165 



CHAPTEE IV. 



FARM-YARD MANURE. 



Tlio fertility of a soil depends upon tlie sum of available food, the continu- 

 ance of the fertility upon the total amount of all food in it — Chemical 

 and agricultural exhaustion of the soil — Exhaustion of the soil by culti- 

 vation, laws regulating its progression ; effect of the transformation in 

 the soil of the chemically fixed into physically fixed elements of food ; 

 effect on the pi'ogress of exhaustion by partial restoration of the -with- 

 dravm food of plants — Progress of the exhaustion by different cultivated 

 plants — Cultivation of cereals, consequence of removing the grain and 

 leaving the straw in the soil ; intervening clover and potato crops ; 

 effect of leaving in the groimd the whole or a portion of these crops ; 

 division of soils ; productive power of wheat fields increased by accumu- 

 lating in them the materials derived from cloA'er and potato fields : cul- 

 tivation of fodder plants ; their food partly derived from the subsoil ; 

 addition of these increases the productive power of the surface soil — 

 Natural connection between the cultivation of cereals and fodder plants, 

 the influence on the fertility of land — Exhaustion of the soil removed 

 by the restoration of the withdrawn mineral constituents ; the excrement 

 of men and animals contains these j their restoration depends upon the* 

 agricultui-ist. 



TO form a correct idea of the effects produced by farm- 

 yard manure in husbandry, it must be remembered 

 that tlie fertility of a soil is always exactly proportionate 

 to tlie amount which it contains of nutritive substances in 

 a state of physical combination ; and that the permanence 

 of its fertility or its productive power stands in proportion 

 to the total quantity of the constituents in the soil capable 

 of passing over into that physical condition. 



The amount of crop reaped from a field in a given 

 time is proportionate to that fraction of the total con- 

 stituents which has passed during this time from the 



