20 PLANT LIFE OK 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 Eoth- 

 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 existence 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, 



