Assessing the indigenous nutrient supplying capacity of soils to Soybean in Vertisols of Northern Karnataka

The study was aimed at validating the Nutrient Expert® (NE) fertilizer decision support tool for soybean with the existing nutrient management practices, and determine soybean response to applied nutrients and seed inoculation in the Vertisols of northern Karnataka. Replicated on-station experiments were conducted at the Agricultural Research Station (ARS) of the University of Agricultural Sciences, Dharwad. The study consisted of nine treatments: 1) NE (27-51-51 kg N-P₂O₅-K₂O/ha) plus seed inoculation with Rhizobium; 2) NE minus seed inoculation with Rhizobium; 3-4) two treatments with omission of N from the NE recommendation, but one with seed inoculation and the other without seed inoculation; 5-6) two treatments with omission of phosphorus (P), and potassium (K) from NE; 7) State Recommendation/RDF (40-80-25 kg/ha); 8) farmer practice/FP (23-58-0 kg/ha); and 9) an absolute unfertilized control.

The average grain yield of soybean in the NE treatment was 2,040 kg/ha and yield response to application of nitrogen (N), P, and K was 470, 395, and 260 kg/ha, respectively. The yield reduction due to omission of N, P, and K was 23, 19.4, and 12.7%, indicating that N followed by P and K are the most limiting nutrients for growing soybean on these medium deep black soils. The average yield recorded for the RDF was 2,170 kg/ha, which was 6% higher than NE. The yield recorded for FP and absolute control were 1,560 and 1,195 kg/ha, which were 24 and 42% lower than the NE treatment. The higher grain yield of soybean in NE treatment was attributed to significantly high dry matter per plant, more number of pods per plant, high test weight of seed, taller plant height, and an increased number of nodules per plant.

The study also offered interesting observations on the effect of omitting N from the Rhizobium-inoculated and the non-inoculated treatments. The results indicate that skipping Rhizobium inoculation from NE, reduced yield by only 3% (66 kg/ha). However, omitting N from inoculated and non-inoculated plots showed a yield decline of 470 and 660 kg/ha, with yield reduction of 23% due to N omission and 32% due to combined omission of N+seed inoculation, indicating that an additional 190 kg/ha of yield was reduced due to omitting seed inoculation. The study also indicated that contribution of N from biological N fixation due to inoculation could be significant with application of N, and skipping both N and seed inoculation could drastically reduce the yield. The higher yield recorded with RDF (40-80-25 kg N-P₂O₅-K₂O/ha) in comparison to NE (27-51-51 kg N-P₂O₅-K₂O/ha), also suggests further investigations on variable rate and time of N and P application to soybean.