Soybean-rhizobium symbiosis is an example of aggregate produced by plants and microorganisms. The interaction of legumes with root nodule bacteria formed nodules where goes the process of nitrogen fixation. Inoculation of soybean seeds with thisbacteria is an integral part of its cultivation technology.

One of the most important indicators of the effective functioning of the soybean nodules is the content of the pigment leg-hemoglobin. Similar to the hemoglobin in the human blood this pigment contained in the cytoplasm of plant cells, not just in the space between the bacteroides and membranes that surround them reversibly capable bind oxygen. The formation of this element is the result of nodule bacteria symbiosis with leguminous plants. It is based on photocolorimetric determination of the content of leg hemoglobin pigment in the nodules of soybean plants. There are three degrees of nitrogen fixation activity: high, medium and low. Legumes have the ability to fix atmospheric nitrogen, while the soil is saturated with available nitrogen nutrients for plants and increases its fertility. A method of determination nitrogen fixing activity, provides a visual analysis of the nodules. The presence of the large number of red color nodules indicates the high activity of soybean symbiotic system. If nodules are small and green or brown, it indicates a lack of pigment leg-hemoglobin, the nitrogen fixation in plants is low or absent. However, this method, which is based only on visual signs is not advisable.