Testing for Soil Nutrients in Iowa

With the rising fertilizer prices and uncertainty of land rental costs in many cases, you may be tempted to cut back, or outright pass on buying dry fertilizer for a season. While there may be sufficient nutrients banked in your soil, you should base any decision to cut or forego dry fertilizer on a recent assessment of what’s in your soils, advises Tracy Blackmer, director of the Iowa Soybean Association On-Farm Network^®. 

 “Soil testing for nutrient content and availability is a must for responsible nutrient management relating to P, K and soil pH,” Blackmer says. The results from samples from more than 900 Iowa corn and soybean fields last season showed that there is a wide range of variability, and therefore, growers need to assess their own fields.

Nutrient Benchmarking Project

In the summer of 2011, the Iowa Soybean Association On-Farm Network launched a new project to gather nutrient data from growers’ fields around the state. Funding for this project came from the soybean checkoff and the Iowa Department of Agriculture and Land Stewardship’s Integrated Farm and Livestock Management program.

More than 400 growers enrolled more than 900 fields in this project. Aerial Imagery was collected for the fields in July and early August. This was followed by growers themselves (in most cases) going out into their fields to collect both soil and plant tissue samples for analysis. Samples were subsequently analyzed for a wide spectrum of nutrients

Presented here is a partial, early snapshot of some of the soil sampling results. Results of tissue testing and overall field analysis of nutrient availability will be released throughout the winter. The topic will also be covered at the annual On-Farm Network Conference on February 16.

The map (above, right) shows the different sampling locations for the Nutrient Benchmarking Project. All soil and tissue samples from the project were analyzed through Midwest Labs in Omaha, NE. The results are reported as percentages in each soil testing category as defined by Iowa State University.

Potassium

Although all soil test results should be viewed with caution, but this is particularly true for results from potassium (K) analysis. There is a fair amount of variability in K test results, and these are strongly influenced by soil moisture. Since most of the state was very dry at the time the samples for this project were taken, we should consider that the K results might be lower than had the samples been taken when soil moisture levels were higher.

From the graph on the right, you can see that almost a third of the samples tested below optimal for K. Keeping in mind the dry soil situation, though, because there is relatively low loss of K from soils after application (and so minimal environmental impact), many growers would likely find it more profitable to apply enough K to bring soil content back to a higher level.

Zinc

Zinc is important in corn and soybean production, but is required in relatively small amounts compared to P and K. The amount of zinc present in our Iowa soils is usually considered adequate and the soil tests confirm this common thought (See graph at top of next page). However, there were some low testing fields in this study, so zinc should not be ignored. Blackmer advises that growers do additional soil testing to determine where zinc is needed rather than a blanket application on all fields. Soils that tend to be low in zinc are generally sandy, with low organic matter content.

Soil Acidity

Soil acidity is reported in terms of soil pH. In general, Iowa fields tend to become more acidic (lower pH) over time from practices like N fertilization. At some point, many fields will need to have lime added to increase the soil pH. Using ISU’s testing guidelines, roughly 20% of the soil samples were below a pH of 6, which is when lime is typically recommended for corn and soybean production. It is important to note that in some areas of the state, soils already contain enough lime (calcium carbonate) to raise the pH naturally to levels as high as 8.3. These areas are referred to as calcareous soils and usually create visual yellowing in soybean plants early in the season.

Phosphorus 

Testing for phosphorus (P) availability in soils is somewhat different than testing for other nutrients because the sensitivity of the testing procedures is influenced by soil pH. Like many other nutrients, not all of the P in the soil is readily available for plant uptake. To predict how much P might be in an available form, a weak acid (Bray 1) is mixed with the soil and the solution is then analyzed for the P that is released. The Bray 1 test is more effective when analyzing neutral to acid soils (below 7.3 pH). The calcareous soils, found in north central Iowa and other parts of the state as well, tend to have pH levels above 7. The Olsen test is generally used for analyzing P content on soils with a pH level of more than 7. The results from the project were split by soil pH into 2 groups and the results are reported according the pH level and P availability test that was conducted on them.

Overall, the results from this season show there are plenty occurrences of fields above or below optimal. The only way you can know where you are at is to have a current soil sample for your fields. Watch www.isafarmet.com for additional information on this project. To get involved in the project for 2012, contact info@isafarmnet.com.