It’s also time to get back to the basics. Determining how often you should soil-sample and using “best practices” are key to a balanced crop nutrition program. In addition, weather patterns can greatly affect yield, opportunities for soil sampling, and fertilizer applications, so be timely with decisions and conscious of the narrow application windows on the calendar. At The Mosaic Company, we believe today’s high-yielding hybrids and varieties remove nutrients at a higher rate than in previous decades. These greater nutrient demands may require more frequent, and perhaps broader soil testing than past nutrition programs. And since pulling representative samples from the field is a delicate procedure, utilizing best-sampling techniques can improve the accuracy of your lab results.
When intensively managed crops are combined with unpredictable weather patterns, like many growers experienced in 2021, monitoring soil nutrient levels is especially critical. Sampling every two years, rather than four, coupled with high-level weather data and building a field-by-field summary of nutrient status can be beneficial for taking yields to the next level. Newer hybrids and higher yields require more nutrients than ever before and need them longer into the growing season. This means that it is critical to review the 4Rs (Right source, Right rate, Right time and Right place) to properly account for removal.
Conducting frequent and accurate soil tests will help in the long run because that information helps you to protect and maintain your largest asset, your land. Soil tests ensure that we are applying the right amount of fertilizer for high-yielding crops in the short term, as well as ensuring sustainability with future crop production in the long term.
The Foundation Is Fertility
With as much as 60 percent of yield dependent on soil fertility, the best growing seasons are built from the ground up. Regardless of how much is spent on other crop inputs, if you haven’t taken care of the foundation — soil fertility — it’s difficult to maximize yield.
Seventeen essential nutrients are required for optimum plant health. They are categorized into primary macronutrients, secondary macronutrients and micronutrients. The Law of the Minimum states that the nutrient present in the least relative amount is the limiting nutrient. In other words, even if all other nutrients are at their optimum level, it only takes one nutrient to be deficient to prevent achieving maximum yield potential.
Understanding soil-sampling and soil-testing concepts as well as your seed variety’s nutritional needs will help provide a basic knowledge of your soil’s crop- production power to drive increased yields.
Consideration Factors when Testing
The introduction of new-and-improved seed genetics, such as rootworm-resistant corn hybrids, suggests that we need to take a closer look and reevaluate the amount of nutrient uptake and removal in order to maximize yield. Studies have shown that new hybrids may need more nutrients and at later stages of growth than their conventional counterparts of the past.
Most growers collect topsoil samples 6–8 inches deep for immobile nutrients like phosphorus and potassium, but there may be situations in which deeper samples should be taken. For instance, mobile nutrients can move down through the soil profile with heavy rainfall, heavy irrigation and in coarse-textured soils. Deeper sampling of 0–12 inches or 0–24 inches may be required to properly account for the mobile nutrients of nitrate nitrogen (NO₃-–N), sulfate sulfur (SO₄²⁻–S), boron (B) and chloride (Cl–). Immobile nutrients are typically found in the soil near the surface (0–6 inches), so a shallower soil sample can boost results in this area. There are also crop-specific situations with alfalfa or other forage crops in which soil labs may recommend a shallower or deeper soil sample than the standard 6–8 inches of top soil. Soil test laboratories provide excellent guidelines to achieve best results.
With soil sampling, the benefits of whole-field, grid and zone testing vary. Your choice should depend on what your needs and goals are based on the results.
Whole-field composite sampling is the most traditional approach, and the results indicate the average nutrient value from an entire field. This generally requires eight to 12 cores to be randomly collected across the whole area. With this strategy, all of the highs and lows from this sample area are mixed together in one bag for analysis. Going one step further, management zone sampling generally groups three to five areas of like soil types or production potential within one field. Each zone is then managed as an independent field. Grid sampling manages fields on an even smaller level, with grid sizes of 1 acre, 2.5 acres, 5 acres or 10 acres. Cores are pulled from the center of each grid, and lab results are tied back to the latitude and longitude associated with that center point. Precision ag software can estimate the values for un-sampled areas to ensure that a soil test value for every area of the field exists. Although grid sampling plays an important role in establishing a baseline across a field, with the appropriate data management, zone sampling can be equally effective in certain soil types and situations. Farm equipment now comes with enhanced technology, including yield monitors and field data collection devices. These technologies use imagery, along with layering in the data collected within the software to help determine management zone delineation. This creates more precise soil-sampling practices.
Depending on the field you are sampling, it might be a benefit to conduct zone or grid sampling to narrow in on specific areas of a larger field. These more detailed sampling methods can cost more, but can be advantageous if used correctly. At a minimum, make sure that you are conducting a whole-field soil test every two years.
Soil test results are only as good as the quality of the soil sample. While it may seem obvious, it is important to stick with basic best practices:
- Use a quality soil-sample probe rather than a spade.
- Pull a minimum of eight to 12 cores to produce a representative sample of each area of interest (e.g., entire field, management zone or grid).
- Always pull core samples from a consistent depth. Standard topsoil depths include 6, 8 and 10 inches.
- Do not angle sample probe when collecting cores. The probe should be placed at a 90-degree angle to the ground.
- Mix sample cores in a clean bucket (galvanized can affect results), and place in a properly labeled soil test bag — one bag for each field/area.
- Write down the crop, a realistic yield goal and other pertinent information requested by your soil test laboratory.
Understanding Nutrient Levels
It is important to understand the basics of soil test levels. Soil levels are categorized from very low (VL) to very high (VH). When nutrient values fall into the range of the VL and low (L) categories, they are considered below the critical level (CL). This means additional fertilizer applications are needed to correct the nutrient deficiency and to push soil test levels into the optimum range. Values and corresponding categories can vary for each nutrient by lab and state, and account for varying agronomic, economic and environmental considerations.
Nutrient levels in the VL category often require a build + maintenance approach in order to bring soil test levels into a more acceptable range for peak production. After soil test nutrients are brought into the optimum range, realistic yield goals can be coupled with removal values to calculate a maintenance nutrient recommendation.
Once soil test levels are between the high (H) and VH categories, special considerations should be taken with fertilizer application. In most cases, applications should be reduced or eliminated for proper environmental stewardship.
Soil Test Analysis
Once you get a soil test report from the lab, the first thing you should look at is the soil pH level. Make sure that it is still in the optimum range for your upcoming crop rotation. Most crops do best with a 6.0 to 7.0 pH, but for alfalfa and other forage crops, a 6.5 to 7 pH level is preferable. A basic soil pH test is conducted by a soil test lab, in which one part soil is mixed with one part water, and then measured with an electrode. When soil pH is low, a buffering solution is added to the sample, allowed to react, and the pH is then measured again. This value tells us the capacity of the soil to change its pH. If the difference between the soil pH and buffer pH is large, the soil pH is easily changed, and will require a smaller rate of liming material. If the soil pH has changed only slightly after the buffer solution has reacted, then the soil pH is harder to change, and more lime will be required.
The organic matter (OM) value on the soil test report is also important to review, because OM is a nutrient reservoir and buffering mechanism for the soil. Often, labs will use percent OM to calculate the nitrogen (N) and sulfur (S) that may be available throughout the season. Increasing levels of OM aid soil structure, water-holding capacity, mineralization, biological activity, and water and air infiltration rates.
Soil test values for immobile nutrients like phosphorus (P) and potassium (K) are analyzed and treated much differently than those for mobile nutrients. Unlike mobile nutrients such as N and S, for which the total amount in the soil is read, immobile nutrients require the use of extractants (chemical solutions to mimic root and soil processes) to simulate nutrient availability throughout the growing season.
Some extractants and methods are better suited for particular nutrients in the soil. For P, Bray-1, Bray-2, Mehlich-3, or sodium bicarbonate (Olsen-P) are typically used; and for K, ammonium acetate, modified Morgan, sodium acetate or Mehlich-3 are typically used. The nutrient-extraction process not only measures current nutrient availability, but also estimates nutrient availability throughout the season.
Cation Exchange Capacity (CEC) is the amount of positively charged cations that can be held by a given weight of soil, and can greatly affect nutrient mobility and uptake within the soil. Base cations include Ca²⁺, Mg²⁺, K⁺ and Na⁺. Oftentimes, a percentage of each of these base cations is represented in the report, and also in relationship to the total CEC. Other ions that may be listed in a soil test report are H⁺, NH4⁺, Al3⁺ and Mn2⁺.
Soil in general has a negative charge. When there are more negative charges, soil can hold more positive cations, such as hydrogen, magnesium, calcium, potassium, ammonium and sodium, which can attach like magnets.
All of these components are important factors in the soil analysis report and are worth the time to analyze. If you need help, ask your lab technician to help you understand the meaning behind the numbers.
The Bottom Line
With today’s high-yielding hybrids, seed varieties are removing more nutrients from the soil than previous conventional hybrids. Nutrient availability can vary greatly by soil type, and is also impacted by unpredictable weather patterns. Knowing this, farmers need to monitor nutrient removal more closely, while being aware that fluctuations can occur in the soil rooting zone.
I would encourage growers to consider your soil-sampling frequency. If you have been soil-testing every four years, ask your agronomist whether sampling every two years is preferable. Local retail agronomists and soil test lab personnel provide excellent resources for reviewing and recommending the necessary nutrients and best sources to meet yield goals. Remember that proper and consistent sampling, along with providing a realistic yield goal and necessary crop information are key to getting the most accurate nutrient recommendation.