Soil scientists research nitrogen efficiency, silicon for plant protection

Randy LaBauve, Gould, Frances I., Blanchard, Tobie M.  |  9/12/2018 7:32:32 PM

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LSU AgCenter soil chemist Jim Wang works with mini-plot water sample collectors, which are used to trace how much nitrogen goes to the plant, soil and water. Photo by Randy LaBauve

National studies of commercial enhanced-efficiency nitrogen fertilizers on row crops have shown some success in improving efficiency of plant uptake and decreasing losses of greenhouse gases while minimizing nitrate runoff.

Until recently these fertilizer compounds have not been adequately tested in the hotter, more humid South, according to Jim Wang, a soil chemist with the LSU AgCenter.

“We’re looking at slow-release, solid fertilizers with urea and ones with coated urea to find the best use efficiency with wheat,” said Wang. “For corn, we’re using controlled-release liquid nitrogen fertilizers, which contain compounds that are urea inhibitors or nitrification inhibitors, and combinations of both, to see how much of these compounds we should apply and when we should apply them, if at all,” said Wang.

The LSU AgCenter started the research of enhanced-efficiency nitrogen fertilizers in 2016 on wheat and expanded the tests to corn in 2017.

One aspect of the study explores possible cost-saving nutrient credits through the U.S. Department of Agriculture Natural Resources Conservation Service. When nitrogen stabilizers are applied to different cropping systems, they could potentially conserve soil nutrients.

“This is very important because it’s a way to improve water quality and decrease greenhouse gases,” said Wang. “Both liquid and solid enhanced-efficiency fertilizers have significantly reduced nitrous oxide and ammonia gas loss.”

Field studies at the Red River Research Station in Shreveport and the Central Research Station in Baton Rouge include tests on new production compounds that have not yet been commercialized. Although the research has shown greater use efficiency in the vegetative part of plants, it has not yet translated to better yields, according to Wang.

“One reason may be because we applied the amount of nitrogen fertilizer at the maximum or sufficient level,” he said. “We changed the strategy and will see if a lower amount of nitrogen is going to make a difference and enhance the use efficiency and directly impact the yield.”

Enhanced-efficiency nitrogen fertilizers currently cost 10 to 15 percent more than regular types. But advances in technology will likely bring those costs down, according to Wang.

“If we’re saving by reducing gas loss and water quality improvement, and if we see an increase of use efficiency, then we will be able to see the direct benefit,” Wang said.

Crops in Louisiana are susceptible to disease and insects because of the long, hot and humid summers. Brenda Tubana, a soil fertility scientist with the LSU AgCenter, has found some success with silicon fertilization in bolstering plant strength against biotic and abiotic stresses.

“Silicon enhances the plant’s physical barrier, which limits pathogen access to plant tissues,” said Tubana. “That’s why silicon fertilization could possibly reduce the need of chemical control for diseases and eventually increase plant production.”

In one LSU AgCenter field test, soybean plants were fertilized with the mineral wollastonite, which has small amounts of iron, magnesium, and manganese substituting for calcium, and others with silicate slag, a byproduct of steel production. Plants treated by both silicon sources had a lower rating of Asian soybean rust disease and better yields compared to non-treated plants.

Research with silicon fertilizer on wheat is being conducted at two LSU AgCenter facilities, the Macon Ridge Research Station in Winnsboro and the Northeast Research Station in St. Joseph.

The silicon fertilization study, now in its fourth year, has shown that light textured soils tend to have less plant-available silicon, and clay soils tend to have higher levels of this nutrient element. Most soils, at some point, need liming to bring down acidity and raise pH. That’s another task where wollastonite and slag excel, Tubana said.

“The demand for silicon by most crops grown in Louisiana, like rice, soybeans and wheat is quite high,” said Tubana. “The idea of using slag may be advantageous because you’re not only correcting the pH, but you’re also adding silicon.”

A spin-off study within the project is happening at the Bob R. Jones-Idlewild Research Station in Clinton. Research on silicon fertilization of soybeans focuses on its ability to help fend off herbivores, such as deer, which eat and destroy large crop portions.

“The deposition of silicon in soybean shoots can create a hard outer layer, making it stronger,” said Tubana. “We’ve seen reports where wild rabbits and locusts preferred unfertilized forage, likely because of difficulty chewing silicon-fertilized grasses.”

Other research for the silicon project is developing robust testing procedures that indicate if plants need silicon applications and investigating the economic feasibility of using slag or wollastonite. While the slag is relatively cheap, transportation costs could be a concern.

Tubana is working on a different Soybean and Grain Promotions Board-funded project, unrelated to the silicon study, which evaluates planting dates and fertilization impact on cover crops in a soybean-corn rotation system. The main focus is determining management practices to optimize cover crops as a way of improving nutrient recycling.

“Cover crops assimilate nutrients that are supposedly left in the field during a fallow period,” said Tubana. “When the cover crop is chemically burned down, its accumulated biomass should benefit the main crop to follow.”

Wang has another distinct research project that started in 2018 and is partially funded with a Soybean and Grain Board grant. The study investigates how the micronutrients molybdenum, boron and iron chelates may benefit soybeans.

“We are testing these separately and in combinations in soybean fields at the Dean Lee Research Station in Alexandria and the Red River Research Station near Shreveport,” said Wang. “These micronutrients can influence nodule formation, and we want to find out if the micronutrients blend with major elements to promote transfer of nitrogen within plants.”

Randy LaBauve

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