Apr. IS. i9i8 Soil Acidity as Influenced by Green Manures 197 



than the unmixed soil, some of these mixtures had a greater Umestone 

 requirement than the soil itself, some a less. 



(23) The green manures applied air-dry were not so largely decomposed 

 as those applied in a fresh state. The former left about i ,000 pounds an 

 acre more of organic residues than the former. 



(24) Of the total organic residue in the untreated soil, only 25 per cent 

 were left as "active humus" or humus soluble in dilute alkali, as deter- 

 mined by the usual method. 



(25) Of the organic residues from the fresh green manures a somewhat 

 larger proportion, 29 and 35 per cent for the legume and nonlegume 

 groups, respectively, were left in "active humus" condition; but of the 

 stable and poultry manures, much less, 14.6 and 8.2 per cent. 



(26) In the mixtures with air-dry green manures the proportion of 

 "active humus" was from one-third to one-fifth less than in the corre- 

 sponding mixtures with fresh green manures. 



(27) Of the "active humus" in these acid soil mixtures, little, if any, 

 exists in ammonia-insoluble combinations with lime or other bases, and 

 probably a considerable fraction can be dissolved by weak acid and water. 



(28) The properties of these variously manured soils show no more 

 definite relationship between their limestone requirement and the "free 

 humic acid" — that is, the humus directly soluble in weak ammonia — than 

 exists between the total organic matter and this requirement. If the 

 material causes of the observed requirement belong to this major fraction 

 of the active humus, either they are of very different composition and 

 degree of influence or else they exist in very different quantities in the 

 several free humic acid residues, a condition very different from that 

 observed in old grass lands upon the same body of soil. 



In general, these experiments have satisfactorily shown that fresh 

 green manures plowed under on this acid silty loam soil reduce its acidity 

 very soon after plowing under, but finally leave a soil of increased acidity ; 

 also that nitrification goes on in them quite vigorously under suitable 

 moisture temperature and aerative conditions and that the green manured 

 soils are rich in nitrates, despite the soil acidity. As to the cause of the 

 increased acidity, beyond showing that it is not largely due to nitrification 

 and indicating that it is in some way associated with the added organic 

 materials or their fermentative residues, the experiments furnished little 

 definite information.^ 



1 The -writer is indebted to Dr. Wm. Freer for his valuable assistance in the interpretation of results 

 herein discussed. 



41813°— 18 4 



