Question

what are the different sources of nitrites and nitrates in the soil? why do plants need them?

Answer 1

Atmospheric N is the major reservoir for N in the N cycle (air is 79% N2 gas). Although unavailable to most plants, large amounts of N2 can be used by leguminous plants via biological N fixation. In this biological process, nodule-forming Rhizobium bacteria inhabit the roots of leguminous plants and through a symbiotic relationship convert atmospheric N2 to a form the plant can use. The amount of N2 fixed by legumes into usable N can be substantial, with a potential for several hundred pounds N per acre per year to be fixed in an alfalfa crop. Any portion of a legume crop that is left after harvest, including roots and nodules, can supply N to the soil system when the plant material is decomposed. Several nonsymbiotic organisms exist that fix N, but N additions from these organisms are quite low (1-5 pounds per acre per year). In addition, small amounts of N are added to soil from precipitation. The amount of N supplied from precipitation averages 5-10 pounds per acre per year in Minnesota.