Soybean Response to Ammonium Sulfate in Louisiana
DR. MD. RASEL PARVEJ
BATON ROUGE, LOUSIANA
Sulfur (S) is an essential nutrient for soybean growth and development, playing critical roles in amino acid synthesis, chlorophyll formation, enzyme activation, and nitrogen (N) fixation. Crucially, three of the nine essential amino acids in soybean protein contain S, including cystine, cysteine, and methionine. Soybean plants obtain S from various sources such as soil organic matter, atmospheric deposition, and the mineralization of S-containing compounds. However, S deficiency has emerged as a significant concern for crop production, exacerbated by a 90% reduction in atmospheric S deposition via rainfall since the “Clean Air Act” was adopted.
Sulfur deficiencies can occur in soils with low organic matter content, acidic pH, or very low S content and availability. Remarkably, around 95% of total soil S is bound within organic matter, with even a mere 1% of organic matter housing over 100 lb S/acre, releasing only 3-5 lb S/acre depending on environmental factors such as temperature, rainfall patterns, and soil management practices. Additionally, trace amounts of S may also be found in pesticides and irrigation water.
The response of soybean to S fertilization depends on a multitude of factors, including soil-test S levels, organic matter content, prevailing environmental conditions, crop management techniques, and the type of S-containing fertilizer utilized. Here are several critical points regarding soybean response to S fertilization:
1. Symptoms of Sulfur Deficiency: Soybean exhibits S deficiency as chlorosis (yellowing) of younger leaves, stunted growth, delayed maturity, and reduced seed yield. Notably, S deficiency symptoms tend to manifest primarily on the upper, younger leaves of soybean plants, often resembling symptoms of N deficiency. However, a distinguishing feature is that while N deficiency primarily affects lower, older leaves due to N’s mobility within plants, S deficiency affects upper, younger leaves owing to S’s immobility within plants.
2. Soil Test S Level: Soil testing plays a vital role in evaluating S availability within soybean production systems. Soil test, sulfate-S (SO4-S) test, offers insights into soil S levels and helps determine the need for S fertilization. According to the current recommendations from LSU AgCenter, soybean requires S fertilization in soils containing less than 10 ppm (20 lb S/acre) of Mehlich-3 S within the 0- to 6-inch soil depth.
3. Sulfur Fertilization: Soybean requires 20 lb S/acre in soils with S concentration ≤ 10 ppm. There are several dry granular S fertilizers available in the market including gypsum (~23% Ca and 16-19% S, depending on purity), sul4R-Plus (23% Ca and 17% S), tiger 90CR (90% S), K-mag (22% K2O, 11% Mg, and 21-22% S), poly4 (14% K2O and 19% S), ammonium sulfate (AMS, 21% N and 24% S), etc. Additionally, some liquid S products are also found in the market including ammonium thiosulfate (ATS, 12% N and 26% S), potassium thiosulfate (KTS, 25% K2O and 17% S), etc.
In recent evaluations of soybean response to S rates and sources in Louisiana, a 1-year study across 21 sites revealed a positive yield response to S fertilization in two sites where Mehlich-3 soil-test S concentrations were < 7 ppm and soil organic matter content was 1.2% at the 0- to 6-inch soil depth. However, no significant yield increment was observed with S fertilization in sites where soil S concentrations were near 10 ppm. Moreover, no yield response was measured in soils with S concentrations exceeding 10 ppm. These preliminary results indicate the necessity of S fertilization in soils with S concentrations less than 10 ppm, while no S is needed in soils with S concentrations exceeding 10 ppm.
In a 2-year study evaluating S sources (gypsum, sul4r-plus, tiger 90CR, ammonium sulfate, K-mag, and poly4), no significant yield response to S fertilization was observed across four sites where soil S concentrations were near or greater than 10 ppm. However, in three out of four sites, a numerically negative yield response (2-5 bu/acre) to S fertilization was observed when using ammonium sulfate (AMS) compared to the no-S control. This negative response may be attributed to the presence of 21% N in AMS, which can adversely affect soybean nodulation. Examination of soybean roots revealed minimal to zero nodules in plants that received AMS compared to gypsum and the no-S control (Figure 1).
Numerous research suggests that AMS has the potential to negatively impact nodulation in soybean under certain conditions. Here's how:
a. Nitrification Effect: Ammonium ions (NH4+) released from AMS can accumulate in the soil solution and eventually undergo microbial conversion to nitrate (NO3-), the process called nitrification. High concentrations of nitrate ions (NO3-) can be toxic to N-fixing bacteria (rhizobia) present in the soil, inhibiting their growth and activity. Consequently, this can interfere with the establishment of beneficial rhizobial populations and the formation of effective nodules on soybean roots.
Some studies conducted in the mid-west have observed a positive yield response in soybean to AMS, possibly attributed to the slower nitrification process. Nitrification, influenced by factors like temperature, soil moisture, pH, and microbial activity, tends to proceed more rapidly in warmer and moister conditions, compared to cooler and drier environments. Therefore, the conversion from ammonium to nitrate may occur at a faster rate in the southern USA than in the northern USA. However, the nitrification process exhibits variability across different years and fields, and its impact on soybean nodulation may not be negative in every instance.
b. Acidifying Effect: When ammonium sulfate undergoes nitrification, it induces soil acidification, thereby lowering soil pH by providing more hydrogen (H+) ions. This decrease in pH creates unfavorable conditions for the survival and nodulation activity of rhizobia. Ideally, rhizobia thrive in soil with neutral to slightly acidic pH levels (pH ~6.5). Excessive soil acidity (pH <6.0) can impede the optimal function of rhizobia.
c. Nitrogen Competition: Ammonium sulfate offers soybean a readily accessible N source. However, excessive N availability from AMS can diminish nodulation and N fixation in soybean plants. In situations where soil contains abundant N, soybean tends to prioritize N uptake from the soil over forming nodules and fostering symbiotic relationships with rhizobia. This phenomenon is particularly concerning in soils with high residual N levels.
To sum up, S fertilization proves to be a valuable approach for addressing S deficiencies and enhancing both soybean yield and protein quality. Nonetheless, it's crucial to consider soil S levels, environmental conditions, and the choice of S fertilizer source when determining S fertilization in soybean production. For soybean producers in Louisiana, opting for S-containing fertilizers with minimal or no ammonium or nitrate content is advisable to fulfill soybean S requirements without adversely affecting nodulation. ∆
DR. MD. RASEL PARVEJ: LSU AgCenter