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Phosphate Supply Pattern of Various Soils¹

ABSTRACT

The pattern of phosphate supply in soils of different phosphate level was investigated by three techniques: continuous soil leaching, successive hourly equilibrations, and intermittent leaching in the presence of growing plants. All three approaches gave similar results.

The pattern of phosphate supply for the four low phosphate-fixing soils could be predicted by assuming the phosphate released was a desorption phenomenon as described by the Langmuir adsorption isotherm. The pattern of phosphàte supply for the three high phosphate-fixing soils could be predicted by assuming that in the presence of water a saturated solution of a chemical compound was formed.

The level of phosphorus in the initial water extract was not a good indication of plant-available phosphorus. Both the intensity of soil phosphate supply and the capacity of the soil to rapidly renew this supply must be evaluated to adequately describe plant-available phosphorus in the soil.

¹ This project was supported in part by the United States Atomic Energy Commission.

² Soil scientists, Soils and Plant Relationships Section, Soil, and Water Conservation Research Branch, A.R.S., U.S.D.A., Beltsville, Maryland.

Received for publication November 10, 1955.