Soil Sampling: One of the Best Investments on Your Farm

Cory De Jong
Agronomic Data Manager, CCA

When harvest wraps up and attention shifts toward the next growing season, soil sampling should be near the top of every grower’s management checklist. Fertilizer prices, commodity markets, and weather conditions may change, but understanding what is happening beneath the soil surface remains essential for making informed agronomic decisions.

At Key Cooperative, we believe accurate soil sampling is one of the most valuable tools growers can use to improve fertilizer efficiency, protect yield potential, and manage input costs. A good soil sample helps answer a basic but important question: What does this field actually need? 
 

Make Every Fertilizer Dollar Count

Without current soil-test information, fertilizer applications may be based on field averages, previous programs, or assumptions. However, nutrient levels can vary significantly between fields and even within the same field.

Regular soil testing identifies areas that need corrective fertility, areas that require maintenance, and areas where additional fertilizer is unlikely to provide an economic return.

According to Iowa State University Extension publication PM 1688, the average probability of a yield response to phosphorus or potassium fertilizer is approximately:

• 80% when the soil test is Very Low
• 55% when the soil test is Low
• 25% when the soil test is Optimum
• 5% when the soil test is High
• Less than 1% when the soil test is Very High

The Optimum category is generally the range to maintain for long-term profitable crop production. High and Very High soil-test levels indicate that the existing soil nutrient supply typically exceeds crop needs, meaning additional phosphorus or potassium seldom provides a profitable yield response.

This is one of the clearest benefits of soil sampling. It helps growers direct fertilizer dollars toward the acres with the greatest opportunity to respond.
 

What Does It Take to Raise Soil-Test PPM?

Soil-test phosphorus and potassium are commonly reported in parts per million (ppm). It is easy to assume that applying fertilizer will produce a quick and predictable increase in soil-test ppm, but the relationship is more complicated.

For a corn-soybean rotation and a 6-inch sampling depth, PM 1688 provides the following average planning estimates:

• Approximately 16 to 18 pounds of P₂O₅ per acre may be needed to raise soil-test phosphorus by 1 ppm.
• Approximately 8 to 10 pounds of K₂O per acre may be needed to raise soil-test potassium by 1 ppm.

Using the midpoint of these ranges:

• Raising soil-test phosphorus by 5 ppm could require approximately 85 pounds of P₂O₅ per acre.
• Raising soil-test potassium by 20 ppm could require approximately 180 pounds of K₂O per acre.

These examples illustrate the significant fertilizer investment often required to change a soil test. However, they should not be treated as exact fertilizer prescriptions.

Because results can vary considerably, the best strategy is often to follow response-based recommendations in Very Low and Low areas rather than applying excessive rates simply to increase soil-test numbers more quickly.
 

Improve Variable-Rate Management

Grid and management-zone sampling provide additional value by revealing nutrient variability within a field. A field average can conceal Very Low-testing areas that need corrective fertility and High-testing areas where additional fertilizer is unlikely to pay.

Combining current soil-test results with yield history and field knowledge allows growers to:

• Apply corrective rates in Very Low and Low areas
• Use crop-removal maintenance rates in Optimum areas
• Reduce or pause applications in High and Very High areas
• Create more consistent fertility levels across the field
• Improve the return on variable-rate technology
• Track soil test trends over time

Following a consistent sampling plan and understanding the laboratory methods used can help growers make meaningful comparisons from year to year. Your local Key Cooperative agronomist can help develop the right sampling strategy for your operation.
 

Monitor Long-Term Soil Fertility

Soil sampling is not only about developing a fertilizer recommendation for the next crop. Consistent testing allows growers to monitor changes in phosphorus, potassium, soil pH, organic matter, micronutrients, and other soil characteristics.

Historical results can help answer important questions:

• Is the fertility program maintaining soil-test levels?
• Are crop-removal rates exceeding nutrient applications?
• Is soil pH limiting nutrient availability?
• Are corrective applications gradually improving deficient areas?
• Are some areas receiving more fertilizer than needed?
• Are micronutrient levels trending toward deficiency as yields continue to increase?

PM 1688 suggests planning a new soil test approximately every two years for most cropping systems. Sampling every three or four years may be acceptable when fields are near the Optimum category and a consistent maintenance fertility program is in place.

Consistent sampling over time provides a clearer picture of whether a fertility program is maintaining, increasing, or drawing down nutrient levels.

Don't Overlook Micronutrients

While phosphorus, potassium, and pH often receive the most attention, soil sampling also provides valuable insight into micronutrient availability. Nutrients such as zinc, sulfur, manganese, boron, copper, iron, and molybdenum are required in much smaller amounts than macronutrients, but they play critical roles in plant growth, nutrient uptake, photosynthesis, enzyme activity, and grain development.

As crop yields continue to increase, micronutrient removal from the field also increases. Deficiencies may not become visible until yield potential has already been reduced, and symptoms are often mistaken for other agronomic issues. Soil testing helps identify fields where micronutrient availability may be limiting performance, allowing growers to make informed fertility decisions rather than relying on visual symptoms alone.

Micronutrients also interact with major nutrients. For example, zinc supports early root development and nitrogen utilization, while sulfur is essential for nitrogen metabolism and protein formation. Even when phosphorus and potassium levels are adequate, a micronutrient deficiency can prevent a crop from fully utilizing those nutrients and reaching its yield potential.

By including micronutrient analysis as part of a comprehensive soil sampling program, growers can identify hidden yield-limiting factors and ensure their fertility investment is delivering the greatest possible return.

The Bottom Line

Soil sampling is more than an agronomic practice. It is a business decision. It can take a significant fertilizer investment to raise phosphorus or potassium by even a few ppm. Knowing the starting soil-test level helps growers determine whether to correct a deficiency, maintain an Optimum level, or temporarily reduce applications in high-testing areas.

Regular soil sampling can help growers:

• Maximize fertilizer efficiency
• Protect potential crop yield
• Improve variable-rate applications
• Monitor long-term soil fertility
• Identify micronutrient deficiencies before they impact yields
• Avoid unnecessary fertilizer expenses
• Support responsible nutrient management

In an environment where every input dollar matters, understanding what is in the soil remains one of the most valuable pieces of information a grower can have. Soil sampling not only helps manage phosphorus, potassium, and pH levels, but also provides insight into the micronutrients that support today's high-yielding crops.

 
Reference: 
PM 1688: A General Guide for Crop Nutrient and Limestone Recommendations in Iowa
Iowa State University Extension and Outreach, Revised February 2023.