The Importance and Difference Between Soil Tests

June 21, 2023

Growing a crop can be difficult, there are many different metrics that need to be correct to get the desired yield. Water management, proper pruning, keeping pests and disease away, and adequate nutrition. Here at Wilbur-Ellis, we are always looking for scientifically sound and meaningful data to help the grower achieve their goals. As a Wilbur-Ellis Agronomist, it’s our job to help the grower and consultant come up with the appropriate nutrition requirements for the season. The best way to help determine what the crop will need is by soil tests.

Why take a soil test?

The purpose of a soil test is to evaluate the soil’s current needs and match the correct nutritional product with that specific need and timing. Soil tests can also be used to evaluate environmental concerns due to soil contamination. We have three main soil samples we like to utilize here at WE and each has a different use and reasoning behind it.

First, we need to understand how crops feed on mineral nutrients in the soil. Crops feed via mass flow, diffusion, and root interception.

  • Mass flow – occurs when a nutrient moves to a plant root with the water that is being absorbed by the plant. This is an important nutrient uptake process for nutrients such as nitrogen, calcium, and magnesium as well as many micronutrients such as molybdenum, copper, boron, and sulfate.
  • Diffusion – is the movement of a nutrient to a plant root due to a concentration gradient between the soil solution and the root surface. The nutrient concentration is higher in the soil solution than at the root surface and, thus, the nutrient moves to the root surface. This is an important nutrient uptake process for nutrients such as phosphorus and potassium.
  • Root Interception – occurs when a root grows next to a clay or organic matter surface and absorbs the nutrient.  Root interception usually means the root has grown near a soil colloid and absorbed the nutrients on or near the colloid surface. This is usually a minor way nutrients are absorbed.

Each soil test is measuring different nutrient pools. The solution pool is what’s immediately available. The labile pool represents the ions in the solid phase that can be exchanged rapidly with elements in solution. The nutrients in the labile pool can be lost from the soil via plant uptake, leaching or other methods. The non-labile pool is a measurement of ions that can’t be exchanged quickly, this includes nutrients that are held on parent material and organic materials that are yet to decompose.


Standard Soil Test (WE-Complete)

The Standard Soil Test (WE-Complete) uses ammonium acetate to anticipate what will be available to the crop during the season. Doing this test makes the solution and labile pools available to measure. Using the complete test is a very common method to test soils throughout the county and is a reliable way to understand what’s in the soil. The complete test uses the Olsen test for checking Phosphorus levels.  This test should be taken at least every three years to have it on record. Having this test will help an agronomist anticipate what might be available through normal seasonal breakdown. Below is an example of our WE-Complete test. Think of this test as what’s in your savings account.

Saturated Soil Test (WE-RAN)

The saturated paste sample uses the grower’s irrigation water instead of an acid to extract the nutrients. The irrigation water mimics the nutrients that will be immediately available during the next irrigation event. This test measures the solution pool of nutrients with the irrigation water. The RAN measures the immediately available crop nutrients strictly in soil solution at a given point in time. This test is an indicator of not only what is immediately available to the crop, but also indicates the levels and release ratios of those nutrients. The RAN test can be used multiple times a year. The test can be run in the spring or fall, as well as in season to determine if in-season nutrition adjustments need to be made. This test is compared to the cash in your pocket, immediately available.


Organic Acid Soil Test (WE-REN)

Organic Acid is a modification of an original soil extractant that mimics the plant roots’ natural process for acquiring nutrients. The soil is mixed with organic acids of 4.4 pH which will mimic the acid a root tip exudate that will be used to mine nutrients. Plants leak organic acids out of their roots, which temporarily lowers the soil pH and makes nutrients more easily accessible. This test uses these same naturally occurring organic acids in an extractant that can be used in soil testing labs. The REN test isn’t as strong as the complete test, but correlates more to what the root can pick up. The REN test, like the RAN is a great in season soil test to determine what is currently happening in your soil.

In all, soil tests are very important to determine what is happening in the soil to help make the best agronomic decision when it comes to fertilizer recommendations. Also, not all soil samples are the same. Determine with your agronomist which soil test would be best to pull depending on the time of the year and what results you are looking for.

Benjamin Sullivan // Nutrition Agronomist