Bioameliorant Mixtures Enhance Soil Nitrogen Status, Nutrient Uptake, and Crop Productivity in Maize–Soybean Intercropping on Suboptimal Land

Suboptimal sandy soils are commonly constrained by low organic matter, limited nutrient-retention capacity, and poor nitrogen (N) availability, all of which restrict crop productivity. This study evaluated integrated bioameliorant formulations, defined here as soil-amendment mixtures combining organic materials, rice husk biochar, and beneficial arbuscular mycorrhizal fungi (AMF), for improving soil fertility, plant nutrient uptake, AMF development, and productivity of maize–soybean intercropping. A randomized complete block design with three replications was used to compare an untreated control (M0) with four formulations containing compost, cattle manure, rice husk biochar, and AMF biofertilizer: M1 (30:30:20:20%), M2 (20:20:40:20%), M3 (25:25:25:25%), and M4 (20:20:20:40%). Initial soil total N was only 0.01%, indicating severe N limitation. Among the tested formulations, M4 produced the most consistent overall response, increasing soil total N, available phosphorus (P), soil organic carbon, plant N and P uptake, AMF spore density, root colonization, biomass production, and grain yield. Maize grain yield increased from 11.56 kg plot−1 in the untreated control to 21.20 kg plot−1 under M4, whereas soybean grain yield increased from 0.13 to 0.73 kg plot−1. Nevertheless, M4 was not statistically superior for every variable. SEM–EDX analysis further indicated that elemental composition alone did not reliably predict agronomic performance. Overall, the M4 formulation showed the greatest potential for improving soil N status, nutrient acquisition, and crop productivity in nutrient-poor sandy soil.