Soil Fertility Best Management Practices with Limited Resources

With low crop prices and increased fertilizer costs, farmers are wondering how best to allocate fertilizer dollars this fall. The following articles were posted in the fall of 2024, but all still apply to today’s situation and provide a list of best management practices (BMPs) in order of significance.  

Soil Sampling

First on your priority list is to take good soil samples so you can adequately assess soil test levels to make decisions on fertilizer inputs. See Taking Good Soil Samples, the First Step in a Good Fertilizer Decision. Soil tests should be taken every 2-4 years. One note of caution this fall, consider how dry soils may impact fall soil sample results, see Beware of Dry Conditions When Soil Sampling and Interpreting Test Results.  

Soil pH

Second on the list is management of the soil pH. Soil pH affects both biological and chemical reactions in the soils, but of concern for this article is the effect on the availability of essential crop production nutrients and their forms. Below are the recommended soil pH levels that are considered optimal for Iowa’s primary crops: 

• 6.9 -  Alfalfa and alfalfa-grass mixed hay or pasture.
• 6.0 - Other forage legumes or legume-grass mixtures and grasses.
• 6.5 - Corn and soybean in areas of the state with low pH subsoil.
• 6.0 - Corn and soybean in areas of the state with high pH subsoil (shaded portion of map).  

Liming recommendations can be found in Table 16 of PM 1688, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa. A free PDF is available to download. 

Figure 15 in PM 1688 showing principal soil associations with high subsoil pH (shaded area). 

Phosphorus (P) and Potassium (K)

Third on the list of BMPs is ensuring that P and K soil test levels are adequate to sustain crop production. Soils with “high” soil test levels for P and K only have a 5% chance to see a positive ROI with additional fertilizer application and a “very high” soil test level has less than a 1% chance of positive ROI. The article Lime, Phosphorus, Potassium Fertilizer Decisions in Times of Limited Resources helps to identify how best to allocate fertilizer dollars. 

Replacing Crop Removal

Our fourth best management practice is a reminder to replace what you remove. If your soil test levels are high or very high, it might be practical to skip P and K applications for a year or two, but remember to account for crop removal. If you remove corn or soybean stover for feed or bedding, remember to account for those nutrients removed in addition to the grain harvested. Nutrient Replacement With Corn and Soybean Stover Removal.

Secondary and Micronutrients

The last discussion point is in regard to secondary and micronutrient needs for crop production. Sulfur is classified as a secondary essential nutrient. Research results show a yield response to sulfur application in corn and alfalfa production (soybean research is on-going).  Who Needs Sulfur? You Might Need Sulfur breaks down need, source, rate and timing.  Current sulfur recommendations for Iowa can be found in CROP 3072, Sulfur Management for Iowa Crop Production. 

Zinc is the only micronutrient for which a calibrated soil test interpretation exists for corn and sorghum production.  Zinc recommendations can be found in Table 15, of PM 1688.  

Allocation of fertilizer dollars to application of micronutrients should be the final decision if there are dollars available. One of the most frequent questions Extension professionals receive regarding soil fertility is Do I Need to Add Micronutrients Into My Soil Fertility Program? The classic answer “it depends” is really true in this case, but research done at Iowa State University and other universities across the upper Midwest shows there is no consistent yield benefit to applying micronutrients. There are many micronutrient products available. Testing if and why a specific product may work for you will ensure you are allocating your nutrient dollar resources in the best way possible. 

Ultimately, having good soil samples should help you plan for P, K and liming decisions. ISU Extension field agronomists offer occasional fertility workshops that can also help you best allocate fertilizer dollars.  

Additional Resources: 

Video: Phosphorus, potassium and lime decisions with low commodity prices.

Video: Recognizing the signs: Identifying common nutrient deficiencies in row crops

ISU Extension and Outreach Soil Fertility Web Page

PM 1688 A General Guide for Crop Nutrient and Limestone Recommendations in Iowa

CROP 3072 Sulfur Management for Iowa Crop Production

Now is the Time to Prepare for Fertilizer Applications