372 UOW CROPS FEED. 



orological conditions. In tte tropics, both ttese processes 

 go on more vigorously than in cold climates. 



Every soil has a certain inherent capacity of production 

 in general, which is chiefly governed by its power of sup- 

 plying plant-food, and is designated its " natural strength." 

 The rocky hill ranges of the Housatonic yield once in 

 30 years a crop of wood, the value of which, for a given 

 locality and area, is nearly uniform from century to cen- 

 tury. Under cultivation, the same uniformity of crop is 

 seen when the conditions remain unchanged. Messrs. 

 Lawes and Gilbert, in their valuable experiments, have 

 obtained from " a soil of not more than average wheat- 

 producing quality," without the application of any ma- 

 nure, 20 successive crops of wheat, the first of Avhich was 

 15 bushels per acre, the last 17^ bushels, and the average 

 of all 161 bushels. {Jovr. Roy. Ag. Soc. of Enrj., XX Y, 

 490.) The same investigators also raised barley on the 

 same field for 16 years, each year applying the same quan- 

 tity and kinds of manui-e, and obtaining in the first 8 

 years (1852-59) an average of 44|^ bushels of grain and 

 28 cwt. of straw ; for the second 8 years an average of 51§ 

 bushels of grain and 29 cwt. of straw ; and for the 16 

 years an average of 48j^ bushels of grain and 28^ cwt. of 

 straw. {Jour, of Bath and West ofEnr/.Ag.Soc, XYI,214.) 



The wheat experiments show the natural capacity of 

 the Rothamstead soil for producing that cereal, and de- 

 monstrate that those matters which are annually removed 

 by a crop of 16^ bushels, are here restored to availability 

 by weathering and nitrification. The crop is thus a 

 measure of one or both of these processes.* It is probable 



♦ In the experiments of Lawes and Gilbert it was found that phosphates, sul- 

 phates, and carbonates of lime, potash, magnesia, and soda, raised the produce 

 of wheat but 2 to 3 bushels per acre above the yield of the unmanured soil, while 

 sulphate and muriate of ammonia increased the crop 6 to 10 bushels. This re- 

 sult, obtained on three soils, viz., at Rothamstead in Herts, Ilolklmm in Nor- 

 folk, and Rodmersham in Kent, the experiments extending over periods of S. 3, 

 and 4 years, respectivelj;. shows tliat tliese soils were, for the wheat croj). rela- 

 tively deficient in assimilable nitroircn. The crop on tlie nnmanured soil was 

 therefore a measure of nitrification rather tlian of mineral disintegration. 



