Studies on potassium nutrition of plants III.—Effects of potassium concentration on growth and mineral composition of vegetable seedlings in solution culture

Abstract

AbstractBecause of diffusion, the effective concentration of nutrients in contact with plant roots in sand culture is much less than the concentration of the nutrient solution supplied. For comparison with earlier observations in sand culture, solution culture experiments have been made with cabbage and red beet seedlings using readily available equipment for the control, within reasonable limits, of the potassium concentration of the root environment.Weight responses and K content of tops and roots, Na content of tops, top: root weight ratios, percentage dry residue of top and root and potassium flux through the roots were influenced in similar ways by increase of K+ concentration in the medium. There were steep slopes to about K0·1 followed by an abrupt change of slope and levelling off or a slight fall at the highest potassium concentrations. The weight responses, K content of top and root and potassium flux increased with increasing nutrient medium K+ concentration whereas the other relationships were in the reverse direction.Root potassium contents increased in a similar manner to those of the tops but were lower than the latter at corresponding medium K+ concentrations. Root sodium contents rose to a maximum at a medium concentration of 0·1 mequiv./1 of K+ and thereafter fell with increasing potassium in the medium. The sums of the cation equivalents (K, Na, Mg, Ca) in the tops and roots were approximately constant.The limiting potassium concentration was determined from the response curves as the lowest concentration at which maximum yield was obtained. For beet in solution culture, the value was 0·15 and the corresponding concentration in sand culture was 0·7 mequiv./1 (ratio, sand/solution = 4·7); the values for cabbage were, respectively, 0·13 and 0·5 mequiv./1 (ratio, 3·8).The rate of removal of potassium by beet seedlings from solutions containing initial concentrations in the range 0·01‐5 mequiv./1 was determined. The flux of potassium through the root surfaces varied from 0·35–1·89 μg K/g root/sec with increasing initial concentration in the medium.