SOIL EXHAUSTION AND ROTATION OF CROPS. 169 



Prof. Johnson. I would ilot assert that to be the fact, ab- 

 solutely or unqualifiedly, but the indications very strongly 

 favor that general conclusion. 



Mr. Gould. That is my own personal impression. I 

 wished to know whether the Professor so understood it. 



Prof. Johnson. I was about to say how much nitrogen 

 was needed in the soil. 



A wheat crop of thirty-three bushels, with straw and chaff, 

 contains fifty-six pounds of nitrogen. If we allow for stub- 

 l)le and roots one-fifth this quantity, we have for the total ni- 

 trogen required in the vegetation of an acre of wheat, say 

 sixty-eiglit pounds.* flellbriegel found, Ijy actual trial, sev- 

 enty pounds of nitrogen to l)e sufficient to produce his maxi- 

 mum wheat crop. 



Mr. Lawes' soil furnished enough nitrogen to yield seven- 

 teen bushels of wheat. Addition of forty-one pounds of ni- 

 trogen, in form of ammonia salts, gave twenty-seven bushels, 

 or an increase of ten bushels. Eighty-two pounds of nitro- 

 gen applied in the same form gave thirty-seven bushels, or 

 twenty bushels increase. 



The reason why Mr. Lawes was obliged to add eighty-two 

 pounds of nitrogen to double the wheat crop, lies in the fol- 

 lowing considerations : 



When ammonia is applied as manure, a portion of it is 

 fixed in a comparatively insoluble condition in a clayey or 

 loamy soil, and a share of this fixed ammonia it is doubtless 

 very difficult for the plant to acquire. Again, nitrification, 



* On examination of wheat roots collectecl by Schubart June 8th, 1855, Stock- 

 liardt found that the roots composed a little more than one-fifth of the entire plant, 

 or twenty-two per cent., and the nitrogen of the roots was a little less than one- 

 fifth that of the entire plant, or eighteen per cent. 



Heiden found the nitrogen of the roots of ripe rye but one-tenth that of the 

 entire nitrogen. Stockhardt's examination was made on the unripe wheat. By 

 ripening, the proportion would doubtless have been reduced. Heiden found, in 

 fact, that the ratio of root to top in blossoming rye was about one to six, but in 

 ripening was reduced to one to thirteen and one-half. 



If, then, the roots alone contain one tenth of the entire nitrogen, the roots and 

 stubble may be fairly reckoned to contain one fifth of the entire nitrogen. 



Weiske, indeed, gives twenty-two pounds of nitrogen per acre for the roots and 

 stubble of wheat, but we are not informed how high the stubble was cut. 



