Soil nitrogen levels are usually higher after growing a pulse crop. Even though the crop uses available soil nitrogen during the growing stage, nitrogen is still available for the next crop through leaking of root exudates that are high in nitrogen and the later decaying of pulse crop residue.
The amount of nitrogen available depends on time, moisture, heat, the amount of residue and how well the crop is nodulated, says Brandon Green, corporate agronomist with Enviro-Test Laboratories.
Cereal and oilseed crops usually return half the nitrogen in their residue to the soil in four years.
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Soil microbes break down the residue but because the residues are low in nitrogen, it must be taken from the soil or fertilizer for this process to occur.
Pulse crop residue, by comparison, is already high in nitrogen, so residue breakdown is faster and half the nitrogen is returned to the soil for the next year.
Soil moisture and temperature during the fall and spring will determine how fast the residue is broken down and the nitrogen released.
If the pulse crop is harvested earlier, there is more time for residue breakdown before freezeup. Breakdown also occurs more rapidly if soil is moist and warm.
Cultivated pulse residue breaks down more rapidly than when it is left on the soil surface. As a result, wait as long as possible before taking soil samples on pulse residue.
Be careful of next year’s crop selection and nitrogen fertilizer rates. A large amount of nitrogen can become available during stem elongation of the subsequent cereal crop, which can lead to lodging, if a big pulse crop was grown the previous year, if the soil is wet and warm, if there’s lots of time from harvest to freezeup and if the pulse residue was cultivated.
By contrast, if time is shorter and the soil is dry and cool, then nitrogen can come available late in the next growing season, which will be used for protein production in the cereal crop. That’s fine for wheat and feed barley, but could prevent malting barley from achieving malt status.
By using figures in the table, expected amounts of nitrogen that will be available following various pulse crops can be determined.
A 2,000 pound per acre (33 bu. per acre) field pea crop would produce (2,000 Ö 100 X 0.5) = 10 lb. per acre of nitrogen while a 3,000 lb. per acre (50 bu. per acre) field pea crop would produce (3,000 Ö 100 X 0.5) = 15 lb. per acre of nitrogen.
While this is not a lot of nitrogen, it helps when considering high nitrogen fertilizer prices.
By contrast, 15 or even 10 lb. per acre of extra nitrogen coming available during stem elongation or protein production can harm an already well fertilized crop.
More relevant nitrogen recommendations can be developed by submitting soil samples, identifying the type of pulse crop previously grown and including yield data. This is an important step in managing nitrogen in a dynamic crop rotation.
