Soil Sampling: Understanding Its Importance & Methods

Make Better Farm Management Decisions With The Results Of Regular Soil Sampling

Producers have many resources they can use to grow outstanding crops. One of the most important is the soil on their farm. To get a complete picture of how soil is doing, you have to look at what’s happening in the ground. One way to do so is with soil testing.

Don’t guess. Soil test.

Getting your soil tested takes the guess work out of your decision making. When a good quality soil test is employed on a farming operation, it answers a wide range of soil characteristics and data points. This includes pH, organic matter, CEC, base saturation of nutrients and microbial activity.

The process and specific metrics vary depending on which type of soil test is used. When done right, soil sampling can provide a representative snapshot of fertility and soil health across an agricultural field–providing growers with critical information to make their best management decisions.

This article covers:

• Why is soil sampling important?
• Four key measurements in soil testing
• Why pH is a critical soil sample measurement
• How to take a soil test
• How to integrate soil test results into input strategies
• Consulting with agronomy experts for soil test interpretations

Why is soil sampling important?

Soil sampling is important because it measures a broad scope of chemical, biological and physical soil characteristics, giving farmers greater insight into the vitality of their soils and cropping systems.

Well-balanced soil provides a wide host of benefits, including:

• Improved input and water-use efficiency
• Reduced soil erosion
• Better soil structure for greater physical stability
• Improved soil aeration
• Higher water drainage and retention
• Greater nutrient absorption
• Increased biological productivity of a field
• Increased yield / net productivity

Soil changes generally occur over the span of several years. It is often difficult to track and identify small changes, but a strong soil sampling strategy can help to capture the variation in soil quality and fertility over time.

One of the most critical elements that testing can reveal is the ability of the soil to support microbial activity. Also known as “microorganisms,” soil microbes perform several complex functions that are essential to crop productivity including:

• Providing free nitrogen to legumes
• Breaking down crop residues
• Cycling nutrients through the soil
• Enhancing water and nutrient uptake
• Stimulating plant growth

Not all microbes are created equal. Fertile soils require a balanced presence of beneficial microbes and an absence or low presence of harmful microbes.

LEARN MORE: Soil Microbiology 101

Four Key Measurements in Soil Testing

There are also several types of soil tests used to analyze the many soil components and properties that may impact crop development. The majority of standard soil tests often focus on four key measurements:

1. pH: Soil pH measures the acidity or alkalinity of the soil, which is critical for nutrient solubility and plant uptake. Most crops perform best in a slightly acidic environment—typically between 6 and 7—ensuring that essential nutrients are available in forms plants can absorb. Monitoring pH also helps guide the application of lime or sulfur to maintain optimal growing conditions.
2. Soil Organic Matter: Soil organic matter is made up of decomposed plant and animal residues that enrich the soil, improve water retention, and enhance soil structure. Higher organic matter levels boost microbial activity and nutrient cycling, which are key for maintaining long-term soil fertility. Tracking organic matter trends over time can signal improvements or declines in overall soil health.
3. Cation Exchange Capacity (CEC): CEC indicates the soil’s ability to hold and exchange positively charged nutrient ions like calcium, magnesium, potassium, and sodium. Soils with a higher CEC can retain more nutrients, reducing the risk of leaching and providing a stable nutrient reserve for crops. This measurement is essential for fertilizer planning, as it reflects the soil’s buffering capacity and its potential response to amendments.
4. Base Saturation: Base saturation expresses the percentage of the soil’s CEC that is occupied by essential base cations—calcium, magnesium, potassium, and sodium. A balanced base saturation generally means improved nutrient availability and better overall soil fertility, while imbalances may signal the need for corrective amendments. Adjusting base saturation through practices like liming can optimize nutrient ratios and enhance crop performance.

In addition to pH, soil organic matter, cation exchange capacity (CEC), and base saturation, soil tests often measure essential nutrients like phosphorus, potassium, calcium, and magnesium, as well as micronutrients such as zinc, manganese, and boron. Other common assessments include soluble salts to monitor salinity and overall soil health indicators that impact plant growth and productivity.

pH: A Critical Soil Sample Measurement

One of the most important measurements that a soil test can give a grower is where the soil solution is on the pH scale. Soil pH is a foundational measurement that influences nearly every aspect of crop performance, from nutrient availability to pesticide effectiveness.

A pH test is simple yet effective, and can give growers three critical insights:

1. Whether fertilizer nutrients will be available or tied up in the soil (see Figure 1)
2. If pesticides will function as intended
3. Whether beneficial microorganisms can survive and thrive (see Figure 2)

pH is just the inverse measurement of hydrogen ions in the soil. Too many hydrogen ions result in a low (more acidic) pH, while few hydrogen ions create a high (more alkaline) pH.

Even small changes in pH can have a significant impact on soil chemistry. The pH scale is logarithmic, meaning each whole-number change represents a tenfold difference in hydrogen ion concentration. For example, a soil with a pH of 5 is ten times more acidic than one with a pH of 6, and 100 times more acidic than a pH of 7.

Figure 1:This chart shows the importance of pH in the availability of nutrients in the soil. www.extension.purdue.edu/extmedia/ho/ho-240-w.pdf

How to Take a Soil Test

Farmers know that soil composition and fertility can vary substantially across the fields of an individual farm. Physical soil samples provide the most comprehensive and accurate data for site specific soil evaluations. The question of when, where, how deep, and how often you should sample your soil remains a running debate among soil scientists. Here’s a general guide to soil sampling steps:

1. Identify and Separate Soil Types: Recognize that soil properties can vary across your fields. Divide your land based on differing characteristics, such as areas with high organic matter versus those with low levels, and sections with varying cation exchange capacities (CEC). Sampling these areas separately ensures that the test results accurately reflect each zone’s unique properties.
2. Choose Proper Soil Sampling Depth: Collect soil samples to a depth that corresponds with the root zone of the crop. According to Perdue University, a depth of 6 to 8 inches is typically recommended for general crops.
3. Determine Sampling Density and Spacing: For each distinct soil area, collect 6 to 8 soil cores using a probe. Ensure that each bagged soil sample represents no more than 10 acres. Spread the sampling points out evenly across the designated soil type area. This systematic approach ensures that the composite sample provides an accurate representation of the entire zone, leading to more reliable soil test results.
4. Collect Representative Soil Samples: Use a soil probe pole to extract thin slices or cores from the soil surface to the determined depth. Place all subsamples from a specific area into a clean, plastic bucket, avoiding contamination from tools made of brass, bronze, or galvanized materials.
5. Ensure Proper Soil Sample Storage: Avoid using plastic bags for storing soil samples, as they can promote mold growth due to moisture retention. Instead, use breathable bags, such as paper bags, to allow moisture to escape and prevent mold development. Avoid contamination from tools made of brass, bronze, or galvanized materials.
6. Select a Reputable Soil Testing Laboratory: Choose a laboratory that participates in recognized proficiency testing programs to ensure accurate results. Examples of reputable soil testing labs include A&L Great Lakes Laboratories, Waters Agricultural Laboratories in Camilla, Georgia, Agri-Labs, Inc. and Brookside Laboratories, Inc.

By following these steps, growers can obtain accurate soil analyses, leading to informed decisions that enhance soil health and crop productivity.

Integrating Soil Test Results into Input Strategies: Soybean Example

Soil test results can significantly impact input strategies. One instance is nitrogen fixation for crops like soybeans. Soybeans require 4 unit of nitrogen to grow a bushel, which equates to around 240 units of nitrogen per acre for a 60-bushel crop. Unlike corn, they don’t need direct nitrogen fertilizer. Why? Because they rely on a beneficial bacterium called Bradyrhizobium japonicum. These little microbes form a symbiotic relationship with the soybean plant, providing up to 90% of the nitrogen needed.

A problem occurs when soil pH is out of balance. If the soil pH is too high or low, it affects the activity of Bradyrhizobium japonicum. In these instances, double inoculation is needed to achieve the correct population of these beneficial microorganisms.

Double inoculation is especially necessary if the soil has been stressed in recent years. Soil stress can come in the form of drought, floods, overheated topsoil, soil erosion, low organic matter (less than 1%), use of chemicals not friendly to Bradyrhizobia japonicum, pH extremes or not having soybeans on the ground in a few years.

READ MORE: 8 Reasons When and Why to Double Inoculate

By testing soil pH and understanding these dynamics, growers can make informed decisions about inoculation, ensuring they achieve the correct rhizobia population for optimal nitrogen fixation and crop productivity.

Consulting Agronomy Experts for Soil Test Interpretation and Input Strategy

The results from soil sampling will likely affect management and input decisions, such as the use of biological soil amendments. However, interpreting soil test results can be complex, and understanding how to leverage that information for optimal crop performance requires expertise.

Locus Agriculture (Locus AG) is an example of a biological supplier that goes beyond by providing access to an agronomy team dedicated to helping farmers interpret their soil test results. This personalized support allows them to develop tailored biological programs that not only improve soil quality but also boost crop productivity.

Locus AG’s agronomy experts work closely with growers to understand the nuances of their soil, recommend specific biological solutions and integrate those recommendations into a comprehensive farm management strategy. This hands-on guidance ensures that growers can make informed decisions, optimizing their inputs and ultimately achieving better yields.

By leveraging soil test results and working with knowledgeable agronomy professionals, farmers can ensure their soil is in optimal condition and make precise management decisions that ultimately lead to more productive crops.