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

Abstract

SUMMARYThe concentration gradients of pa2 labbelled phosphate were measured in a Coral Rag Clay soll, placed in contact with planes of onion and ryegrass roots for up to 12 days' growth. After contact with the roots the soil block, 3 × 2 • 1 cm was quick-frozen and sliced into sections 0.1 m m thick, on which the p3~ was directly determined. The concentration : distance relationship showed only a small depletion profile, because the soil had a high power for buffering changes in phosphate concentration. There was also an unexpectedly large seatter ( i 5% of mean) in the initial distribution of p3~ in the undepleted soll; detailed analyses was, therefore, not possible. However, a eonstant reduced concentration at the root: soll boundary was indicated in all cases, corresponding to uptake (by onion) increasing linearly with square root of time. Calculations gave an effeetive diffusion coefficient in the range 0.29-0.71 X 10 -8 cm~/sec for a 10% depletion of the initial phosphate content at the root surface. These findings agree with previous inferences t h a t suppIy of phosphate to plant roots in this soil could be accounted for b y a simple diffusion proeess in which only N 10% of the long-term isotopically exchangeable phosphate was depleted; there was no evidence t h a t the phosptlate diffusion characteristics of this soil were altered b y the presence of plant roots.