This study dealt with the extent and dynamics of a thin zone of soil that interacts with rainfall and overland flow in releasing soil chemicals to runoff. A relatively immobile tracer, 32P, was applied at 0.0 (soil surface), 0.5, 1.0, 1.5, or 2.0 cm depths in duplicate soil boxes of three different soils. Simulated rainfall of 6.5 cm/h was applied to each soil box for two separate 30‐min periods. The degree of interaction decreased very rapidly, more or less exponentially, with depth below the surface. An effective average depth or zone of interaction, within which the degree of interaction equals that of the soil surface, was assumed to exist. The effective average depth calculated from the data ranged between 0.2 and 0.3 cm, depending more upon the period of rainfall than upon the type of soil. These average depths were used, along with values of total desorbable P and the fractions of the 32P applied on the soil surface that appeared in runoff, to predict the P concentrations in runoff, which agreed rather well with the measured values. The assumption of an effective average depth was thus valid for P. Transient changes in the effective average depth of interaction during a rainfall period were calculated by using simultaneous P and 32P concentrations in runoff, where the 32P was applied at the soil surface. The effective average depth increased somewhat with time during a rainfall period, especially during the first 30‐min rain. The 30‐min mean effective average depth of interaction calculated by this method agreed well with that obtained by the first method described above.

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The release of P from surface soil to runoff water under simulated rainfall conditions was investigated for five soils at several rates of fertilizer P application. A simplified kinetic model proposed earlier, describing the desorption of soil P, adequately described both the concentration of soluble P in runoff during an event and mean values for successive events. During an event the logarithm of soluble P concentration decreased linearly as the logarithm of time increased. Mean soluble P concentrations of individual runoff events increased linearly with amount of desorbable P in the surface soil (top 0.5 cm), while the effect of increased rain intensity on concentrations could be explained by a power function related to water/soil ratio. The average depth of runoff‐surface soil interaction calculated from the data using the kinetic model ranged from 1.5 to 3.0 mm for the five soils, and was significantly related to degree of soil aggregation. These depths agreed closely with those determined using 32P as a tracer in earlier studies. The depth of interaction increased with increase in soil slope, kinetic energy of the raindrops, and, to a lesser extent, rainfall intensity. The use of the kinetic model would improve modeling of soluble P loading in runoff.

The adsorption of soluble P by surface soil and suspended sediment material during transport in surface runoff under field and simulated laborato ry conditions was investigated. The soluble P concentration of surface runoff from several Southern Plains cropped and grassed watersheds decreased with an increase in sediment concentration. A linear inverse relationship between soluble P and sediment concentration was significant over a wide range in sediment concentration. The slope values of the relationship were similar for different watersheds on the same major soil type. Using soil from these watersheds in simulated surface runoff, sorption of soluble P added in rainfall was found to occur during transport. The magnitude of this sorption was more closely related to the sorptive capacity of the sediment in the surface runoff than to that of the surface soil material. The results suggest that for unfertilized watersheds and for watersheds where fertilizer P is incorporated into the surface soil, away from the zone of immediate removal in surface runoff, the leaching of P from the vegetative cover can contribute significant amounts of soluble P to runoff, and that soil material may act as a P sink rather than a P source.

Seven cropland watersheds and four rangeland watersheds in central Oklahoma were monitored for surface hydrology and discharge of nitrogen, phosphorus, and sediment over a 1 year period. Precipitation and runoff were much above normal during the study. Sediment losses from the continuously grazed rangeland watersheds ranged from 18 to 23 metric tons/ha during the study. None of the sediment losses from the other watersheds exceeded 10 metric tons/ha.Total nutrients discharged in runoff ranged from 2 to 15 kg/ha of N and 1 to 11.5 kg/ha of P. Flow‐weighted mean concentrations ranged from 1 to 6 ppm of total N, 0.2 to 1.9 ppm of nitrate‐N, 0.5 to 4.8 ppm of total P, and 0.04 to 0.9 ppm of soluble P. Runoff losses of soluble inorganic nitrogen were generally less than those quantities received in rainfall. Concentrations of soluble phosphorus in runoff from the cropland watersheds were much greater than from the rangeland watersheds. Losses of fertilizer nitrogen and phosphorus did not exceed 5% of the most recent applications, although surface runoff was 4‐ to 10‐fold greater than that observed in previous years.

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The kinetics of P desorption for several soils was investigated at different water/soil ratios, during a short period of time, so that the results could be related to P release from agricultural soil to rainfall and runoff water. For all soils the logarithm of P release (Pd) was linearly related to the logarithm of contact time (t) at any given water/soil ratio (W) and P amendment and to logarithm of W at any given contact time and P amendment. The amount of P released was also directly proportional to the amount of desorbable P (Po) in the soil initially. The following simplified empirical model was developed to describe the desorption of soil P,Pd = KPotαWβ,where K, α, and β are constants. The simplified model gave a reasonably good description of P desorption from five southwestern soils and provided similar values of the constants K, α, and β, for each soil, over a range of experimental conditions. Consequently, we suggest that average values of the constants for each soil can be used in the model to describe P release for general applications.