CHAPTER XXXII 



LOSSES OF PLANT FOOD FROM SOILS 



THERE are four ways in which plant food may be lost or re- 

 moved from the soil: (i) by removal in crops as already explained, 

 (2) by leaching after solution in the rain water or soil water, (3) by 

 mechanical erosion, either by surface washing or by wind action, 

 and (4) by volatilization, a factor of minor importance, represented 

 chiefly by the slight loss of nitrogen in ammonia or by denitri- 

 fication, a process which may occur to a limited extent under some- 

 what abnormal conditions. 



Loss of plant food from soils by the process of leaching is a matter 

 of very great consequence, chiefly because large amounts of nitrogen 

 may thus be lost every year in humid sections. Even under the 

 best systems of farming more or less nitrogen is likely to pass off 

 in drainage waters. The annual loss of lime by leaching is large 

 (see Tables 27 and 28), and when long periods of time are con- 

 sidered, the amounts of phosphorus, potassium, and other elements 

 removed from the soil by leaching (see Table 74) become very 

 significant. 



The only practical method of preventing or reducing the loss by 

 leaching is by the use of growing plants, the roots of which may 

 absorb the plant food about as rapidly as it is made soluble. If 

 desired, it may be then returned to the soil in the form of organic 

 matter, afterward to become available when required to meet the 

 needs of regular crops. The use of rye or rape as a green manure, 

 by seeding in the fall and plowing under the next spring on land that 

 would otherwise lie bare during the fall, winter, and early spring, 

 is often profitable, in part because of the conservation of plant 

 food that would otherwise be lost by leaching. This fact and 

 principle is well illustrated by the following data from that great 

 source of positive agricultural information, the Rothamsted Ex- 

 periment Station. 



556 



