166 FARM-YABD MANURE. 



The soil will still continue productive for new wheat 

 crops in the following years ; but the amount of prod- 

 uce 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 climatic conditions, the temperature, and the fall of 

 rain remain the same, there will be reaped from the 

 field in the second year only 1980 kilogrammes of grain, 

 and 4950 kilogrammes of straw ; and in each succeed- 

 ing year the crop must fall off in a fixed ratio. 



If the wheat crop in the first year took away 250 

 kilogrammes 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 culti- 

 vation 18,492 kilogrammes of nutritive substances. 



Whatever variations in the amount of produce 

 may have been caused by climatic conditions during 

 the intervening years, it is evident that in the thirty- 

 first year, if there has been no restoration of mineral 

 matters, the field will produce, even under the most 

 favourable circumstances, only iff 0*74, or some- 

 what less than three-fourths of an average crop. 



If these three-fourths of an average crop do not give 

 the farmer a sufficient excess of income over expen- 

 diture, if they barely cover his outlay, the crop can no 

 longer be called remunerative. He calls his field ' ex- 

 hausted ' for the cultivation of wheat, although it con- 

 tains seventy-four times the quantity of nutritive sub- 

 stances required by an average crop for the year. 

 ,Owing to the presence of the entire sum of nutritive 

 substances, in the first year of cultivation each root 

 found, in the parts of the soil in contact with it, the 

 requisite amount of mineral food for its complete devel- 

 opement ; but, owing to the continuous crops, only 

 three-fourths of this quantity is found in the thirty- 

 first year in the same portions of the soil. 



An average crop of rye (1600 kilogrammes ( = 



