Apr. IS. i9i8 Soil Acidity as Influenced by Green Manures 193 



In case of both legume and nonkgume green manures, therefore, the 

 organic residue from the material applied fresh contains a larger pro- 

 portion of alkali-soluble humus than does that from the air-dried manure. 

 The percentage of alkali-soluble humus in the organic residue from the 

 fermentation of stable manure in the soil is much less and that from the 

 poultry manure is even lower. 



Finally, we may consider whether these examinations exhibit any 

 definite relation between the limestone requirement due to the added 

 green manure and the alkali-soluble organic residues from these additions. 

 For such purpose the results from the determination of alkali-soluble 

 humus by method B are possibly best suited. 



It will not be needful to make an extensive comparison in order to 

 discover that no definite relation of the kind stated is here apparent. 

 Table II shows 8 cases out of 15 in which the limestone requirements of 

 the manured soils was less than that of the untreated soil at the end of 

 the 9-month period; Table VIII shows one such case out of 10. On the 

 other hand, Table XVI shows no case in which the alkali-soluble humus 

 determined by method B in the manured soils is not greater than that of 

 the untreated soil. 



Therefore, if the chief acid effect in these cases is due to the alkali- 

 soluble humus constituents as a whole, the influence of a unit weight of 

 these parts of the several organic residues must be very dififerent in 

 degree in the respective cases. 



SUMMARY 



Under conditions corresponding for nine months to those of a summer 

 fallow with fairly frequent, thorough cultivation and well-distributed 

 rainfall of amount just sufficient to keep the soil well moist without 

 setting the drains aflow, the following facts were observed with respect 

 to the add soil of the ammonium-sulphate plot 32, both as fallowed by 

 itself, with stable manure and with various leguminous and nonlegu- 

 minous manuring crops, applied fresh and also in an air-dry condition, 

 respectively. 



I. — THE SOIL WITHOUT MANURIAl, ADDITION 



(i) Its limestone requirements changed little, if at all. 



(2) Its nitrate content increased sharply at two periods, first, during 

 the second and third months, and at a less rate after the fifth month. 

 If these increases represented a change from ammonia to nitric acid, 

 without a simultaneous replacement of the transformed ammonia from 

 organic nitrogenous sources, they would be equivalent to increases in the 

 limestone requirement of 104 and 120 pounds, respectively, to the acre 

 7 inches (2,000,000 pounds). 



(3) At the conclusion of the experiment the amount of nitric nitrogen 

 was 36.16 pounds greater than at the beginning, or somewhat more than 

 double. 



