Phosphates in Soil

The distribution, dynamics, and availability of phosphorus in soil are controlled by a combination of biological, chemical, and physical processes. These processes deserve special attention, as a considerable proportion of the applied phosphate is transformed into insoluble calcium, iron, or aluminum phosphates. On average, only a small proportion, perhaps 15-20% of the total amount of phosphorus in the plant, comes directly from the fertilizer applied to the crop. The remainder comes from soil reserves. For most of the twentieth century, farmers in Western countries were advised to add more than double the amount of phosphate required by a crop, because these immobilized calcium, iron, and aluminum phosphates had been assumed to be permanently unavailable to plants.

The primary source of phosphorus taken up by plants and microorganisms is dissolved in water (soil solution). The equilibrium concentration of phosphate present in soil solution is commonly very low, below 5 p,mol. At any given time, soil water contains only about 1% of the phosphorus required to sustain normal plant growth for a season. Thus, phosphates removed by plant and microbial uptake must be continually replenished from the inorganic, organic, and microbial phosphorus pools in the soil. These continuous processes dominate contemporary agricultural production to remove about 8.2 kgP ha- from cropland each year on a world average (based on our own estimate), and commonly 30 kgP ha-1 from the US and European fertile agricultural soils.

Each phosphate mineral has a characteristic solubility under defined conditions. The solubility of many compounds is a function of acidity (pH; Figure 2). An increase in pH can release sediment-bound phosphorus by increasing the charge of iron and aluminum hydrous oxides and therefore increasing the competition between hydroxide and phosphate anions for sorption sites. Also, organic acids can inhibit the crystallization of aluminum and iron hydrous oxides, reducing the rate of phosphorus occlusion. The production and release of oxalic acid by fungi explains their importance in maintaining and supplying phosphorus to plants.

The relative sizes of the sources and stocks of phosphate in soil change as a function of soil development (Figure 3). The buildup of organic phosphates in the soil is the most dramatic change. As time goes on, this becomes the chief reservoir of reserve phosphate in the soil. In most soils, organic phosphates range from 30% to

Figure 2 The solubility of phosphorus in the soil solution as a function of pH. Adapted from Schlesinger WH (1991) Biogeochemistry: An Analysis of Global Change. San Diego, CA: Academic Press.

Figure 2 The solubility of phosphorus in the soil solution as a function of pH. Adapted from Schlesinger WH (1991) Biogeochemistry: An Analysis of Global Change. San Diego, CA: Academic Press.

Figure 3 Phosphates in the soil vary with soil development. Adapted from Emsley J (1980) The phosphorus cycle. In: Hutzinger O (ed.) The Handbook of Environmental Chemistry: The Natural Environment and the Biogeochemical Cycles, pp. 147-167. Heidelberg: Springer.

65% of total phosphorus, and it may account for as high as 90%, especially in tropical soils. The reasons for this are their insolubility and chemical stability. It has been noted that acid soils tend to accumulate more total organic phosphorus than do alkaline soils. This is almost certainly because organic phosphates react with iron and aluminum under acid conditions and become insoluble. Being the salts or metal complexes of phosphate esters they release their phosphate by hydrolysis, but only very slowly. Phosphate esters can have half-lives of hydrolysis of hundreds of years. This process can be greatly speeded up by the action of phosphatase enzymes in the soil whose function is to facilitate reaction by catalyzing it. At later stages of soil development, phosphorus is progressively transformed into less-soluble iron- and aluminum-associated forms, and organic phosphorus contents of the soil decline. At this stage almost all available phosphorus is found in a biogeochemical cycle in the upper soil profile, while phosphorus in lower depths is primarily involved in geochemical reactions with secondary sediments.

When the supply of dissolved phosphates to growing biomass is abundant, a net immobilization of inorganic phosphorus into organic forms will occur. Vice versa, inadequate inorganic phosphorus supply will stimulate the production of phosphatases and the mineralization of labile organic forms of phosphorus for microbial uptake. A continuous drain on the soil phosphorus pools by cultivation and crop removal will rapidly deplete both labile inorganic and organic phosphorus in soils.

Allowing soil reserves of readily available phosphorus to fall below a critical value, determined by field experiments, can result in a loss of yield. The turnover of available phosphates by plants in soil solution is determined by rates of releases of phosphorus from insoluble forms to soluble phosphates. All kinds of soil particles can contribute to this process and in some cases it is not only chemical balance that maintains the supply but also the action of microbes and enzymes that release phosphate from organic debris in the soil. It is believed that the biogeochemical control of phosphorus availability by symbiotic fungi is a precursor to the successful establishment of plants on land.

However, our existing knowledge - briefly discussed above - cannot yet provide a comprehensive understanding of the complex movements and transformations of phosphorus, especially of its organic forms. This hampers the efficient application of phosphate fertilizers, and the efficient control of phosphorus losses.

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