Soil and Symbiotic Nitrogen Requirements for Optimum Soybean Production1

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Abstract
Field and outdoor hydroponic studies were conducted to determine the relative potential of soybeans (Glycine max L. Merr.) to utilize nitrate and atmospheric nitrogen as sources of nitrogen. Comparisons of nodulating and nonnodulating isollnes coupled with enzymatic assays of nitrate reductase (in vivo) and nitrogenase (in situ acetylene reduction) were used as indexes. Seasonal profiles indicate maximunmit rate utilization at the full‐bloom growth stage, with symbiotic N2(C2H2) fixation peaking some 3 weeks later during pod fill. Nitrogen fixation estimates based on comparison of nodulating and nonnodulating isolines are not valid under growth conditions of low nitrate levels because growth of the nonnodulated isoline is stunted and nitrate utilization is also impaired. Seed yield of plants totally dependent on atmospheric nitrogen was less than one‐half the yield of plants utilizing both nitrate and atmospheric nitrogen under hydroponic growth conditions. Plants grown on a low nitrate level had higher symbiotic N2(C2H2) fixation rates than those grown on no nitrate. Similarly, seed yield of plants grown in hydroponics on high nitrate levels, which inhibited symbiotic fixation, was less than yield of plants utilizing both nitrate and atmospheric nitrogen. Thus, both symbiotic N2 fixation and nitrate utilization appearede ssential for maximum yield.

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