Validation of fertilizer recommendations for soybean under high yielding crop production systems in Central and South of Russia

Two field experiments were conducted in 2015 in Krasnodar Krai, South Russia, on leached chernozem soils located in the central and northern agroecological zone of the region, respectively. Both soils (0 to 20 cm soil layer) had high clay content, neutral pH, low organic matter (3.3 to 3.5%), high Olsen P (16 to 17 ppm) and a very high exchangeable K (358 to 376 ppm). The following six treatments were studied in 2015: 1) Control (zero fertilizer); 2) 18 kg N/ha; 3) 6-26 kg N-P₂O₅/ha; 4) 6-26-18 kg N-P₂O₅-K₂O/ha; 5) 12-25 kg N-P₂O₅/ha; 6) 12-52-18 kg N-P₂O₅-K₂O/ha. All fertilizers were applied at planting (2 cm × 2 cm spacing). Seeds were inoculated and treated with Mo.

Pod filling of an early maturity variety was practically finalized before a drought period occurred in the end of July, however, a medium maturity variety was negatively affected and the genetic potential could not be realized this season. The best yield of an early maturity variety (2.1 t/ha, 7% increase over control) was produced with the starter application of 6-26-18. The highest yield of the medium maturity variety (1.7 t/ha, 5% increase over control) was achieved with the 6-26 treatment. Omitting of K fertilizer had no statistical effect on soybean yield at either locations. Results obtained in 2015 show that both varieties required moderate starter rates of nutrients.

In Belgorod oblast, trials were organized on a large industrial farm in two locations with dark grey forest soil and typical chernozem soil. In both locations, experimental plots had low plant-available S (3.2 to 4.1 ppm). The experiment tested the following treatments: 1) control, no fertilizers; 2) 18 kg N/ha; 3) 9-39 kg N-P₂O₅/ha; 4)9-39-60 kg N-P₂O₅-K₂O/ha; 5) 18-78 kg N-P₂O₅/ha; 6) 18-78-60 kg N-P₂O₅-K₂O/ha; 7) 9-40-60-10 kg N-P₂O₅-K₂O-S/ha; 8) 18-80-80-20 kg N-P₂O₅-K₂O-S/ha.

In spite of unfavorable weather conditions (relatively dry summer with only 75% of the average precipitation), soybean yields were higher than in 2014. On the chernozemic soil, the highest yield (2.6 t/ha) was achieved with 18-80-80-20. However, the same yield was achieved with lower fertilizer application 9-40-60-10. Sulfur content in seeds was not sufficient in all treatments. On the grey forest soils, yields were 2.6 to 2.7 t/ha. The highest yield was achieved with 9-40-60-10 and 18-80-80-20. The content of S in seeds was only sufficient (0.32% of dry matter) in treatments providing S. The identified crop response to S has very important implications since nearly all arable soils in Belgorod oblast are low in S and this nutrient is likely the yield-limiting factor for high-yield soybean productivity in this region.