Potassium Uptake from the Subsoil by Green Manure Crops

ABSTRACT

The aim of this experiment was to compare crops commonly used as green manure or forage crops in temperate climatic regions in terms of their total K uptake and the proportion of K taken up from the subsoil. Two techniques were used to determine K uptake from the subsoil: The ‘open-ended pot’ technique based on a decrease in the K-to-Rb ratio of plants grown in Rb-enriched topsoil compared with plants grown in pots without access to the subsoil, and a technique based on injection of Rb, as tracer for K, at different soil depths. The green manure crops tested were chicory (Cichorium intybus L.), red clover (Trifolium pretense L.), perennial ryegrass (Lolium perenne L.), lucerne (Medicago sativa L.), barley (Hordeum vulgare L), birds-foot trefoil (Lotus corniculatus L.), yellow sweetclover (Melilotus officinalis L.) and lupine (Lupinus angustifolius L.). The latter four crops were grown for one season, the others two seasons. In the first year of establishment, all green manure crops, except chicory, took up K from the subsoil and topsoil in much the same proportion as the cash crop barley, with 41–67% of the K taken up originating from the subsoil. K uptake from the subsoil was mainly determined by differences in the crop's total K uptake. Chicory had the highest total uptake amounting to 124 kg ha⁻¹ in the first year and twice that in the second year. A period of drought in the second year reduced growth of most crops, except chicory and luceme. This did not result in a higher uptake of Rb injected at 60 and 90 cm relative to uptake at 10 cm, but it is possible that chicory and lucerne took up substantial amounts of K from depths greater than 1 m, not accessible to the other crops.