80 CROP PRODUCTION AND SOIL FERTILITY 



highest possible yields. We can speak of degrees of fertility in a 

 similar sense as degrees of richness, for example. 



Very fertile soil or very high fertility implies a very high pro- 

 ductive power. Low fertility implies that the yielding power is 

 not very good. ^^ Infertility^^ means the lack of productive power. 



Fertility as applied to a soil in Wisconsin, for example, does 

 not imply that that soil has the ability for producing good or high 

 yields of all kinds of crops to be found in the world; but it has 

 meaning only in relation to those crops grown in that particular 

 climate or section. • ^, . ^^- — ^^_ ^ \ 



FiQ. 29.— Poor soil, poor crop. A poor crop shows the soil is not fertile. 



Crops Indicate Fertility. — Crop growth is the only indicator of 

 soil fertility, and the use of good seed is the only proper means in 

 testing fertility. Since the requirements and adaptability of 

 different crops vary, a soil may be fertile for one or several kinds 

 of crops and not for another (Figs*. 30 and 31); for example, a soil 

 may grow excellent crops of clover, potatoes (Fig. 32), corn, 

 and grains, but fail absolutely to grow alfalfa, for lack of 

 proper inoculation. 



Factors Determining Fertility. — The factors which determine 

 soil fertility are discussed in the first pages of this chapter. They 

 may be thus enumerated as positive and negative factors: 



