136 SOME PRACTICAL LESSONS FROM SOIL BACTERIOLOGY 
tance, for it adds to the permanent fertility of the soil. Many 
fertilizers are of benefit only to the few succeeding crops; while 
others merely act as stimulants which cause the plants to exhaust 
more of the food supply of the soil than if no fertilizers were added. 
But by the use of leguminous crops, not only may the immediate 
crop-bearing power of the soil be improved; but the total supply 
of nitrogen for future crops may be greatly increased. 
There is no need of purchasing nitrate fertilizers. The money 
may be better spent on phosphorus and potash. Of these two, 
phosphorus is the more important, because soils are much more 
likely to become deficient in phosphorous than in potassium. 
Phosphorus can be applied in a fairly cheap form (raw rock phos- 
phate) which is slow in becoming available, but increases the 
phosphorous content of the soil as much as the more expensive 
forms. Raw rock phosphate is made available by the action of 
bacteria, and bacterial activity is greater in the presence of farm 
manure or green manure. 
The cultivation of legumes, with the use of phosphatic and 
occasionally potassium fertilizers, seems to be the secret of the 
continuance of agriculture. If the farmer will only learn the 
principles of this practice, and alternate legumes with his other 
crops, he may maintain indefinitely a high fertility in his soil, in 
spite of long-continued cultivation. 
