UNEQUAL DISTRIBUTION OF FOOD IN SOILS. 169 



sion of crops is determined by this principle, that the 

 second plant must always take away from the soil less 

 than the first, or possess a greater number of roots, or 

 generally a larger absorbent root-surface. After the 

 average crop of the first year, the crops would go on 

 yearly diminishing. 



The farmer, to whom uniform average harvests are 

 the exception, and an alternation of good and bad crops 

 dependent upon change of weather is the rule, would 

 hardly notice this constant diminution, even supposing 

 his field to be actually in that favourable chemical and 

 physical condition which would enable him to cultivate 

 wheat, rye, and oats for seventy years in succession, 

 without restoring any of the mineral constituents re- 

 moved from the soil. Good crops approaching the 

 average in favourable years, would alternate with defi- 

 cient crops in bad seasons ; but the proportion of un- 

 favourable to favourable returns would go on in- 



creasing. 



Most of the land under cultivation in Europe is not 

 in the physical condition assumed in the case of the 

 field which we have been considering. 



In most fields the phosphoric acid required by the 

 plants is not all distributed in an effective condition, 

 and accessible to the roots ; a part of it is merely dis- 

 seminated through the soil in the form of small gran- 

 iiles of apatite (phosphate of lime) ; and even where the 

 soil contains altogether a quantity more than sufficient, 

 yet in some parts of it there is much more and in 

 others less than the plants require. 



If we suppose our field to contain 25,000 kilo- 

 grammes of the ash-constituents of wheat equally dis- 

 tributed through the soil, and five, ten, or more thou- 

 sand pounds of the same constituents, unequally dis- 

 tributed, the phosphoric acid as apatite, the silicic acid 

 and potash as decomposable silicates ; and, further, if 

 every two years a certain quantity of this second por- 

 tion of food elements becomes, in the manner stated, 

 soluble and distributable, so that the roots of plants in 

 all parts of the arable soil could find as much of these 



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