The supply of nutrient ions by diffusion to plant roots in soil

Summary

Phosphate uptake by onion roots over 18 days' growth, and the resultant phosphate loss from a sandy soil, were considerably larger than predictions using independently estimated diffusion coefficients and the soil's phosphate desorption isotherm. Soil pH was lowered by the plant roots; decreases of about 0.5 pH units could be measured close to the root. Separate experiments on pH effects on the phosphate desorption isotherm showed that the phosphate buffering power of this soil decreased and the concentration in solution increased as pH fell. These pH effects explain, in part, the 2.5-fold increase of the measured effective diffusion coefficient over 18 days' uptake and the larger depletion. When the soil solution contained mostly NO₃ besides phosphate, soil pH near roots increased by about 0.4 pH units during 10 days' uptake; this rise would increase the phosphate buffer power and so decrease the effective diffusion coefficient, as observed experimentally in this weakly buffered sandy soil. Theoretical predictions of plant uptake and of the concentration: distance relationship in the soil should take account of pH gradients near roots and the consequent effects on the phosphate desorption characteristics of the soil.