How Often Should You Test Your Soil?

Some farms take soil tests every year. Some pull samples every 4-6 years. Others choose not to soil test at all. While soil testing seems straightforward, the answer is not simply “the more often, the better.”

In fact, decades of university research suggest something surprising: for most fields, annual soil testing provides little added value. The real question is not how often you can test but how often you can gather information that guides a management decision.

What Does the Science Say?

In Wisconsin agricultural fields should be retested at least every four years.

Four years may seem like a long time. But there is solid agronomic reasoning behind it. Over a typical four year period, crop removal usually does not change soil phosphorus (P) or potassium (K) levels enough to move a field into a completely different fertilizer recommendation category. Second, the four year interval also helps maintain accurate lime recommendations by keeping track of gradual pH changes. Wisconsin farms with nutrient management plans are required to test their field every 4 years. For all farms, four years is not a compromise. This timing strikes a balance between two costly situations: allowing deficiencies to develop due to underapplication of a nutrient and overapplying nutrients that offer little return on investment.

Not Every Field Is the Same

Of course, farming would be easier if every field behaved the same way. Some situations warrant more frequent monitoring because nutrients can change much more rapidly. The University of Wisconsin recommends more frequent sampling when you are growing:

• Corn silage
• Alfalfa
• High value vegetable crops

The recommendation also applies to:

• Sandy or sandy loam soils
• Irrigated fields
• Fields experiencing rapidly changing nutrient conditions
• Or when crop rotation recently changed to include higher demand level crops

In these situations, testing every two years is justified. In intensive vegetable production systems, annual sampling may be the best management choice. The key principle is simple: the faster soil fertility conditions can change, the more frequently you should measure them.

What Are Other States Doing?

Wisconsin’s recommendations are not unusual. A 2023 review published in the Soil Science Society of America Journal examined soil fertility recommendations across the United States and found considerable consistency. The study reported:

• 20% of states recommend annual sampling.
• 16% recommend sampling every 2 years.
• 18% recommend it every 3 years.
• 12% recommend it every 4 years.
• Several states adjust recommendations based on crop type or soil characteristics.

The overall range was between one and four years, with most states clustering around three to four year intervals for routine fertility monitoring. In other words, Wisconsin’s four year recommendation falls well within mainstream agronomic practice.

Why Not Sample Every Year?

This is where many farmers are surprised. If soil testing is good, shouldn’t more testing be better? Not necessarily. Most university research agrees that annual routine testing for phosphorus, potassium, and pH is often not cost effective because these properties typically change slowly. Soil pH moves gradually over time. Phosphorus changes slowly because it is immobile in the soil. Potassium decreases gradually, except in crop rotations with exceptionally high crop removal such as forage production systems. As a result, fertilizer recommendations for these nutrients often look similar from one year to the next.  The lack of differences in soil tests taken annually may lead to difficulty in making management decisions to profitability grow the crop.

When Does Annual Soil Testing Make Sense

It does not mean annual testing is never worthwhile. Certain situations benefit significantly from yearly monitoring. Examples include:

• Soil nitrate testing
• High value crop production
• Intensively manured systems
• Investigating problem areas within fields

In these cases, nutrient levels can fluctuate enough from season to season that annual information directly influences management decisions. The goal is not testing more frequently. The goal is to test frequently enough to improve decision making and meet crop needs.

The factors that justify more frequent soil sampling.Across university extension publications, several themes emerge related to the factors that justify more frequent soil sampling than every 4 years. Every factor on this list below increases either nutrient removal, nutrient movement, or management complexity. When variability increases, uncertainty increases. And when uncertainty increases, more data becomes valuable.

Key Takeaways

So, let us return to the original question: How often should you test your soil? For most Wisconsin corn and soybean farms, the answer remains straightforward: routine fertility soil tests every 4 years.

The factors that justify more frequent soil sampling.Across university extension publications, several themes emerge related to the factors that justify more frequent soil sampling than every 4 years. Every factor on this list below increases either nutrient removal, nutrient movement, or management complexity. When variability increases, uncertainty increases. And when uncertainty increases, more soil test data becomes valuable.

• Sandy soils with low cation exchange capacity
• Intensive Corn silage or alfalfa systems
• Vegetable production
• Irrigated fields
• Large manure applications
• Significant yield variability
• Precision nutrient management programs
• Suspected nutrient deficiencies or pH problems: Sample whenever needed rather than waiting for the regular cycle

Soil testing is not about collecting more samples. The best soil testing program is one that helps you make better decisions, protect profitability, and manage nutrients responsibly for the long term.