Abstract
In this paper some observations on the estimation of organic phosphorus in mineral soils are reported. The fact is emphasized that the accuracy of all the methods available is relatively poor. Usually, there are no reasons to pay attention to differences less than about 20 ppm. of organic P. Analyses performed on 345 samples of Finnish mineral soils by the extraction method of MEHTA et. al. (10) and by a simple procedure adopted by the author (successive extractions with 4 N H2SO4 and 0.5 N NaOH at room temperature in the ratio of 1 to 100) gave, on the average, equal results. It seemed to be likely that the MEHTA method removed the organic phosphorus more completely than did the less vigorous method, but in the former the partial hydrolysis of organic phosphorus compounds tends to be higher than in the latter. An attempt was made to find out whether the differences between the respective values for organic phosphorus obtained by an ignition method and the simple extraction method could be connected with any characteristics of the soil. No correlation or only a low correlation coefficient could be calculated between the difference in the results of these two methods and e. g. the pH-value, the content of clay, organic carbon, aluminium and iron soluble in Tamm’s acid oxalate, the indicator of the phosphate sorption capacity, or the »Fe-bound» inorganic phosphorus, respectively. The absolute difference tended to increase with an increase in the content of organic phosphorus. For the 250 samples of surface soils analyzed, the ignition method gave values which were, on the average, about 50 ppm. higher than the results obtained by the extraction procedure. The corresponding difference for the 120 samples from deeper layers was about 20 ppm of organic P. The author recommends, for the present, the determination of the total soil organic phosphorus as an average of the results obtained by the ignition method and the extraction method.
Highlights
For 28 per cent of the samples the ignition method gave values which were more than 50 ppm. higher than those obtained by the extraction procedure
Analyses performed on 345 samples of Finnish mineral soils by the extraction method of Mehta et al [10] and by a simple procedure adopted by the author gave, on the average, equal results
It seemed to be likely that the Mehta method removed the organic phosphorus more completely than did the less vigorous method, but in the former the partial hydrolysis of organic phosphorus compounds tends to be higher than in the latter
Summary
The determination of the total content of organic phosphorus in soil samples still seems to form a problem which is not yet quite satisfactorily resolved. This is due to the fact that we do not know what, we are trying to estimate: at present only about one half of the compounds forming the organic phosphorus complex in soils is identified. We must resort to indirect methods in which the difference between the corresponding contents of »total» and inorganic phosphorus is taken to represent the organic phosphorus content of the soil sample. It is likely that the composition of the organic phosphorus complex varies, at least to some extent, in various kind of soils. In lack of any absolute standard, each of us usually likes to consider his or her own procedure the most reliable one
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