I recently read a publication that stated pulse crops add 40 pounds of nitrogen to the soil. The statement was referring to the fertilizer N-credit that can be taken by producers for the crop after an annual legume or pulse crop. In other words if you grow an annual legume, most soil testing labs would give N-credits (approximately 40) when making nitrogen fertilizer recommendations for that field the year after the pulse crop is grown. Does this mean that pulse crops add nitrogen to the soil? In fact annual legumes do possess the unique ability to fix atmospheric nitrogen gas into plant useable forms through a symbiotic relationship with rhizobium bacteria. This nitrogen is utilized to satisfy the plant's nutritional demands during growth and then produce seed.
Annual legumes, which include soybeans, peas, lentils and chickpeas, actually have very high demand for nutrients (especially nitrogen) during their life. Producers typically treat all annual legume seed with rhizobia just prior to planting or apply a granular product that contains peat and rhizobia in the seed trench to ensure that the young plants have ample rhizobia and can produce the nitrogen they need to grow. Much of this nitrogen is then removed in the seed when the crop is harvested. As with any crop, some of the nitrogen is left on the field in the straw or residue. Nitrogen contained in crop residue is not available to plants until microbes decompose the plant material. The microbes in the soil, that break the crop residue down and convert it to soil organic matter, use nitrogen from the soil as an energy source to aid the process if the residue is low in N (high C:N ratio).
So why is it that a 40 pound N-credit is allowed the next crop year? The N-credit allowed for crops following an annual legume in a rotation results because the legume straw does not tie up as much soil nitrogen as it breaks down. There are two reasons for this. Legume straw has a lower carbon to nitrogen ratio (C:N) when compared to wheat straw or corn stover. This allows the legume residue to break down more easily and not tie up as much soil nitrogen in the process. In addition, legumes and other broadleaf crops often produce less residue than crops like wheat and corn. This further reduces the amount of mineral nitrogen immobilized in the residue decomposition process. The additional amount of residual nitrogen in the soil can be used by the following year's crop for growth, rather than as a source of energy for the microbes that are busy breaking down the previous crop's residue. In essence we do not have more nitrogen after the annual legume; it just is not tied up by microbes breaking down residue. Legume crops put more than 40 lbs of N/acre into the soil, but most of it is used to produce that crop, it is not the source of the credit.
More information regarding N credits for legumes can be found at the following web sites:
Gardening 101: Alternative Gardening
The first Gardening 101 spring workshop is being held April 27, from 9 a.m. - 4 p.m. in Huron at the Department of Health building (1110 3rd St. SW). The workshop will cover alternative gardening techniques like raised beds, container planting, and no-till gardening as well as mulching and watering techniques. There will be hands on demonstrations and participants will plant a one gallon pot with spring vegetable seeds to take home. The registration fee is $40 and covers all supplies that participants get to keep. To register, contact Mary Roduner by phone at 605-394-1722 or email firstname.lastname@example.org.