Early soil chemical indicators and okra growth under k-bearing rock dust–organic amendment systems

Abstract Short reaction periods after soil-amendment incorporation may reveal changes in potassium (K) availability and exchange properties before they are evident in crop performance, but such early responses are rarely evaluated for mineral-organic systems in tropical Alfisols. This study tested whether K-bearing rock dust (RD), poultry manure (PM), Tithonia diversifolia biomass (TD), and their combinations altered early soil chemical indicators and whether the indicators were associated with cumulative okra vegetative growth. We hypothesized that RD-organic combinations would increase exchangeable K and operational cation exchange capacity (CEC), moderate acidity, and support greater vegetative vigour than unamended or singly amended soil. A 3 × 3 split-plot field experiment with three replications was conducted at one site during one growing season in southwestern Nigeria. Baseline soil samples were collected before treatment application, and early samples were collected 14 days after RD, PM, and TD incorporation (2 weeks after amendment application; 2 WAA). The 2-WAA sampling preceded okra sowing and conventional NPK fertilizer application; observations from NPK-designated plots therefore represented pre-NPK conditions. Okra height and leaf number measured from 2 to 12 weeks after planting were integrated as the area under the vegetative vigour index curve (AUVVI). RD increased exchangeable K and operational CEC to 1.67 and 8.54 cmol kg⁻¹, respectively, compared with 0.49 and 4.63 cmol kg⁻¹ in plots without an early mineral input. RD × PM recorded the highest early exchangeable K (1.90 cmol kg⁻¹) and operational CEC (9.69 cmol kg⁻¹), whereas RD × TD produced the highest AUVVI (24,949.0 cm·leaves·week). Associations of AUVVI with early exchangeable K (R² = 0.039, p = 0.323) and the baseline-referenced H⁺ ratio (R² = 0.057, p = 0.230) were weak and non-significant. Thus, RD-organic systems modified early K and cation-retention indicators, but these indicators did not predict cumulative vegetative growth. The findings are treatment-associated evidence from one site and season and require validation across seasons, locations, and soil conditions.