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How does drought affect soil nitrate levels?

Curt Arens Drought stricken corn field
HARDEST HIT: Parts of the Great Plains states have been hit hard by drought this season, which will have important implications on soil nitrate levels, and potentially pH, going into next season, says Fred Vocasek. ServiTech senior laboratory agronomist.
A senior laboratory agronomist shares insights into drought impacts on soil nitrates and pH.

The drought conditions gripping much of Nebraska and the Great Plains this growing season will have wide implications when we look at nitrogen recommendations and soil nutrient needs going into next year.

Fred Vocasek, senior laboratory agronomist with ServiTech, works with growers across Kansas, Oklahoma, Iowa and Nebraska. Vocasek’s career in agronomy began in northeast Nebraska in the Neligh area, so he is familiar with one of the regions in the state hit hard by drought this season.

He uses his experiences from 2011 to talk about this year’s conditions. That year, the drought line broke at Interstate 70 in Kansas and Colorado. Comparing soil test nitrates received that summer — south versus north of I-70 in the area that included the Oklahoma panhandle, western Kansas and eastern Colorado — the hardest-hit drought areas had soil test nitrates at double the rate of those in areas that were not hit so hard by drought.

More nitrogen

In other words, the areas hit by drought were showing soil test nitrates at 15-25 pounds of nitrogen more per acre than in the areas not hit as hard that year. “That’s easily worth $10 to $15 per acre as nitrogen fertilizer,” Vocasek says.

Drought affects microbial activity, with optimum soil temperatures between 90-95 degrees F for microbes. “When we accumulate more heat units faster and for a longer time, which is what happens during drought, microbes in the soil work longer and harder to decompose organic matter,” Vocasek says. “This results in more nitrate accumulation.”

He sees a similar situation setting up this year in southwest Nebraska, the Panhandle and other drought-stricken parts of the Great Plains. The lack of precipitation keeps nitrates from being moved into the subsoil, so surface nitrate accumulations can be higher than normal. It isn’t uncommon to have up to 50-100 pounds of nitrate per acre, even in dryland fields that did not receive fertilizer nitrogen, Vocasek adds.

“We’ve lost customers from these kinds of soil test results,” Vocasek says. “They call us and can’t believe that it is right. We might run another test, and it comes back the same. It is hard to believe, but it is an unexpected, unplanned benefit to drought.”

Having ample fibrous roots systems building biomass below the ground helps to build organic matter to improve the soil and help with fertility. Reduced tillage and no-till systems preserve that root structure, Vocasek says.

“I relate it to feeding cattle, because we don’t really feed cattle, but we feed the microbes in the rumen of the cattle,” he explains. “It’s the same with the soil. Microbes need sources of nitrogen, and they need different sources.”

As crop yields go up, there is more biomass to feed the microbes, and organic matter goes up.

Soil pH

Drought can also affect soil pH levels in soil tests. Again, using 2011 as a benchmark, there were places in Texas where soil pH levels were 0.7 to 0.8 units lower than normal, Vocasek says.

“The depression of soil pH is due to natural changes occurring as soil dries down during long-term drought,” he says.

That’s why ServiTech uses a Sikora-2 method of testing pH, because it stages the soil analysis slightly differently in order to get a more accurate pH result, especially on sand soils, for instance. With anomalies like these so common after extended periods of drought, Vocasek says that it will be important this fall for farmers in the drought areas to look closely at their soil test results.

Vocasek recommends looking at smaller zones this year when testing soil in drought areas, because there will most likely be greater variability in nitrate levels. Understanding and knowing the nitrate levels in the soil can save farmers money going into next season, because some of the zones may require very little fertilizer nitrogen to grow a crop in 2023.

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