Natural zeolite enhances tomato yield, reduces nitrate accumulation, and immobilizes heavy metals in fertilized dark chestnut soil

Tomato (Solanum lycopersicum L.) is a widely cultivated vegetable crop in Kazakhstan, yet its productivity and quality are often limited by soil degradation, nutrient imbalances, and the excessive use of mineral fertilizers. Natural zeolites, particularly clinoptilolite, offer potential as soil amendments due to their high cation exchange capacity, water retention properties, and ability to regulate nutrient availability. This study aimed to evaluate the effects of natural zeolite (2 t/ha), alone and in combination with two mineral fertilizer doses (N₄₅P₄₅K₄₅ and N₉₀P₉₀K₉₀), on tomato yield, fruit quality, soil heavy metal content, and economic profitability under dark chestnut soil conditions in southeastern Kazakhstan. A field experiment was conducted using a randomized complete block design with six treatments and three replications. Results showed that all treatments increased yield compared to the control (21.7 t/ha), with the highest yield (29.1 t/ha) observed under the zeolite + N₉₀P₉₀K₉₀ treatment. Fruit quality improved in terms of dry matter (up to 5.46%) and sugar content (up to 3.80%) with zeolite and fertilizer combinations. Nitrate accumulation in fruits was highest under N₉₀P₉₀K₉₀ alone (78 mg/kg) but decreased significantly when combined with zeolite (64 mg/kg), indicating the mineral’s capacity to reduce nitrate uptake. Heavy metal analysis revealed that zeolite reduced the bioavailability of cadmium and lead in soil, keeping concentrations below permissible limits. Economic evaluation indicated that the zeolite-only treatment provided the highest profitability (171%) due to relatively low input costs and moderate yield gains. Overall, the results demonstrate that zeolite, especially when integrated with moderate fertilizer inputs, enhances tomato productivity, improves fruit safety, and supports sustainable soil management. Its use can be particularly beneficial in resource-limited and environmentally sensitive agricultural systems.