Apr. IS, i9i8 Soil Acidity as Influenced by Green Manures 177 



except in the case of the redtop and sorrel mixture, with a majority of 

 slight increases during the sixth and seventh months, and a slight but 

 usually greater decrease during the next two months. 



It is particularly notable that there was no difference in tendency at 

 these periods among the legumes, nonlegumes, and stable manures. 



The several averages of Table III show the net effects of the several 

 additions upon the reaction of the acid soil used as a basis. 



The control soil without manurial addition gradually but slightly 

 increased in acidity after five months' exposure under the conditions 

 of this experiment. The addition of either the acid green manures or 

 of alkaline stable manure caused, within two weeks, a very great decrease 

 in limestone requirement. This lowered requirement was very pro- 

 nounced for a very short time, however; and in a month the requirement 

 had returned nearly to the level of that for the control soil. A later 

 phase, manifest after the third month, showed a greater acidity or lime 

 requirement for the manurial mixtures than for the soil; and with a 

 few exceptions this continued until the end of the experiment. 



The average conditions of lime requirement throughout the experi- 

 ment of the individual manurial substances, after the fermentations had 

 well started, were as follows : 



Pounds per acre. 



Red clover 2, 648 



Sorrel 3, 271 



Stable manure 4, 275 



Rye 4,317 



Alfalfa 4, 326 



Redtop 4,349 



Timothy 4,359 



Control soil 4, 741 



Pounds per acre. 



Hairy vetch 4, 767 



Canada field pea 4, 987 



Oats 5, 308 



Soybean 5, 053 



Sweet clover 5, 450 



Rape 5,500 



Corn 5, 919 



Wheat 5, 926 



In general, red clover as a green manure was the best corrective o* 

 acidity, taking the whole period into account, and sorrel was the 

 only green manure whose addition kept the soil, at all times after fer- 

 mentation had started, below the level of that exhibited by the soil 

 alone. On the other hand, wheat and corn increased the average lime 

 requirement more than any other of the manurial substances tried. 



CAUSES OF THE CHANGES IN LIMESTONE REQUIREMENT 



When the causes to which the marked changes are due are considered, 

 we may narrow the field of inquiry by these considerations: 



(i) The periodicity of the changes is not a result of variation in the 

 environment, in moisture relation, or in aeration, but is probably inci- 

 dent to successive series of fermentations with corresponding chemical 

 products. 



(2) The general similarity in the changes exhibited by all the manurial 

 mixtures indicates that these changes are related to the constituents in 

 which they are alike rather than to those in which they differ. 



