This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
It has been known from the earliest times that leguminous plants (Peas, Beans, Clovers, Vetches, etc.) had a beneficial effect upon cultivated soils - crops of a different nature grew better after a leguminous crop. But it was not until 1886 that Hellriegel discovered how the nodules on the roots of leguminous plants were really storehouses of nitrogen-fixing bacteria, and further investigations are being made by other scientists.
Arising out of these discoveries Professor Bottomley conceived the idea that, as the bacteria could be cultivated and isolated and kept for a long time under certain conditions, it would be possible to "inoculate" a barren soil - a heap of slag or clinker even - and bring it into a fertile condition by the aid of these bacteria, and especially one called Bacillus radicicola. In this way the "nitrogen famine", predicted by Sir William Crookes a few years ago at a meeting of the British Association, was to be staved off.
Experiments in "soil inoculation" have been carried out more or less carefully in several places, chiefly by non-cultivators, but it is not possible to draw any definite conclusions from them. But one thing at least appears to have been demonstrated by experiments carried out by Professor Nobbe, of Germany, and that is, that the nodule bacteria are likely to become overfed and lazy if there is already a good supply of nitrogenous food at their disposal in the soil; whereas, if there is a deficiency, it excites them into greater activity and virulence, exactly as if they were millionaires on the one hand and peasants on the other.
The practical lesson to be learned from these investigations would appear to be that (1) it would be a mistake to apply nitrogenous manures to leguminous crops in any great quantity, as they would prevent the healthy working of the bacteria; and (2) as the soil bacteria generally are in greater numbers in the first six inches of soil, they could be utilized to fertilize or inoculate the soil to a greater depth by trenching the soil, and burying the top spit, containing the trillions of bacteria, lower down. The subsoil rich in mineral foods (see p. 111) but lacking in bacteria, would thus become a medium in which the bacteria would exercise their activity and virulence to the utmost. That is a far more simple and expeditious method of inoculating the soil than cultivating the bacteria in gelatine or powder. One crop of Broad Beans, French Beans, Clover, or Peas would produce trillions upon trillions of nitrogen-fixing bacteria to the acre in the top spit in the course of a season, and cultivators would do well to bear this fact in mind.