Murphy and Riley proposed use of a single reagent for determining P. The method was found to be accurate for determining P in soil extracts. This method is based on reduction of the ammonium molybdiphosphate complex by ascorbic acid in the presence of antimony. The color produced is stable for 24 hours. It is less subject to interfering substances than are methods involving reduction by SnCl 2 .

Adsorption of phosphorus by soils from dilute solu tions showed a closer agreement with the Langmuir iso therm than with the Freundlich isotherm. Constants calculated from the Langmuir isotherm and interpre tations based upon the meaning of these constants permit a sound theoretical approach to some of the problems of phosphorus retention in soils.The adsorption maximum calculated from the Lang muir isotherm was closely correlated with the surface area of soils as measured by ethylene glycol retention. The correlation coefficients and regression equations were r = 0.98 and y = 0.276x f 3.47 for 10 alkaline soils, and r -0.96 and y = 0.641x f 5.73 for 12 acid soils, where y -mg. P per 100 g. soil and x = rag. ethylene glycol retained per g. of soil. For a given surface area, i.e., 30 mg. glycol per g. soil, the acid soils held 2.17 times as much phosphorus' as the alkaline soils.The average values of a second constant, k, derived from the slope and intercept values, were 0.92 and 4.39 for the alkaline and acid soils, respectively. As the value of this constant increases, .the bonding energy of the soil for phosphorus increases. Thus, the acid soils re tained more phosphorus per unit of surface area and also held the phosphorus with a greater bonding energy than the alkaline soils ''HE reaction of fertilizer phosphorus with soils depends upon the nature and amount of adsorbing surface as

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Studies on the nature of phosphate sorption by calcium carbonate indicate that when soluble phosphate fertilizers are added to calcareous soils, the reactions with calcium carbonate consist of rapid monolayer sorption on CaCO3 surfaces and, at high phosphate concentrations in the vicinity of fertilizer particles, the precipitation of dicalcium phosphate, or a compound with similar properties. The initial products of these reactions are characterized by very high specific surfaces and greater phosphorus solubility than the stable hydroxyapatite or fluorapatite. Dynamic equilibrium in calcareous soils involves all of these forms of phosphate. Some of the effects of the ionic environment on solubility of calcium phosphates were calculated and experimentally verified.

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Self‐diffusion of sodium and calcium ions was measured in bentonites containing various amounts of water and the respective chlorides. The contribution of mobile exchangeable cations to diffusion was calculated from the data by subtracting measured rates of free electrolyte diffusion from the total rate of self‐diffusion measured in these systems. Mobile exchangeable cations accounted for from 3 to over 500 times as much diffusive movement as was accounted for by free electrolyte cations in these systems. The fractions of the exchangeable cations that were in the diffuse layer were estimated from the contribution of exchangeable ions to diffusion, the fluidity of water near clay particles and the calculated concentration distributions of the adsorbed cations. Approximately 70% of exchangeable sodium ions were mobile. Approximately 25% of exchangeable calcium ions were mobile.

Four determinations of phosphorus—Olsen‐P, Colwell‐P, total exchangeable P, and resin‐extractable P—were evaluated in terms of total plant P uptake in a 3‐year continuous greenhouse study of 23 high P calcareous and neutral soils. All methods were highly correlated with the total P taken up from the soils by 5–8 successive greenhouse crops. The Olsen‐P procedure extracted an average of nearly 50% of the total plant P while the Colwell procedure extracted nearly 80%. Resin‐extractable P and total exchangeable P values approximated the total plant P uptake, and served as good biological measures of the total plant‐available P in the soil. No significant differences were observed between the carrier and carrier‐free 32P methods on the 23 test soils.