PHOSPHATE AND POTASSIUM MOVEMENT AND UPTAKE AS INFLUENCED BY TRANSPIRATION LEVEL AND FREQUENCY OF SOIL WATERING

This study was conducted to determine the effects of different oscillations in water levels on solute (phosphate and potassium) movement and uptake by plant roots. It also provided values for soil characteristics and plant parameters to be used in a mathematical simulation of plant nutrient uptake. Corn (Zea mays L., \u27NB611\u27) was grown in a growth chamber for two weeks at 35% relative humidity (RH) or at 55% RH. The soils were all watered to \u27field capacity\u27 ((theta)(,max)) and allowed to dry to the following minimum values ((theta)(,min)) before rewatering: .130, .115 and .100 for the Boelus LS, 5% (Udorthentic Haplustoll); .145, .130 and .115 for the Boelus LS, 2% (Udorthentic Haplustoll); and .317, .294 and .271 for Plano SiL (Typic Hapludolf). The water treatments were chosen such that the plants were never under water stress during the entire growing period. Transpired water was measured gravimetrically. Following harvest, the plants were measured for phosphate and potassium uptake, root length, root diameter, root surface area and leaf surface area. The soils were analyzed for texture, bulk density, moisture release characteristics, effective diffusion coefficient and solute adsorption-desorption functions for phosphate and potassium. Increase in (theta)(,min) within a soil caused increased watering frequency and hence higher mixing of solutes. At 35% RH, significant effects were observed of differences in (theta)(,min) on phosphate and potassium flux in the sandy soils and on phosphate flux in the SiL soil. The effect of water treatment on potassium flux in the SiL soil was nonsignificant. All water treatment effects were non-significant at the 55% RH. The significant effects of water treatment (hence mixing action) on phosphate flux to the roots at low RH but not at high RH shows that phosphate diffusion directly to the roots may not be the only mechanism for phosphate movement in soils. Rather, transport to the transpiration stream represents a new mechanism. This is the result of diffusion and mixing that is perpendicular to the direction of the convective flow. This mechanism controls the solute concentration in the convective flow