U^TSQUAL DISTRIBUTIOX OF FOOD IX SOILS. 171 



from tlie soil. Good crops approaclnno- tlie average in 

 favourable years, would alternate with deficient cro})s in 

 bad seasons ; but the proportion of unfiivourable to 

 favoui'able returns would go on increasing. 



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 requii^ed by tlie 

 plants is not all distributed in an effective condition, and 

 accessible to the roots ; a part of it is merely disseminated 

 through the soil in the form of small granules of apatite 

 (phosphate of hme) ; 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 requii^e. 



If we suppose our field to contain 25,000 kilogrammes 

 of the ash-constituents of wheat equally distributed 

 through the soil, and five, ten, or more thousand pounds 

 of the same constituents, unequally distributed, the phos- 

 phoric acid as apatite, the silicic acid and potash as 

 decomposable silicates ; and, further, if every two years a 

 certain quantity of this second portion 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 nutritive substances 

 as in the preceding years of cultivation — sufficient, there- 

 fore, for an average crop ; we should, in that case, be able 

 to obtain full average crops for a number of years by 

 always letting a year of fallow intervene after a year of 

 cultivation. Instead of thirty progressively decreasing 

 crops, we should in that case reap thirty full average crops 

 in sixty years, if the excess of mineral matter in the soil 

 were sufficiently large to replace everywhere the phos- 

 phoiic acid, silicic acid, and potash taken away in each 



