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Effective Volume of Soil Around Plant Roots Determined from Phosphorus Diffusion¹

ABSTRACT

The effective volume of soil contributing P to the roots was measured from a knowledge of the diffusion coefficient of P, a capacity factor for renewal of the soil solution, and the P concentration in solution. Rate of P uptake per square centimeter of surface area by excised corn roots was measured from P³²-tagged soils at -bar moisture suction. The rates of release of resin-extractable P from the soils were determined. These rates of release of extractable P from the effective volume of soil were much greater than the rate of uptake by the roots. The data indicate strongly that the rate of release of solid phase soil P to the soil solution is a nonlimiting step in the absorption mechanism of P by roots.

¹ Contribution from Northern Plains Branch, Soil and Water Conservation Research Division, ARS, USDA, in cooperation with the Colorado Agr. Exp. Sta., Scientific Journal Series 1090.

² Research Soil Scientists, USDA, Fort Collins, Colo.

Received for publication March 4, 1966. Accepted for publication July 1, 1966.