Past practices of soil sampling every four years may not be enough in high-yield systems. High-removal systems may warrant sampling every two years. Consulting with a retail provider, consultant and soil-testing laboratory can help ensure proper fertilizer sources, rates and help monitor fluctuations in soil test levels over time.
Mobile vs. Immobile Nutrient Samples
Most growers collect topsoil samples that are 6 to 8 inches deep for routine phosphorus and potassium analysis, but there are situations in which deeper samples should be taken. For instance, mobile nutrients can move down through the soil profile with heavy rainfall or coarse-textured soils. Nutrients such as nitrate nitrogen (NO3-N), sulfate sulfur (SO4-S), boronBoron (B)Boron (B) is a micronutrient that is essential for cell wall formation and rapid growing points within the plant, such as reproductive structures.
Learn More (B) and chloride (Cl) may require deeper sampling. 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 to 8 inches of topsoil.
Modernizing Your Strategy
Traditional soil sampling represents nutrient values from the whole field, and requires 8 to 12 cores to be collected across an entire area. With this strategy, all of the highs and lows from sampling are mixed together in one bag for analysis. Although traditional methods of soil sampling (whole-field composite sample) may include some variation, it is still a common practice, as shown in the graph below.
Types of Soil-Sampling
Source: 2015 Precision Ag Services Dealership Survey
Grid sampling is more repeatable and strategic than traditional soil-sampling strategies, and can help establish an unbiased soil test approach. The most common grids are 1-acre, 2.5-acre, 5-acre or 10-acre. Cores are generally pulled from the center of the grid, and lab results are tied back to each latitude and longitude associated with that center point. Precision ag software can mathematically 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 baselines across a field, with appropriate data management, zone sampling can be equally effective in certain soil types and situations. Harvest equipment now comes with enhanced technology, including yield monitors and field data collection devices that help determine and create these management zones for more precise sampling. Imagery and other data can be layered within the software to help determine the management zone delineation.
Once you get a soil test report back, make sure that soil pHSoil pHSoil pH is a measure of the acidity and alkalinity in soils. pH levels range from 0 to 14, with 7 being neutral, below 7 acidic and above 7 alkaline. The optimal pH range for most plants is between 5.5 and 7.0; however, many plants have adapted to thrive at pH values outside this range.
Learn More is still in the optimum range for your upcoming crop rotation. Most of our cropping situations do best with a 6 to 6.5 pH, but for alfalfa and other forage crops, a 6.5 to 7 pH level is preferable. Soil pH is a basic test 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 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 (O.M.) value on a soil test report is also important to review because it is a nutrient reservoir and buffering mechanism for the soil. Often, labs will use percent O.M. to calculate the nitrogenNitrogenNitrogen (N) is an essential macronutrient for plant growth, development and reproduction. Despite nitrogen being one of the most abundant elements on earth, N deficiency is probably the most common nutritional problem affecting plants worldwide – N from the atmosphere and earth's crust is not directly available to plants.
Learn More (N) or sulfurSulfurSulfur (S) is a part of every living cell and is important to the formation of proteins. Unlike the other secondary macronutrients like calcium and magnesium (which plants take up as cations), S is absorbed primarily as the SO42- anion. It can also enter plant leaves from the air as dioxide (SO2) gas.
Learn More (S) that may be available throughout the season. Increasing levels of organic matter aid soil structure, water-holding capacity, mineralization, biological activity, and the water and air infiltration rates.
Soil test values for immobile nutrients like phosphorusPhosphorusOne of three primary macronutrients, phosphorus (P) is essential for plant growth, and a plant must access it to complete its normal production cycle. Plants absorb P from the soil as primary and secondary orthophosphates (H2PO4- and HPO42-).
Learn More (P) and potassiumPotassiumPotassium (K) is one of the essential macronutrients nutrients and is taken up in significant amounts by crops. Potassium is vital to photosynthesis, protein synthesis and many other functions in plants. It’s classified as a macronutrient, as are nitrogen (N) and phosphorus (P). Plants take up K in its ionic form (K+).
Learn More (K) are analyzed and treated much differently than those for mobile nutrients. Unlike mobile nutrients like 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.
The soil test value of immobile nutrients is not a measure of the total quantity of nutrients in the soil, but simply what will be available within a growing season. Some extractants and methods are better suited for particular soils. For Phosphorus, Bray 1, Bray 2, Mehlich 3, or Sodium Bicarbonate (Olsen) 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)Cation Exchange Capacity (CEC)CEC helps to explain why certain fertilizer elements such as positively charged potassium (K+), calcium (Ca2+) and magnesium (Mg2+), as well as ammonium nitrogen are not as easily leached from the soil as the negatively charged ions, or anions, of nitrate nitrogen, sulfates or chlorides.
Learn More 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+. Often, a percentage of each of these base cations is represented on the report, and then in relationship to the total CEC. Other ions that may be listed on a soil test report are H+, NH4+, Al and Mn. 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 report, and are worth the time to analyze or ask your lab technician for help with when it comes to understanding the meaning behind the numbers.
CEC Soil Analysis
CEC of Soil Types
0 to 10
Coarse Loams (Medium)
11 to 20
Fine Loams (Medium)
20 to 30
Clays/Clay Loams (Fine)
if OM > 20%
What’s the Bottom Line?
Today’s high-yielding hybrids and varieties remove more nutrients from the soil than previous conventional hybrids. This requires us to more closely monitor nutrient removal and fluctuations that can occur in the soil profile. I would encourage most growers to soil-sample more frequently. If you are currently soil-testing every four years, discuss the value of sampling every two years with your retailer or crop consultant.
If your advisor does not have local nutrient recommendations tailored to your region, most, if not all, soil test labs will provide you with nutrient recommendations based on the intended crop, yield goal and current soil test value. Remember to be attentive when it comes to providing a realistic yield goal and any other information in order to achieve the most accurate nutrient recommendation.
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