168 FARM-YARD MANURE. 



If these tliree-fourtlis of an average crop do not give 

 the farmer a sufficient excess of income over expenditure, 

 if they barely cover his outlay, the crop can no longer be 

 called remunerative. He calls his field ' exhausted ' for 

 the cultivation of wheat, although it contains seventy-four 

 times the quantity of nutritive substances requked by an 

 average crop for the year. Owing to the presence of the 

 entire sum of nutritive substances, in the first year of cul- 

 tivation each root found, in the parts of the soil in 

 contact with it, the requisite amount of mineral food for 

 its complete developement ; 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 ( = 31 J 

 cwts.) of grain, and 3800 kilogrammes ( = 74^ cwts.) of 

 straw) takes away from the ground per hectare only 180 

 kilogrammes ( = 3^ cwt.) of ash-constituents. 



If the production of an average wheat crop requires 

 the presence in the soil of 25,000 kilogrammes of the 

 ash-constituents of wheat plants, a soil with only 18,000 

 kilogrammes of such constituents will prove sufficiently 

 rich to give an average and a succession of remunerative 

 crops of rye. 



By our reckoning, a field, though exhausted for the 

 cultivation of wheat, stiU contains 18,492 kilogrammes of 

 mineral constituents, the same in properties as those 

 which the rye plant requires. 



If it is asked after how many years continuous rye- 

 cultivation the average crop will sink down to a three- 

 quarter crop, assuming this to be no longer remunerative, 

 we find that the field wiU produce 28 remunerative rye- 

 crops, and after 28 years wiU be exhausted for its cultivation. 



The nutritive substances yet remaining in the soil will 

 still amount to 13,869 kilogrammes of ash-constituents. 



