Rate of Uptake of Potassium by Three Crop Species in Relation to Growth

Barley, ryegrass, and fodder radish were grown in flowing nutrient solutions at four potassium concentrations, [K+], from 0-05 to 4 mg l-1. During the first 2 weeks after germination the response to [K+] (fodder radish > barley > ryegrass) depended on the potential relative growth rate, the ratio of root surface area to plant weight, and on the K+ flux into the roots. Subsequently, there was no effect of [K+] on growth rate within the range tested. The K+ flux decreased from 4-23 x 10"12 mol cm-2 s"1 in the first 2 weeks after germination, when it was concentration-dependent, to 2-5 x 10~12 mol cm-2 s"1 after 4-5 weeks, when it became independent of [K+] down to 0-05 mg 1_I. The results explain the importance of high [K+] and rapid root growth during the first 2 weeks after seed germination.