Effects of soil bulk density and water regimen on root growth and uptake of phosphorus by ryegrass

The effects of soil bulk density (ρ) and water content on root extension, phosphorus (P) uptake and plant growth were studied with ryegrass in pot experiments with a sandy loam soil. Uptake was considered in terms of a root cylinder model with parameters of root length and uptake per unit length. Increasing ρ (from 1.0 to 1.54 g cm) slightly increased root diameter and reduced root hair length, but the main effect on uptake per unit length of root was related to the increased mass of soil and hence the increased mass of P within the root hair cylinder. The effect of ρ on diffusion to the surface of the cylinder appeared to be small. Increasing ρ also increased soil strength, causing a reduction in root length which was linearly related to P uptake. Thus the effect of ρ on total P uptake was a product of the opposing effects on uptake per unit length and root length. The importance of these two effects was confirmed by calculating potential P uptake using a simple root cylinder model in which ρ was a variable. Predicted and observed values were highly correlated (r = 0.94). Drying the soil reduced P uptake per unit length of root, but, in compacted soil, this effect was small compared with the effect of drying on root extension. It appears that, when this soil type is not cultivated, drying will reduce P uptake mainly because mechanical resistance reduced root extension, and root extension ceased well within the available water range in intact soil cores