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Increasing Sensitivity of Organic Matter and Nitrogen Analysis Using Soil Separates¹

ABSTRACT

Little is known about the use of soil separates in the determination of soil organic matter and total-N content. To achieve uniformity and complete oxidation, soil samples for use in these analyses are commonly finely ground. The present investigation was initiated to study organic matter and total-N contents in sand (2.0-0.050 mm), silt (0.050-0.002 mm), and clay (< 0.002 mm) fractions of selected Alfisols, Aridisols, Entisols, Mollisols, Spodosols, Ultisols, and Vertisols. A further aspect of this study was to establish the correlation with the respective contents obtained by analysis of whole soil (< 2 mm) samples. Soil organic carbon was determined by the Walkley-Black wet oxidation method, while total-N was analyzed by the semi-micro Kjeldahl method. All analyses were done in four replications. The results indicated that particle size separation had concentrated most of the organic matter in the soil fraction < 2 µm. Generally lower amounts of organic C and N were detected in the silt and sand fractions. In most cases, summation of organic C and total-N percentages derived from soil separates yielded values higher than those obtained from analyses of whole soils, indicating that, by the use of whole soils, a considerable part of C and N was not analyzed. The higher-organic C contents in clay fractions, exceeding in a number of cases those measured by the use of whole soils, and the detection of appreciable amounts of organic C and N in silt and sand, pointed to a more complete oxidation and better sample uniformity. The higher recovery of organic C by the use of soil separates also implied an increase in sensitivity of the analysis, improving reliability of results, which was of special importance with B horizon samples, where low levels of organic C were involved. The usual positive correlation between organic C and total-N content was also obtained by using soil separates. The high correlation (r = 0.929, df = 23), together with increased sensitivity and complete oxidation resulting in higher values and improved reliability, makes the use of soil separates desirable in organic C and total-N analyses.

¹ Contribution of the Univ. of Georgia, College of Agric., Exp. Stn., College Stn., Athens, Ga. This research was supported by State and Hatch funds allocated to the Ga. Agric. Exp. Stn.

² Professor and former Graduate Research Assistant, respectively. Dep. of Agron., Univ. of Georgia, Athens, GA 30602. Present address of junior author: Southern Forestry Research Center, Weyerhaeuser Co., Hot Springs, Arkansas 71901.

Received for publication August 14, 1980. Accepted for publication January 21, 1981.