A method is described which allowed the quantification of the potential uptake of P and Mg from the subsoil (> 30 cm) by spring wheat. Wheat was grown on an artificial topsoil (sand with no plant available P or Mg) which was superimposed on loess subsoils in N. Germany. The supply of P and Mg in the topsoil was varied by application of different quantities of P and Mg fertilizer. Uptake of P and Mg from the subsoil was calculated as the difference between total plant uptake (determined by plant analysis) and the quantities of P and Mg removed from the topsoil (determined by soil analysis). P uptake from the subsoil increased from 37% to 85% of total P uptake, with decreasing P supply in the topsoil. Calculations of potential supply by diffusion showed that, with a CAL-extractable P2O5 content in the subsoil of 9 mg 100 g-1, supply from the subsoil was only possible if the influence of root hairs was considered. The method also showed that the total demand for Mg by spring wheat could be satisfied from the supply of Mg from the subsoil of typical loess soils. Mg uptake from the subsoil decreased to 33% of total uptake with increasing Mg supply in the topsoil.
