Predicting Phosphorus Pollution Potential
Does phosphorus risk assessment or soil test provide better information?
The key to minimizing agricultural P impacts on surface waters is being able to identify fields within a watershed that are most likely to contribute P to water bodies, and then to manage those areas using BMPs which will reduce P runoff from those sites.
Soil test P levels are one option for predicting P runoff. However, soil tests such as Olsen or AB-DTPA extractants were developed to predict P availability to crops. They were not designed to predict the potential for runoff from soils. Therefore, we have to be careful about interpreting soil test P levels in terms of environmental impact.
Research studies have shown that as soil test P levels increase, the concentration of P in runoff increases. However, the slope of the line is different for each soil, depending on the soil texture, its ability to fix P, field slope, and management factors that affect residue cover and erodibility (Figure 2).
Since the slope of the line is different for each soil, it is impossible to predict runoff P concentration from soil test P unless there is a database on that soil type available which includes the slope and management conditions that you are dealing with. Accumulating such a database will require a huge effort, and Natural Resource Conservation Service (NRCS), Agricultural Research Service (ARS), and land grant universities (including CSU) are involved in this nationwide effort.
The P Index is another tool for predicting P pollution from agricultural fields. The P Index factors in the potential for P transport off-site in addition to the amount of P present. In other words, P runoff depends on two criteria: how much P is present and the potential for transport of that P into surface waters. Site factors used to characterize P supply include soil test P, manure and fertilizer application rates, and methods of application. Site characteristics used to estimate P transport potential include soil erosion, irrigation erosion (based on flow rate and slope), and runoff class (based on permeability and slope). Each characteristic is rated, multiplied by a weighting factor, and then added together to develop a rating for P runoff risk.
We are working with NRCS to develop a P Index for Colorado. In 1998 and 1999, Nicole Harry, Reagan Waskom, Troy Bauder, Jim Valliant, Wayne Cooley, and I sampled runoff from 38 fields and measured the site characteristics described above and the P concentration in the runoff. Additional factors like length of run (for furrow-irrigated fields) and the use of polyacrylamides were also included. We are currently using this database to develop a P Index specific for Colorado for predicting P runoff from agricultural fields. NRCS intends to use the P Index to evaluate P hazards and to make decisions regarding whether manure application rates should be based on the N or P needs of the crop.
| Figure 2. Runoff P concentration for 3 hypothetical soils. |
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