Starter fertilizer toxicity mitigation by maximizing starter nitrogen and potassium fertilizer rates in subsurface placement for enhanced early development of maize ( Zea mays L.)

Fertilizer salt injury and ammonia toxicity are major constraints to seedling emergence and early plant development. This study evaluated the ecotoxicological effects of varying rates of subsurface-applied starter nitrogen (N) and potassium (K) fertilizers on maize emergence across different soil types. Fertilizer treatments included 45, 55, 65, and 75 kg N ha−1 and 74, 95, 107, and 120 kg K ha−1, with a control. Significant interactions among soil type, fertilizer source, and application rate influenced maize emergence and seedling development. Key factors such as fertilizer salinity, ammonium-N (NH4+ -N), and soil pH significantly affected seed ecotoxicology (p < 0.05). Soil-specific toxicity thresholds were established: in silt-clay soil, 75 kg N ha−1 was the critical threshold for all N sources, while in silt-loam soil, thresholds were 25 kg N ha−1 for DAP and urea, and 75 kg N ha−1 for MAP. Both silt-clay and silt-loam soils tolerated up to 105 kg K ha−1 irrespective of K source. In sandy soil, subsurface application of urea and KCl negatively impacted 90% relative seed emergence. Observed toxicity symptoms included delayed emergence and leaf chlorosis. Strong correlations between emergence and early biophysical traits indicate that this research offers practical diagnostic guidelines for managing fertilizer toxicity in maize production.