Abstract

South American countries are huge grain producers, primarily cultivating soybean (Glycine max), wheat (Triticum aestivum), and maize (Zea mays). The long-term maintenance of prolific grain production systems largely depends on soil fertility. Beyond liming, which is a necessary common practice due to the low soil acidity prevailing in most of the arable lands, preserving adequate soil availability of the macronutrients nitrogen (N), phosphorus (P), potassium (K), and sulfur (S), throughout a single or successive cropping cycles, has become a considerable challenge. Starter fertilizer blends frequently fail to support the anticipated crop yield and grain quality. Polyhalite is a natural marine sediment, which consists of 14% potassium oxide (K₂O), 48% sulfur trioxide (SO₃), 6% magnesium oxide (MgO) and 17% calcium oxide (CaO). As a fertilizer, polyhalite releases nutrients considerably slower than other K-containing fertilizers, thus suggesting additional means to improve soil K availability. The main objective of the trials set in Argentina and Paraguay was to compare, under field conditions, the agronomic efficiency of bulk fertilizer blends that include polyhalite with other formulations currently in use.

Three field trials were conducted in Argentina - one experiment at Nueve de Julio with wheat and two at Mercedes with soybean and maize - and were designed to evaluate the direct effects on a single crop. The treatments included mono-ammonium phosphate (MAP) alone (control), single super phosphate (SSP), MAP + gypsum (34/66%), and MAP + polyhalite at three different ratios 37/63%, 22/78%, and 16/84% that provided increasing levels of K, magnesium (Mg), and S. The crop responses to S were obvious at all growth stages and the average yield increases were 1,371, 1,303 and 754 kg ha^-1, (29%, 24% and 39%) for wheat, maize and soybean, respectively. Among the three crops, only soybean yield increased significantly in response to elevated polyhalite rates.

In Paraguay, a single trial was carried out at Itapúa with soybean as the initial crop grown using the starter fertilizer blend, and maize as the succeeding crop testing the residual soil effects. Treatments included MAP (control), compared with two common fertilizer blends which differed in their phosphorus pentoxide and potassium oxide ratio (P₂O₅:K₂O) - 3:1 vs. 2:1 - comprising MAP, SSP, and a K donor (KCl or polyhalite). Both crops demonstrated significant yield increases in response to the higher K dose applied with the 2:1 P:K ratio. The use of polyhalite also gave rise to a slight but significant soybean yield increase at the 3:1 P:K ratio.

In conclusion, polyhalite is an effective S source on S-deficient soils. In addition, it can successfully replace KCl fertilizers on K-deficient soils. However, the long-term impact of polyhalite is quite limited and cannot be accounted for under successive cropping cycles. Moreover, the advantages of supplemented S, K, or MgO are observable only where the requirements of other essential macronutrients, such as N and P, are adequately met. Otherwise, polyhalite or other corresponding nutrient donors are prone to fail in supporting grain production systems in South America.

Keywords: Glycine max; potassium; starter fertilizers; sulfur; Triticum aestivum; Zea mays.