Oxidation of Elemental Sulfur in Granular Fertilizers Depends on the Soil-Exposed Surface Area

Abstract

Elemental S (ES) is the most concentrated S source and does not leach readily in soils but only becomes available to plants after oxidation to SO 4 -S. Commercial ES fertilizers are usually in granular form (e.g., as ES prills or ES-fortified N-P-K fertilizer), but studies on ES oxidation have mostly focused on ES powder mixed through soil. We studied the oxidation of commercial granular ES fertilizers and powdered ES in a column incubation experiment at 25°C with three soils (two Mollisols and an Oxisol). The columns were leached at periodic intervals, and ES oxidation was evaluated by measuring S in the leachates and by analyzing ES remaining in the soil at the end of the experiment (56 or 36 wk). Sulfate immobilization in organic matter was taken into account in the derivation of the ES oxidation rate. For all three soils, the oxidation rate was around 0.02 d −1 for ES particles (diameter <100 mm) mixed with soil, around 0.005 d −1 for ES-fortified monoammonium phosphate granules with up to 7.5% (<100 mm) ES, and around 0.0006 d −1 for ES-bentonite pastilles (90% ES). The slower oxidation of granular than of powdered ES was attributed to the reduced surface area in contact with the soil. Knowledge of the oxidation rate of fertilizer ES is important for improving fertilizer management. This study shows that the oxidation rate of granular ES fertilizers differs greatly depending on their composition and provides the first quantitative assessment of the oxidation rate of ES in S-fortified N-P fertilizers.Abbreviations: ES, elemental sulfur; ESD, equivalent spherical diameter; MAP, monoammonium phosphate. S ulfur is an essential element for plant growth, with concentrations in most plants ranging between 1 and 3 g S kg −1 . In cropping systems, the yearly export in harvested products ranges from 4 to 30 kg S ha −1 (Zhao et al., 2002). Despite these relatively high values, S has received little attention as a plant nutrient because inputs through atmospheric deposition and S-containing fertilizers or amendments such as single superphosphate (SSP) or (NH 4 ) 2 SO 4 were usually sufficient to fulfill the crop demand (Ceccotti, 1996). However, atmospheric S deposition has substantially decreased in many regions during the last decades because of stricter pollution control. Furthermore, a shift from SSP to high-analysis P fertilizers has also resulted in reduced inputs. On the other hand, crop removal of S in new high-yielding cultivars has increased (Bender et al., 2013). As a result, S deficiency has become more common (Scherer, 2001), and the demand for S fertilizers has increased (Ceccotti et al., 1998).In the majority of inorganic-S fertilizers, S is present either as SO 4 -S or as ES (S 0 ). Sulfate-based fertilizers include (NH 4 ) 2 SO 4 , single superphosphate, K 2 SO 4 , gypsum, and (potassium) magnesium sulfate. Sulfate fertilizers have the benefit that the S applied is readily available to the plant, but because of the high mobility of Core Ideas• Judicious use of elemental S fertilizers requires knowledg...

Availability of fertiliser sulphate and elemental sulphur to canola in two consecutive crops

“…These values are similar to the ones determined in a column incubation experiment with three soils from US, Canada and Brazil (Degryse et al 2015), in which oxidation rates were around 0.5 % per day for Sfortified NP fertilisers with 5-7.5 % ES and around 0.06 % per day for ES pastilles.…”

“…The oxidation of ES in the S-fortified NP fertilisers was much faster than for the ES pastilles, but has been shown to be slower than for ES particles of same size (as the particles in the fortified fertiliser) mixed through soil (Degryse et al 2015). This is most likely due to the reduction in the effective ES surface area when ES is cogranulated (Friesen 1996).…”

“…Solberg et al (2005) measured SO 4 -S in soils incubated with different ES fertilisers and found very low SO 4 -S recoveries for various ES-bentonite products (<3 % of applied ES after 12 weeks), while fine ES powder mixed through soil showed SO 4 -S recovery around 50−60 % after only 6 weeks. The slow oxidation of ES-bentonite pastilles can be explained by these pastilles behaving as a single large ES particle, resulting in very slow oxidation (Degryse et al 2015).…”

“…The pastilles are supposed to disintegrate under field conditions, aided by rainfall and freeze/thaw cycles. Disintegration of the pastilles appears to be essential, otherwise each pastille acts as a single large ES particle, resulting in very slow oxidation (Degryse et al 2015). Boswell et al (1988b) found that easily dispersible surface-applied ES prills disintegrated quickly and already had a large contribution to S uptake in the first year of application.…”

“…In a recent study, we evaluated the oxidation rate of ES in commercial fertilisers in three soils at 25°C, and found that the (first-order) oxidation rate of ES in ES-fortified MAP fertiliser with 5-7.5 % ES was around 0.5 % per day (i.e. half-life of oxidation~140 d), compared to around 2 % per day (half-life of 35 d) for ES particles of similar size (diameter <100 μm) and around 0.06 % per day (half-life~1200 d) for ES pastilles (Degryse et al 2015). However, further evaluation of these products in pot trial and field trials is still needed to assess their agronomic effectiveness.…”