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An Evaluation of the Mechanisms Governing the Supply of Ca, Mg, K, and Na to Soybean Roots (Glycine max)¹

ABSTRACT

Three mechanisms, root interception, mass-flow, and diffusion, govern the rate of supply of nutrients from the soil mass to the plant root. Two experiments were conducted to evaluate methods developed for calculating the supply of ions to the root by mass-flow and by root interception. Root interception values were calculated by assuming that all the exchangeable and soluble ions were in the pore space and that the roots intercepted an amount in proportion to their volume. Mass-flow was calculated by multiplying the amount of water uptake of the plant by the concentration of ions in this water as determined from analysis of a saturation extract.

Contributions of mass-flow were varied by creating three rates of transpiration. The results indicated that mass-flow was significant for supplying Ca and Mg but was of little importance for Na and K in the soil used. Increases in mass-flow of Ca and Mg were highly correlated with increases in the uptake of Ca and Mg by the plant.

In a second experiment, root interception of Ca and Mg was varied by using soil-sand mixtures varying from 100% soil to 20% soil and 80% sand. Mass-flow was minimized by leaching the soil before use. The results indicated that the calculated Ca uptake by root interception gave a reasonably close approximation to uptake of Ca minus mass-flow Ca.

¹ Journal paper No. 2576, Purdue University Agr. Exp. Sta., Lafayette, Indiana. Contribution from the Department of Agronomy. Presented before Div. S-4, Soil Science Society of America, Nov. 17, 1964, Kansas City, Mo.

² Formerly post doctoral fellow Purdue University, now Soil Scientist, Instituto de Edafologia, Madrid, Spain; and Professor of Agronomy, Purdue University, respectively.

Received for publication June 24, 1965. Accepted for publication September 29, 1965.