102 NOT ALTOGETHER ABOUT PLANTS 



depends. It is evident that if the supply of any one of 

 the soil-substances necessary to plant life should give out, 

 our own lives would very promptly be affected. 



Now it is a fact that old farms whose soil has not been 

 improved by the addition of fertilizers appear to lose their 

 fertility. As the years go by, the amount of crop per acre 

 becomes reduced. In recent years the question has been 

 raised as to whether this is due to the gradual exhaustion 

 in the soil of certain substances necessary to plant life. It 

 is evident that this soil problem is of vital importance to the 

 human race. Each year the population of the world in- 

 creases. Each year there is more hunger to be satisfied. 

 Are we sure that each year there will be more food ? Most 

 of the fertile land in the world is already under cultivation, 

 and if fertility is decreasing while food-consumption is 

 increasing, the outcome is evidently going to be very bad 

 for the human race. 



To solve the problem of maintaining the food supply of 

 the human race it is evidently necessary to know just what 

 are the properties of soil which cause it to be fertile in the 

 production of food plants. It formerly was thought that soil 

 fertility was due to the presence of certain substances, and 

 soil sterility due to the absence of these substances. But 

 now it is known that the relation of soil to plant growth is 

 not so simple a matter as that. In fact it is a very complex 

 matter, concerning which scientists are more inclined to 

 speak of what is yet to be discovered than of what is 

 already known. 



It is only in cultivated soil that we note decrease of fer- 

 tility. Farmers speak of the " exhaustion " of soil, but the 

 exhaustion which they observe in their fields is not observ- 

 able in nature. The plants which grow upon uncultivated 



