Decomposition of Sweet Clover as a Green Manure 137 



Thus the amount of active nitrogen increased with the amount of the 

 reagent. On examination it was found that the supernatant hquid in 

 the Kjeldahl flask after digestion was colorless in each case. This showed 

 that all the permanganate had been used up; thus it, and not the organic 

 matter, became the limiting factor in determining the amount of nitrogen 

 obtained. It became evident that the reagent was reacting with other 

 substances besides the organic matter, notably the iron of the soil. A 

 trial on the portion of the soil remaining after ignition showed this to 

 be true. It was thus seen that the alkaline permanganate method was 

 not applicable to the determination of the availability of soil-organic 

 nitrogen unless the organic matter could be separated from the mineral 

 part. In this connection the method of separating the organic from the 

 mineral constituents of commercial fertilizers as devised by Jones and 

 Anderson (1914) suggested itself. Jones has applied this method to 

 muck soils high in organic matter. It was not found to work satisfactorily 

 for light soils, and was not applicable to the work with Volusia silt loam. 



Attention was next turned to the methods previously mentioned as 

 used in measuring rate of decay ; and, inasmuch as these experiments were 

 conducted under well-defined conditions, it was decided to adopt the 

 method of determining the nitrates formed. It was further decided to 

 study only the pots in which the crops had been turned under at the 

 end of the second and third periods, since the dry matter produced at 

 the end of two months growth proved to be too little to make profitable 

 the tuming-under of a crop at this period. At harvest, the nitrates were 

 determined in all the pots of the last two series. The pots were then kept 

 under constant conditions for four months and the nitrates again deter- 

 mined. At the beginning of the period an effort was made to have the 

 soil at the same degree of compactness so that the oxygen supply might 

 not vary, and a moisture content of 30 per cent was maintained thruout. 



It is realized, of course, that nitrates acciunulated under a green 

 manure may not necessarily have resulted from its decomposition. It 

 may be considered that the green manure is changed to nitrates, or it 

 may be considered that the organic matter increases the nitrification 

 of sources of nitrogen already present in the soil. Doubtless both these 

 processes obtain. However, where check pots having no green manure 

 are used, as in these experiments, the net gain in nitrates as a result of 

 adding the green manure can be shown. The production of available 

 nitrogen as a result of the addition of the organic matter is thus shown, 

 and this is really the object of an experiment of this kind. 



The samples taken for the nitrate determination were allowed to 

 air-dry before the analysis was made and the results were then figured 

 to the moisture-free basis. It is realized that in drying in air, nitrate 

 formation is accelerated, and that as a consequence the results so obtained 



249 



