N test effective, use grows slowly
P-Index introduced
How have things changed?

Soil testing has become an integral part of today’s agriculture. Whether done by a farmer or a crop advisor, the economic and environmental importance of soil tests has been linked with public policy that requires careful management of nitrogen (N) and phosphorus (P).

ISU agronomy professor Alfred Blackmer, who died in 2006, worked directly with hundreds of farmers to promote soil testing. He received the 2006 Master Farmers Exceptional Service Award for his efforts. [Photo by Rod Swoboda, Wallaces Farmer]

Most recently, the Center has supported efforts to educate farmers regarding the P-Index, a tool developed by scientists from ISU, the United States Department of Agriculture (USDA) National Soil Tilth Laboratory, and the USDA Natural Resources Conservation Service (NRCS). The Leopold Center funded research by ISU agronomy professor Antonio Mallarino and the now-retired James Baker (ISU Agricultural Biosystems and Engineering) for the development of the P-Index and explored soil P thresholds.

Iowa’s N-management programs have been in place since 1982, with initiation of the Big Spring Basin, Integrated Crop Management, and Model Farm demonstration projects. The Iowa Groundwater Protection Act of 1987 created the Leopold Center and as well as other initiatives to protect water resources.

Through the years, the Leopold Center has been a key player in development and implementation of improved soil tests for various nutrients, following its legislative charge to “identify and reduce the negative environmental and socio-economic impacts of agriculture” and “research and assist in developing emerging alternative practices that are consistent with a sustainable agriculture.”

The N and P management tools have markedly different histories but both have the potential to reduce costs and off-site environmental effects of fertilization while maximizing crop benefits.

N test effective, use grows slowly
Groundwater testing in the early 1980s showed alarming rates of nitrate in Iowa. Nitrogen moves easily in the soil profile and is water soluble so careful and timely application of the nutrient is crucial. Acknowledging the public health consequences of such elevated concentrations, efforts were initiated to encourage reduced rates of fertilization.

An important focus of these efforts was to move farmers away from simple goal-based recommendations to a system that accounts for influences such as weather, and the form or timing of N fertilizer application, all of which figure heavily into losses and availability of N to the crop.

Among the Center’s early efforts was development of the late-spring soil test for nitrogen, developed at Iowa State University by the late Alfred Blackmer and based on prior work done by the University of Vermont’s Fred Magdoff. Throughout the early 1990s, the Center supported a variety of work by Blackmer and others evaluating the effectiveness of the test and exploring producer attitudes about the test.

The N-Trak soil test kit developed by Blackmer was marketed by Hach Company to test soil nitrate when corn reaches six to 12 inches tall. Research showed that this time frame was optimal for achieving corn response from N application.

Blackmer’s work also confirmed that losses of N fertilizer soon after application were important, that the yield goal has little or no impact on the optimum N fertilization rate, and that carryover N was less important than generally believed. That research and other projects resulted in current ISU guidelines for N fertilization that do not consider yield goals.

Although the late-spring soil test was introduced in 1990, it was not readily adopted by Iowa farmers. A Leopold Center-sponsored study showed that less than one-half of 1 percent of farmers had purchased the test kit, and many of those didn’t use the kit even though they owned it.

By 2002, Paul Lasley’s Iowa Rural Life Poll showed that 12 percent of farmers were testing at a moderate to heavy rate. There are no recent surveys available on use of the test, but it appears that some inroads have been made in changing producer attitudes about testing for the amount of N needed at a time when
response will be most dramatic.

One of the barriers that kept farmers from testing soil samples in late spring was the time it took to get results back from a lab. They need a quick turnaround to permit timely sidedressing. The Hach Chemical kit allowed growers or consultants to analyze their own samples in a matter of minutes. Now most commercial labs provide the analysis in 48 hours or less, thus eliminating the need for individual test kits. Many different types of kits also are commercially available and used by crop consultants. In recent years, government programs such as EQIP and CSP have begun to provide incentives to growers who use this test.

To date, no other method of making N fertilizer recommendations has shown as high an accuracy as the late-spring nitrate test in Iowa. The development of this test has helped increase growers’ profitability, but at the same time it often has reduced the total amount of N applied. The biggest benefit is that it has provided a data-based system for N management and evaluation.

A review of Leopold Center grants shows that since 1992, the Center has invested about $260,000 in projects aimed at developing, fine-tuning, and assessing adoption of the late spring soil nitrate test.

P-Index introduced
Over the years, the Center also has supported efforts to improve soil testing for phosphorus. The effort included research by Mallarino, who conducted the first statewide field calibrations of a better P test for high-P soils. These tests are being recommended for these soils by ISU and state agencies.

Although soil testing for P is important in being able to predict P fertilization needs for crops, research in many states, including Iowa, has shown that soil testing alone is not effective for assessing the risk of phosphorus loss to water resources.

Therefore, in April 1999, the NRCS issued a policy that directed states to develop a tool that includes soil-test P levels and other factors for assessing the risk of P being moved from fields to surface waters. Concerns arose from movement of P into water where it increases aquatic vegetation and algae blooms. Methods and rates of manure application, commercial fertilizer application, soil erosion and surface runoff influence the risk of P loss from fields. The P-Index integrates these and other factors to determine risk of P loss from fields.

Current comparative data isn’t available for phosphorus, but in a 2005 Agricultural Resources Management Survey (ARMS), P was applied to 70 percent of corn acres at an average rate of 64 lb/A. This contrasts with P being applied to 81 percent of 19 corn states at an average rate of 58 lb/A.

No agency is charged solely with tracking use of the P-Index, but the index is required by the NRCS for most livestock producers who submit nutrient management plans or those who request financial or technical assistance. The Iowa Department of Natural Resources (DNR) has required P-Index assessments as part of the state-mandated manure management plans for feeding operations with more than 500 animal units, for which renewals are being phased in over three stages.

Gene Tinker, coordinator of Animal Feeding Operations for DNR, estimates that one-quarter to one-third of the 4,600 anticipated plans have been renewed to date, and between 1,150 to more than 1,500 plans have used the index.

“Some farmers do the P-Index themselves and many have used consultants,” Tinker explains. He says that providing the index has not been a problem for producers, though some whose samples were older than four years or were not done at the required level of one sample for every ten acres needed to acquire new and additional samples. “The index is appropriate in that it rewards people who have good management practices,” Tinker adds.

How have things changed?
Many national sources note the significant reduction of more than 12 percent in Iowa’s N use on corn between 1985 and 1995. And while per acre use varies greatly depending on rainfall and other environmental factors from year to year, Iowa’s rate of application has remained below that of other neighboring agricultural states. That rate showed only a 3 percent differential from 1985 compared to 2005, according to data from the ARMS.

However, according to data available from the ISU Agronomy Department, in 2003 Iowa corn growers applied an average 133 lb of nitrogen per acre, contrasted with an average 152 lb/A throughout the Corn Belt. States in the comparison were Illinois, Indiana, Nebraska, Ohio and Missouri.

The benefits of using soil tests as well as other assessment tools such as the P-Index and management practices to reduce movement of N and P into surface and groundwater are many. Reducing nitrate levels protects drinking water and reduces the cost of water treatment for cities. Fertilizers and pesticides derived from fossil fuels make up 75 percent of the energy inputs in Iowa corn production. Finally, reduced movement of P into Iowa’s lakes and streams will diminish algal blooms and improve habitat for fish and other wildlife.

While the wheels of change move slowly, the tools are there to help Iowa’s farmers make informed decisions about farming practices.

EDITOR’S NOTE: Many thanks to Antonio Mallarino, who reviewed and helped prepare this article, and to the Iowa Soybean Association, which also contributed information used in this report.