Modern agriculture must raise yields while safeguarding soil fertility. We evaluated the effect of a vermiculite-based organomineral amendment (GumiVer) on maize (Zea mays L.) growth and biomass under semi-arid soil conditions. A controlled-environment pot experiment (5-L pots) using field-collected light-chestnut soil representative of semi-arid conditions in southeastern Kazakhstan. Treatments included soil-only and vermiculite-only controls and soil:GumiVer ratios (v/v) of 19:1, 9:1, and 4:1. Pots were arranged in a randomized complete block design (n = 4 per treatment), maintained at 25 °C, 12-h photoperiod, and irrigated to 70% field capacity; the pot was the experimental unit. GumiVer significantly improved growth relative to the control, with the 9:1 ratio yielding the largest gains: shoot mass up to 135%, leaf area ~155%, and total biomass ~140%. Expanded vermiculite alone produced moderate improvements. Root growth also increased but less than shoot growth, indicating a shift in biomass allocation toward aerial parts. Responses increased with GumiVer dose up to 9:1 (≈0.5 L GumiVer per 5-L pot) and then plateaued at higher additions; benefits at 19:1 (≈0.25 L) were present but smaller. Incorporating GumiVer at ~9:1 soil:GumiVer (v/v) can enhance early maize growth and biomass in light-chestnut, semi-arid soils, likely via complementary aluminosilicate–humate effects on substrate structure and nutrient dynamics. These results support the use of scientifically formulated organomineral amendments in sustainable management of degraded, nutrient-imbalanced soils.