Determination of organic phosphorus in samples of peat soils

Authors

• Armi Kaila Department of Agricultural Chemistry, University of Helsinki
• Oili Virtanen Department of Agricultural Chemistry, University of Helsinki

DOI: https://doi.org/10.23986/afsci.71371

Abstract

Attention was paid in the present paper to the fact that the precision of the values obtained by different methods for the total organic phosphorus in soil cannot be very high. Even the variation caused by the treatment of the extracts and connected with the colorimetric estimation of phosphate in the solution makes it impossible to report the results more accurately than by 10—20 ppm organic P, at least if routine analyses are in question. Although the somewhat modified methods of Dean, Wrenshall and Dyer,. Pearson, and Mehta et al. yielded equal results for the organic phosphorus content of the respective mineral soils and of most of the peat soils analyzed, the treatment with cold alkali in the Wrenshall and Dyer procedure apparently failed to extract the organic phosphorus from two peat samples as quantitatively as the treatment with hot alkali in the other methods. On the basis of this observations a new modification of the method of Wrenshall and Dyer was proposed. It consists of an extraction of 1-g sample with 25 ml of 4 N sulphuric acid at room temperature for 18 hours, followed by washing with water and two successive extractions with 100 ml of 0.5 N sodium hydroxide, the first of them for 18 hours at room temperature, the second for 4 hours at 90°C. This method was found to extract from 40 peat soil samples on the average about 97 per cent of the total phosphorus dissolved by the Kjeldahl digestion and about 9 per cent more organic phosphorus than the method of Pearson. Experiments concerning the ignition and acid extraction procedures indicated that the method of Ghani was not suitable for the determination of organic phosphorus in the twelve samples analysed. The extraction with sulphuric acid showed no marked differences between the increase in the soluble phosphorus due to the ignition when the ratio of extraction was varied from 1:40 to 1:200, and the extractant from 0.2 N acid to 5 N acid. The results obtained for 40 peat samples by ignition for one hour at 600°C and extraction of the ignited and untreated samples with 0.2 N sulphuric acid in a ratio of 1:100 for half an hour were on the average 8 per cent higher than those given by the proposed acid-alkali extraction. The total organic phosphorus content of soil may probably be somewhat higher than the figure yielded by the acid-alkali extraction and slightly lower than the value obtained by the ignition method. For the present, the most reliable result seems to be found in the average of the data given by these two methods.