POTASSIUM AVAILABILITY IN RELATION TO SOIL-MOISTURE .1. EFFECT OF SOIL-MOISTURE ON POTASSIUM DIFFUSION, ROOT-GROWTH AND POTASSIUM UPTAKE OF ONION PLANTS

The objective of this research is to evaluate the influence of soil water content on -the mobility of potassium in soil, -plant growth and -K uptake of plants. The mobility of K increased with soil moisture. Increasing the volumetric water content (.theta.) from the 0.1 to 0.4 resulted in a rise of the effective diffusion coefficient (De) by a factor of about 10. This is mainly due to the increase of the tortuosity or impedance factor with higher soil moisture. In order to relate K mobility in soil to the availability of K for plant uptake, onion plants were grown in special containers under constant water content in the range of 0.1 to 0.4 cm3 H2O cm-3 of soil. Results are -both K content and growth of the plants increased with soil moisture, -water content below .theta. = 0.1 reduced root growth -K inflow per unit of root surface increased with soil moisture. Maximum rate of inflow occurred with .theta. = 0.25 in the soil used. It is therefore concluded that soil moisture affected K availability by affecting both K mobility and root growth.