THE SOIL AS RELATED TO PLANTS. 85 



of the desired crop are present in the soil. This 

 question can be answered by chemical analyses 

 of plants and of soils. 



I. Chemical Analysis of Soils. 



If the required elements for a certain crop 

 are not present in the soil they must be sup- 

 plied by a fertilizer, or some other crop sown. 



But if chemical analysis does show the neces- 

 sary elements to be present, it does not satis- 

 factorily answer the question as to whether that 

 food is available for the use of the plant ; that 

 is, whether conditions are such that the plant 

 can and will use this food. 



As has already been shown, the chemical 

 composition of the rock from which the soil 

 is obtained, the texture, drainage, temperature, 

 tillage, ventilation, and water content of the 

 soil which determine the delicate and little- 

 understood life processes of the plant all are 

 factors in the productiveness of the soil. 



There are so many conditions, then, that enter 

 into the productiveness of the soil which chem- 

 ical analysis cannot take into account that it is 

 generally of little practical use to the farmer. 



II. Vegetation Experiments. 



These are of much value in determining just 

 what fertilizer is needed, but they require time. 

 If, however, the farmer will do as the United 

 States Department of Agriculture advises, 



