Linking Soil Properties to Grain Quality: Individual and Combined Effects of Rice Husk Biochar and Potassium Humate in Paddy Rice Systems

The integrated effects of biochar and humate on soil nutrient status, shoot nutrient accumulation, and grain composition and quality remain underexplored. This field study evaluated four treatments—control, rice husk biochar (RHB; 15 t/ha), potassium humate (KH; 6.3 kg/ha), and RHB + KH—on soil properties, shoot nutrient accumulation, grain nutrient composition, yield, and grain quality in a premium Thai rice production system. Compared with the control, RHB significantly increased soil pH and shoot accumulation of nitrogen (N; +93%), phosphorus (P; +321%), and potassium (K; +75%) and resulted in a greater grain P concentration and milled grain yield (+56%). RHB also increased volumetric expansion (+42%), with a significant RHB main effect detected in the factorial analysis. KH significantly increased soil total N (+11%) and ammonium (+19%), shoot N (+79%) and P (+363%) accumulation, and grain N concentration relative to the control, while also increasing milled grain yield (+144%) and grain protein content. KH also produced numerical increases in volumetric expansion (+21%) and grain elongation (+4%), although these responses were not significant in the factorial analysis. The RHB + KH treatment significantly increased soil total N (+18%), shoot N (+70%) and P (+423%) accumulation, and the grain K concentration relative to the control and produced treatment-specific changes in the amino acid profile. The amendment treatments generally decreased residual soil nitrate and available P concentrations, although the factorial response of nitrate included a significant RHB effect and RHB × KH interaction but no significant KH main effect. Grain carbohydrate content decreased under the amendment treatments, whereas TPC showed a treatment-specific response characterized by a significant KH effect and RHB × KH interaction. Principal component analysis revealed associations among soil properties, shoot nutrient accumulation, grain nutrient composition, yield, and grain quality attributes. These findings demonstrate distinct effects of RHB and KH, individually and in combination, on soil properties, shoot nutrient accumulation, and grain composition and quality. Overall, the amendments showed potential for modifying soil nutrient dynamics and shoot nutrient accumulation while altering selected grain nutritional and market-relevant quality attributes, although responses varied among individual parameters.