108 THE PEOPLE'S FARM AND STOCK CYCLOPEDIA. 



just the conditions needed for those chemical changes by which 

 plant food is changed from inert into available forms. 



There is also always a risk of loss of fertility by drainage. 

 As the plant food is rendered soluble by chemical action, it may 

 be washed out by a sudden heavy rain. Clover saves this waste 

 by taking up the material as rapidly as it is rendered soluble, 

 organizing it into forms which will not waste, but can be readily 

 used by the succeeding crop. As this waste from drainage is 

 most likely to occur in the latter part of the summer and early 

 fall, the great advantage of sowing clover with wheat becomes 

 apparent. Experiments have shown that, if the soil is left bare 

 after the wheat is cut, there will often be more fertility lost by 

 drainage than was taken up by the crop. 



If the theory referred to in the beginning of this chapter 

 that, under certain conditions and in some climates, the nitrogen 

 in the soil may be increased by chemical action then it is prob- 

 able that, in warm climates, on suitable soils, clover actually 

 causes an increase of nitrogen in the soil. This point, however, 

 is not yet established, and, while it favors clover, should not be 

 depended on. 



Another question will probably occur to the reader: "If 

 clover adds no plant food to the soil, but only enables the suc- 

 ceeding crop to draw more heavily on the original supply, will 

 not the constant use of green manures, unaccompanied by the 

 addition of plant food, either in barn-yard manures or commer- 

 cial fertilizers, bring about the ultimate exhaustion of the soil 

 more rapidly than continuous cropping without clover?" The 

 answer must be that, with regard to phosphoric acid and potash, 

 it certainly will ; and, unless the theory already referred to con- 

 cerning nitrogen is true, the result will be the same with it. 



Sir J. B. Lawes, the great English experimenter, found that 

 a plat which he cropped alternately with clover and wheat, 

 through a long term of years, showed at the end of the period 

 a much greater reduction in the percentage of plant food in the 

 soil, than another plat which he cropped continuously with 

 wheat, no manure being used in either case. 



But the ultimate exhaustion of the soil is not a thing so 



