3872 HOW CROPS FEED, 
orological conditions. In the tropics, both these 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 which was 
15 bushels per acre, the last 173} bushels, and the average 
of all 164 bushels. (Jowr. Roy. Ag. Soc. of Hng., XXV, 
490.) The same investigators also raised barley on the 
same field for 16 years, each year applying the same quan- 
tity and kinds of manure, 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 48! bushels of grain and 28} cwt. of 
straw. (Jour. of Bath and West of Eng. Ag.Soc., XVI,2i4.) 
The wheat experiments show the natural capacity of 
the Rothamstead soil for producing that cereal, and de- 
menstrate 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, inaynesia, 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, Holkham in Nor- 
folk, and Rodmersham in Kent, the experiments extending over periods of §, 8, 
and 4 years, respectively. shows that these soils were, for the wheat crop, rela- 
tively deficient in assimilahle nitrogen. The crop on the nnmanured soil was 
therefore a measure of nitrification rather than of mincral disintegration. 
Py 
