Troubleshooting Soil Fertility Problems

Follow the recommendations, keep soil pH in the proper range and know what to look for when problems arise.

• By Amanda Huber •

Soil is a living ecosystem and is a farmer’s most precious asset. A farmer’s productive capacity is directly related to the health of his or her soil — that’s a quote by Howard Warren Buffett, farmer and former special assistant in the U.S. Department of Agriculture, who is also the grandson of American investor and philanthropist Warren Edward Buffett.

If something is alive, it has to be fed and taken care of, and that’s true of the soil. To get the best out of it and make a top-yielding peanut crop, the first place to start is with a soil test, says Glen Harris, University of Georgia soil scientist.

Harris’ general peanut fertilization recommendations are as follows: soil test and follow those recommendations; add lime as needed to have a soil pH at 6.0 to 6.5; build soil phosphorus and potassium; inoculate if the field has been out of peanuts for more than four years; provide calcium to the pegging zone; and apply 0.5 pounds of boron per acre.

“If you do these things, soil fertility should not be your limiting factor from making good yields.”

However, there will be times when troubleshooting field issues will be necessary. Harris says when investigating problems in the field, the process of elimination always seems to be herbicide, disease, nematode, a seed issue, insects, and when those are ruled out, fertility is likely the answer.

Catch Problems Early

“Try to catch problems within 30 days after planting, especially things like nitrogen or inoculant failures. Look for overall patterns in the field. If it’s streaked or ‘to the row’ that would indicate problems with the planter or spreader truck. Is the problem on the field edge or in weak spots?

“Look for symptoms on the plants. Is it on new leaves or old ones, top or bottom of the plant? Look at the roots to see if they have been inoculated and are nodulated by bacteria to fix nitrogen.

“Make sure you are keeping a record of planting date, varieties, timing of symptoms and whether you had unusual weather patterns. All of this information can be helpful when trying to figure out a field problem.”

Consider Grid Sampling, VR Lime

Harris says the No. 1 soil fertility problem in Georgia is low soil pH.

“Dr. Eric Prostko would agree with this. He gets a lot of calls for what the farmer thinks is herbicide damage, and it turns out to be low soil pH.

“In fact, there are not many problems in soil fertility that will flat out kill plants, but one that will is a soil pH below 5.5.

Especially at a level below 5.0 to 4.5, peanut plants will certainly die,” he says.

Harris says grid sampling and variable-rate lime applications are a good way to create a more uniform soil pH in the field, although it is not an official recommendation yet.

“Grid sampling and variable-rate lime is a good way to fix a lot of problems we’ve been seeing. There tend to be spots in most fields that will be lower in pH, and if you keep putting out the same rate of lime every year, then the bad spots keep getting worse over the years. Fields tend to be more variable in pH than one might think.

“When you grid sample and apply lime by variable rate, you might use the same amount of lime, but you’re concentrating it in those areas that really need it and not putting it in areas that don’t or where soil pH is holding.”

Why pH Matters

The No. 1 soil fertility problem in Georgia is low soil pH. This field shows a pH of 5.7 on the right and 4.9 on the left.

Peanuts are susceptible to both aluminum and zinc toxicity, both of which are more available at a lower pH.

“In fact, aluminum is not available above soil pH of 5.5 at all, but below that it becomes available and kills most plants,” Harris says.

Another reason to make sure soils are in the proper pH range is to create conditions where Rhizobia bacteria thrive to complete the nitrogen-fixation process to provide plants with the needed nitrogen. Fertilizer applications reduce soil pH.

“Ammonium sulfate brings down soil pH,” Harris says. “It takes 4 pounds of lime for every pound of nitrogen. For example, for every 60 pounds of nitrogen applied, it takes 240 pounds of lime.”

What about elemental sulfur as a fungicide? Harris says the sulfur recommendation is 5 pounds per acre times three sprays which equals 15 pounds of sulfur per acre.

“According to data from the ‘Soil Fertility Handbook,’ it would take 172 pounds of sulfur per acre to lower the pH from 5.5 to 5.0, and we’re only putting out about 15 pounds per acre.”

Troubleshooting Calcium Problems

Since calcium problems are usually not discovered until the peanuts are dug, troubleshooting calcium isn’t easy and is usually a matter of thinking about what might have gone wrong.

“Did you apply gypsum too early and it leached out? Was it applied too late? Did you reduce the rate of gypsum? Did you apply lime at early bloom instead of planting? Was there not enough irrigation or rain to move gypsum into the soil? Finally, did you apply a foliar calcium, which doesn’t work?”

By adhering to the peanut fertilization recommendations, keeping the optimum soil pH and knowing what to look for when troubleshooting, fertility will not be a limitation to great yields.

No In-Furrow Fertilizers

That’s the message this spring — no in-furrow fertilizers — from UGA’s Scott Monfort. Research shows in-furrow fertilizers affect germination, reducing emergence and stand uniformity.

Monfort says because of the increased interest in using fertilizers in furrow but the lack of data on this practice, it was necessary to conduct research to see if this causes a problem.

“In 2020, we had several fields that had problems with in-furrow fertilizer that we were able to document. Based on what the grower did and a problem with stopped-up tubes, it helped us figure out the cause. However, we also had a lot of germination issues last year. The question is, ‘How do you break those two issues apart?’”

Fortunately, Monfort says, a greenhouse was available for use covering bare soil with types representative of peanut fields.

“The objective was to evaluate in-furrow fertilizers on peanut. Companies are recommending products like Riser and other in-furrow fertilizers at a rate of 1 to 3 gallons per acre.

“If you think about it, we are usually at a rate of 5 gallons per acre of carrier liquid for the products applied in furrow. Think about how much water is being replaced by fertilizer in the application.

“In our test, the treatments were an untreated check and then Riser applied at 0.5, 1, 2 and 3 gallons per acre with a total carrier volume of 7 gallons per acre.”

The plots were planted at six seed per foot 2 inches deep with optimum temperature and soil moisture.

“Emergence was checked every day from day six after planting to day 14. We wanted to know how quickly peanut seed would emerge, but also the total amount emerged at the 14-day mark.

“At six days after planting, we had 18% emergence in the untreated plot, but the best we had for the Riser was 5% at the 1/2 gallon rate, followed by 1% to 2% for the other rates. At day seven, emergence was up to 53% on untreated plots, 36% at the 1/2 gallon rate, 20% at 1 gallon, 7% at 2 gallons and 5% at 3 gallons.”

Although the 1/2-gallon and 1-gallon rate start to catch up, Monfort says the 2- and 3-gallon rates stayed considerably behind on emergence.

“You might say, ‘Well, I can still put out 1 or even 1/2 gallon and still do well.’ But what I want to point out is that any rate of Riser slowed that early emergence compared to the untreated plot. Slow and erratic emergence favors the expression of tomato spotted wilt virus.”

Monfort also points out that in the greenhouse, they were able to have optimum temperature and moisture for germination, which is often not the case in the field. Additional stress on peanut seed will reduce germination further.

“We have confirmed our UGA recommendation to not apply in-furrow fertilizers at planting because of the problems it can cause with slower emergence.”