DLMIXl'TIOX OF FOOD IX A WHEAT SOIL. 1G7 



An average wheat crop of 2000 kilogrammes ( = 30 

 cwts.) of grain, and 5000 kilogrammes (=98 cwts.) of 

 straw, receives from a hectare (=2^ acres) of ground 250 

 kilogrammes ( = 5 cwts.) of ash constituents, on an average. 

 Now, upon the supposition that a field, to give an average 

 crop, must contain 100 times that quantity (or 25,000 

 kilogrammes) of ash constituents in a perfectly available 

 state, it follows that such a field gives 1 per cent, of its 

 total store to the first crop. 



The soil will still continue productive for new wheat 

 crops in the following years ; but the amount of produce 

 will gradually decrease. 



If the soil is most carefully mixed, the wheat plants 

 will, in the next year, find everywhere upon the same 

 field 1 per cent, less nutriment, and the produce in corn 

 and straw must be smaller in the same proportion. If 

 the chmatic conditions, the temperature, and the fall of 

 rain remain the same, there wiU be reaped from the field 

 in the second year only 1980 kilogrammes of grain, and 

 4950 kilogrammes of straw ; and in each succeeding year 

 the crop must fall ofi" in a fixed ratio. 



If the wdieat crop in the first year took away 250 kilo- 

 grammes of ash-constituents, and the soil contained per 

 hectare to the depth of 12 inches one hundred times that 

 quantity (25,000 kilogrammes), there would remain in 

 the ground at the end of the thirtieth year of cultivation 

 18,492 kilogrammes of nutritive substances. 



Whatever variations in the amount of produce may 

 have been caused by climatic conditions during the inter- 

 vening years, it is evident tliat in the thirty-first year, if 

 there has been no restoration of mineral matters, the 

 field wiU produce, even imder the most favourable cir- 

 cumstances, only i§|i=0'74, or somewhat less than three- 

 fourths of an avei-age crop. 



