Exploitation of K near roots of cotton, maize, upland rice, and soybean grown in an Ultisol

Native soil potassium (K) has received increased attention as a K source for plants to reduce fertiliser input. Our objective was to compare the ability of different crops to utilise native K. We also wanted to study the exploitation and transport pattern of soil K influenced by plant uptake. Cotton, maize, soybean, and upland rice were cultivated in rhizoboxes. The system permitted sampling of 1-mm-thin soil layers at increasing distances from the plant roots. Both exchangeable and nonexchangeable K was determined and compared with plant uptake of K. The upland rice was superior in K uptake, and took up some nonexchangeable K. Soybean and cotton grew poorly, and K was accumulated in the root zone due to excess supply by mass flow. The importance of mass flow over diffusion of K was verified by calculations and is contrary to accepted principles of K transport in soil. The reasons were high transpiration and restricted root growth. This indicates that mass flow of K in some situations is more important than generally assumed. Mass flow also caused the accumulation of Ca and Na in the root zone, especially that of rice. Accumulation of K in the root zone of rice did not cause K fixation, possibly due to an unknown K-releasing mechanism of upland rice.