Letter: Minding your peas

From Rob Wetselaar

According to “Let’s hear it for the pea” (29 April) the cultivation of the pulse pigeon pea “can improve the land”. The operative word here is “can”, which needs to be put into perspective.

It is generally believed that wherever a legume is grown, the soil nitrogen (N) content will be increased. This belief is justified for pastures that incorporate effectively N²-fixing legumes, because most of the N contained in the above-ground part of the plants that are removed by the grazing animals is returned to the soil after ingestion and excretion. However, the more this N is removed from the soil/legume system the less likely it becomes that the soil’s N status will be improved.

In the case of the pigeon pea, if it is harvested as a green vegetable, all above-ground N is removed, leading to soil N depletion. Well-growing pulses such as pigeon pea will fix 100 to 150 kilograms of N ha⁻¹ per crop. When the pods, with a yield of 3000 kg ha⁻¹ and a protein content of 28 per cent, are harvested, then 134 kg N ha⁻¹ will be removed. In that case, there is likely to be neither gain nor loss of soil N.

Yet, it has very often been found that non-leguminous crops such as millet will grow exceptionally well after a pulse crop. My own field experiments have shown conclusively that this is in most cases due to the generation of nitrate that originated from the topsoil N. During heavy rainfall periods, this nitrate can be leached to the sub-soil out of reach of many root systems. This occurs especially where the pulses are grown in roots, with the inter-row areas leaving the soil free of roots during the early part of growth.

Of course, when the stems are also used, for example for fuel or basket making, more N is removed from the soil/plant system, leading to further soil N depletion.