How To Turn Soil Samples Into Profitable Decisions, One Test at a Time

Soil is full of secrets. And the answers to crop potential can be unearthed with one simple tool: a soil test.

As you fill the management toolbox with key strategies for maximizing crop success, making sure your soil tests are up to date, properly sampled and analyzed is critical. Implementing a strategic approach to soil testing can help you collect better data, make more accurate decisions and use the latest technologies to ultimately reduce costs, enhance productivity and improve sustainability.

Why Soil Test?

You know you need to soil test. But why and what can it tell you? A soil health test measures biological, chemical and physical properties. From soil texture and pH levels to nutrient levels, organic matter content and soil health parameters, its real potential is found in the details of a soil sample. Let’s look at what each of these soil components might reveal:

1. Nutrient Levels: Cation exchange capacity (CEC), effective cation exchange capacity (ECEC) and anion exchange capacity (AEC) determine the soil’s ability to hold nutrients. “Soils with high values will have more reserve fertility, while soils with low values will require more frequent fertilization to meet crop needs,” explains Zach Larson, a sustainable systems agronomist with Bayer.
2. pH Levels: Nutrient availability and microbial activity are both affected by soil pH.
3. Organic Matter Content: Water retention, nutrient holding capacity and microbial activity can all be improved by the soil’s organic matter. According to Larson, monitoring soil organic matter levels can help assess soil health and determine the need for organic matter amendments such as compost or cover cropping.
4. Soil Health Parameters: This includes the tests that measure microbial activity, labile carbon sources and nutrients available because of biological activity.
5. Soil Texture: The composition of soil particles, including sand, silt and clay is drawn from soil texture. It affects water-holding capacity, drainage and nutrient availability. “Soil surveys are a good starting point for knowing surface soil textures,” says Larson. “However, to obtain more precise measures such as carbon storage, a known value through a sample is a better starting point.”

What Types of Soil Tests Are Available?

All soil is not created equal, and neither are the tests that analyze its properties.

A comparison of soil health tests can help you understand the ins and outs of each one. Here’s a brief overview of soil test options to consider.

• Comprehensive Assessment of Soil Health (CASH) Test: A popular test available from Cornell Soil Health Testing Lab that is designed for growers who want a more in-depth look at nutrient levels. The test measures and rates 12 different soil attributes before combining them to form an overall quality score (out of 100).
• Haney Soil Health Nutrient Tool Test: Working to estimate the availability of nitrogen (N), phosphorus (P) and potassium (K), the Haney Soil Health Nutrient Tool Test reveals several different measurements of each to estimate their availability over the course of the growing season. The Haney Test measures the portion of organic matter that dissolves in water, which is therefore more accessible to soil microbes. It also provides a soil health score.
• Phospholipid Fatty Acid (PLFA) Tests: Structural molecules found in the membranes of all living organisms are called PLFAs. The levels of certain fatty acids are used to indicate activity and the size of bacterial, fungal or other types of microbial populations. The higher the levels, the better soil health might be. PLFA test results can differ depending on the lab.
• Soil Food Web Biology Test: The Soil Food Web Biology test primarily focuses on the same groups of microbes as the PLFA test. However, microscopy is used with this test to determine the relative abundance of soil microbes and their activity. This test distinguishes active bacteria and fungi from totals of both groups.
• Active Carbon (Permanganate Oxidizable Carbon — POXC) Test: The Ohio State University developed this affordable soil health test to help farmers measure the biologically active organic matter in the soil. This test can be especially important in high-organic-matter soils because the active pool of organic matter plays a key role in contributing to the fertility needs of a growing crop year to year. It might also be used to measure the microbial biomass and particulate organic matter as it changes over time.

How are Soil Samples Taken?

Soil probes are one of the most common and traditional methods for soil sampling, but emerging technologies are taking the stage and offer growers a key starting point for defining management zones.

Soil Probe Method: Lime needs, organic matter, phosphorus, potassium, sulfur, zinc and soil pH can be determined from surface samples collected 0 to 8 inches deep. Samples collected deeper than 8 inches typically results in lower test values for organic matter, phosphorus and zinc. Potassium and pH could increase, decrease or remain the same with deeper samples. Sampling to the proper depth of 8 inches is essential to ensure accurate representation of soil properties. (1)

Soil samples collected in late fall or early winter is acceptable except when testing nitrate-nitrogen on coarse textured soils. Fall sampling allows laboratories more time to evaluate results and gives growers ample planning time in designing management programs for the following year. Spring sampling prior to planting is the preferred option for soil testing. Doing gives time for soil moisture in the root zone to be replenished making it easier to sample many soils. Additionally, spring sampling allows for greater distribution of nitrate-nitrogen in the subsoil to be more representative of conditions during the growing season. (1)

Emerging Technologies: Some older and some newer technologies test soils in place. With this method, several probes using electromagnetic technology can be used for testing soils without taking a sample. One example is using Soil Electrical Conductivity (EC) to delineate field variation.

“These tools are being replaced, at least in part, through smart planter technologies such as SmartFirmer, a product that gathers information with the planter,” Larson says. He adds new technologies that provide more traditional soil test information using real-time in situ analysis, such as Mobile Soil Analysis.

While traditional soil testing has the benefits of being the most “accurate” on the farm, according to Larson, that doesn’t discount the reliability of other methods in determining soil nutrient availability.

“Lab-based sampling methods have the advantage of calibration of nutrient recommendations to the values obtained from those tests,” Larson says. “Newer methods generally lack the same amount of rigor in determining nutrient recommendations relative to the values read by the testing unit.”

He also says lab-based methods can be more time consuming, while sampling is generally limited to when a crop isn’t present in the field.

What Can Soil Test Data Mean for My Farm?

Knowing what fields might be deficient, optimum or even excessive in nutrient levels could indicate what nutrient and manure application needs to be prioritized on your farm.

Larson says it’s best to base nutrient needs on land-grant guidelines as they are based on the response of crops to a given fertility level and should reflect what’s happening with the soils in your state.

“Comparing soil test results over multiple years allows you to evaluate the effectiveness of your nutrient management practices,” Larson notes. “Look for trends in nutrient levels and adjust accordingly.”

He adds that soil testing can be used to evaluate the outcomes of nutrient applications or changes to nutrient plans.

Soil testing is an investment in your operation. And Larson says it’s a valuable tool in the management toolbox regardless of whether fields are treated on a field-by-field basis or a zone or grid-based variable rate prescription.

“Variable rate will provide a quicker ROI if fields have a lot of variability in soil types, textures or organic matter contents,” Larson explains. “Additionally, it will likely ROI faster if fields are larger as it increases the likelihood of more variability.”

Individual samples on smaller acreage can help address some of the variability because there are more samples and smaller fields can be managed similar to zones of a much larger field, Larson adds.

University of Wisconsin-Platteville research proved precision nutrient management based on soil testing resulted in a 10% to 15% reduction in fertilizer costs while either maintaining or increasing crop yields.

Additionally, soil testing and subsequent nutrient management resulted in a 10% to 15% increase in corn yields and a 5% to 10% increase in soybean yields as shown in a Purdue University study. (2)

As growers continually put their best foot forward to maximize ROI and sustainability, starting from the ground up with soil testing can be key to long-term profitability.

This article was written by Trust In Food in collaboration with Bayer.

Resources:

1. G1740, Guidelines for Soil Sampling, University of Nebraska, https://extensionpublications.unl.edu/assets/html/g1740/build/g1740.htm.
2. Purdue/Indiana Soil and Water_How to Understand and Interpret Soil Health Tests.pdf.