14 MANUFACTURE OF FERTILIZING MATERIALS 



needed plant food. The chief value of their 

 use lies in the fact that they can change unavail- 

 able into available forms of plant food. It can 

 readily be seen that, when stimulant fertilizers are 

 used exclusively for a term of years, the soil each 

 year loses nitrogen and phosphoric acid, which are 

 not replaced. The inevitable result of such treat- 

 ment is the exhaustion of these important food 

 constituents from the soil. This affords an explan- 

 ation of the question often raised now as to why 

 the application of land-plaster does not give 

 such results in crop yields at present as in former 

 days. When land-plaster was the only fertilizing 

 material added to soils for years in succession, 

 it was possible to produce increased crops so 

 long as there were in the soil enough compounds 

 of nitrogen, potassium and phosphorus to be 

 rendered available by the action of the land- 

 plaster. When, therefore, these forms of plant 

 food were largely removed, there was nothing 

 for the land-plaster to act upon in order to 

 increase the supply of available food material. 

 The land-plaster furnished no needed food, but 

 simply helped the crop to use more rapidly the 

 store of plant food present in the soil. 



Direct fertilizers contain forms of plant food, 

 which contribute directly to the growth and 

 substance of plants. Such materials, as have 

 already been mentioned, contain nitrogen, potash, 

 and phosphoric acid compounds, or any two, or 

 all three of these materials. 



