There is potential for P to leach below the root zone and be transported to surface waters through subsurface flow (Figure 1). Because P is strongly adsorbed to soil particles, P leaching would occur only when the percentage P saturation of the soil is increased to very high levels through continued applications of P exceeding crop requirement. Although P leaching is frequently greater in sandy soils (low P adsorption capacity), P leaching can occur in some clay soils through transport in macropores (Djodjic et al., 1999; Laubel et al., 1999). Estimating P leaching potential requires quantifying drainage water volume and the P concentration in the drainage water.
Phosphorus Concentration in Drainage Water
Many recent studies have demonstrated downward movement of P in soils where high rates of manure were applied over extended periods. Ham et al. (2000) demonstrated P leaching in a sandy soil when Mehlich-3 P (0 to 20 cm depth) was 250 mg P/kg (Figure 12). At this Mehlich-3 P level, the P adsorption capacity was nearly saturated (100% P saturation at 270 mg/kg). These authors also showed the influence of continued waste application on increasing P leaching potential (Figure 12). Application of 1,600 kg P/ha to a fine loam soil over 5 yr resulted in P leaching to 76 cm in depth (Figure 13). In this soil, the P adsorption capacity was 100% saturated at 250 mg/kg Mehlich-3 P (Reddy et al., 1980).
Based on numerous leaching studies, the P concentration in the leachate can be estimated using Mehlich-3 P levels, similar to the relationship used for estimating soluble P in runoff. For example, in North Carolina, if Mehlich-3 P levels (0 to 20 cm in depth) are less than the threshold values in Figure 8, then P leaching does not likely occur. If Mehlich-3 P levels exceed these threshold values, however, the soil profile to a depth of 75 cm should be sampled to determine whether P leaching has occurred. Depending on the Mehlich-3 P level at a 75-cm depth, the leachate P concentration can be estimated from Figure 14.
Using the Olsen soil test, Heckrath et al. (1995) reported that P concentration in the drainage water substantially increased at Olsen soil test P levels >60 mg/kg. These researchers forwarded the change point concept, or the soil test P threshold above which dissolved P in the drainage water greatly increases (McDowell and Sharpley, 2001). In similar work, Maguire and Sims (2002) reported greatly elevated dissolved P in drainage water when Mehlich-3 P levels exceeded approximately 200 mg/kg. In development of practical P loss assessment tools, the change point or soil test P threshold should be established for the major soil groups in each state.
Drainage Water Volume
When P leaches below the root zone, intensive subsurface drainage will increase potential for subsurface transport (Figure 1). Intensive subsurface drainage also decreases surface runoff that reduces potential runoff P loss (Skaggs et al., 1982; Gilliam and Skaggs, 1986; Evans et al., 1995; Gilliam et al., 1999). The factors affecting subsurface P transport are subsurface drainage intensity (rate of drainage water leaving a field) and P adsorption capacity of the soil. Subsurface drainage occurs when the capacity of surface water storage has been satisfied. Subsurface drainage is influenced by soil hydraulic conductivity, profile depth, water table depth, and characteristics of artificial drainage (drain depth and spacing) if present.
For purposes of estimating P leaching potential, when the estimated leachate P concentration equals 0 mg/kg, calculating leachate volume is unnecessary. For well-drained soils with a calculated leachate P concentration of >0 mg/kg, the average annual precipitation (PPT), runoff volume calculated from the curve number method, and evapotranspiration (ET) based on site-specific data on climate, crop, and soil type can be used to estimate subsurface drainage volume by the following equation:
Average subsurface drainage (cm) = Annual PPT (cm) - Runoff (cm) - ET (cm)
As with estimating runoff volume in drained soils discussed previously, the DRAINMOD model (Skaggs, 1978) is the most accurate method to estimate water balance of shallow-water-table soils. DRAINMOD calculates evapotranspiration, infiltration, surface runoff, subsurface drainage, deep drainage, water table depth, and soil water distribution. Model descriptions and applications are presented by Evans and Skaggs (1989), Skaggs (1999), and Skaggs and Chescheir (1999). DRAINMOD can be readily incorporated into practical P loss assessment tools used by technical service providers and other practitioners. Once drainage water volume and P concentration are estimated, the quantity of P leached can be determined.