic States and Gulf States are lowest in total phosphorus. High concentrations occur in Tennessee, Kentucky, and the Pacific Northwest.

Virgin surface soils and their subsoils contain similar concentrations of total phosphorus, but many tilled soils do not. The liberal use of phosphate fertilizers has resulted in accumulations of phosphorus in the surface soil in some areas notably in potato-growing areas and places close to phosphate deposits. Some soils in potato areas have received as much as 100 pounds of phosphorus an acre for each potato crop. Since less than one-fourth of that amount may be removed by the crop, accumulations have resulted.

As phosphorus does not move appreciably in the soil, the accumulations are primarily in the first foot of soil.

On the other hand, some soils have been in agricultural production long enough to deplete the total phosphorus supply, and unless fertilizer phosphorus is added equivalent to the amount removed by crops, the phosphorus in the surface layer will decrease further.

Most of the total phosphorus supply is tied up chemically in a form that is not usable by the crop in a single growing season it is not available to the growing plant. The available soil phosphorus originates from the breakdown of soil minerals, from soil organic matter, or from the previous addition of phosphate fertilizer. The available soil phosphorus usually is only about 1 percent of the total soil phosphorus.

The available soil phosphorus is not necessarily related closely to the total soil phosphorus, partly because the chemical nature of the phosphate minerals and organic compounds is not the same in all soils. The differences in agricultural soils are due primarily to management practices, which affect the available phosphorus much more than they affect the total phosphorus. While high fertilization increases available soil phosphorus more than total soil phosphorus, crop removal without fertilization rapidly lowers the available phosphorus without changing very much the total content.

Phosphorus occurs naturally in soils in the form of the calcium phosphates, hydroxyapatite and fluorapatite; iron phosphate and aluminum phosphate; various primary and secondary minerals, in which a phosphate group substitutes for a silicate group in a crystal lattice; and as organic phosphorus, which may constitute as much as 75 percent or as little as 3 percent of the total soil phosphorus.

As the soil matures by weathering processes and plant growth, these natural forms of phosphorus change into other forms, which accumulate in the surface layers and the clay fraction of the soil.

The original source of most of the inorganic phosphorus is the apatite group of minerals. These forms tend to disappear as weathering proceeds and the soil becomes more acid. The content of iron phosphate and aluminum phosphate increases because they are more stable under acid conditions. The clay minerals and hydrous oxides of iron and aluminum, which result from weathering, can adsorb phosphorus on their surfaces. Thus the phosphorus content of the clay fraction increases with weathering unless clay is lost by erosion. The exact nature of the phosphorus associated with the clay fraction is unidentified, but it is probably in iron, calcium, and aluminum phosphates that are present as tiny particles.

The amount of organic phosphorus increases as the organic nitrogen increases. Soil micro-organisms change the organic phosphorus to inorganic phosphorus at a faster rate as the soil PH increases. Therefore the amount of organic phosphorus tends to be larger in acid soils than in alkaline soils. Organic phosphorus occurs in soils as inositol hexaphosphate, other inositol phosphates, phospholipids, and unidentified compounds.

Phosphorus applied to soil as fertilizer is changed into forms similar in some respects to the native forms already present. A characteristic feature of soil phosphorus is its low solubility in water or the soil solution.

Acid soils contain a large excess of iron and aluminum. Alkaline and calcareous soils contain calcium. All three readily combine with water-soluble phosphates (such as superphosphate) and convert them into sparingly soluble forms.

The process of changing soluble phosphates into less soluble phosphates in soils is called fixation, or reversion. The nature of fixation may affect the efficiency of the phosphorus fertilizers differently on different types of soil.

Hydrated iron and aluminum oxides in acid soils are known to adsorb soluble phosphorus from fertilizers to form iron and aluminum phosphates. The amounts of hydrated aluminum and iron oxides in soils increase in general as the weathering processes continue. It is highest in regions of high temperature and rainfall (where kaolinite is the dominant clay mineral) and lowest in regions of low rainfall.

Acid soils with their higher content of hydrous oxides will fix phosphate to a greater extent than alkaline soils of similar texture. Within a group of acid or alkaline soils, the fixed phosphorus is less available on clay loams than on sandy loams, because of their higher clay mineral content. The amount of phosphorus adsorbed by a given soil series of varying textures is related closely to the surface area of the soil particles.

DIFFERENCES in the solubility of fixed forms of phosphorus result from differences in the pH of the soil at the time the fertilizer is applied.

Iron and aluminum phosphates are least soluble at pH 4. Their solubility increases as the pH increases from 4 to 8.5. J. F. Fudge, at the Alabama Agricultural Experiment Station, found that liming an acid soil to pH 6.3 increased tenfold the phosphorus concentration of the displaced soil solution.

The calcium phosphates begin to form around pH 6. Their solubility decreases as the pH increases to about 7.5. In the presence of excess calcium carbonate, the solubility of the calcium phosphates increases between pH 7.5 and 9. This effect of pH holds true for the native and fixed forms of phosphorus. The availability is at a maximum in the PH range 6.5 to 7. As the PH increases from 7 to 8.5 in alkaline soils, the availability of phosphorus again drops. This may be due to a change in the rate or capacity of the roots to adsorb phosphorus or to a decrease in the concentration of the H₂PO₄ ion.

In neutral, alkaline, and calcareous soils, soluble phosphorus from fertilizers reacts with exchangeable calcium ions, soluble calcium salts, and calcium carbonate to form slightly soluble calcium phosphates. The form of calcium phosphate that appears first may be dicalcium phosphate, but this form is unstable in the presence of excess calcium ions and changes into hydroxyapatite or similar calcium phosphates. A calcium fluorphosphate has been found on the surface of large limestone particles in a soil that had been limed and then treated with superphosphate for many years.

A MAJOR PROBLEM of concern to the farmer involves the availability of the native and fixed forms of phosphorus to plants and the soil-management practices that can be followed to get the maximum benefit from fertilizer.

The availability of the soil phosphorus depends primarily on its degree of water solubility, since the plant obtains its phosphorus from the soil solution. Repeated restoration of the water-soluble phosphorus is necessary to meet the phosphorus requirements of the plant.

One has a clearer idea of the relation of forms of soil phosphorus to their availability if he knows how the plant gets its phosphorus from the soil.

The immediate source is the phosphorus dissolved in the soil water or solution. At least three events occur as the plant grows and absorbs phosphorus from this soil solution.

First, the amount or concentration of phosphorus in the soil solution immediately around the root drops. The amount of phosphorus in the soil solution is insufficient to meet the needs of the plant.

Second, the solid-phase phosphorus in contact with the soil solution enters the soil solution to replace the phosphorus absorbed by the root.