The Composition and Use of Fertilizers. 249 



done to some extent by the use of indirect fertilizers, like gypsum, 

 lime and salt; by the use of organic manures; by growing deep- 

 rooting crops; by judicious use of drains, and by vigorous and 

 thorough tillage. 



It cannot be too strongly emphasized that, before one can ad- 

 vantageously approach the question of the economic use of com- 

 mercial fertilizers, he must make sure that all those conditions of 

 soil are present which are essential to the favorable growth of 

 crops, in addition to plant-food supply. 



It will thus be seen that it is not always so simple a matter to 

 tell when one should use commercial fertilizers. But the general 

 rule will be to use them when their use is attended with increased 

 profit coming from increased crops, and this can be learned with 

 certainty only by actual trial. Attention will be called later to 

 specific methods for making field tests. 



2. What Constituents of Plant Food are Needed. 



When it has been clearly settled that a soil needs the addition 

 of plant food in order to grow crops more successfully, the ques- 

 tion at once presents itself as to what kinds of plant food are re- 

 quired. Does some form of nitrogen need to be supplied, or is it 

 phosjmoric acid that is needed, or is it potash? It may be only 

 one, it may be some two, or it may be all three of these constitu- 

 ents are necessary to grow crops to the best advantage. 

 How can we ascertain what hind of 'plant food is required? 



Analysis of Soils. — It was formerly thought that an analysis 

 of the soil would answer this question beyond doubt. But it is 

 now known not to be especially helpful except, perhaps, in a nega- 

 tive way by making known the fact when there is little or no plant 

 food in the soil. A chemical analysis may show an abundance of 

 plant food in the soil, and yet this may be mostly unavailable, a 

 fact which is not readily ascertained by- our methods of chemical 

 analysis. Again, there may be an abundance of nitrogen, potash 

 and phosphoric acid compounds in the soil in an available condi- 

 tion, and yet the soil may be unproductive from lack of humus, 

 from need of proper drainage, from lack of porosity or from some 

 other cause which affects the mechanical or physical condition of 

 the soil in such a way as to unfit it for plant growth. Again, we 

 may put upon an acre of soil an amount of fertilizing constituents 

 that will greatly increase the crop, and yet an analysis of the soil 

 before and after the application will show no appreciable differ- 



