Effects of Nitrogen Fertilizer Levels on the Growth and Yield of Kale and Collards in Soil Infested with Clubroot

Clubroot disease, caused by the obligate biotroph Plasmodiophora brassicae, severely impairs root function in Brassica crops, leading to stunted growth, nutrient deficiencies, and yield losses of 30–70% in infested soils. In acidic soils (pH < 6.5), resting spores germinate readily, releasing zoospores that infect root hairs. Nitrogen (N) fertilization modulates plant vigor through protein and chlorophyll synthesis but interacts with soil pH and pathogen dynamics: ammonium-based sources acidify soil via nitrification (releasing H⁺ ions), potentially favoring the pathogen, while calcium ammonium nitrate (CAN) provides a liming effect via Ca²⁺ that raises pH and suppresses spore activity. Two field experiments were conducted using a randomized complete block design with split-plot arrangement (3 replications) from June to October 2025 in Wundanyi and Mwatate Sub-Counties, Taita-Taveta County, Kenya. Main plots were kale (Curly and Dinosaur) and collard (Vates and Morris Heading) varieties; subplots were N rates (0, 10, 20 g/plant) from CAN or sulfate of ammonia (SA). Key findings included: 10 g N from CAN significantly maximized plant height (kale: 42.6 ± 2.5 cm, +68% over control), leaf area (+113%), chlorophyll content (50 ± 2.5 SPAD units), fresh biomass (+100%), and reduced galling index (1.5 ± 0.2 vs. 2.8 ± 0.3 control; p < 0.05, ANOVA, Tukey HSD). CAN raised soil pH (6.3–6.4), lowering disease severity, while SA acidified it (5.5– 5.6), increasing susceptibility. Biochemical analyses showed enhanced Ca, Zn, Fe, phenolics, and flavonoids at the optimal rate. These results demonstrate that moderate CAN application optimizes sustainable production by balancing nutrition, disease suppression (via pH and Ca effects on pathogen biology), and nutritional quality while minimizing nitrate risks. Recommendations include farmer adoption of 10 g CAN/plant with soil pH monitoring for clubroot-infested regions.