Initial Validation of NPK Fertilizer Rates and Plant Spacing for Morkhor 60, a New Soybean Variety, in Sandy Soils: Enhancing Yield and Economic Returns

Soybeans (Glycine max (L.) Merr.) are a vital global crop; however, Thailand currently imports 99% of its domestic requirement, highlighting the critical need for enhanced domestic production. Morkhor 60, a new high-yielding variety, lacks optimized agronomic management for cultivation in the challenging sandy soils of Northeast Thailand. This study evaluated the effects of NPK fertilizer rates and plant spacing on Morkhor 60 growth and yield through two independent experiments conducted in sandy soils over a four-season period (2022–2023). Results demonstrated that 23.44 kg ha−1 NPK provided optimal cost-effectiveness for Morkhor 60, achieving yields of 1238 kg ha−1 statistically comparable to higher rates (1286 kg ha−1) while reducing input costs by 50%. Plant spacing significantly affected productivity, with 30 × 20 cm spacing producing the highest yield (1775 kg ha−1), representing 41% improvement over the narrow spacing (20 × 20 cm: 1257 kg ha−1). The integrated management system (23.44 kg ha−1 NPK with 30 × 20 cm spacing) achieved 87.6% ground cover for moisture conservation and delivered net profits of 29,850 THB ha−1, with a benefit–cost ratio of 3.1. This research provides evidence-based agronomic recommendations for Morkhor 60 cultivation in sandy soil environments, contributing to Thailand’s soybean self-sufficiency through sustainable and economically viable production practices.