The sorption of added inorganic phosphate (P) by three soils which varied appreciably in their ability to sorb P was evaluated using the Langmuir adsorption equation. When the sorption data were plotted according to the conventional Langmuir equation, two linear relationships were obtained, indicating the presence of two populations of sites which have a widely differing affinity for P. Previous workers have obtained a single linear Langmuir relationship over the same equilibrium P concentration range (up to 14 µg/ml). The sites in part I (straight line corresponding to lower equilibrium P concentrations) had a very much higher (between 33 and 91 times) binding energy constant (K1), determined by regression analysis, than those in part II (straight line corresponding to higher equilibrium P concentrations). Between 40 and 52% of the total adsorption maxima was sorbed by sites in part I. A plot of the sorption data according to a rearranged form (Eadie‐Hofstee plot) of the Langmuir equation gave a curve which could not be resolved satisfactorily into two straight‐line components, suggesting the existence of three populations of sites. The rearranged form of the Langmuir equation was found to be preferable for evaluating P sorption at low equilibrium P concentrations. Predictions, based on the addition of high levels of added P, of the interrelationships between sorbed P and equilibrium P concentrations typical of the soil solution and soil‐water systems, such as runoff, may be in error because of the failure to consider the existence of sites with appreciably higher K1 values than those which sustain relatively high concentrations of inorganic P in solution.

An improved method to assay phosphodiesterase activity in soils is described. It involves extraction and colorimetric determination of the p‐nitrophenol released when 1 g of soil is incubated with 5 ml of 1mM buffered [0.05M tris (hydroxymethyl) aminomethane (THAM), pH 8.0] bis‐p‐nitrophenyl phosphate solution at 37°C for 1 hour. The reagents (0.5M CaCl2 and 0.1M THAM pH 12) used for extraction of the p‐nitrophenol released give quantitative recovery of p‐nitrophenol added to soils and do not cause chemical hydrolysis of the substrate, bis‐p‐nitrophenyl phosphate. Results showed that this soil enzyme has its optimum activity at buffer pH 8.0. The initial rates of p‐nitrophenol release obeyed zero‐order kinetics. The phosphodiesterase activity of six surface soils studied ranged from 16 to 147 µg p‐nitrophenol released · g−1 soil · hour−1. Steam sterilization and formaldehyde destroyed and toluene increased the activity of this enzyme. At 5mM, PO43‐, EDTA, and citrate inhibited phosphodiesterase activity. The inhibition by orthophosphate showed competitive kinetics. The temperature dependence of the rate constant conformed to the Arrhenius equation up to the point of enzyme inactivation (70°C). The activation energy of phosphodiesterase of the six surface soils studied ranged from 7,860 to 10,390 (avg. = 8,720) calories · mole−1. By using the Lineweaver‐Burk plot, the Km values ranged from 1.26 to 2.02 (avg. = 1.69)mM and the Vmax ranged from 52 to 530 (avg. = 303) µg p‐nitrophenol released · g−1 soil · hour−1. Phosphodiesterase activity was significantly correlated with organic C in each soil profile and in the surface soils studied.

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SUMMARYEight soil Mn tests, namely, CH3COONH4-, Mg(NO3)2-, H3PO4-, hydroquinone-, 3m-NH4H2PO4-,1·5m-NH4H2PO4- and EDTA-extractable Mn and total Mn were evaluated for their ability to predict Mn uptake by maize (Zea mays L.) on 63 soils of diverse origin. Regression equations were derived by comparing Mn uptake with each test in combination with pH. The best prediction of Mn uptake was provided by a combination of CH3COONH4-extractable Mn and pH; 52·9 % of the variability in Mn uptake was accounted for by these two variables. EDTA and H3PO4-extractable Mn were superior to the other soil Mn tests in predicting Mn uptake.The effect of soil properties on the extractability of soil Mn by each extractant was analysed statistically. Only with hydroquinone-extractable Mn did the soil variables investigated account for a major portion of the variability in the Mn test. The soil variables most often correlated significantly with the Mn test were pH and total Mn. Organic carbon and clay contents were significant only in determining EDTA- and Mg(NO3)2-extractable Mn, respectively.

A theoretical fractionation of urban runoff phosphorus (P) according to its chemical mobility and potential biological impact is presented and the P fractions feasible for routine analysis established. Urban runoff P from two separate storm sewer systems draining residential areas in the Lake Wingra basin (Madison, Wis.) was characterized in detail. Flow‐weighted mean concentrations of dissolved inorganic P (Pi) for individual runoff events ranged from 0.10 to 2.11 mg P/liter and generally comprized ≥ 79% of the total dissolved P (Pt), allowing optimization of routine P characterization by the determination of dissolved Pi (or dissolved Pt) and total P. Flow‐weighted mean concentrations of total particulate P (Pt) ranged from 0.14 to 2.37 mg P/liter. However, while the composition of the particulate Pt at the lower concentrations was widely variable, at the higher concentrations particulate Pt was constituted mainly of organic P (Po). Flow‐weighted mean concentrations of dissolved Pi were more consistently correlated at a significant level with particulate Pt and particulate Po than with particulate Pi. The higher concentrations of dissolved and particulate P were associated with leaf and elm fruit fall, in the fall and spring, respectively, and with longer dry periods immediately before runoff events. A significant proportion (35 to 50%) of the particulate Pt occurring during the first flush and high flow phases of runoff events would remain suspended in the lake photic zone for several days. The upper limit for potentially available P in urban runoff can be given by dissolved Pi (or dissolved Pt) plus 0.25 × particulate Pt, for the watersheds studied.