Leopold Center - Phosphorus work fine-tunes Iowa P-Index

Iowa livestock producers have been keeping a watchful eye on government regulators this year as they develop rules for future manure management in the state. Of particular interest has been the Phosphorus Index, or P Index, and how it will be used to determine application rates in producers’ manure management plans.

Developed by scientists from Iowa State University, the USDA National Soil Tilth Laboratory and the USDA Natural Resources Conservation Service, the P Index estimates the risk of phosphorus loss from a field. Research has shown phosphorus that moves from farm fields and into water bodies can cause algae blooms and other water quality problems.

The index, released in 2001, uses several factors to calculate a site-vulnerability ranking that includes erosion, soil P tests, farmer management practices and location of the field. The ranking is then used to determine how manure can be applied to fields, at what rates, and whether applications are based on nitrogen (N) or phosphorus (P) content.

Even with the advent of this management tool, there are still many unresolved questions about the role and movement of phosphorus in the soil. Leopold Center-funded work has both informed the development of the P Index and tried to identify soil P thresholds.

In the most recently completed Leopold Center project, members of the ISU Departments of Agronomy and Agricultural and Biosystems Engineering examined the relationships of P to crops, soils and water. Experiments were designed to compare different application methods, placement methods, and types of P tests (agronomic and environmental) to each other, to yields and to P loss.

Research questions posed
Some of the questions going into this work included:
• How is soil P affected by fertilizer or manure applications?
• Can variable rate application help manage soil P?
• What are the relationships between environmental P and runoff or tile flow?
• What’s the threshold value for soil P and how can farmers use it to manage P?

Several field experiments were conducted over three years, some on ISU research farms and some on private farms. Five long-term trials evaluated the effects of P application methods (deep banding, injecting, broadcast) on crop yield and soil P for no-till and chisel-disk tillage systems. Three long-term on-farm trials assessed the value of variable-rate application of fertilizer and liquid swine manure to achieve better P distribution over the area of the field.

Three other long-term trials evaluated the impact of fertilizer and manure P applications of total and dissolved P loss through tile drainage and surface runoff. Soil phosphorus was measured using routine agronomic tests that are considered appropriate for soils in the region, and also with what are called ‘environmental’ tests.

The commonly used agronomic tests emphasize evaluation of plant-available P. The environmental tests focus on leachable and mobile P and bioavailable (algae-available) P. Investigators thought the environmental tests would provide a better assessment of phosphorus that would move into water supplies, rather than plant-available P.

What researchers found
Current guidelines for soil phosphorus levels, measured in parts per million (ppm), are on a scale of Very Low, Low, Optimum, High and Very High. Although farmers may have an ‘optimum’ test result, they may add P over the recommended rate to help manage risk and to assure higher yields. This research, however, confirmed a very low probability of corn and soybean yield response at soil test P levels higher than 16 ppm.

The research also found that placement of phosphorus is important, even though it has little or no effect on grain yield. In this project, deep banding or injecting P greatly reduced the accumulation of P at or near the soil surface when compared with applications to no-till soils or soils managed with chisel-disk tillage. Further, incorporating P by injection significantly reduced both dissolved P and total P loss. This makes sense because P at the soil surface tends to move more easily off the land in rainfall events.

Farmers have questioned potential yield gains or risk that might be associated with using fertilizer P versus using liquid swine manure as a source of P. In this work, using variable application, there were seldom significant yield increases when the researchers compared variable rate applications of P fertilizer with variable rate applications of P-based liquid swine manure.

However, the variable-rate method itself managed the P application much better, that is, less P is applied to the fields overall and within-field soil-test P variability is always reduced. This is important for overall cost of P application, and for more sustainable and environmentally sound P management.

What researchers did not find
Researchers looked for a relationship between P concentrations in tile drainage and soil test P

Generally, P losses in tile drainage rose with increasing P application rates and with the amount of soil-test P in the surface soil. However, the actual concentration in tile drainage in the test sites was low, and could not be significantly related to the soil-test P until these levels exceeded 80 to 100 ppm, which is four to five times higher than the optimum levels for crops. Even then, the drainage concentrations were quite low. The environmental P soil tests proved no more useful in identifying a specific relationship than the common agronomic tests.

One objective of this work was to determine a threshold – a turning point to trigger specific increases in P loss for specific amounts of soil test P. Researchers looked for a clear threshold where soil surface and total loss would dramatically increase or decrease given specific soil test levels, even those that were two and four times above the recommended levels.

That elusive relationship was not identified by this work. Additionally, it was hoped that environmental P tests might be more useful than common agronomic soil tests to establish relationships between soil-test P and P concentration in surface runoff. No findings could confirm any relationship.

Overall the work was useful in validating the Iowa P index as an all-purpose tool for statewide farmer use. It reinforced some management issues and provided new insights into others.

It may, however, not go as far as possible in achieving the site-specific sustainability that would contribute even further to improved statewide P management. Lead researcher Antonio Mallarino said there may be other models that would allow farmers to further tailor agronomic and environmental P management practices to their own lands, and he is committed to make these other options a reality for farmers.

More about the P Index

More than 150 people attended five public hearings in March conducted by the Iowa Department of Natural Resources to gather comments about proposed rules regarding phosphorus and manure management plans. The proposed rules and a fact sheet on the use of the P Index are available at the Iowa DNR web site at: www.state.ia.us/epd/watewtr/feedlot/feedlt.htm, or on the Iowa NRCS web site at: www.ia.nrcs.usda.gov/