How Often Should Farmers Test Soil for Better Crop Yields?

By Blake Jackson

Soil testing is an important tool for managing crop nutrients, but testing more frequently does not always produce better results.

According to Chris Clark, Crops and Soils Outreach Specialist at the University of Wisconsin-Madison, the most useful testing schedule depends on how quickly soil fertility conditions change and whether the results will support a management decision.

For most Wisconsin agricultural fields, soil should be tested at least once every four years. University research indicates that this interval generally provides enough information to monitor phosphorus (P), potassium (K), and soil pH without the expense of unnecessary annual testing.

Over four years, crop nutrient removal typically does not change P or K enough to move a field into a substantially different fertilizer recommendation category. The interval also helps track gradual changes in soil pH and maintain appropriate lime recommendations.

Some fields, however, require more frequent monitoring. Clark notes that testing every two years can be appropriate for corn silage, alfalfa, irrigated fields, sandy or sandy-loam soils, high-value crops and fields where nutrient conditions are changing quickly.

Fields recently placed into rotations with crops that have greater nutrient demands may also benefit from more frequent testing. In intensive vegetable production, annual testing may be appropriate.

Wisconsin's approach is consistent with practices in other states. A 2023 review of state soil-fertility recommendations found that sampling intervals generally range from one to four years, with many states recommending testing every three or four years for routine fertility monitoring.

Annual testing can still be valuable when nutrient levels change rapidly or when precise information affects crop-management decisions. Examples include soil nitrate testing, high-value crop production, heavily manured fields, and investigating unusual or problem areas within a field.

The main consideration is not simply how often soil can be tested, but whether the information will improve fertilizer, lime or crop-management decisions.

More frequent sampling becomes more valuable as nutrient removal, nutrient movement, field variability, or management complexity increases.

Practical Recommendations

• Test routine fields at least every four years to maintain current fertility information.
• Consider testing every two years for high-demand crops and rapidly changing soil conditions.
• Sample sandy and irrigated fields more frequently because nutrients can change faster.
• Use annual testing for intensive vegetable production when nutrient management is especially critical.
• Consider annual nitrate testing when nitrogen availability can vary substantially between seasons.
• Use soil tests to guide decisions, rather than testing simply because more data is available.

Photo Credit: pexels-binyaminmellish