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How to unlock the secrets in soil test data

Soil test results are often a mystery. Farmers mainly look at fertility recommendations for nitrogen (N), phosphorous (P), and potassium (K) with little thought to the remaining test results. But the rest of the numbers tell a lot about the field.

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“A soil test is the foundation of your cropping system,” says Brian Arnall, precision nutrient specialist at Oklahoma State University. Soil test components can give a fairly complete DNA of a soil’s function and capability to produce crops, he adds.

Let's take a look.

A Soil Test Roadmap

The sample results accompanying this story were taken from a 160-acre field, which was sampled in 2.5-acre grids at a 6-inch depth. Each sample contains eight “cores” taken with a soil sampling tool. (For simplicity’s sake, we used the field average of all 63 samples in the test results listed below.)

Soil Test Results

Field Average (63 samples taken)

5.7 pH
3 Organic Matter
63 Phosphorous Bray 2
30

Phosphorous Mehlich 3

275 Potassium
12 Sulfur (parts/million)
20.1 Cation Exchange Capacity
55 % Calcium
1.2 % Sodium
3.5 % Potassium
17.4 % Magnesium

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“The nice thing about grid sampling is the resolution,” says Landon Oldham, owner of Heartland Soil Services in Cunningham, Kansas. “Rather than taking a grainy snapshot of the field’s fertility, the high-resolution picture gives much more detail.”

Grid sampling may seem expensive until you amortize that $10-an-acre cost over the four years a soil test lasts. 

“That’s $2.50 per acre per year, giving you the information to make hundreds of thousands of dollars’ worth of input decisions,” Oldham says.

Consider this: After soil test laboratories dry and grind soil samples, they will use just 10 grams from each bag of soil, so it is imperative that samples are representative of the area being tested.

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The Basics: pH and Organic Matter

Soil pH is the degree of soil acidity or alkalinity a soil has, based on a graduated scale. A pH of 7.0 is neutral. Below 7.0 is acidic; above it is alkaline. If soil pH varies too far from neutral, other soil nutrients are not as readily available.

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Commercial fertilizer and herbicide also are optimized when soil pH is balanced, says Noah Goza, soil fertility specialist with Heartland Soil Services. “Our goal is to be between 6.0 and 7.0 on the pH scale, which is usually the prime area for release of all the other nutrients,” he says.

If soil pH is too low, ag lime (calcium carbonate) may be applied to reduce the acidity, like taking a Tums to reduce heartburn. If soil pH is too high, sulfur is the answer.

A soil’s organic matter is an indicator of how much living material is in the soil. Organic matter is 58% carbon and holds water and plant nutrients.

Most soil organic matter ranges from 1% to 5% of soil by weight, with each percent containing 1,000 pounds of nitrogen, 220 pounds of phosphorous, and 150 pounds of sulfur in the top 6 inches of soil per acre, according to Ray Ward, founder of Ward Laboratories. Balancing pH to neutral — or between 6.2 and 7.2 — is the first priority.

“It’s the first thing farmers should strive to do,” Oldham says. 

Cation Exchange Capacity and Base Saturation

The cation exchange capacity (CEC) is a chemical property of each soil, measuring the soil’s ability to hold and exchange cations, or elements with a positive charge.

Cation Exchange Capacity

and Soil Texture

Sands 3 to 5
Loams 10 to 15
Silt loams 15 to 25
Clay and clay loams 20 to 50

“The soil is a net negative charge and is attracted to positively charged things,” Arnall explains. Nutrients containing a positive charge include calcium, magnesium, ammonium, plus sodium and potassium. “We want to have as much CEC as possible because we want to hold onto as many of those plant nutrients as possible.”

In a soil test, CEC values are given in milliequivalents per hundred grams (meq/100g), but Oldham suggests looking at CEC as a “fuel tank.”

“The larger that fuel tank, the more nutrients and water the soil can hold,” he says.

Why is CEC important? This is where high school chemistry comes in handy. You may have thought you would never need to know about an element’s positive and negative charges, but soil chemistry is full of them.

“CEC is a measure of the soil’s ability to hold positively charged ions, including magnesium, potassium, sodium, and hydrogen,” Oldham explains.

In our soil test sample, the average CEC, or fuel tank, is 20.1. That’s the total of negatively charged sites, where positively charged cations can bind. The percent of each of those cations is “base saturation.” Therefore, if 55% of the CEC is occupied by calcium, that is the calcium base saturation.

“Base saturations and pH are closely linked,” Goza says. “If the base saturation of hydrogen is high, soil is more acidic. If the others are high,  then the soil will be more basic. Base saturation is just the percentage of each of those nutrients. We talk about base saturations when we are talking about the percentages in general, but there is not just one base saturation for a soil.” 

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Potassium, calcium, magnesium, and sodium percentages should add up to at least 70% or more, says Ward.

“Hydrogen should be less than 30%, and if sodium is greater than 5%, there is a problem,” he says.

If hydrogen is excessive, applying ag lime is necessary. Think about basic chemistry: Calcium has two positive charges; hydrogen has one positive charge. The calcium will drive hydrogen away, Oldham says.

Base saturation percentages should be taken in context with the soil test report’s parts per million values. In the example soil test, K ppm appears to be very high. However, because the soil is a silt loam, the percent K in the base saturation is ideal.

“That tells me the soil is fairly heavy, and potassium base saturation is about right,” Oldham adds. Here are the ideal base saturation percentages, according to Michigan State University scientists:

  • Calcium: 60% to 80%
  • Magnesium: 10% to 20% (the heavier the soils, the lower the amount)
  • Potassium: 2% to 6%
  • Hydrogen: 10% to 15% (the higher the hydrogen, the higher the pH)
  • Others (iron, manganese, copper, zinc, and sodium): 2% to 4%

A soil test report also details levels of other macro- and micronutrients, usually with a guide of how much of each to apply for the next crop.

Nutrient Needs Per Crop Unit

(pounds per bushel or bale)

Nitrogen

Phosphorous

(P2O5)

Potassium

(K2O)

Magnesium Sulfur

Corn

1.1 0.36 0.25 0.08 0.08
Wheat 2.4 0.48 0.29 0.15 0.1
Soybeans 4.8 1 1.2 0.18 0.18
Cotton 50 14 19 3.5 n/a

Nitrogen (N)

Nitrogen (N) fuels photosynthesis, which converts sunlight to power growth and development. Nitrogen also is a prime ingredient in the amino acids within plant structures, which are the building blocks in cell development. Plants use a form of nitrogen called nitrate (NO3), which must be converted from commercial fertilizers by microorganisms in the soil. Nitrate is soluble and will readily move through the soil with water, Ward says. Some soil test reports don’t contain an N recommendation because it is soluble and may leach by the time the next crop is planted. 

Phosphorous (P)

Phosphorous stimulates root growth and improves stalk and stem strength. Growers often tend to apply too much P. It is needed at a much lower rate than nitrogen. It is attached to soil particles and moves very slowly — just 0.75 inches per year in silt loam soils and 2 inches in sandy soils, Ward says.

Potassium (K)

Potassium activates dozens of enzymes that influence photosynthesis, energy metabolism, nitrate reduction, and more. Potassium is vital when helping plants cope with weather stress. In dry conditions, potassium reduces water loss from leaf tissue and increases the ability of root cells to take up soil water. In a soil test, potassium levels are indicated by parts per million. However, growers should look at the percent base saturation, Goza says.

“Potassium levels depend on how heavy the soil is. Soil with more clay needs more potassium, while a lighter, sandier soil needs less potassium and can hold less potassium,” he explains.

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Other Important Nutrients

Here are other nutrients common to a soil test report and their function in plants.

Calcium (Ca)

Essential in early root growth, Ca boosts the strength of cell walls and is required to regulate nutrient uptake and movement throughout the plant.

Magnesium (Mg)

Mg helps seed germination and the seedling’s ability to use phosphorous.During photosynthesis, it is essential in chlorophyll production.

Sulfur (S)

In the days before Environmental Protection Agency clean air requirements, S was essentially a free nutrient deposited during rainfall events. Now, however, growers are finding that supplemental S applications are necessary. Sulfur is a critical component of amino acids and proteins, helps with plant sugar production, and stabilizes nitrogen. It is leachable.

Zinc (Zn)

Needed for plant development, including growth hormone production and internode elongation, Zn also helps calcium translocation.

Boron (B)

Essential for early growth and protein tube formation along with healthy plant cells, boron is leachable.

Copper (Cu)

Enzyme function depends on adequate Cu. The element also is necessary for photosynthesis and seed development.

Iron (Fe)

Plant greenness requires adequate Fe; deficient plants show “iron chlorosis,” or interveinal yellowing of leaves with eventual dieback and possible plant death.

Manganese (Mn)

Necessary for formation of sugars and starches, Mn also helps the crop use nitrogen more efficiently. Mn values appear to be getting lower in soils, Ward says. An in-season tissue test could prove to be valuable, Goza adds.

Rich, black soil in the hand of a farmer
Gil Gullickson

Too Much Is Never Enough

Conventional wisdom holds that a four-year rotation between grid sampling of soils is frequent enough. Antonio Mallarino, fertility specialist at Iowa State University, disagrees.

“Come on. That’s old stuff,” says Mallarino, who advocates testing every two years. “When prices are high, investing in good soil sampling to know what to cut and what not to cut is important,” he adds.

He emphasizes good soil sampling. Eight to 10 samples per 2.5- or 5-acre grid are ideal, he says. Growers who don’t want to invest that much time or money on grid sampling should use a zone sampling approach, based on yield maps. Pulling soil cores can be monotonous but is the most critical component of soil sampling. Errors in depth and mixing dramatically impact the amount of money spent on fertilizer.

An Iowa farmer kneels to dig in the soil on his farm
Photo credit: Iowa Corn

Which Phosphorous Test?

Phosphorous is such an important nutrient in crop production that it must be tested. Farmers may choose from three tests when selecting a soil testing package: Mehlich 3, Bray, and Olsen. Each determines how much phosphorous is in the soil, ready to be taken up by plants.

Which one is right for you? The short answer: It depends. Each test is designed to work in a specific soil pH. They all use liquid solution to extract phosphorous from the tested soil. The readings, however, may be different depending upon the soil pH, says Brian Arnall, precision nutrient management specialist at Oklahoma State University.

  • Bray: best for neutral soils (pH of 6.5 to 7.5) or acidic soils (below 6.5 pH)
  • Olsen: best for calcareous soils, with pH of 7.5 or greater
  • Mehlich 3: best for slightly acidic soils, or neutral soils

“In a neutral soil, they’re all OK,” Arnall explains. “If I get a soil sample that comes back at a pH of 6, I’m going to look at Mehlich or Bray,” he continues.

“If the pH is above 7.4, I ignore those two and focus on the Olsen test,” Arnall says.

Some laboratories will adjust the phosphorous test based on the soil pH reading. However, growers should have an idea of the best phosphorous test based on field location. Here’s a little secret: Most laboratories will focus on one of the available tests but can conversion to give a score on another.

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