THE SOIL. 7 



to estimate with any accuracy, although we know that 

 they are frequently large, even greater than the losses 

 in ways which we can estimate. 



Limiting Factors in Soil Fertility. As previously 

 stated, there are a number of factors which go to de- 

 termine the fertility of the soil, such as moisture-hold- 

 ing capacity, aeration, tilth or penetrability of the 

 roots, and supply of essential elements. A lack in 

 any one of these requirements will limit the power of 

 any particular soil to produce good crops. If, for 

 instance, the supply of nitrogen, potash, or of water 

 be inadequate the amount of this substance that is pres- 

 ent becomes the limiting factor in crop production. 

 This principle is illustrated in the accompanying dia- 

 gram, Fig. 3, which was devised by Dr. Dobenec, of 

 Germany. The amount of water which the barrel 

 can hold is determined by the height of the lowest 



FIG. 3. 



