ALTHOUGH no great progress has been made in the study of soil organic matter since Waksman's comprehensive review of the subject in 1938 (I), much useful information has been obtained and several points have been clarified. Since the literature published during this eriod is too extenthe important developments in the field. Progress in other directions will be reviewed later.
The Uronic Fraction of Soil Organic MatterUronic acids are commonly determined by the method of Lefcvre and Tollens (2) which de ends upon the fact that uronic acids are quantitaThis method has been applied to soils by various workers (3-13) and the results have been taken to indicate that a considerable fraction of soil organic matter is in the form of uronide material. Shore and Martin ( ), per cent. of the organic carbon in nine soils examined could be accounted for as uronic carbon. Results obtained by Norman and Bartholomew 7) indicatd that about 1-15 er cent. of the organic carbon of surface with depth, particularly in podzolized soils, in which a marked accumulation was found in the B horizon. The a parent uronic carbon in the B, Fuller (10) founJthat about 9-23 per cent. of the organic carbon in surface soils and 19-30 per cent. of that in subsoils a peared to be uronic. Data obtained by Fuller, Bartholomew, and d r m a n (9) on samples of a single soil type indicated that in spite of wide variations in clay content and organic carbon the roportion of uronic carbon to total carbon remained rather constant. puller (11) determined the ap arent continuously to a single crop for 25 or more years or to a stable crop rotation and found that the proportion of uronic to total carbon in a sin le soil type changed only slightly as a result of different cro ping related to the total carbon content t an to the nature of the crop or cro pin system; the lower the total carbon content of a given soil, the hig R f l er t e percentage of uronic carbon in the organic matter, regardless of the crop. As much as 3-40 per cent. of the total carbon in several of the soils appeared to be uronic.Although the term 'apparent uronic carbon' has been used in the sive for adequate treatment here, this article will dea P with only a few of tively decarboxylate B when boiled with 12 per cent. hydrochloric acid.who first used the method for soil analysis, found that Y rom 12.8 to 22.4 in a stud of the distribution of apparent uronic carbon in 20 soil profi I es soils is in the form of uroni B es. The proportion was found to increase sample of a sand loam represented 3 t) per cent. of the total carbon. uronic carbon in the surface soils of 9 soil types that had been p f anted an fertilizer treatments. The chan es appeared to be more c P osely c f P