Influence of legumes on microbial activity in soil

An attempt has been made in the present investigation to elucidate the influence of growing legumes upon the microbial population and upon its activities in the soil. Annual legumes, pea and vetch were compared with oats, and red clover with timothy. At the beginning and at the end of the summer the number of bacteria, calculated through various culture media, appeared to be greater in the rhizosphere of the pea plants than under the vetch or oat plants. The rhizosphere flora of the young pea plants showed a capacity for more active nitrogen fixation than that of the young vetch or oat plants, but later the superiority of the pea plants disappeared. A higher rate of nitrification under and for some time after the legume crops compared with the corresponding non-legume crops was noted. Nitrification of added ammonium sulfate did not occur more actively under the legumes, due either to the lack of a stimulation of the nitrifying flora, or to the higher acidity in the legume soils. Compared with the non-legumes, the legumes could not be found to stimulate carbon dioxide production from the soil in any other way than through their more easily decomposable residues. Within the bounds of this investigation no special effect of legumes upon the soil flora could be established that cannot be explained on the basis of their nitrogen economy, either their utilizing soil nitrogen to a smaller degree than non-legumes, or due to composition of their roots and stubble. Under the field conditions, however, several other factors not demonstrable from this material may exist