OPTIMIZING INORGANIC FERTILIZER APPLICATION FOR OFF-SEASON SAFE TOMATO PRODUCTION IN VAN SON COMMUNE, PHU THO PROVINCE

Abstract

This study aimed to determine the optimal inorganic fertilizer rate for safe off-season tomato (Solanum lycopersicum L.) production under the agroecological conditions of Van Son commune, Phu Tho province - a highland area suitable for temperate vegetable cultivation. A field experiment was conducted using the tomato variety Savior under a randomized complete block design (RCBD) with three N, P, K treatments CT1 (120-90-120 kg N-P₂O₅-K₂O ha⁻¹), CT2 (150-110-150 kg ha⁻¹) and CT3 (180-135-180 kg ha⁻¹), following VietGAP production standards. Results showed that CT3 significantly improved plant height, leaf number, fruit number and fruit weight, leading to the highest actual yield of 36.8 t ha⁻¹ with 15,2% higher than CT1. Fruit vitamin C content and soluble solids (°Brix) increased under CT3, while nitrate accumulation remained within the safe limit (< 150 mg kg⁻¹ fresh weight). Soil pH, organic matter, and heavy metal contents remained stable, indicating no adverse environmental effects. The study suggests that applying 180-135-180 kg N-P₂O₅-K₂O ha⁻¹ is optimal for safe off-season tomato production in the Van Son commune highland.