286 SOILS 



been cropped for many years is occasionally due, 

 in part, to the exhaustion of soluble plant food. 

 Usually it is due, wholly or largely, to the way in 

 which the soil has been handled. It is more apt 

 to be a problem in improving the physical con- 

 dition of the soil than in enriching it. This fact 

 has been proved on thousands of American farms, 

 where better tillage, more thorough drainage, 

 rotation of crops, green manuring and other 

 methods of improving the texture of a soil have 

 been practised. These matters are discussed at 

 length in other chapters. 



LOSS OF FERTILITY BY EROSION 



Not all of the plant food that is lost each year from 

 farms is carried off in crops. A far more damag- 

 ing cause of reduced yields on some farms is 

 erosion, or the washing of soil. This removes the 

 best part of it the surface soil that has been made 

 fine and has had its plant food made soluble by 

 weathering. 



The loss of fertility by erosion is one of the 

 greatest leaks on American farms. The chief 

 reason why erosion is so dangerous is that it is 

 insidious. Its ravages are not very conspicuous 

 until it has done much damage. Erosion does not 

 ruin a soil in a single night, or a single season; it 

 starts from small beginnings, usually unnoticed, 

 and creeps stealthily upon the land. Every tiny 

 rill trickling down the slope carries off some of the 

 finest and richest soil on the farm. After a heavy 

 rain the puddles in the hollows are muddy. The 

 deep furrows left up and down the slope by the 

 cultivator teeth become miniature watercourses, 

 and the trickling water exacts a tribute of rich 



