Grain Legumes need Potash

Potassium uptake and removal by legumes

Beans are quite deeply rooted but peas have a much less extensive root system. It is important that the topsoil is well supplied with potassium to support uptake by peas. Legumes can take up very large amounts of potassium as shown by work in France on white lupins. By late August at peak canopy, the crop had produced more than 20 t/ha dry-matter and had taken up around 400 kg K₂O/ha. The grain yield of 3.3 t/ha removed 45 kg K₂O/ha.

Dry matter production White Lupin		Uptake of Potash White Lupin
SCPA Centre de Recherches Aspach, France

Combined peas remove around 10 kg K₂O per tonne of dry peas (fresh weight) and beans and lupins remove around 12 kg K₂O/t of dry seed. Removal for a 4 t/ha crop will be around 40 kg K₂O/ha for peas and 48 kg/ha for beans and lupins.

Rothamsted Research

Yield response to potassium

In field trials at Rothamsted on silty clay loam soils, field beans responded up to around 200 mg/l soil K (upper soil index 2).

Experiments on a sandy clay loam soil at two levels of soil potassium examined the response of wheat and field beans to two rates of fertiliser potash. Wheat yields were not affected by the level of soil potassium in the trial and gave no response to added fertiliser. Beans however yielded much less on the lower potassium soil even though the addition of fertiliser potash gave a large yield increase. The best bean yield was on the higher potassium soil.