22 Fertilizers 



DIRECT AND INDIRECT EFFECT OF MANURES 



It is obvious, therefore, that any substance which con- 

 tains nitrogen, phosphoric acid or potash may serve 

 as a direct manure, and any substance which contains 

 no plant-food, but which possesses the power of improv- 

 ing the physical character of soils, may also serve as a 

 manure, though the one effect is quite distinct from the 

 other. The first adds to the soil the essential constitu- 

 ents ; the other helps to make the constituents already in 

 the soil serve as food to the plant. 



The use of the one will tend to increase both the poten- 

 tial and practical fertility in the soil, while by the use of 

 the other the active fertility is increased as the potential 

 fertility is decreased. That is, when actual plant-food 

 is added in the form of nitrogen, phosphoric acid or potash, 

 and crops are removed, the exhaustion of the soil is in 

 proportion to the amounts of these removed over and 

 above the amounts which have been added. Whereas, 

 in the other case, when no plant-food is added, the exhaus- 

 tion is measured by the amount of the constituents re- 

 moved. It is clear, therefore, that the addition of only 

 indirect manures has a tendency to rapidly reduce the 

 fertility of soils of low natural strength, or those that do 

 not possess large stores of food constituents, whereas, 

 on soils that are rich in the fertility elements, the indirect 

 manuring may result in an increased yield for a long 

 period, though ultimately the soil will become exhausted 

 if not completely, to such a degree as to render further 

 cropping by this method unprofitable. 



There are a number of substances which act in both 

 capacities directly and indirectly and in order to 

 understand thoroughly the value of such materials they 



