20 PLANT LIFE ON THE FARM. 
other hand, the most highly manured plot has yielded 
for the same period an average of seven thousand one 
hundred and sixty-eight pounds of hay per acre, varying 
in separate years from four thousand four hundred and 
eighty to eight thousand nine hundred and sixty pounds, 
according to season. These figures will suffice to illus- 
trate the amount of food derived from the soil and from 
the atmosphere, and the beneficial effects of suitable cli- 
matal conditions. The decline not only of produce, but 
also in mineral and nitrogenous ingredients in the soil, 
in the case of the continuously unmanured plots at Roth- 
amsted, is very marked. To insure continued fertility, 
therefore, and obviate exhaustion, some restitution must 
be made ; and this is effected by the addition at the right 
time, in the right condition, and in the right quantities, 
of an appropriate manure ; or the exhaustion may be 
compensated by suitable rotation, or the growth in alter- 
nate periods of plants having different requirements, as 
wheat after potatoes or clover after wheat. 
Apparent Power of Selection, how Explained.—The 
circumstance that certain crops are specially benefited by 
particular manures, though they contain relatively little 
of the substance in their composition, would seem to in- 
dicate the existgnce of a power of selection, as also would 
the fact that plants of such very different constitutions 
grow on the same soil, but these facts are better explained 
by the varying osmotic conditions of the plants. Cereal 
crops and grasses generally are, for instance, specially 
benefited by nitrogenous manures, though they contain 
relatively little nitrogen as compared with clover and 
other leguminous crops, but which, although they con- 
tain so large a proportion of nitrogen in their constitu- 
tion, are not particularly benefited by nitrogenous ma- 
nures. Beet roots and potatoes, which contain a con- 
siderable proportion of potash in their constitution, are, 
