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
The 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.
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) is a micronutrient that is essential for cell wall formation and rapid growing points
within the plant, such as reproductive structures.
(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 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
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) 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.
Sulfur (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.
(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
One 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-).
Potassium (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+).
(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)
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.
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+.
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%
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.
We are all hoping for an early spring to allow for fieldwork and fertilizer applications prior to planting. There are
farmers across the region who were not able to get their fertilizer applied during the fall. For them, it will be important
to be prepared to take advantage of even the smallest application window this spring. It will be a challenge to let soil
properly dry out before beginning fieldwork, given the compressed fall that many of us encountered in 2018.
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.
©2019 The Mosaic Company. All rights reserved.
Related Reference Topics: