Liquid Sulfur Fertilizer Sources Containing Nitrogen or Potassium Applied to Soybeans

INSIGHTS

Graph 1. Seasonal K₂O uptake in soybean. Peak K₂O uptake occurs between flowering and the end of seed filling (Bender et al., 2015)

• Decreased atmospheric sulfur (S) deposition is resulting in more frequent corn and soybean sulfur deficiencies.

• Potassium (K) uptake by soybeans is 2.5 lbs of K₂O per bushel.

• In this study, soybean response to N, K, or S containing liquid fertilizers was minimal suggesting those nutrients were not the yield limiting factor.

Introduction

Recently, sulfur has started to become yield limiting in many geographies, as atmospheric sulfur deposition has decreased with improved air quality standards and as crop removal rates have increased with yields. Sulfur mineralizes from organic matter in the soil into sulfate (SO42-) which makes it more subject to leaching, similar to nitrate nitrogen. Deficiencies are often noticed in coarse, eroded, or low organic matter soils that are less able to mineralize the plant-available sulfate form. Mineralization will often slow with cool soil conditions, sometimes making soils that otherwise test high in sulfur show deficiency symptoms until warming and sulfate mineralization speeds up. Due to this, soil testing procedures for sulfur are often unreliable and typically only recommended for use on sandy soils. Plant tissue samples are often needed to differentiate from other nutrient deficiencies.

Soybean demand for potassium is substantial, accumulating 2.5 lbs of K₂O/bushel or roughly 175 lbs of K₂O for a 70 bu/A soybean crop (Graph 1). There is a large reserve of K in the soil, however, a relatively small amount of K is available for plant growth at any one time. Most of the K is tied up in the structural components of the soil and the availability of this K is environmentally dependent. Potassium has limited mobility in the soil and is taken up by the plant through diffusion. As uptake of K occurs, the concentration in the soil solution near the root decreases. This creates a gradient for the nutrient to diffuse though the soil solution from a zone of high concentration into the depleted solution adjacent to the root. Peak K uptake occurs between flowering and the end of seed filling (Graph 1). Soil moisture, soil temperature, and soil oxygen levels are all key factors that affect K uptake.

2023 Sulfur Fertilizer Trial in Soybeans

In 2021, sulfur trials were conducted at nine locations across the Midwest. Ammonium thiosulfate (ATS, 12-0- 26) was surface dribbled 3-inches to each side of the row during planting at 20 lbs/A of S. Non-sulfur treated plots were treated with 9 lbs/A of nitrogen (N) in the form of urea ammonium nitrate (UAN, 32-0-0) using the same application method and timing to provide an equivalent amount of nitrogen as was applied to the ATS treated plots. Sulfur applications at two of the nine locations significantly increased soybean yield by 8 and 16 bu/A.

In 2023, Golden Harvest Agronomy in Action Research expanded on the sulfur trial to answer the question: When making a sulfur application to soybeans, is there additional yield to be gained using a sulfur source containing potassium, such as potassium thiosulfate (KTS), compared to using a sulfur source containing nitrogen, like ATS?

*significant difference between sulfur treatment and the check at α = 0.10
Graph 2. Effect of sulfur treatment on yield for 8 Golden Harvest soybeans averaged across early and late relatively maturity locations.

Trials were established at 7 locations across Illinois, Iowa, Kansas, Nebraska, South Dakota, and Wisconsin. ATS or KTS was applied 2×2×2 with the planter (Figure 1). ATS was applied at 7 gal/A suppling 20 lbs/acre of S and 9 lbs/acre of N. KTS was applied at 9.6 gal/acre suppling 20 lbs/acre of S and 29 lbs/acre of K. Check plots did not receive any N, K, or S.

Treatments were applied to 4 soybean varieties at each location to measure any potential response interactions. All treatments were replicated 4 times within each location.

Figure 1. ATS and KTS applied with liquid fertilizer banding attachment on planter.

Soybean Response to ATS or KTS Fertilizer

There was no significant difference between ATS or KTS and the check at any of the 7 locations. Only at Bridgewater, SD, Clinton, IL and Waterloo, NE was there small numerical yield increases. Bridgewater experienced a 1.4 and 1.6 bu/acre response to ATS and KTS, respectively. Clinton had a 1.1 bu/acre response to ATS fertilizer. Waterloo responded to the KTS by 2.4 bu/acre. When averaged across all locations and varieties, soybeans yielded 70.0 bu/acre treated with ATS and 70.3 bu/acre treated with KTS, compared to 69.9 bu/acre when no additional fertilizer was applied.

Statistically there was no interaction between fertility treatment and soybean variety meaning all varieties responded similarly to the fertility treatments (Graph 2). GH2004XF, GH2083E3S, GH2884XF brand soybeans were the only soybean varieties that tended to respond to KTS although were not statistically significant. No varieties responded to ATS (Graph 2).

Soil Test Values

Pre-plant soil tests show adequate K levels at all locations except Clinton, IL and Slater, IA (Table 1). Both locations were the highest yielding locations ranging in yield from 79-93 bu/acre meaning K demand was elevated. However, there was no yield increase when KTS was applied at either location. Sulfur soil test values were low for all sampled locations despite no response to S fertilizer. None of the locations contained a sandy soil type where most S deficiency symptoms are typically observed. Likely, there was enough S mineralized through organic matter to meet the demand of the soybean crop.

Table 1. Soil test values for 6 locations across the Midwest.

Summary

When evaluating crop response to nutrient applications, the crop must be deficient in the applied nutrient to achieve a response. Leibig’s Law of the Minimum states crop yield is limited by the lowest resource level. In this study, N, K, or S was likely not the limiting factor, and adding additional fertilizer had no effect on soybean yield.

Despite no response to the S and K fertilizers in this study, both nutrients are critical for soybean production. A proactive fertility management program should be utilized to mitigate the risk when environment conditions are not conducive to nutrient release and availability. Coarse or sandy soils are prone to S leaching and reduced mineralization often leading to deficiencies. In addition, large amounts of K are removed with the grain each year in both corn and soybeans and must be managed to maintain maximum yield potential.

Reference

Bender, R.R., Haegele, J.W. and Below, F.E. (2015), Nutrient Uptake, Partitioning, and Remobilization in Modern Soybean Varieties. Agronomy Journal, 107: 563-573