It seems obvious that the lighter and poorer soils would benefit more than the heavier or richer soils by the extended growth of leguminous crops.
The nodules on the roots of leguminous plants are induced by the presence of a minute organism now known to do no injury to the plant.
Leguminous crops take some of the nitrogen which they require from the air, but most plants obtain it from the nitrates present in the soil.
It is, in fact, fully established that these leguminous crops acquire a considerable amount of nitrogen by the fixation of the free nitrogen of the atmosphere under the influence of the symbiotic growth of their root-nodule-microbes and the higher plant.
Plants of the gramineous, the leguminous and of other families were operated upon.