Importance of mass flow and diffusion on the potassium supply to cotton plants as affected by soil water and potassium

Late season potassium (K) deficiency has been observed quite frequently for cotton crops in the Brazilian cerrado region. One possible reason for such a problem could be the low water availability at this period of the season. Thus, an experiment was conducted in order to quantify the relative contribution of mass flow and diffusion in supplying K to cotton roots, as affected by soil water and K availability. The arable layer of a typic Red Latosol (Haplortox), with 630 mg kg(-1) sand, 40 mg kg(-1) silt, and 330 mg kg-1 clay was sampled and filled into 5 L pots. The treatments were two K rates (15 and 121 mg dm(-3)), applied as potassium chloride, and four levels of soil water (-0.03, -0.1, -0.5, and -1.0 MPa). Two cotton plants were grown in each pot and harvested 53 days after plant emergence. Diffusion was the main transport mechanism of K to cotton roots, contributing from 72 to 96% of the total uptake. The effect of soil water upon the K transport to the root surface was greater in soils with a higher K concentration. That is the reason why mass flow is more important in dryer soils.