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Nathan Slaton gets his hands dirty filling in soil-test data gaps

In this modern age, immediacy may be prized but patience is still a virtue (and necessity) in the field of soil fertility research. Nathan Slaton, who has held countless handfuls of delta dirt during his tenure at the University of Arkansas, knows this well.

“Most of the work we do takes a while,” says the agronomist and professor. “It’s rare that definite conclusions can be drawn after one or two years of research. So, at the end, we need to produce something worthwhile, something farmers will really benefit from.”

Right now, the bulk of Slaton’s research is aimed at improving soil test recommendations. As in past years, this spring his team put out “as many tests as we could possibly keep track of and monitor properly.”

Most years, that means at least five or six sites each for rice, soybeans and wheat scattered around the state. “We put out fertilizer rate trials — one with phosphorus and one with potassium — to see whether or not the soils respond. When we complete 20 or 30 of those tests, the dots can start to be connected. Positive yield increases to fertilization is what we’re looking for.”

Those results allow the researchers to study soil samples and confidently tell a farmer whether or not he’s likely to get a good yield response to fertilization.

Unfortunately, there are gaps in soil fertility databases. Agriculture changes quickly and frequently. Farming techniques, varieties, and yields shift constantly. With diminishing budgets and staff, researchers can struggle to keep up.

Until recently, no-till systems were hardly known in Arkansas. Now, Slaton points out, reduced-till, stale seedbed, and no-till are commonly accepted practices. Such production practice shifts can influence how soil samples need to be collected and/or how they’re interpreted.

“So correlation/calibration work is important. We need to understand the relationship between crop yield response to a particular fertilizer and a particular soil test. And that relationship can clearly change between production systems and soils.”

And not only agriculture is impacted positively by soil studies. The environment benefits, as well.

“Take phosphorus for example. Soil test phosphorus may be optimum or above optimum in some fields. From an agronomic perspective, we probably don’t need to apply phosphorus fertilizer in those fields to optimize yields.

“If we can figure out where fertilizer needs to be applied, that, in itself, is doing a lot to conserve natural resources — phosphorus and potassium must be mined and there’s a limited quantity in the world — and keep excess nutrients from becoming a problem downstream. So this soil work is, indeed, environmentally beneficial.”

One of the big concerns right now is there’s such an environmental emphasis on nutrient management that agronomics is lost.

“People are very concerned about soil and fertilizer nutrient movement into streams, rivers and eventually down to the Gulf of Mexico. The focus of a lot of environmental research has been to examine nutrient loss from fields with above-optimum nutrient levels. Our research is focused on the agronomics of nutrient management which can also be considered best nutrient practices for the environment.”

For example, when does rice or soybeans respond to phosphorus or potassium fertilizer? What’s the appropriate nutrient rate to apply when soil tests are low, medium or optimum?

Soil fertility data is important across nearly every aspect of agriculture. Precision farming is one newer practice that relies on a solid soil data foundation.

“With grid-soil sampling in precision agriculture, getting 20 or 30 soil samples from a single field is great. But if the soil-test information can’t be interpreted with confidence, what use is it? You need to know if a certain 5-acre block needs fertilizer while the block next to it doesn’t need it.

“If we can gain a better understanding of what soil test numbers actually mean regarding fertilization, all the other technology can fall in line and be used in a much fuller manner.”

Producers don’t want to spend money unnecessarily and input costs are increasingly expensive.

“Like everyone else, farmers have house payments and need college money for their kids. So, if we can tell them, ‘You don’t need fertilizer on this field. You can make 200-bushel rice without it,’ they want to know. They don’t want to apply those nutrients unless warranted.”

Slaton is involved with a soil test summary published annually by the university. There are trends that show up. For example, for soils with rice in the rotation, soil test phosphorus tends to be quite low.

“That, I think, is due to a combination of factors. First, over the last 20 years, recommendations for phosphorus and potassium on rice have changed considerably. Twenty years ago, we didn’t recommend phosphorus on rice. We do now.

“Back then, we said, ‘If you think you have salt problems on your soils, don’t apply potassium to rice.’ So historical fertilization practices of that crop, along with the flooded soil environment (which has an effect on how available soil nutrients are) tend to show rice soils have lower test P and K than soils used exclusively for cotton production.

“In the Mid-South, we kind of take rice for granted. When it’s thrown into a rotation, that flooded environment — through the summer and also in the winter for waterfowl — influences how the soil releases nutrients and how much fertilizer is needed to optimize crop production. That only increases the amount of data we need to collect.”

Slaton and colleagues also continue to research micronutrients like zinc and boron.

“We keep trying to identify problem (boron) fields with soil tests. Frankly, the chances of being able to do that are very low. Soil testing simply has limits for some nutrients.

“Over the last few years, we’ve conducted 16 to 20 boron tests. We’re doing five more this year. Hopefully, after this year — or maybe next — if there’s a relationship, we’ll be able to figure it out. We have made headway on being able to tell farmers, ‘Here’s how to apply boron to soybeans,’ or ‘Here’s what to do if you see a boron deficiency.’”

As Arkansas is the largest poultry producer in the nation, poultry litter is a huge issue in the state. Currently, some of the litter is taken to eastern Arkansas and more could be.

“If that happens, we need to know how available the nitrogen is so the inorganic nitrogen fertilizer rates can be adjusted. We also need to know how available P and K are in the litter. That research has been going on for a couple of years.”

Slaton is also looking at slow-release nitrogen fertilizers. A couple of years ago, he saw a blurb in a trade magazine about ESN (a polymer-coated urea called Environmentally Smart Nitrogen). At that time, it was being evaluated on some Midwest corn acreage.

“Since then, a lot of research has gone on. We got interested and the company provided us with some product for tests. We’re now looking at it on rice to try and figure out how quickly, or slowly, the nitrogen in this product needs to be released. If we could find those answers, the nitrogen could be put out preplant with a truck.

Could the company manipulate the product to make it melt quicker or slower depending on what Slaton’s team determines the crop needs?

“That’s right. We just need to know what nitrogen release rate coincides with crop uptake. How rice takes up nitrogen is already known. What we need now is a nitrogen source that sort of mimics rice’s growth curve.

“Last year, we worked with just one company. This year, two other companies with similar products approached me. These products may, or may not, end up working. But it’s something new to check out that could really benefit growers.”

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