Not all soil types are created equal, especially when it comes to fertilizer requirements, says Glen Harris, University of Georgia Extension Agronomist — Soils and Fertilizer.

UNIVERSITY OF GEORGIA Extension agronomist Glen Harris discusses cotton and peanut fertilization during the recent Stripling Irrigation Research Park Field Day held in southwest Georgia.

“If you look at the most common soil type across the Coastal Plain where we grow peanuts and cotton in Georgia, it’s probably a Tifton-type soil. It has subsoil clay within the top 6 or 9 inches of soil,” he explained during the recent Stripling Irrigation Research Park Field Day.

The soils in the irrigation park, which is located in southwest Georgia, are different from the most common type, but still common to that particular area of the state where crops such as peanuts and cotton are grown.

“It’s a deep sand soil, which does not have any subsoil clay within the top 20 inches, typically. It makes a big difference, especially when it comes to fertilizing because there are some nutrients that are mobile in these sandy soils under irrigation, and they can move whenever there’s a rainfall event or water is applied,” says Harris.

This marks the second year of fertilizer research on the deep sand soils, he says, looking specifically at the top issues in both cotton and peanuts.

“First, with cotton, I don’t think there’s any doubt the main issue during the past couple of years has been potassium. We had a lot of rainfall, going back to last year. And potassium prices have remained high while nitrogen and phosphorus fertilizer prices have come down after peaking in 2008. But potassium has remained high, and we’ve seen a number of problems,” says Harris.

A potassium deficiency shows up in cotton as a leafspot, he says. Cercospora, alternaria and stemphylium leaf spot all have been linked to potassium deficiency. These leaf spot diseases are considered secondary to potassium deficiency, and if potassium deficiency is avoided then these leaf spots should be prevented.

Not all cotton leaf spots are related to a potassium deficiency, he notes, and if potassium nutrients are adequate, there’s probably no need for fungicides.

Harris says his plots are irrigated, four rows wide, and 40 feet long, with varying rates of potassium. The potassium is applied by hand to insure the exact rate. There are also split applications.

“This is the only soil type where we recommend you put out some potassium at planting and then some at side-dress, just like we do with nitrogen. A lot of growers in this area do it. I don’t think it has been checked by the university in a long time, so we’re trying to confirm that it is a good recommendation, and based on last year’s data, it looks right,” he says.

Currently, foliar K applications should automatically be considered on deep sands (more than 18 inches to subsoil clay), low-K soils, high-Mg soils, high-yielding conditions, short-season varieties and especially where severe K deficiencies and leafspot have been observed in the past, says Harris.

Many of the new varieties that will be grown in 2010 are earlier in maturity and high-yielding compared to DP 555 B/R so they may require some foliar K. Two foliar applications of 5 to 10 pounds of K in each application during early bloom (first through fourth week of bloom) should be considered in these situations.

“You will not see potassium deficiencies on a young plant that’s just beginning to square. It really starts hitting at about first bloom, when the bolls start getting green. We’re also looking at foliar potassium on cotton.”

Switching to peanuts, Harris says it’s a totally different plant from cotton, as it fixes nitrogen and is a good scavenger of P and K.

“The big issue on peanuts is calcium and getting calcium into that pegging zone so you don’t have pops and pod rot and low yields. We’re looking at calcium rates on peanuts. This has become an issue because we’ve shifted away from small-seeded runner peanuts, primarily Georgia Green, to a larger-seeded runner peanut, something like Georgia-06G, and those are the two varieties we have in our tests, with varying rates of calcium, basically gypsum at bloom-time.”

The general recommendation is that if you have 500 pounds of soil-test calcium in the pegging zone, you don’t need gypsum, says Harris. “We had five locations in our research last year, and that recommendation pretty much held true. As long as you had 500 pounds, you did not need gypsum, except in one location, and that was here on these deep sand soils. There’s something about these soils, and there’s something about these large-seeded varieties, especially 06G. We had almost 1,000 pounds of soil-test calcium in the pegging zone, and we got a good response to the gypsum on those peanuts.

“And not only did that carry through to yield, but working with the state lab in Tifton, we shelled the peanuts and we got warm germination, cold germination and also percent calcium in the peanut, and we correlated all of that. This information is especially interesting to the peanut seed companies, as we work to figure out exactly where we need to be on these large-seeded varieties, so we have good germination when we plant the seed next year.”

There also seems to be more interest among peanut producers in putting out fertilizer through center pivot irrigation systems, says Harris. “There are actually some liquid calciums. We don’t normally recommend liquid calcium, especially foliar-applied. But there are some liquid calcium products that are recommended at 20 to 30 gallons per acre. Just when those plants are really putting on a lot of peanuts, and the calcium should be going into the pods, the irrigation system is putting out liquid calcium.

“We simulated that last year, but we’re not to the point of recommending it. We still need to look at the cost compared to gypsum. In theory, it makes sense.”

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