Observed and Calculated Potassium Uptake by Maize as Affected by Soil Water Content and Bulk Density

Potassium uptake of field crops may be restricted when the upper soil layer, which is usually high in available K, dries out or is compacted. To evaluate the factors involved, the effect of soil water content and bulk density on maize (Zea mays L.) root growth, soil solution K concentration, and K mobility in soil was studied. Plants were grown in soil-filled pols where roots could penetrate the bottom into a K-free nutrient solution underneath. The soil was adjusted to bulk densities of 1.2, 1.4, and 1.6 g and gravimetric water contents (ω) of 10.7 to 19.0% wlw (corresponding to pF of 4.2 to 3.0), with a low (0.2 mol m⁻³) and a high (1.5 mol m⁻³) soil solution K concentration (CLᵢ). Plants were harvested 11 and 19 d after sowing and root length, shoot dry weight, and K content were determined. High bulk density reduced root growth up to the first harvest. Thereafter, relative root growth rate varied from 1.0 × 10⁻⁶ s⁻¹ at low to 2.3 × 10⁻⁶ s⁻¹ at high water content independent of soil strength. Potassium uptake decreased with decreasing soil water content, because both root length and K influx were reduced at low water content by about 50%. Increasing soil bulk density tended to increase K influx because the volumetric water content (θ) increased when the gravimetric water content (ω) was kept constant. Drying the soil increased CLᵢ, but its influence on K uptake was counteracted by the decrease of both θ and the impedance factor for diffusion (f). Model calculations simulating K transport in soil and K uptake by roots agreed fairly well with observed data, except at high soil water content, low K application, and high bulk density. Under these conditions, measured influx was lower than the calculated influx. It is concluded that this was due to uneven root distribution in compacted soil, which led to interroot competition. At high CLᵢ, K influx was almost unimpaired by low water content because, as model calculations showed, the reduced K mobility was compensated by an increase of the K concentration gradient towards the root.