MAINTAINING SOIL FERTILITY 321 



for the plant food they contain, but also for their 

 cleansing action upon the soil. Manures benefit 

 the soil chiefly because the humus in them cleanses 

 the soil. 



7. The practical application is to rotate crops, and 

 to use farm manures and green manures, which 

 supply the humus that cleanses the soil of plant 

 excretions. In the wild the soil cleanses itself by 

 the constant addition of decaying plants. 



The clash between the current belief and the new 

 belief is mainly this : The prevailing belief explains 

 the unproductiveness of soils that the chemist finds 

 to be very rich in plant food by saying that the soil 

 is in poor texture and hence has not the conditions 

 of warmth, aeration and moisture that are essential 

 to plant growth. The new conception explains 

 the same situation by saying that the worn-out soil 

 has become unsanitary because of an accumulation 

 of excretions from the roots of plants, not enough 

 humus being present to absorb them. 



These two interpretations of the cause of infer- 

 tility are radically different, but there is no dispute 

 about the remedies. In either case they are good 

 tillage, a rotation of crops and the addition of humus 

 to the soil in the form of farm manures and green 

 manures. In either case these remain the most 

 valuable means of maintaining the fertility of 

 farm soils. So the farmer will continue to rotate 

 crops, and to use barn and green manures, unmind- 

 ful of the controversy that is being waged in the 

 scientific world concerning the exact way in which 

 they benefit the soil. 



