While the application of manure to improve soil quality has attracted attention, the effect of pig manure application rates on soil acidity remains poorly understood. In this study, we analyzed the changes and correlations between soil acidity, pH buffer capacity (pHBC), soil chemical properties, and crop yields after 8 years of pig manure application at different rates (i.e., 0, 7.5, 15, 30, and 45 Mg ha⁻¹) in a red upland soil (Ultisol). With an increase in the application rates, the crop yields were 0.77–8.85 times higher; the pH was enhanced by 0.4–0.8 units; and the soil organic matter (SOM), pHBC, iron activation (Feo), exchangeable calcium (ExCa), and exchangeable magnesium (ExMg) contents increased by up to 74.1%, 92.7%, 36.7%, 62.2%, and 48.7%, respectively, whereas that of total exchangeable acid (ExAcid) decreased by 17.2–52.9%. The crop yields were positively related to the soil pH but negatively correlated with ExAcid. Redundancy analysis revealed ExAcid and pHBC were more sensitive than pH was to the other chemical indicators. ExAcid was negatively correlated with SOM and ExCa; pHBC was positively correlated with ExMg, TN, SOM, and Feo. In conclusion, the crop yield could be improved by adjusting the soil acidity characteristics, and the application of pig manure reduced the soil acidity, with an optimal application rate of 15 Mg·ha⁻¹.

Soil acidification is a significant threat to agricultural sustainability, particularly in paddy fields, where acidic conditions can limit crop productivity and soil health. This study aimed to explore the combined effects of alkaline amendments—lime, magnesia, and silicon fertilizer—on the acid-neutralizing capacity (ANC) of paddy soils and the rice yield, with the objective of identifying effective strategies to mitigate soil acidification and enhance agricultural productivity. From 2018 to 2021, a four-year field trial in Hunan tested lime, magnesia, and silicon fertilizers. Soil samples (0–20 cm depth) were collected once post-harvest in 2021 to evaluate the cumulative treatment effects. After four years, the control soil pH was 6.12. Lime and light magnesia treatment increased it to 6.70 and 6.99, respectively. Silicon fertilizer showed no significant difference (pH 6.05). ANC analysis revealed the following anti-acidification capacity ranking: light magnesia > lime > control > silicon fertilizer. Light magnesia boosted the rice yield by 13.02% over the control. Statistical analyses indicated a significant positive correlation between the soil acid-neutralizing capacity and pH (ANC4.0 = 7.53 × pH − 30.00, R²adj = 0.70; ANC5.0 = 6.96 × pH − 37.49, R²adj = 0.58). The rice yield was correlated with exchangeable magnesium (yield = 0.42 × Ex-Mg + 24.54, R²adj = 0.44). The continuous application of lime and light magnesia enhanced the nutrient availability and soil anti-acidification, with light magnesia also improving the rice yield. These findings provide insights to aid in enhancing soil quality and agricultural productivity in acid-affected regions.

While the application of manure to improve soil quality has attracted attention, the effect of pig manure application rates on soil fertility and acidity remains poorly understood. This study analyzed the changes in and correlations between soil acidity, pH buffer capacity (pHBC), soil chemical properties and crop yields after 8 years of pig manure application at different rates (i.e., 0, 7.5, 15, 30, and 45 t·ha^-1) in red soil. Crop yield, soil nutrients, pH, pHBC, iron activation, and exchangeable base cations increased with increased pig manure application rates. Soil fertility and pHBC were significantly enhanced when the application rate was larger than 15 t·ha^-1. At 30 and 45 t·ha^-1, pH values increased significantly while exchangeable acid decreased (41.3% and 52.8%) compared to no pig manure input. Crop yields were positively related to soil pH, but negatively correlated with soil exchangeable hydrogen and exchangeable aluminum. Redundancy analysis revealed pH and pHBC to be positively correlated with exchangeable base cations, total phosphorus, total nitrogen, soil organic carbon, and amorphous iron oxide. In conclusion, crop yield could be improved by adjusting soil acidity characteristics, and the application of pig manure reduced soil acidity, with an optimal application rate of 15–30 t·ha^-[1]. [1]Hunan Soil and Fertilizer Institute, Changsha, 410125, China. ²Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences/ Key Laboratory of Agricultural Environment, Ministry of Agriculture and Rural Affairs of P.R.China, 100081, Beijing, China. ³Hunan Agricultural University, Changsha, 410128, China. *email: longzd313@hunaas.cn; sungeng@hunaas.cn.

Rice is a staple food crop that feeds billions globally. Addressing Zn deficiency in rice is crucial for improving nutrition and food security. Zn deficiency in rice is a widespread issue, especially in purple tidal mud substrates, which often exhibit low Zn availability. The objective of this two-year pot study was to explore the relationship between Zn content, yield components, and Zn absorption in rice grown in purple tidal mud substrate with varying amounts of Chinese milk vetch (Astragalus sinicus L.) incorporation. The experimental design consisted of seven treatments: an unfertilized control, a Chinese milk vetch control, a chemical fertilizer control, and four treatment variations incorporating Chinese milk vetch alongside chemical fertilizer applications. The results indicated that planting and applying Chinese milk vetch improved the grain yield of rice in purple tidal mud substrate, and the yield increased with higher levels of Chinese milk vetch applied. The increased grain yield resulted in higher Zn absorption in rice grains. The application of Chinese milk vetch, both solely and in combination with chemical fertilizers, had varying effects on zinc uptake and grain zinc formation efficiency in early and late rice, with the control and low-level Chinese milk vetch treatments generally exhibiting the highest performance across the two-year period. By introducing Chinese milkvetch following the use of chemical fertilizers, the Zn content in rice grains increased starting from the second year. The treatment with Chinese milkvetch applied at a rate of 2.25 t/hm² showed the best results in increasing the Zn content in rice grains. The increase in Zn content and Zn uptake by the rice plants gave rise to a lowering of the DTPA-extractable Zn content in the purple tidal mud substrate. Sole Chinese milk vetch application and using Chinese milk vetch following chemical fertilizer application both increased Zn content extracted by DTPA in purple tidal mud substrate.