390 



SOIL PHYSICS AND MANAGEMENT 



being present, are not under favorable conditions for carrying on 

 their work, the plant suffers and the soil appears as if exhausted. 



Plant Food Elements. Ten elements are essential to the 

 growth of plants. (See the table on page 2.) Of these, sulphur, 

 calcium, magnesium, iron, nitrogen, phosphorus, and potassium are 

 furnished by the soil. The last three are the ones most liable to be 



Plant Food in Crops * 



* Compiled from various sources. 



deficient in soils. There are many indications, however, that cal- 

 cium may become so low that legumes, which require large amounts 

 of it, may suffer in their growth because of an insufficient supply. 



Removal of Plant Food. Crop Requirements. Removal of 

 plant food from the soil by the crop is one of the common causes 



