Food security

Why do farmers need to test their soils?

Have you ever wondered what is in the soil that supports a plant to survive, reproduce, and complete its life cycle? Where do all those nutrients present in your breakfast corn flakes, or the bread for your sandwiches come from? Nutrients come mostly from – the soil!

Plants need 17 essential nutrients to function and carry their routine physiological processes. Of these nutrients, three are found in air and water: carbon (C), hydrogen (H), and oxygen (O). The remaining 14 nutrients come from the soil.

soil core in tray next to ruler in lab
This is a soil core before analysis. Farmers take these cores and other soil samples to be sent into labs for nutrient analysis. This allows them to use the optimum amount of fertilizer for the best yields. Credit: Rishi Prasad

Not all plants need these nutrients in the same quantities. Some nutrients such as nitrogen (N), phosphorus (P) and potassium (K) are required in greater quantity than others. They are called primary macronutrients. Plants require other nutrients in lesser quantities such as magnesium (Mg), calcium (Ca), and sulfur (S) which are known as secondary macronutrients. There is a third category of nutrients known as micronutrients which are required by plants in tiny amounts. They are boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), and zinc (Zn).

Since humans cannot see, touch, or count nutrients through their naked eyes, they rely on scientific instruments found in soil testing laboratories to measure them.

How do farmers make sure that there are enough nutrients in the soil to make a good crop?

Farmers rely on soil testing to make decisions on how much additional nutrients they need to apply to make a good crop. Limitation of one nutrient can have a detrimental effect on overall crop yield.

two people using auger to dig up soil core in field with trees and sky in background
A soil auger is used to dig up a soil core in a field. The core is then pulled out, creating a replica of soil at each increment of the core, versus digging and mixing the soil into one homogenous group. Credit: Rishi Prasad

German scientist Justus von Liebig proposed the “Law of the Minimum” in the 19th century. It states that if one of the essential plant nutrients is deficient, plant growth will be affected, even when all other essential nutrients are abundant.

That is why every fall after the harvest of cash crop, farmers collect representative soil cores from 4 to 6 inches in depth at several locations on their farm. They send them to a soil testing laboratory for analysis.

Once a soil testing laboratory receives the soil, the lab dries, grinds, and sieves the sample to make it uniform before running the tests. Then they perform the requested tests designed to quantify nutrients in the soil. The results provide information on the soil’s nutrient supplying capacity – primarily phosphorus, potassium, and micronutrients.

two people wearing lab coats and safety glasses in science lab using various equipment
In the lab, scientists dry the soil, grind it, and sieve it to allow for testing of nutrients. They then create a report that is sent to farmers to help them determine if they need to fertilize. Farmers send in samples for testing every 1 or 2 years. Credit: SSSA Staff

The results showing the nutrient concentrations is then evaluated. Upon completion of analysis, an expert such as a soil scientist interprets the result as well as information about which crops need what types of nutrients to grow best. They then make recommendations on what is present in the soil and how much additional fertilizer would be needed to achieve optimal yields based on the crops that will be grown.

The laboratory then sends the report to the farmer with their findings and provides recommendations for nutrient applications. Farmers also recruit or consult an agronomist who looks at the soil test report and makes recommendation on how much and when to apply fertilizers. Many times, farmers collect and send soil samples every one- or two-year period. Maintaining a record of soil test reports also provides valuable information on long-term changes in soil fertility. This allows farmers to make better decisions on fertility management to get optimum yields.

Answered by Rishi Prasad, Auburn University

Read a story about Dr. Prasad’s research here: One Soil Test to Rule Them All

About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.

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