Crop growth across a toposequence controlled by depth of sand over clay

Abstract

Sand soils containing, by definition, less than 5% clay, may give substantial variation in crop growth, even where they have apparently similar chemical and physical properties. This work tests the proposition that crop growth in a winter-rainfall environment is dependent upon the depth of a uniform sand profile, and that crop growth can be negatively correlated with depth to clay because of the low water and nutrient storage capacity of such soils. Spring wheat and barley, cereal rye, and narrow-leaf lupin were grown at three adjacent sites across a toposequence having a 3° slope with increasing sand depth (from 0.6 to >2.5 m) downslope. The experiments were conducted over two years, the first wetter than average, the second drier. In both years there was a significant decline in dry matter accumulation and crop yield with increasing sand depth in which the wheat biomass and lupin grain yield were most affected. Water uptake and nutrient distribution patterns within the profiles showed that available water was least and leaching most severe on the deepest sand, where no fine-textured materials at depth occurred to trap water and nutrients for later-season use within the rooting zone. Maximum rooting depths were estimated to average 2.0 m for wheat, and 2.4 m for lupin and cereal rye. However, on the deepest sand where roots grew to >2.5 m, yields were reduced to half, even under the high fertilizer conditions of the experiments. It is concluded that depth of sand provides the most reliable prognosis for crop growth and yield where there are no distinctive differences in clay content or chemical reaction.