10 SEASONAL NITRIFICATION AS INFLUENCED BY CROPS, ETC. 



soil solutions from soil that was first dried and from undried samples 

 direct from the field. The results agreed within the limits of experi- 

 mental error, and as better mixing was possible by using the dried 

 samples this procedure was an advantage. 



The determination of nitrates under ideal conditions does not 

 present any serious difficulty if the procedure is carefully followed 

 out aiitl precaution against errors observed. As is well known, 

 however, difficulties are likely to be encountered when nitrates 

 are to be determined in soil extracts, especially if the extract 

 is highly colored with soluble organic matter or if it contains con- 

 siderable soluble inorganic salts, especially chlorids. There are thus 

 three main sources of error in nitrate determination in a soil extract : 

 Foreign organic color, which is superimposed on the yellow color 

 developed on nitration of the phenoldisulphonic acid, which renders 

 the colorimetric reading more difficult; the soluble organic matter 

 may be acted upon by the nitrates, or possibly by the acid, thus 

 introducing a chemical error ; the presence of considerable amounts of 

 alkali salts, of which chlorids seem to be the most serious ones. 



The soil extract obtained in the work here reported did not contain 

 enough chlorids to cause error, as the amounts were too small to 

 measure accurately by means of titration; only traces were ever 

 observed, and frequently none at all. 



The errors which might be introduced because of the presence of 

 soluble organic matter are believed to be too small to seriously con- 

 sider, as the only soil samples which gave appreciable organic colors 

 were those from the fallow plat, and no serious difficulty was en- 

 countered in making the colorimetric readings of these extracts, the 

 organic color being of quite a different shade from the yellow color 

 due to nitration. The error which may have been introduced 

 through the chemical disturbance of the organic matter was neglected, 

 it being considered too small to be of serious consequence; as stated 

 above, the organic color was not strong. All points considered, it was 

 decided that under the circumstances and conditions less error would 

 be involved in using the solutions directly as obtained than by trying 

 to free them of organic matter by oxidation or reduction. 



It should be kept in mind that the chief value of the results 

 obtained lies in their relativity more than in the absolute 

 amounts found. This weekly relation of the amounts of nitrates in 

 the various soil layers in the various plats could not be seriously 

 disturbed by any slight error which might possibly have been intro- 

 duced because of the presence of soluble organic matter in the soil 

 extracts. 



It has been quite well made out that no appreciable error was 

 introduced in quickly drying the soil samples in the oven at 100° C. 



173 



