Potassium uptake from the subsoil by green manure crops

The aim of this experiment was to compare crops commonly used as green manu re 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 we re used to determine K uptake from the subsoil: The 'open-ended pot' techni que 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 soi l depths. The green manure crops tested were chicory (Cichorium intybus L.) , red clover (Trifolium pretense L.), perennial ryegrass (Lolium perenne L. ), lucerne (Medicago saliva L.), barley (Hordeum vulgare L.), birds-foot tr efoil (Lotus corniculatus L.), yellow sweetclover (Melilotus officinalis L. ) and lupine (Lupinus angustifolius L.). The latter four crops were grown f or 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 tops oil 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 mai nly 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 twic e that in the second year. A period of drought in the second year reduced g rowth of most crops, except chicory and lucerne. This did not result in a h igher uptake of Rb injected at 60 and 90 cm. relative to uptake at 10 cm, b ut 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.