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Managing Sulfur Fertility to Enhance Soybean Yield and Protein Quality

Ray Weil

Sulfur (S) is an essential macronutrient and a key component in the essential amino acids methionine and cysteine (METH+CYST) that are the building blocks of protein. The nutritional value of protein from soybeans, like that from most grain legumes, is limited by relatively low levels of METH+CYST. Sulfur deficiencies are becoming more widespread as soil reserves of S are depleted greater removals in high yielding crops in combination with much lower inadvertent S input. Sulfur deficiencies are most common on sandy soils that are low in organic matter and anion exchange capacity. Sulfur management has largely been ignored in soil fertility because (1) historically many fertilizers and pesticides contained S “impurities”; (2) until the late 1990s, sufficient S was supplied through atmospheric deposition of sulfur dioxide emissions from coal-fired power plants; and (3) until the inventions and widespread laboratory use of sophisticated and expensive instrumental methods (Inductively Coupled Plasma-Atomic Emission Spectroscopy, ICP-AES), the measurement of S in plant and soil samples was cumbersome and unreliable.

In order to compare two common sulfate sources of sulfur and determine whether sulfur fertilization could enhance the amino acid make-up of soybean protein, we conducted a series of eight field experiments. The experiments had a randomized split plot design with two levels (applied or not) of S sources factors (Gypsum, Epson salt) to give four treatments: 1) control (G0E0), 2) 560 kg/ha gypsum (17% S) broadcast at planting (G1), 3) 86 kg/ha Epsom salt (13% S) as a foliar spray at soybean R1 growth stage (E1), and 4) the combination gypsum + Epsom (G1E1). Soybean yield, seed S concentration, seed S yield were measured on all plots to determine the effects of S fertilization. In addition, selected samples were analyzed for contents of amino acids. In each of two years (2017 and 2018) this experiment was conducted using two types of soybean crops (full season and double crop) and two soil types (relatively coarse and fine) for a total of eight site-years. Soybean seed yield, seed S content, and S yield were significantly (p<0.1) increased with the S treatment in three out of the eight site-years. Sulfur-containing amino acid (METH+CYST) content of the seed was significantly increased by all three S application treatments (p<0.1). Results of this experiment show that applied S, on low available S soils, can produce significant yield increases (up to 35%) and stimulate dramatic increases (up to 90%) in the METH+CYST content the seed. Thus our work demonstrated that managing sulfur fertility can indeed enhance the value of soybeans as a source of protein.

We also attempted to evaluate four alternative soil test methods for predicting where S application would be beneficial. Soil S levels were analyzed using four different extraction protocols: (1) 0.01 Molar calcium chloride shaken with soil at a ratio of 5:1, (2) 500 ppm calcium phosphate in water shaken with soil at a ratio of 2.5:1, (3)500 ppm calcium phosphate in 2 Molar acetic acid shaken with soil at a ratio of 2.5:1, and (4) Mehlich-3 extracting solution shaken with soil at a ratio of 10:1 (referred to as CaCl2, CaHPO4, CaHPO4-HOAc, and Mehlich-3, respectively). For the four S soil test protocols, the measured soil S levels in the 0-10 cm A1 layer, in the subsoil below the A horizon to 30 cm, and the weighted average of all layers 0-30 cm were compared to the soybean crop responses to applied S at up to 23 sites (122 individual blocks). Relative soybean seed yield (kg soybean seed/ha) and relative soybean S yield (kg S in seeds/ ha) were the crop responses used to calibrate the soil tests. The calibration involved calculation of a critical value intended to demarcate fields so deficient in S that soybeans would likely respond positively to S applications from fields sufficient in S such that applying additional S would not be expected to have an effect. When considering the weighted average of the full 0-30 cm sampled soil depth, the critical values were 5.5, 4.4, between 9.9 and 11.3, and between 16.2 mg S/kg soil, respectively, for the CaCl2, CaHPO4, CaHPO4-HOAc, and Mehlich-3 soil test protocols.

The exploration of sulfur soil test protocols was only partially successful. The CaCl2, CaHPO4(water) and the Mehlich-3 all proved to have some value in identifying soils on which soybeans are likely to respond positively to applied S. However, all the protocols also mis-identified some fields as not needing S when the soybean did respond and some as needing sulfur when the soybean did not respond. The Mechlich-3 test was as good as any of the others and correctly identified about six or seven of the nine responsive soils in the study. Including soil from below the A horizon did not seem to improve the soil test accuracy. More research should be done to refine these findings on a wider range of soybean varieties, soils and environments. As with any nutrient, significant responses to sulfur application are expected only where the soil S availability is limiting to plant growth or function. Therefore, more work is needed to improve soil testing methods that can reliably identify S-responsive soils.