Grain Legumes need Potash

Grain legumes in the UK

Field beans and peas are the main grain legume crops in the UK with, between them, around 250,000 ha grown. In recent years, lupins have become a significant crop and small areas of broad beans, soya beans and navy beans are grown. Field beans (including tick beans and horse beans) and broad beans are all sub-species of Vicia faba. Navy beans are a different species, Phaseolus vulgaris. Average yields of beans, peas and lupins in the UK are 3.5 - 4.0 t/ha but potential yields are much higher. These crops will repay attention to husbandry generally and to nutrition in particular.

Legumes need a good K-start!

Potassium is an essential nutrient for legumes and all other crops. It is an activator for many enzymes, especially those involved in protein synthesis. Potassium also has an important role in maintaining water balance in the plant. In legumes, potassium is necessary for the proper development and functioning of root nodules. Rhizobium bacteria in the nodules fix atmospheric nitrogen and make it available to the legume plant. If nitrogen fixation is affected by potassium deficiency, the legume will suffer a nitrogen shortage and reduced yield.

Deficiency symptoms

Serious potassium deficiency causes yellowing or scorching of leaf margins, forward curled leaves and, in beans, shortening of internodes (stem between leaves). As potassium is involved in nitrogen fixation in the root nodules, deficiency can also appear as an apparent nitrogen shortage, with stunted plants. Where there are visible symptoms of potassium deficiency, crop yield will have been significantly affected - less severe deficiency, which still affects yield, will not be visible. It is important therefore to ensure a good potassium supply and to avoid the occurrence of deficiencies. Concentrations in pea and bean leaf tissue of 1.1 - 1.2% K in dry matter have been associated with optimum potassium nutrition.