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Do You Really Know Your Soil?

Get the inside scoop.

Ever been told by your doctor that you need to lower your blood pressure or cholesterol? Without a physical exam or a blood test, you wouldn’t have a clue these steps are needed.

Ditto for your crop fields.

They may be low in phosphorus. Acidic soils in need of lime may be crimping yields. Fields could even be teeming with pests like soybean cyst nematode (SCN).
Without a soil test, though, you just don’t know.

“Soil sampling helps you make decisions based on what is currently in that field,” says Fabian Fernandez, University of Minnesota Extension nutrient specialist. “If you don’t know what the fertility in the field is and you’re applying a standard rate, you could be underapplying or overapplying fertilizer.”

Underapplication of vital nutrients can pinch your pocketbook by lowering yields and returns. Environmentally, overapplication can put you in the crosshairs of regulators monitoring fertilizer runoff into rivers, lakes, and streams. By religiously soil-testing his 50/50 corn and soybean farm, John Motter is able to avoid both ends of the application spectrum.

“Dad raised me to leave the farm better than when I found it,” says the third-generation farmer from Jenera, Ohio. “These inputs are too expensive to mismanage them."

Soil testing can also enable you to discover maladies (like SCN) that stealthily steal yields – often without your knowledge. “Soil sampling allows me to discover abnormalities I never knew about,” explains Motter. “The only way I know is to go out there and sample. It’s the right thing to do.”

Not An Automatic Cure-All

Don’t assume soil tests can give you all the answers, though. Soil health test results, unlike test results for nutrients and SCN, can be variable and difficult to interpret. “The most useful tool for evaluating soil health in your field is still your shovel,” says Abbey Wick, Extension soil health specialist at North Dakota State University.

Below you’ll read about soil sampling for nutrients, sampling for SCN, and soil health tests. You’ll also learn how you can get the most bang for your soil-testing buck.

Nutrient Soil Sampling

First, key in on the big three nutrients: nitrogen (N), phosphorus (P), and potassium (K). In regions with high precipitation, the fall soil nitrate test is not very useful for predicting availability the following spring, as the potential for loss is greater, says Fernandez.

For example, fall soil nitrate testing is only recommended in the western and northern parts of Minnesota, where there is lower rainfall and more fine-texture soils. If you have a lot of N left in your field, you know you overapplied, says Fernandez. In the southern and eastern parts of the state, the fall soil nitrate test is not recommended because of higher rainfall and coarse-texture soils more prone to N losses.

In those situations, the in-season soil nitrate test can be a more useful tool. “Sampling before sidedressing allows you to look at the amount of N in the soil and decide if you need to apply more or if there are adequate amounts,” says Fernandez.

Sampling for N preplant is less reliable because N is extremely moveable, says Fernandez. If there’s significant rainfall after testing, the results may not be accurate.

It’s important to test for both P and K. By doing so, you may find that one of the nutrients is at an adequate level while the other is not, says Fernandez. That’s an opportunity to save money on the adequate nutrient and invest in the inadequate one.

“Soil tests for P and K are not perfect, but they are useful, especially with unfavorable commodity prices,” says Antonio Mallarino, Iowa State University soil scientist. “Soil tests are a very small investment that can save a lot of money.”

Then there’s soil pH.

Optimal soil pH is essential to key proper nutrient cycling, soil microbial activity, and soil structure. If your soil pH is 6.5 or above, don’t worry. That’s a sufficient pH level for all crops, even alfalfa, says Mallarino. If you farm soils with high pH subsoils (calcareous), liming to raise soil pH to 6.0 for corn and soybeans is sufficient.

"Below those levels, though, lime is recommended. “If your soil pH is 5.4 or 5.5, you need to apply lime, even with current prices,” says Mallarino. On the other side, though, corn and soybean yield decreases can even result when soil pH is raised up to 7 to 7.4, he says.

How to Sample

Stick with a soil probe rather than a shovel, says Fernandez. Unlike a shovel, a soil probe will collect a uniform sample across the entire sampling depth. Collect samples 6 to 8 inches deep, depending on your state’s recommendations, says Fernandez. Sampling at the incorrect depth can lead to wrong interpretations. Timing is important, too, especially for K. Sample for P and K at the same time every year, as the values will change throughout the year, says Fernandez. Comparing fall results one year to spring the next year is like comparing apples to oranges.

“Typically for soil sampling, the more cores, the better,” says Fernandez. He suggests making a composite of at least 10 to 12 cores.

When collecting your cores, be sure to collect GPS data, says Fernandez. “It’s important to be able to go back to the same location to look for trends over time.”
Different fertilizer application methods will affect how samples should be collected.

When fertilizer has been broadcast, collecting random cores is sufficient. However, when fertilizer has been banded, for every sample you take in the fertilizer band, you need to take two or three cores outside of the band, advises Fernandez.

Move Beyond

One way to improve on classic sampling by soil-type method is with either grid sampling or zone sampling. A Sampling Efficacy Index examines the capacity of a sampling method used to identify field areas with different response to P or K. The higher the percentage, the more accurate it is.

Here’s how various sampling methods stack up based on Iowa State University analysis:

  • 0.3- to 0.5-acre grids: 100%
  • 2.5-acre grids: 50%
  • Zone sampling: 39%
  • Soil-type sampling: 22%

Grid sampling, for example, is more costly because more samples are required, says Mallarino. “It describes P, K, and pH variation better,” he says. “It can be used with yield maps to estimate P and K removal. It adapts so well to variable-rate application."

“Sampling every two years for P and K is a good investment, and you avoid guessing,” says Mallarino. That’s the approach Motter takes. He hires grid sampling every two to three years, depending on rotation, availability of the technician, and weather conditions.

Micronutrients Testing

While soil testing is crucial for P and K, the same can’t be said for micronutrients. “The challenge with micronutrients, other than with zinc, is that soil tests aren’t very reliable,” says Fernandez.

You can take a soil sample, get it analyzed, and get a number, but it’s not going to be reliable to help you get a recommendation, explains Fernandez.

It may be useful as a diagnostic tool, though, adds Fernandez. “If you have a pocket in the field where you have a problem and another area that’s fine, take samples in both areas. You might find that all nutrients are about the same except for one,” he says. “That, coupled with plant material and tissue tests, can help you know what’s going on in the field.”

Sample For SCN

What costs farmers $1.2 billion annually and can cause 30% yield loss in soybeans without visual symptoms? If you say soybean cyst nematodes (SCN), you’d be correct.

These pests live in the soil and invade plant roots, all while robbing soybeans of nutrients and water, and reducing N-fixation efficiency. Visually, SCN above-ground symptoms can include yellow and stunted plants, says Amanda Howland, Extension plant nematology coordinator for the University of Missouri.

The tricky part about this pest is that all symptoms can mimic nutrient deficiencies, drought, or diseases, explains Howland.

“If you very carefully dig soybean plants up, you can see tiny yellow cysts on the roots,” says Howland. That’s the only sign of SCN you’ll spot on the soybean plant.

The best way to know if you have SCN is to collect samples and test for SCN, says Howland. After harvest is the ideal testing time. Because populations have had all summer to reproduce, infestations will be highest in the fall. However, you can still sample during the spring to get a general idea of population levels. Field sizes for sampling should be no larger than 40 acres; the ideal size is just 10 to 20 acres, says Howland. When collecting soil samples, remember, like sampling for nutrients, the more samples you collect, the more accurate the test. Collect samples at an 8-inch depth. When collecting the samples, walk in a Z- or W-pattern, as it will increase the odds of finding SCN in the field. Howland recommends collecting 20 cores and mixing them into the composite.

Nematodes are sensitive to heat, says Howland. Keep samples out of the sun – preferably in a cool location – before sending them to the lab for the SCN egg-count test.  

Keep in mind that SCN can adapt to the SCN-resistant varieties on the market. If you notice soybean yields decreasing, switch to a different line of resistance and rotate to nonhost crops. You can also use the HG-type test to identify the race of SCN. Use this information when purchasing resistant varieties.

Humans are the main cause of SCN movement. To avoid spreading SCN, keep field equipment as clean as possible during the season, and clean equipment at the end of the season to avoid infecting a different field the following year.

Soil Health Tests

If you think soil health tests tell you everything you need to know about the soil health of your field, think again. Unlike testing for nutrients and SCN, these results can be highly variable and difficult to interpret.

The most useful tool for evaluating soil health in your field is still your shovel, says Abbey Wick, Extension soil health specialist at North Dakota State University.

At this point, soil health tests are far from perfect, says John Lee, soil scientist at AGVISE Laboratories.

Currently, soil health tests will examine the biological activity in the soil, but Lee says results are variable. Soil health test results depend on many different factors, such as:

  • The time of year samples are collected.
  • The environmental conditions where the sample is collected.
  • Lab conditions such as temperature and rewetting methods.  

Remember, results from the same soil sample will vary from lab to lab. “You might think because a number comes out of a laboratory, it’s golden, but you should understand that with soil health tests, the methods just aren’t there yet,” says Lee.

“The soil health tests that are trying to get at soil biological activity measure carbon dioxide coming off of the sample,” says Wick. “You have to compare two extremes for it to work well. For example, comparing a heavily tilled soil with a soil that has been in no-till for 35 years. However, when you’re looking for slight changes in management, this type of test really doesn’t tell you that much.”

If you’re looking to see if your cover crop made a difference or if your first year of no-till changed the biology, it’s not going to give you reliable answers, says Wick. Instead, she recommends going into the field and sliding your shovel into the soil. Check poor parts of the field for soil health issues like compaction, soil aggregation, and earthworms. Check where there are relatively high yields, and then check the soil in the fencerow or tree line, says Wick.

“You will see a drastic difference in what the soils look like, and that will show you more than any soil test out there,” Wick says.

“One of the best measurements for soil health is aggregate-size distribution,” says Wick. “Aggregates are sand, silt, and clay that are wrapped together or held together with microbial glues. They have organic material on the interior; they’re a mechanism for protection and timely turnover of organic matter. Aggregates help increase water movement, air exchange, and root penetration in the soil because of the pore sizes they create. They are a reflection of biological, chemical, and physical soil properties. That makes them a great indicator of soil health.”

Macroaggregates (soil aggregates greater than 2 mm in size) are a sign of a healthier system, because they are formed by fungal hyphae and roots, says Wick. Macroaggregates persist when they are not disturbed by tillage and there’s enough rooting material and organic material to build those aggregates.

“They are the first to be destroyed with tillage,” says Wick. “We have a lot of macroaggregates in grassland systems and very few in a conventional-till system.”

In a system somewhere between grasslands and conventional-till, like a no-till system with a diverse rotation, you’d have an even distribution of all the aggregate classes: large macroaggregates, some small macroaggregate-size class, and microaggregates.

In a conventional-till system, there are a lot of microaggregates and very few large macroaggregates or small macroaggregates.

“When you pull out a shovelful of soil, you can see how it breaks down,” says Wick. “Is it all fine, small particles? Or can you see the large macroaggregates where it looks like cottage cheese?”

The Future of Soil Health Tests

In the future, these tests may be really useful, agree Wick and Lee. “We need to make sure the science is backing them up,” says Wick.

“We need to feel comfortable making recommendations off of the results, and so do you if you are going to reduce fertilizer rates from the results,” says Wick. “Before you go and bet the farm on it, we want to make sure those numbers are right.”

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