Effects of Conservation Tillage Method, Phosphorus Level, and Variety on Soil Organic Carbon and Cowpea Performance at Minna in Nigeria’s Southern Guinea Savanna

Abstract

The study was carried out at the Teaching and Research Farm of the Federal University of Technology Minna, Gidan Kwano Campus in Latitude 9° 41' N, Longitude 6° 31' E, and Altitude 258.5 m above sea level. This study was conducted across 2 seasons within the cropping season of 2017 to determine the effects of tillage method, phosphorus level, and cowpea variety on soil organic carbon and cowpea performance in Minna. The experiment was therefore a 3x3x3 factorial arrangement in a split-plot design with tillage methods (zero, reduced, and manual ridging) and cowpea varieties (IT93K-452-1, IT99K-573-1-1, and IT90K-277-2) forming the main plot while phosphorus levels (0 P kg ha-1, 30 P kg ha-1, and 60 P kg ha-1) formed the sub-plot with 3 replicates. Results obtained revealed that soil organic carbon was not significantly affected by tillage method and phosphorus level alone, regardless of the planting season. Cowpea variety alone significantly affected organic carbon only in the late planting season. Zero tillage, 60 kg P ha-1, and IT93K-452-1 produced the highest soil organic carbon content, respectively. In the early season, N fixed was highest (0.98 g plant-1) when cowpea was fertilized with 60 Kg P ha-1 under zero tillage. The closest to this value (0.92 g plant-1) was recorded in the late season when 60 kg P ha-1 was supplied to cowpea under manual tillage. Regardless of the planting season and varietal difference, cowpea recorded the highest N fixed when fertilized with 60 kg P ha-1. Across varieties, cowpea fixed more N under zero tillage than the other tillage methods, especially in the early season. In the late season, only IT90K-277-2 fixed N (0.49 g plant-1) under zero tillage that was statistically not different from the highest N fixed due to manual tillage of soil under the cultivation of IT93K-452-1 and IT99K-573-1-1 (0.62 g plant-1, respectively). In the early season, IT99K-573-1-1 cultivated under zero tillage produced the highest grain yield (404.11 kg ha-1). Fodder yield followed a different trend compared to grain yield. In the early planting season, cowpea fertilized with 60 kg P ha-1 produced the highest fodder yield, while in the late season, the application of 30 kg P ha-1 produced the highest fodder yield. Regardless of the planting season, grain yield was highest when IT99K-573-1-1 was fertilized with 60 kg P ha-1. Averagely, soil organic carbon content, N fixed, and grain yield values were highest during the late planting than the early planting season. The reverse was, however, the case for fodder yield, seedling emergence, root length, root, and shoot biomass, respectively. For the best performance in cowpea production, farmers are encouraged to plant IT99K-573-1-1 on zero-tilled soil with phosphorus application at the rate of 60 kg ha-1. IT90K-277-2, which is the most genetically stable variety across seasons, will be most ideal in controlling negative climate change and should therefore be considered for further investigations.