Integrated Assessment of Vermiculite Enriched with Humic Substances or Chlorella vulgaris: Soil Fertility, Maize Nutrition, and Microbial Indicators

Maize production in semi–arid irrigated systems depends on soil fertility and an active rhizosphere. We hypothesized that vermiculite enriched with humic substances (HS) or Chlorella vulgaris (CV) would outperform vermiculite alone by improving soil fertility, maize nutrition, and rhizosphere-associated microbial indicators. A field experiment was conducted in southern Kazakhstan under medium–loam sierozem using a randomized block design with three replicates and seven treatments: control, vermiculite at 1 and 2 t ha−1, vermiculite + HS at 1 and 2 t ha−1, and vermiculite + CV at 1 and 2 t ha−1. Amendments were incorporated before sowing, and soil, plant, and microbial measurements were taken before sowing, at V6–V8, and after harvest. Compared with the control, all amendments improved early maize growth, leaf area development, biomass accumulation, and nutrient status, and increased grain yield. The strongest response was obtained with vermiculite + HS at 2 t ha−1, which increased grain yield from 6.48 to 10.24 t ha−1 (+58%). Microbial indicators differed between bulk soil and the rhizosphere, while Pearson correlation and PCA revealed coordinated soil–plant–microbe responses and productivity–linked variables across treatments. Taken together, these results indicate that enriched vermiculite, especially HS–enriched vermiculite at 2 t ha−1, is a promising amendment for improving maize productivity and rhizosphere functioning in semi–arid irrigated systems.