Soil tests are a good investment, agronomist says
Regardless of whether it is for a row crop, pasture or cover crop, one expert agrees that a soil test is a must.
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| Fred Vocasek |
pasture conditions can change in a short amount of time and producers need to take that into account. Consistency in samples and depth allows growers to compare “apples to apples.”
“This year it has been so dry that depending on the soil sample depth they have been using, it has been hard to get to that same depth by hand probing,” Vocasek said. “If you are used to taking an 8-inch sample and you can only pull a 6-inch sample that’s going to skew your historical average results.”
Tools that can help
Some producers are using cordless drills to ensure they are reaching the right depth, he said. Oklahoma State University developed a “Sweatless Soil Sampler” that a grower can build for himself and has been endorsed by Clemson University.
The developers of the tool said the SSS works well in hard compacted soils where it is difficult to take a sample with a conventional soil probe when the soil is dry. The SSS also works well in sandy soils where the conventional soil probe has difficulty retrieving the soil core. Another advantage to using the SSS is that unlike the tip of conventional soil probes the bit will not quickly dull. Information is available at https://agriculture.okstate.edu/departments-programs/plant-soil/soil-testing/site-files/docs/publications/pt2003-6-sweatless-soil-sampler.pdfsampler.
Proper depth
It is common for soil pH to drop during a drought, often by a 0.4 to 0.5 units. In 2011, some Texas Panhandle growers were seeing their soil pH drop by 0.6 and 0.7 units which triggered a first-time recommendation to apply lime.
The best advice Vocasek has for growers is “do not panic” because it may be drought-related, but to make sure to sample the following year. One of his concerns is seeing a long-term downward trend in soil pH levels across the High Plains from normal crop production.
Pennsylvania State University Extension notes in a bulletin that when the soil pH is between 6.2 and 6.8 most plants will do well. The pH is a measure of acidity and alkalinity of a material and 0 is extremely acidic and 14 is extremely alkaline with 7 being considered neutral. When acidic soil is neutralized by liming, soil nutrients are made more available for the plants to absorb through their roots, the PSU bulletin notes.
Nitrate levels
Soil nitrates can also be elevated during drought, sometimes as much as 100 pounds of nitrogen per acre. A frequent comment Vocasek receives is, “The soil test can’t be right. I didn’t have a crop out there but you’re saying I have this much nitrogen out in the field.” Vocasek’s response, “Yes, you probably do.”
Producers experience reduced yields during drought and that means less nitrogen is removed by harvest, he said. Also—without rainfall—nitrates are not being moved downward into the soil profile, even under irrigation.
Higher temperatures during drought increases microbial activity which breaks down more organic matter, releasing nitrates. For example, Vocasek surveyed summer-time soil samples from the severe drought years of 2011 and 2012 in the western Plains, finding the nitrates averaged about 20 to 25 pounds higher than in 2009 and 2010. By the time the drought ended in 2014 and 2015, nitrate levels returned to normal.
He says growers may need to expect more variability in nitrates within a single field because of drought.
“When you stand at the edge of a field, you see tall and short plants and potentially differences in growth. That may reflect back in equally variable soil test levels.”
Taking a single sample and applying a single rate across the entire field may be frustrating. “There is no rulebook to gauge variability as it depends on crop growth and water patterns,” Vocasek said. “The bottom line is we’re likely to see more variability this year than we saw before.”
Hay fields
Hay fields should be tested regularly and in some regions that can also apply to range and pastures, he said.
Forages are going to be a premium this year, he said, which is another reason to test. Nitrogen is important, but don’t overlook phosphorus and potassium, he said. “Those are important in forage production whether it is alfalfa or grass.”
Continuous forage production works the ground harder because more nutrients are removed from a field by forage harvest than if cropped to grain, he said, which increases the need to monitor soil test levels.
If a grower harvested his corn crop for feed because of the 2022 drought, he should also make sure that nutrient levels are correct for the 2023 season.
Why consistency?
Maintaining a consistent sampling schedule is his recommendation to growers about testing. “We work to get the individual test as accurately as we can. Even more valuable is looking at trends.”
Vocasek says the downward drift of soil pH in the Plains may show up first in wheat. Soil acidity of 5.0 to 5.5 or lower can dissolve the aluminum out of clay minerals. That damages the young root system and affects growth. This can look like a phosphorus deficiency in the plant even though the soil test phosphorous level is high. The problem is the damaged root system can’t take up the soil phosphorus, he said.
Declining soil pH is one concern, but declining soil phosphorus is another. It is harder to rebuild a soil test than to maintain it. Vocasek said it typically takes about 15 to 20 pounds of P2O5 as phosphate fertilizer to increase the soil test by 1 part per million. He noted that a University of Kentucky study suggested different numbers.
“If the phosphorus test is down in low single digits – 4 or 5 ppm—it may take 35 or 40 pounds P2O5 phosphate to make that change,” he said, “But if I’m up 40 to 50 ppm it might only take 4 to 5 pounds of P2O5 phosphate per 1 ppm fertilizer.”
It takes time to build the soil test, but once it is established it becomes easier to maintain, he said.
With strip-till practices growers may switch from anhydrous ammonia to surface applications of liquid nitrogen or urea nitrogen when switching to reduced tillage or strip-till. Over time, nutrients can stratify and creates layers in the soil profile.
Some soil tests show high phosphorus and zinc levels in the top 2 or 3 inches, but a low, or acidic, pH. “We’re creating an acid surface layer. So even though you have higher fertility, it is right where you’re laying the seed. The seedling root systems of wheat or corn can be damaged if the pH level is low enough.” That can affect emergence of individual plants and interfere with establishing uniform stands.
Vocasek’s advice in these current condition is to continue to take routine soil samples at normal depth to maintain the history. Then consider spot checking to look at pH or phosphorus levels in the 3-inch to 4-inch depth to see if a layer has developed, so action can be taken.
“The cost of a soil sample should be considered an investment and not an expense item,” Vocasek said. “Investing in a soil test is money well spent.”
— By Dave Bergmeier, originally published in the High Plains Journal, April 7, 2023
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