New Soybean Tissue Sampling Protocol Has Potential to Boost Producer Profits

The Problem

Soybean producers surveying their fields may see green leaves and no problems. But an invisible enemy may be lurking in the plants — “hidden hunger.” Soybean fields in Arkansas and elsewhere are prone to suffer from this condition, which describes plants that look healthy but are experiencing a nutrient deficiency that can harm yield and eat into potential profits. Potassium — also called potash — is the yield-limiting nutrient for soybean plants.

According to the U.S. Department of Agriculture’s National Agricultural Statistics Service, Arkansas harvested 3.15 million acres of soybean in 2022, with an average yield of 52 bushels per acre.

Developing recommendations for producers to identify hidden hunger during the growing season and course-correct to help recover lost yield and improve the bottom line.

The Work

Researchers from the Arkansas Agricultural Experiment Station have been investigating the effects of potassium deficiency in soybean plants since 2016. As part of this ongoing work, Trent Roberts, professor and the Endowed Chair in Soil Fertility Research for the experiment station, led a recent study to determine the best protocol for collecting in-season tissue samples of soybean plants, which can be used to identify hidden hunger.

Tissue testing involves analyzing soybean plant material in a lab to determine the concentration of potassium in the plant. If plants test below a certain threshold — called the critical concentration — the plants are suffering from nutrient deficiency and will lose yield potential.

The researchers conducted grid sampling in one-acre grids at five producer-managed soybean fields in Arkansas in 2020 and 2021. The samples were analyzed in a lab to determine the average concentration of potassium in each sample at a given time and field location. The measured concentrations were then compared against the critical potassium concentrations.

The study was conducted by Roberts and doctoral student Carrie Ortel. Other collaborators included doctoral student Kyle Hoegenauer, assistant professor of precision agriculture Aurelie Poncet, professor and soybean extension specialist Jeremy Ross, and Nathan Slaton, assistant director of the experiment station.

The Results

Through spatial analysis of the data, Roberts said the team found no benefit to high-resolution grid sampling. The data showed no indication that concentrations of potassium in soybean tissue varied within a given field management area — a segment of land with similar conditions defined to help fine-tune management decisions. In other words, Roberts said, the data demonstrated no benefit to grid sampling.

Instead, composite sampling was found to be sufficient to capture the variability of soybean tissue samples within management areas. The researchers recommend that a composite sample consisting of at least 18 of the uppermost fully expanded trifoliolate leaves will be representative of the management area. Trifoliolate leaves are compound leaves composed of three leaflets.

Roberts said one surprising finding that wasn’t an objective of the study was that 90 percent of the fields they evaluated were deficient in potash but had no visual deficiency symptoms. In other words, they were suffering from hidden hunger.

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The Value

Roberts sees a great opportunity to address potassium deficiency in soybean plants, noting that producers could increase yields by as much as 5 to 15 bushels per acre. As an example, Roberts said that an 80-bushel soybean crop that’s losing 10 percent of yield from hidden hunger would be losing 8 bushels of yield. At a price of $14 per bushel, recovering those 8 bushels per acre could earn back more than $100 per acre for farmers.

This new soybean tissue sampling protocol sets the stage for the next stage of Roberts’ research, which is to develop a set of calibrated potassium fertilizer rates to give producers site-specific recommendations for in-season applications. The goal is to give producers a calibrated, site-specific in-season fertilizer rate to help producers maximize yield and ultimately take the guesswork out of production systems.

About the Researcher