FOR many years what is known in agriculture as the nitrogen problem has received considerable attention both from men of science and from practical men. It lias two aspects. Few soils contain nitrogen compounds in sufficient quantity for the needs of non-leguminous crops, and the application of nitrogenous manures is one of the commonest, as well as one of the more costly operations of modern agriculture. On the other hand, leguminous crops not only need no nitrogenous manure for themselves, but actually increase the store of nitrogen compounds in the soil, and dispense with the necessity of adding more for the succeeding crop. The problem would obviously vanish if leguminous crops could be grown every other year, but unfortunately they are liable to “sickness,” and can only be grown once in four or even six years. Even as it is, however, any method that increases the nitrogen-fixing power of a leguminous crop is a welcome addition to the resources of a farmer.