SOIL EXHAUSTION AND ROTATION OF CROPS. 217 



form ol' ammonia salts, gave twenty-seven bushels, or an increase 

 of ten bushels. Eighty-two pounds of nitrogen 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 following 

 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, or conversion of ammonia 

 into nitrates goes on, and the nitrates are freely soluble and wash 

 out of the soil. Then we know that the roots of the plant cannot 

 come into contact with the whole of the soil, so that we should 

 not expect that all the available nitrogen there would be taken 

 up. The figures show that from seventy to eighty pounds is suffi- 

 cient, provided it is in a form and in a position in which the plant 

 can appropriate it. ' In stable manure we appear to waste a 

 considerable quantity simply because it is not present in a form in 

 which the plant can use it. 



Now, stable manure, when it is put into the soil, may be com- 

 pared with clover roots or any other vegetable matter put into 

 the soil. Stable manure consists very largely of vegetable matter 

 which has passed through animals, and of more or less litter which 

 we mix with it. There is a small portion of the nitrogen of the 

 manure actually formed into the ammonia salts which Mr. Lawes 

 applied, but most of the nitrogen, in order to be used by the plant, 

 must be transformed, must pass into some other state than that in 

 which it exists in the manure itself; must probably either be con- 

 verted into ammonia or nitrates. 



Mr. Lyman. Suppose I wish to use all the liquids of my stock, 

 and absorb it all, and do not pay so much attention to the coarser 



/ 



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. Stock hard t'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 mots alono contain one-tenth of the entire nitrogen, the roots and stubble 

 may be fairly reckoned to contain one-fifth of the ontire 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. 



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