AgCenter scientists explore diverse strategies for soil improvement

For five years, LSU AgCenter scientists have been evaluating the effectiveness of silicon fertilization in strengthening plants against disease and other stresses that damage plant structure.

Two wheat cultivars susceptible to scab disease were planted at the LSU AgCenter Central Research Station in Baton Rouge and fertilized with three silicon-based fertilizers — wollastonite, which is a more soluble source for soil applications, and two different foliar solution sprays.

Silicon-fertilized crops showed no visible evidence of scab and, particularly with wollastonite, had bigger stalks that had less lodging than non-silicon treated plants.

“This indicates that silicon reinforces the mechanical structure of the plant,” said Brenda Tubaña, a soil fertility scientist with the AgCenter. “We’re recording disease ratings to see if there are any disease problems, and we’re evaluating yield data.”

In 2016-17 results, silicon-treated soybean plants showed a lower rating of foliar diseases and better yields than untreated soybeans.

In a different study that has been ongoing since 2004, Tubaña and her researchers found consistent long-term results that indicate a standard threshold for soil phosphorus levels (35 ppm) for corn and soybean rotations.

But even when proper amounts of phosphorus, potassium and other essential nutrients had been applied, improper pH consistently led to significant decreases in yield.

“Correcting pH of the soil is very important, and we’ve found that if you lime and apply only minimal units of phosphorus and potassium to soybeans, we have about a 9% to 10% average increase in yield,” said Tubaña. “If you don’t correct pH, you will have excessive amounts of trace minerals that can lead to toxicity and yield declines.”

The LSU AgCenter started researching enhanced-efficiency nitrogen fertilizers in 2016 on wheat and, in 2017, expanded those tests to corn.

The most recent nitrogen field tests on corn and wheat were completed at the AgCenter Central Research Station. A combined fertilization of urea ammonia nitrate, a nitrification inhibitor and a urease inhibitor resulted in a 25% increase in corn yield over urease ammonia nitrate, a standard practice for most farmers.

Application of that same combination with wheat resulted in an 18% increase in yield over the standard fertilization practice, and use of only urease inhibitors resulted in a 12.9% harvest improvement.

These combinations also showed a dramatic increase in nitrogen-use efficiency when applied at certain times and rates for both corn and wheat.

“This is very important because it’s a way to improve water quality and decrease greenhouse gases,” said Jim Wang, a soil chemist with the LSU AgCenter. “Through several years, almost all the experiments showed that use of stabilizers has reduced loss of nitrous oxide emissions and decreased nitrate leaching.”

Wang’s team also analyzed the effects of micronutrients — boron, molybdenum, iron and sulfur — on soybeans at three AgCenter research stations, the Central Research Station, the Red River Research Station in Bossier City and the Dean Lee Research Station in Alexandria.

The most noticeable results were with a fertilizer combination of molybdenum and ammonium sulfate (4Mo+SN), resulting in a 20% increase in yield over standard fertilization. A foliar molybdenum spray (FoliarMo0.25) also resulted in a 20% increase.

Iron fertilization (4Fe) resulted in a 13.5% increase in yield, but boron and combinations of boron and iron resulted in yields significantly below standard fertilization yields.

Finally, sulfur fertilization showed a significant 7.5% increase in yield on alkaline soils while exhibiting less of an effect in acidic soils. According to Wang, blending sulfur with other major nutrients might further improve yields.

Soybean plants fix nitrogen in the soil, so the common assumption was there’s already enough nitrogen in the soil without fertilization. However, Wang’s 2018 tests showed that an application of urea-ammonia nitrate on alkaline soils at a particular rate resulted in an 8.4% increase of harvested beans.

AgCenter researchers have also studied cover crops to find ways to keep important biomass and nutrients in the ground, making them available for plant growth.

Field experiments were done at the Red River Research Station on hairy vetch, rapeseed, ryegrass and different mixtures of these cover crops.

In laboratory incubation studies, soil with a combination of hairy vetch and winter wheat showed a significant increase in total phosphorus and potassium concentration compared to the control treatment, according to Changyoon Jeong, an LSU AgCenter agronomist.

Hairy vetch holds phosphorous well, which decreases leaching of that element, and has a relatively low carbon content in its biomass — meaning it has a relatively high nitrogen content. Winter wheat, on the other hand, contains a high carbon ratio compared to nitrogen, meaning it has a relatively low nitrogen content in the biomass.

The cover crop mixture of hairy vetch and winter wheat could have the benefit of extra nitrogen from hairy vetch to supply crop soil, while reducing nitrogen leaching due to the high carbon content of winter wheat, according to Jeong.

Conservation tillage is another potential strategy for improving soil health for crop production. The process leaves a majority of the crop residue on the soil surface and minimizes nutrient loss.

“Conservation tillage systems for crops may need different nutrient management strategies compared to conventional till,” said Syam Dodla, an LSU AgCenter agronomist.

In field tests from spring 2018, Dodla found corn had higher yield in fine sandy loam when urea-ammonia nitrate liquid was injected below the soil compared to surface application of granular urea and ammonium nitrate. A split timing application of nitrogen also improved yield.

These same applications for corn in clay soil showed no yield benefits, Dodla said.

For soybeans grown in very fine sandy loam and clay soils, the application of either phosphorus or potassium increased grain yields under conservation tillage. The same applications showed no yield gain with conventional tillage.

“These first-year results showed that optimum fertilization practices for corn and soybeans under conservation tillage may be different than for conventional tillage,” Dodla said. “But additional research is needed.”

Randy LaBauve

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LSU AgCenter soil scientist Brenda Tubaña, center, works with a postdoctoral researcher and graduate students to take soil samples in a soybean field at the AgCenter Ben Hur Research Farm in Baton Rouge. Photo by Olivia McClure

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Jim Wang, a soil chemist with the LSU AgCenter, holds a handful of soil used for fertilization tests on soybeans at the LSU AgCenter Central Research Station in Baton Rouge. Analyses of the soils were used to determine the effects of micronutrients on soybean yields. Photo by Randy LaBauve

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A conventional tillage field used to compare nutrient programs for different types of tillage and to develop optimum fertilization rates specific to conservation tillage. The tests were conducted in 2019 at the LSU AgCenter Red River Research Station in Bossier City. Photo by Syam Dodla
11/6/2019 11:22:28 PM
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