INFLUENCE OF INTEGRATED NUTRIENT MANAGEMENT ON SOIL PROPERTIES AND PRODUCTIVITY OF POTATO (Solanum tuberosum L.) IN MASHA DISTRICT, SOUTH-WESTERN ETHIOPIA

Abstract

Nutrient depletion has become a major constraint for potato production in Masha district, Sheka Zone, South-western Ethiopia. Thus, studies on different fertilizer management options appear to be a vital agronomic approach to increase the production of the crop and to improve soil properties. Accordingly, three field experiments were conducted. The first experiment was on the effects of the combined application of cattle manure (CM) with inorganic nitrogen and phosphorus fertilizers (NP); the second experiment was on the effects of combined use of vermicompost (VC) and inorganic NP fertilizers and in the third experiment, the promising results of six treatments from the 1st and the 2nd experiment were considered for further evaluation using three rates of blended nitrogen, phosphorus, sulfur, boron (NPSB) fertilizer recommended. The treatments for experiment-1 included three rates of CM (2.5, 5 and 7 5 t ha-1), each with 25%, 50% and 75% of recommended dose of inorganic NP fertilizers (RDF), 100% of recommended dose of inorganic NP fertilizers (165 kg N ha-1 and 60 kg P ha-1) and no-fertilizer treatment(control) making a total of 11 treatment combination. For experiment-2, the treatments consisted of combinations of three levels of VC (2.5, 5 and 7.5 t ha-1) with three levels of NP (25%, 50% and 75%), 100% of recommended dose of inorganic NP fertilizer and a no-fertilizer treatment (control). In experiment-3, treatments [7.5 t VC ha-1 + separately 25%, 50% and 75% of NP, 5 t VC ha-1 +75% of NP), 7.5 t CM ha-1 + separately 50% and 75% of NP from Experiment I and II, 100 kg NPSB + 61.5 kg P +129 kg N ha-1, 150 kg NPSB+138 kg N+53.7 P ha-1, 200 kg NPSB+129 kg N +46 kg P ha-1, 100% of NP and no-fertilizer treatment(control) were evaluated in 2017 cropping season. The design used for all the experiments was a Randomized Complete Block with three replications. Analysis of the results revealed that the combined application of 7.5 t CM ha-1 with 75% of NP gave potato tuber yield 40.20 in Belg and 34.22 t ha-1in Meher which increased by 55.9% and 43.45% in Belg and by 51.19% and 36.64% in Meher season respectively over the no-fertilizer treatment (control) and 100% of NP. Among the soil physico-chemical properties OC, total N, available P, pH, CEC, exchangeable Ca+2, Mg+2 and K+ and total porosity were increased significantly, while bulk density, exchangeable acidity and exchangeable aluminum, significantly decreased. Further, the application of 7.5 t CM ha-1 along with a balanced dose of 124 kg N ha-1 + 45 kg P ha-1 enhanced significantly the plant growth, delayed 50% flowering, and 70% maturity, and increased plant height, marketable and total tuber numbers hill-1, average tuber weight (g tuber-1), dry biomass yields (g hill-1) and, in turn, leads to the maximum marketable tuber yield (t ha-1). The partial budget analysis also gave the use of CM combined with inorganic NP fertilizers indicated that there were maximum net benefits of 91704.6 Birr ha-1 in Belg and 119887 Birr ha-1 in Meher cropping season of 2016 in response to the application of 7.5 t CM ha-1 combined with 75% NP. The result of the second experiment indicated that the combined application of 7.5 t VC ha-1 with 75% NP gave a yield advantage of 41.64 t ha-1 in Belg and 33.7 t ha-1 in Meher i. e. improvement in 63.73% in Belg and 55.46% in Meher seasons over the no-fertilizer treatment (control). Similarly, soil properties after crop harvest also improved with respect to OC, total N, available P, pH, CEC, exchangeable Ca+2, Mg+2 and K+ and the total porosity whereas bulk density, exchangeable acidity, and exchangeable aluminum were significantly reduced with increased rate of VC. The application of 7.5 t VC ha-1 +75% NP increased plant height, improved marketable and total tuber yield (t ha-1), marketable and total tuber numbers hill- xxv 1, average tuber weight (g tuber-1), specific gravity and dry matter content over the no-fertilizer treatment(control) with the highest net benefits of 115091 and 118045 Birr ha-1 in Belg and Meher season from the application of 7.5 t VC ha-1 combined with either 75 % or 25 % of NP respectively in 2016 cropping season. Generally, the combined application of 7.5 t VC ha-1 with 75% NP was more effective in improving soil physico-chemical properties and enhancing growth, yield components of potato. Likewise, the result of third experiment indicated superiority of the application of 7.5 t VC ha-1 combined with 75% of inorganic NP(RDF) and mineral NPSB rates, even though this treatment has shown difference in yields between 2016 and 2017 cropping seasons. The treatment like 7.5 t ha-1 VC combined with 75% of inorganic NP(RDF) gave the highest tuber yield of 36.82 t ha-1 in Belg and 36.7 t ha-1 in Meher. Similarly, the same application rate increased OC, total N, available P, pH, CEC, exchangeable Ca+2, Mg+2, K+ total porosity and increased plant height, marketable and total tuber numbers hill-1, average tuber weight (g tuber-1), dry biomass yields (g hill-1) and, in turn, improved marketable and total tuber yield (t ha-1), starch content, total soluble solids on the one hand, but the same rate of fertilizers application significantly decreased BD, exchangeable acidity, exchangeable aluminium and delay 50% flowering and 70% maturity, however, 200 kg NPSB ha-1 gave relatively higher crude protein content. It was observed that the application of 7.5 t VC ha-1 combined with 75% inorganic NP resulted in the highest economic return of 89017.5 Birr ha-1 in Belg and application of 7.5 t VC ha-1 combined with 75% of inorganic NP (RDF)gave the highest economic return of 131443.5 Birr ha-1 in Meher. Generally, the results of these studies indicated that the use of VC as organic fertilizer source along with judicious use of inorganic N and P fertilizers could improve soil physico-chemical properties and enhanced the productivity of potato in the study areas. Therefore, it can be concluded that potato fields can be supplied with the combined application of 7.5 t VC ha-1 with 75% of recommended dose of nitrogen and phosphorus fertilizers (124 kg N ha-1 and 45 kg P ha-1) to enhance the yield of potato and the soil physico-chemical properties whereas, when the economic return is considered, the application of 7.5 t VC ha-1 with 25% of NP(41.25 kg N ha-1 and 15 kg P ha-1) in Belg and 7.5 t VC ha-1 with 75% of NP(124 kg N ha-1 and 45 kg P ha-1) in Meher season, respectively would be better option in the study areas.