Part II.] :MAIXTEXAXCE OF SOIL FERTILITY. 39 



of corn at a cost of $300 for fertilizers, and in rotation we have 

 produced 959 bushels at a fertilizer cost of S40, so that the 

 extra 100 bushels of corn produced in the continuous cropping 

 has cost S260, or S2.60 per bushel. 



The chief difference between rotative and continuous cropping 

 is that in continuous cropping we must furnish all or nearly 

 all the nitrogen required by the crop, whereas in rotative 

 cropping we may secure a large part of this nitrogen through 

 the clover crop without cost. 



I have already said that in Broadbalk Field a sixty-one-year 

 average yield of wheat of 36.6 bushels per acre has been main- 

 tained by chemical fertilizers alone, but at a cost for fertilizers, 

 including nitrogen, equal to the value of the crop, saying noth- 

 ing about rental of land, seed and labor. 



At the Ohio Experiment Station the same average yield of 

 wheat has been maintained for twenty-three years in a rotation 

 of potatoes, wheat and clover on each of three different tracts 

 of land, and of 37.8 bushels on each of three other tracts, in 

 which the increase of potatoes and clover has more than paid 

 all cost of fertilizing, leaving the increase in wheat as clear 

 gain. 



In this experiment the potatoes are grown on clover sod, 

 which seems to furnish all the nitrogen required by the potatoes, 

 but the wheat has been able to use a little more nitrogen than 

 that left over by the crops preceding it. In short, our experi- 

 ments indicate that clover cannot be expected to furnish much 

 more than enough nitrogen for the crop immediately follow- 

 ing it, if the hay is removed and only the roots and stubble are 

 left, and that if more than thjs is required the entire crop must 

 be plowed under or else nitrogen must be provided from other 

 sources. Under some conditions it may be justifiable to plow 

 under the clover crop, but ordinarily much more may be made 

 of it by feeding it to live stock and returning the manure to the 

 land. 



Barnyard Manure. 



The latter method is the one followed in one of the Ohio 

 station's experiments, which has been in progress for eighteen 

 years, the manure being applied to corn in a three-year rota- 

 tion of corn, wheat and clover. The outcome has been that 



