38 BOARD OF AGRICULTURE. [P. D. 4. 



trebling the ammonium salts, to 36.6 bushels, while 16 tons of 

 barnyard manure applied every year has produced 35.2 bushels. 

 This experiment has demonstrated the possibility of main- 

 taining the largest yield of wheat, considering both amount and 

 duration, of which the world has any record, by the use of 

 chemicals alone, without any addition of organic matter or 

 humus-forming material except the roots and stubble of the 

 wheat itself. But the increased yield of 24 bushels has cost 

 S34, — $8 for the minerals, assuming the sulphates of soda 

 and magnesia to have been unnecessary, and $26 for the nitro- 

 gen. Evidently some cheaper source of nitrogen must be found 

 than chemical fertilizers if wheat is to be grown altogether on 

 chemicals. Nor is there much more encouragement for de- 

 pendence upon manure alone, as it has required 16 tons of ma- 

 nure to produce 23 bushels of wheat. 



Crop Rotation. 



At the Ohio Experiment Station corn, oats and wheat have 

 each been grown continuously for twenty-three years. Where 

 no fertilizer or manure has been used the yield of corn has 

 fallen from 26 bushels per acre for the first five years to 7^ 

 bushels for the last five years; that of oats from 28 to 20 

 bushels, and that of wheat from 10 to 7 bushels. 



The only treatment which has maintained the yield of the 

 continuously grown corn without reduction has been the annual 

 application of a fertilizer carrying 50 pounds of nitrogen per 

 acre in nitrate of soda, together with 160 pounds of acid phos- 

 phate and 100 pounds of muriate of potash, the whole costing 

 over SI 3 per acre annually. This treatment has maintained an 

 average yield of 46 bushels per acre for twenty-three years, at a 

 cost of about 40 cents for each bushel of increase over the un- 

 fertilized yield; but with less than one-half this quantitj' of the 

 mineral fertilizers, and without any fertilizer nitrogen, corn has 

 been grown on the same land and for the same period in rota- 

 tion with other crops, including clover, and the yield has been 

 43^ bushels, costing only 10 cents a bushel for all the fertilizers 

 required to produce a bushel of grain. 



Comparing the two systems for the twenty-three years we 

 find that in continuous culture we have produced 1,057 bushels 



