Research Highlights

Supplying Micronutrients Based on Soil Conditions Increases Soybean Yield

By Laura Temple

Soybeans adapt well to a wide variety of soil types, but they need key micronutrients, like molybdenum, iron, boron and manganese, to thrive. How those micronutrients interact with other elements and soybean roots depends on the soil environment.

“Soil type and pH determine if soybeans can take up these micronutrients or not,” says Dr. Jim Jian Wang, a professor for the School of Plant, Environmental and Soil Sciences, Louisiana State University Ag Center. “For example, molybdenum is commonly deficient in acidic soils, while several other micronutrients tend to be deficient in alkaline soils.”

He notes that very little work has been done to learn how to address deficiencies of these micronutrients based on soil type, especially with today’s varieties. The benefits of meeting these micronutrient requirements for soybeans are also largely unknown. To figure out optimum application rates, methods and blending options with macronutrients, Wang is leading a multi-year research project funded by the Louisiana Soybean and Grain Research and Promotion Board. 

“In Louisiana, we have acidic soils along much of the Mississippi River, but our Red River soils tend to be alkaline,” he explains. “This research is helping us develop soybean micronutrient fertilization management plans that boost yields, while fitting with current fertilization practices.”

Addressing Molybdenum Deficiencies in Low pH Soils

Molybdenum is an important part of the enzyme soybean root nodule bacteria use to fix nitrogen. No adequate soil test exists to measure molybdenum levels in soil, so it is difficult to determine its availability.

However, in acidic soils, those with a low pH, molybdenum tends to bind to iron and aluminum oxides or form iron molybdate, all forms that are insoluble. That means it is bound up in compounds that soybean roots are not able to absorb from the soil.

Wang conducted molybdenum trials in two locations, with low rates of the micronutrient. The trials included soil applications at planting, and foliar applications at the R1 growth stage.

“Our trials showed that adding molybdenum to a fertility program in acidic soils increase yield noticeably,” he reports. “Soil applications averaged an 11 percent yield increase, while foliar applications averaged a 17.4 percent yield increase.”

This research is benchmarking molybdenum needs, so that recommendations can be developed to incorporate it into fertilizer programs in acidic soils, where it can be a problem. 

Delivering Iron in Alkaline and Saline Soils

Iron deficiency chlorosis is a known issue. Soybeans use iron in the development of chlorophyll for photosynthesis and root nodule formation to fix nitrogen. The risk of iron deficiency chlorosis increases in saline soils, or those with a high salt content. It also increases in high pH, or alkaline, soils.

Iron oxidizes quickly, forming insoluble compounds that plant roots can’t take up. For micronutrient applications, the form of iron applied is a chelate, meaning it is bound in a way to make it more stable in the soil, less likely to oxidize, and more available for plants to absorb. A couple of the most common chelates for iron fertilizer either offer only short-term iron availability in the soil, or do not remain stable in soils with a high pH, above 7. 

“We tested a new iron chelate, Fe-EDDHA, which keeps iron available in the soil longer, and can maintain stability up to soil pH 11,” Wang explains. 

Iron trials in two locations used soil applications of the new chelate, and achieved soybean yield increases between 3 and 13 percent.

“The new chelate appears to improve soybean nutrition in alkaline soils,” he says.

Additional Micronutrient Challenges in High pH Soils

Alkaline soils can also create deficiencies of boron and manganese. Boron supports node number, plant height, flowering, pollen viability, pod formation and seed set. Manganese plays a key role in photosynthesis. 

Through three years of trials in two locations, Wang saw a response to added boron only in 2020 in soils with a pH of 7.2, where it provided a 12 percent yield increase. His team will continue to study this micronutrient in 2021 to better understand the inconsistent results.

In 2021, his team will start manganese trials, as well. Wang plans to study both soil and foliar applications of this micronutrient in high pH soils to see if soybean yields respond. And as with all the micronutrients, he will look at the best way to blend it with macronutrients.

“This project is helping farmers understand how micronutrients can boost yields in specific soil types,” Wang says. “At the same time, we are minimizing application costs by blending these micronutrients with macronutrients like phosphorus and sulfur.”

“We have had a lot of progress in fertility research. So much so, that a lot of input retailers have a wide variety of products to address a myriad of nutrient needs,” says Scott Wiggers, a farmer from Winnsboro, Louisiana, who serves as secretary/treasurer for the Louisiana Soybean and Grain Promotion Board. “For the farmer, it is important to see research like Dr. Wang’s to know how these micronutrient supplemental products could – or could not – benefit you on your farm and with your soils. And it is great to have a land grant university act as that third party to answer those questions for us to maximize the return on our dollar.”

Published: May 24, 2021