26 August 1998
Legumes for organic nitrogen
I WOULD like to grow a legume that fixes large amounts of nitrogen to produce hard wheat that has a high protein content and does not require high amounts of inorganic nitrogen. Does anyone have any suggestions as to what species I should consider?
IT doesnt matter whether you grow peas, beans or soya, their nitrogen contribution is basically the same. Its what you do afterwards that determines its actual availability.
If you plough down to 25cm (10in) or more, youll be moving N to an unavailable part of the soil profile where it will be potentially more liable to be leached as the nodules decompose.
However, if you employ non-inversion tillage and incorporate the top layers, then you should be able to make adjustments to the inorganic N.
But in wheat after peas, for example, the downward adjustment ought to be around 40kg/ha. This will depend on the yield potential of the field: if you push the crop too hard youll lose N in the grain.
One way weve found to achieve more consistent breadmaking quality in wheat after legumes is mechanical weeding in the spring. This allows us to defer the first top-dressing of 40kg/ha until flag leaf emergence, after the main mid-April application of 100-120kg/ha.
LUPINS are widely believed to contribute large quantities of nitrogen to the rotation. In fact, they may not have the benefits traditionally associated in terms of N economy. They do fix a lot of N, but the amount they return is quite small.
For peas and beans, about 60% of the N fixed goes to the grain while the remaining 40% returns to the soil in the form of haulm. With lupins the harvest index is higher, leaving only about 15% residue, no greater than a high-yielding wheat crop.
However, while lupins are inferior to pulse crops in terms of N contribution, theyre probably superior in terms of the phosphorus they return, though this benefit hasnt been measured yet. Parts of their roots secrete acid which dissolves phosphorus from normally insoluble sources.
A series of MAFF-sponsored demonstrations this autumn should provide a good opportunity to see how the new types of dwarf determinate lupins perform.
Further away in commercialisation terms, work on lentils at Reading University suggests that following wheat crops would have lower N requirements. Research there is assessing the impact on breadmaking quality.