Effect of Climate and Soil Properties on Sulfate and Elemental Sulfur Recovery from MicroEssentials

Introduction

Sulfur (S) is an essential element for all crops. Sulfur deficiency has become more common due to decreased atmospheric inputs, higher yields, and a shift to high-analysis fertilizers with little or no S. Commonly used S fertilizer sources contain either sulfate-S (SO4-S) or elemental sulfur (ES). Sulfate-S is readily available to plants but is vulnerable to leaching in most soils. On the contrary, ES is not prone to leaching and must be oxidized into plant-available SO4-S. The rate of oxidation depends on several factors, including climate, soil properties, and fertilizer granule characteristics. Oxidation of ES is a biological process and it generally increases with an increase in temperature, soil pH, organic matter content, and microbial activity. Among fertilizer granule characteristics, ES particle size and %ES concentration within the granule greatly affect the oxidation rates. Oxidation is dependent on surface area and decreases dramatically as the particle size increases. The surface area available for oxidation also depends on the total concentration of ES in the granule. As the ES concentration within the granule increases, the oxidation rate decreases due to decreased contact with the soil (Degryse et al, 2016a, 2016b; C rop Nu t r i t ion, 2017).

Climate and soil properties also have a large effect on the fate of SO4-S and ES fertilizers. Leaching of SO4-S depends on average annual precipitation and soil type, whereas the oxidation of ES is highly temperature and soil pH dependent. This study aimed to evaluate the effect of climatic and soil conditions on the recovery and residual value of SO4-S and ES applied from MicroEssentials using stable isotope tracing (Degryse et al, 2020). Three field trials were conducted for 2 years in Argentina, Brazil, and Canada (Fig. 1). Crops commonly grown in these geographies were planted with MicroEssentials applications made in the first year as a broadcast application to measure the S recovery for two years (Table 1). Plant samples for S uptake were collected at harvest.

The recovery of plant S from SO4-S and ES is shown in Figure 2. For Argentina, total recovery in the harvested material at the end of the second year was 85.7% (77.6+8.1) for SO4-S compared to 25.7% (12.3+13.4) for ES. More SO4-S recovery can be explained by low rainfall in the first two months after fertilizer application (Fig. 3) and low ES recovery is due to slower oxidation in the slightly acidic soil with low organic matter content. For Canada, total recovery was 65.6% (59.1+6.5) for SO4-S and 19.2% (5.8+13.4) for ES. In the colder climate, slower conversion of SO4-S to organic S led to higher SO4-S uptake and slower ES oxidation lead to reduced ES recovery. The trends over time were similar for both sites, where recoveries of SO4-S were considerably greater than those of ES in the first-year due to the rapid availability of SO4-S for plant uptake and slower oxidation of ES. For these situations, the remaining ES continues to oxidize over time and contributes to plant uptake. However, the total uptake of S in the second year was considerably lower at both sites, indicating there was insufficient S available for plants for the second crop. Therefore, additional S application to the next crop in rotation is necessary to meet the crop’s S needs.

For the Brazilian site, the recovery from applied fertilizer was more from ES than SO4-S for all crops except soybean in the first year, where both forms of S had a similar recovery. The total recovery of S at the end of the second year was 9.3% for SO4-S compared to 15.9% for ES. Lower SO4-S recovery was due to faster immobilization and excess rainfall because nearly 600 mm rainfall occurred in the first 2 months after S application (Fig. 3). Higher recovery for ES was due to faster oxidation in a warmer climate. Lower total S recovery can also be due to leaching of SO4-S that was oxidized from ES during the 2-year period in the warmer climate. More contribution from ES in Brazilian conditions thus suggests the need for ES as an S source for better utilization of applied S fertilizers.

Recoveries of fertilizer S varied quite dramatically across different climatic and soil conditions. The SO4-S recovery in the year of the application was much smaller for the Brazilian site than for Argentinean and Canadian sites due to high leaching potential at the location in Brazil, indicating the importance of ES source in Brazil. Therefore, products like MicroEssentials^®S9^® with SO4 (2%) and ES (7%) are a great fit for Brazil’s climate and soil conditions. For colder climates like Canada, crop S needs for early growth stages can be met by including higher SO4-S amounts along with ES, like in MicroEssentials S15 (7.5% SO4 and 7.5% ES). Therefore, depending on climate and soil conditions, a fertilizer containing both forms of S in suitable amounts will help reduce leaching risks, provide readily available S, and supply season-long S. The study also showed a trend of decreased contribution from total fertilizer S in the second year, demonstrating that the fertilizer applied only once in the first year is not sufficient to meet crop demand. Therefore, the application of S every year, and to every crop, is very critical.

Suggested Readings

Oxidation of Elemental Sulfur in soils. Crop Nutrition 2017, 13. https://www.cropnutrition.com/resource-library/oxidation-of-elemental-sulfur-in-soils

Fien Degryse, Roslyn Baird, Rodrigo C. da Silva, Christopher B. Holzapfel, Claudinei Kappes, Monica Tysko and Mike J. McLaughlin. Sulfur Uptake from Fertilizer Fortified with Sulfate and Elemental S in Three Contrasting Climatic Zones. Agronomy 2020, 10, 1035

Degryse, F.; Ajiboye, B.; Baird, R.; da Silva, R.C.; McLaughlin, M.J. Oxidation of elemental sulfur in granular fertilizers depends on the soil-exposed surface area. Soil Sci. Soc. Am. J. 2016a, 80, 294–305.Degryse, F.; da Silva, R.C.; Baird, R.; McLaughlin, M.J. Effect of cogranulation on oxidation of elemental sulfur: Theoretical model and experimental validation. Soil Sci. Soc. Am. J. 2016b, 80, 1244 –1253.

Acknowledgement: Fertilizer Technology Research Centre, University of Adelaide.