It’s time to take the guesswork out of crop nutrition planning. This starts with asking yourself a couple of important questions:

Are you testing your soil frequently enough?

At The Mosaic CompanyThe Mosaic CompanyThe world's leader in crop nutrition, dedicated to exploring and advancing crop nutrition knowledge and practices.
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, we believe today’s high-yield systems, which come with greater nutrient demands, require more frequent, and perhaps broader, soil testing than the traditional approach.

Are your soil-sampling practices providing the most accurate results?

Pulling representative samples from the field is a delicate procedure, and ensuring best sampling techniques, can drastically impact the results from the lab.

Conducting frequent and accurate soil tests helps in the long run, because the information provided helps to protect and maintain our farms and other large investments. When combined with routine fertilizer applications, soil tests ensure peak performance for the short term and help maintain the land for 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 base — soil fertility — it’s difficult to maximize yield.

There are 17 essential crop nutrients. Nutrients that are required for optimum plant health 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-testingSoil-testingThe purpose of soil testing in high-yield farming is to determine the relative ability of soil to supply crop nutrients during a particular growing season.
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concepts will help provide a basic understanding of your soil’s crop production power to drive increased yields.

Soil-Testing Approaches

Soil-testing categories range from Very Low (VL) to Very High (VH). When values fall into the VL and Low (L) ranges, nutrients must be applied to correct the nutrient deficiency. Soil test values that fall into this area of the curve are said to be below the critical level (CL), and need additional fertilizer applications in order to push soil test levels into the optimum range. These categories in relation to percent yield are displayed in Chart 1. Values for each category often vary by state, and account for various levels of agronomic, economic and environmental considerations.

Levels in the VL category displayed in Chart 1 often require a Build + Maintenance approach in order to bring soil test levels into more acceptable ranges 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 values are between High (H) and Very High (VH) levels (dashed-line B), special considerations should be taken with fertilizer application. In most cases, applications should be reduced or eliminated for proper environmental stewardship.

Chart 1. Soil-Testing Concepts

Soil Testing Graph

Sampling Basics

Soil test results are only as good as the quality of the soil samples. 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 8 to 12 cores to produce a representative sample of each area of interest (e.g., entire field, management zone, or grid). Pulling six to eight cores per grid in grid-sampling situations is common.
  • Core samples should always be pulled 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° angle to the ground.
  • Mix sample cores in a clean plastic bucket (galvanized can affect results), and place in a properly labeled soil test bag — one for each field/area. Write down the crop, realistic yield goal and other pertinent information as requested by your soil test laboratory.
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As a general rule of thumb, the more samples you take and the more consistently you take them, the better the analysis you will receive from the lab.

As a general rule of thumb, the more samples you take and the more consistently you take them, the better the analysis you will receive from the lab.

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.
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(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

Types of Soil Testing

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.

Soil Analysis

Soil Analysis

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.
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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.
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(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.
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(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-).
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(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+).
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(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.
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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 Soil Analysis CEC Soil Analysis
CEC of Soil Types
Estimated Texture CEC
Sands (Coarse) 0 to 10
Coarse Loams (Medium) 11 to 20
Fine Loams (Medium) 20 to 30
Clays/Clay Loams (Fine) 31+
Peat/Muck if OM > 20%
CEC Soil Analysis

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.

©2016 The Mosaic Company. All rights reserved.

by Curt Woolfolk

Curt Woolfolk is a senior agronomist with The Mosaic Company. His role involves developing the North American agronomic strategy to support Mosaic's commercial team.

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