An acid‐free vanadate‐molybdate reagent was developed for the spectrophotometric determination of P in HClO4 digests of soils. The optimum acidity in the analysis is derived from the residual acidity of the HClO4 digest of soils. A comparative study of the proposed reagent with a commonly used molybdenum‐blue method on ten widely different soils indicated that the proposed method gave highly reproducible results and was not affected by high amounts of Fe3+ which is invariably present in soils. In rare cases chromium interference can be serious because of the similarity of the color of the chromate ion to that of the molybdivanadophosphoric complex. For soils low in chromium, chromate interference can be removed by volatilization as chromyl chloride during the digestion. The modified reagent greatly simplifies the determination of total P and is admirably suited for automated analysis.
Increases in soil test values (Bray P‐1) resulting from annual applications of P fertilizer were approximately 1 pp2m for each 4 pp2m of P added in fertilizer during a 3‐year period on six field experiments. In another field experiment, soil tests were made over a 5‐year period following a single fertilizer application, and again the addition of approximately 4 pp2m of fertilizer P was required to increase the P‐1 soil test by 1 pp2m. These results also indicated that the soil test values, though still changing, were sufficiently stable to be reliable 1 year after the application. When a more variable group of soils were incubated in the laboratory for 49 weeks, additions ranging from 2 to 10 pp2m of P were required to increase the soil test value by 1 pp2m.
The source of most of the isotopically exchangeable P in widely different soils was found to be in the Al‐ and Fe‐P fractions. The relative contribution of Fe‐P to exchangeable P was greater at longer exchange periods, showing that Fe‐P had more slowly‐exchangeable components than Al‐P. The rate of exchange of Al‐ and Fe‐P varied from 28 to 162 ppm/hr and 22 to 72 ppm/hr, respectively, in the first half‐hour and was in no case greater than 2 ppm/hr after 24 hours of exchange. Ratio of specific activities of Al‐ or Fe‐P were used to compare the surface activities of the P fractions. The term “Relative Activity Number” (ratio of specific activities of Al‐ to Fe‐P in a soil) is proposed as an index of the relative activity of Al‐ and Fe‐P. RAN for soils studied varied from 1.11 to 2.80 showing that Al‐P was 1.11 to 2.80 times as active as Fe‐P. RAN in soils which had received P was smaller than RAN in untreated soils showing greater exchangeability of the reaction products in the Fe‐P form. In a few soils as much as ½ to ⅔ of Al‐ and Fe‐P was not exchanged in 147 hours. Thus, a considerable proportion of these P fractions is relatively “inactive.”.
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