Some factors which affect the inoculation of soybeans

Various factors which affect the inoculation of soybeans were studied and the following results were secured. Considerable difference exists in the ability of various cultures of soybean bacteria to produce nodules on soybean plants. A low soil moisture content at time of planting may be an important factor in reducing nodule formation. This is probably due to the fact that the root-hairs on the main stem disappear before the bacteria have an opportunity to come in contact with them. Keeping a soil at a high moisture content after it was held at a low moisture content for 10 days after soybeans were planted produced a lower percentage of inoculated plants and fewer nodules per plant in 9 out of 13 varieties of soybeans studied. Preparing a milk suspension of soybean bacteria before adding the culture to the seed did not give any better inoculation than when a water suspension was used. The addition of superphosphate (acid phosphate) and potash to the soil used in these experiments did not affect nodule formation. The use of chlorinated water from the city water supply instead of distilled water to keep the soil at an optimum moisture content resulted in a considerable depression in nodule formation but did not completely prevent inoculation. Field and greenhouse studies on the inoculation of a large number of soybean varieties, using both pure and mixed cultures, indicate that there is considerable variation in the ability of different varieties of soybeans to resist infection by a particular strain of soybean bacteria. If the nitrogen content of plants can be used as a measure of the benefits derived from the association of soybean bacteria with the soybean plant, data are presented to show that under certain conditions uninoculated plants may contain a higher percentage of total nitrogen than inoculated plants. This would indicate that some soybean bacteria may be of less value than others from the standpoint of nitrogen fixation and the subsequent utilization of this nitrogen by the soybean plant. Well-inoculated soybean plants grown on soil treated with 2 tons of wheat straw per acre contained less nitrogen than inoculated plants grown on untreated soil. Various factors which might cause such a condition were considered.