RESULTS

Best management practices, including reduced and no-tillage management, for different cropping systems on the same soil has led to continued vertical stratification of Bray P (Figure 1). The control reflects a pre-1989 history of heavy manuring and minimal tillage. Cropping systems 1 and 2 receive starter fertilizer, while system 4 receives manure applications and system 3 receives absolutely no fertilizer input. Comparison of systems 1, 3, and 4 shows the effect of intentional P drawdown, as well as vertical stratification persisting despite repeated chisel plowing. However, cropping system 2 shows no stratification in the top 20-cm, probably due to conventional tillage between 1989-94. Where vertical stratification has occurred, the range of measured P values is so large that the meaningfulness of a mean is uncertain.

Langmuir sorption isotherms differ systematically with soil depth, including within the plow layer, and reveal the soil solution P concentration as well as the soil capacity to sorb additional phosphorus (Figure 2). Soil profiles may become oversaturated with respect to P and, depending on location and mobility, the phosphorus may be more susceptible to overland runoff or leaching below the root zone.

Bray P is inversely correlated to soil buffer capacity (Figure 3). As Bray P increases in a soil profile, the ability of the soil to sorb additional phosphorus is reduced. Additional data, not reported here, shows a positive correlation between Bray P and soil solution P concentrations, indicating a change of P mobility within a profile.

Soil solution P concentrations (C₀) from sorption measurements are strongly correlated with phosphorus concentrations in soil solution collected at natural soil:water ratios by heavy liquid, immiscible displacement (results not shown). Use of microplate methodology (Avila-Segura et al., 2004) quickly and efficiently determines phosphorus sorption parameters which can be fit to Langmuir equations for C₀ determination.

As root growth transects the soil profile, individual roots encounter different nutrient levels and P buffer capacities, none of which are identical to the “average” for the bulk sample. When roots reach a depth of 20-cm, they are no longer in contact with adequate phosphorus levels (Figure 4).