Biochar and Quicklime Co-Application Enhances Soil Fertility and Nut Yield in Acidic Pecan Orchards

Soil acidification and low nutrient availability in acidic red soils are major constraints on pecan (Carya illinoinensis) productivity and fruit quality. However, the integrated effects of quicklime and biochar application in pecan orchards on acidic red soils remain poorly understood. In this context, a field experiment was conducted in an 8-year-old pecan orchard in an acidic red soil region to evaluate the effects of sole and combined applications of biochar and quicklime at different ratios on soil properties, fruit yield, and quality. The results showed that the combined application of biochar and quicklime showed greater benefits for soil fertility, fruit yield, and kernel quality than single-amendment treatments. The combined treatments significantly increased soil pH by 0.47–2.15 units relative to the control and markedly improved soil nutrient status. After 12 months of application, SOM contents under L1B–L3B were 20.8–23.2% higher than those under the corresponding quicklime-only treatments, reaching 37.57–43.37 g·kg⁻¹. The combined treatments also maintained higher total nitrogen, total phosphorus, and available potassium levels than the corresponding quicklime-only treatments, with TN under L1B–L3B reaching 1.65–1.78 g·kg⁻¹, representing a 126.0–166.7% increase over their respective quicklime-only treatments. The combined treatments also generally enhanced soil biological activity and improved fruit yield and quality. Their effects on fruit traits varied with application ratio: the low-dose treatment (L1B, 2 kg biochar + 1 kg quicklime) was more effective in improving physical traits such as dry kernel weight and kernel percentage; the medium-dose treatment (L2B, 2 kg biochar + 1.5 kg quicklime) produced the highest single-tree yield, reaching 26.80 kg·tree⁻¹, which was 24.25% higher than the control and significantly higher than all single-amendment treatments (23.43–25.07 kg·tree⁻¹); and the high-dose treatment (L3B, 2 kg biochar + 2 kg quicklime) was more favorable for improving nutritional quality, increasing amino acid and vitamin E contents to 1267.01 μg·kg⁻¹ and 153.22 μg·g⁻¹, respectively, which were 45.41–91.90% and 5.02–78.77% higher than those under the single-amendment treatments. Overall, the combined application of biochar and quicklime effectively alleviated soil acidification, improved soil fertility, and promoted higher fruit yield and quality, providing a scientific basis for the efficient, high-quality, and sustainable development of pecan orchards in acidic red soil regions.