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Legumes need a good K-start!

May 2020

Potassium is an essential nutrient for legumes as it is for all crops. It is an activator for many enzymes, especially those involved in protein synthesis and 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. In legumes, rhizobia bacteria in the nodules fix atmospheric nitrogen and make it available to the plant. Nitrogen fixation can be affected by potassium deficiency, as the number and size of nodules per plant is reduced as is the nodule nitrogen production. As a result, plants will suffer a nitrogen shortage and yield will be reduced.

Potash and broad beans.

  Potash supply
  Low K Medium K High K
No. of nodules/plant 233 250 251
Size of nodules, % 100 111 129
Nodule N production, % 100 147 195
Source: The influence of potassium on the activity of Rhizobium bacteria. Proc. 11th Int. Colloquium, IPI.

Yield response to potassium

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 despite a large yield increase from the addition of fertiliser potash.

  Soil K level, mg/kg
  110   170
  Potash applied kg/ha   Potash applied kg/ha
  0 53   0 53
Wheat yield t/ha 8.5 8.5   8.5 8.6
Field beans yield t/ha 2.5 3.6   4.4 4.4
Rothamsted Research – Saxmundham data.

Don’t forget Sulphur for Legumes

Sulphur is an essential nutrient as it is a constituent element of some amino acids. Sulphur deficiency decreases plant growth, photosynthesis, and seed yield in all plants, including legumes. In nodulated legumes, sulphur supply is positively linked to nitrogen fixation and sulphur deficiency results in a decrease in nodulation, inhibition of nitrogen fixation, and slowing down of nodule metabolism.

Impact of sulphur application on nodules per plant and nodule weight