Effect of tillage, biochar, poultry manure and NPK 15-15-15 fertilizer, and their mixture on soil properties, growth and carrot (Daucus carota L.) yield under tropical conditions

Tillage, biochar, poultry manure, NPK fertilizer and their combined application could improve soil quality, sustainability and carrot productivity. The effects of two tillage treatments: conventional tillage (CT) and reduced tillage (RT) each combined with 30 Mg ha−1 biochar (B), 10 Mg ha−1 poultry manure (PM), 300 kg ha−1 NPK 15-15-15 fertilizer, 150 kg ha−1 NPK 15-15-15 fertilizer +15 Mg ha−1 biochar +5 Mg ha−1 poultry manure and a control (no biochar/poultry manure/NPK fertilizer) on soil properties, growth and carrot yield were investigated. The research was carried out for two consecutive growing seasons (2018 and 2019) at Owo in the forest-savanna transition zone of Nigeria on a sandy loam. The experiment was laid out in a randomized complete block design in a factorial combination of ten treatments and replicated three times. Reduced tillage had relatively lower soil bulk density, penetration resistance, dispersion ratio and temperature, and had significantly higher (p = 0.05) soil aggregate stability, mean weight diameter, porosity and water content than conventional tillage and these resulted in higher soil pH, organic C, N, P, K, Ca and Mg, growth and fresh root yield of carrot compared with conventional tillage. Reduced tillage increased fresh carrot root yield by 2.3 Mg ha−1 and 2.6 Mg ha−1 for the first and second growing seasons, respectively, compared with conventional tillage, which corresponded to a 11.1% increment for both years. Application of biochar alone, poultry manure alone and complementary application of NPK fertilizer, biochar and poultry manure decreased soil bulk density, penetration resistance, dispersion ratio and temperature and increased soil water content, porosity, aggregate stability and mean weight diameter whereas NPK fertilizer did not improve these soil physical properties. Biochar alone, poultry manure alone, NPK fertilizer alone and combined application of NPK fertilizer, biochar and poultry manure increased soil total N, available P, and exchangeable K, Ca and Mg concentrations compared with the control. Application of biochar alone improved soil pH, OC, K, Ca and Mg better than the NPK fertilizer. Poultry manure improved soil pH, OC, N, K, Ca and Mg better than the NPK fertilizer. Combined application of NPK fertilizer, biochar and poultry manure at sub-optimal rates gave higher soil N, P, K, Ca and Mg concentrations, higher plant, number of leaves, root length, root diameter and fresh carrot root yield compared