The Fertilizer and Fertilization Technology Innovation Team at the Institute of Agricultural Resources and Regional Planning, in the Chinese Academy of Agricultural Sciences, recently discovered something really big in something small. It found that Chinese milk vetch — a small purple-flower plant that farmers in China grow to make the soil healthier — can activate low-grade phosphate rock powder to supply phosphorus to rice paddies, while reducing the risk of the highly toxic metal cadmium. These findings were then published in the Journal of Hazardous Materials (JCR Q1, IF 11.3).

Rice paddies in southern China have long faced challenges such as excessive chemical phosphorus fertilizer applications, low utilization efficiency, increased runoff losses and soil contamination from cadmium — posing safety risks to rice. Although low-grade phosphate rock is abundant and theoretically offers a slow-release phosphorus source and cadmium fixation, its low solubility limits direct agricultural use. Existing activation methods often rely on external agents, are energy-intensive, or have unstable microbial colonization — necessitating exploration of in situ activation methods that can integrate into existing crop rotation systems. Furthermore, the mechanical application of powdered phosphate rock requires granulation to address dust and uniformity issues. Green manure presents a promising natural solution for activating low-grade phosphate rock. Chinese milk vetch is a major leguminous green manure crop in the rice-growing regions of southern China. After incorporation, it provides organic substrates and stimulates phosphate-solubilizing microbial activity, thereby promoting the dissolution of poorly soluble phosphate rock. It can also reduce soil Cd availability by promoting iron plaque formation, enriching anaerobic microorganisms, and increasing humic-like components. However, systematic validation is lacking on whether green manure-mediated phosphorus activation and phosphate-based cadmium fixation can synergistically achieve phosphorus reduction, cadmium reduction, runoff control and yield maintenance in the field.

The IARRP study first conducted a nationwide meta-analysis to clarify the spatial patterns of phosphorus utilization efficiency, phosphorus runoff losses, and cadmium pollution in Chinese rice paddies. Based on this, two-year field plot trials were conducted in East China’s Anhui and Jiangxi provinces, along with three-year large-scale demonstrations using granulated phosphate rock powder, to verify two core hypotheses. These were: (1) Biological activation by Chinese milk vetch can enhance the agronomic effectiveness of low-grade phosphate rock powder, allowing significant reduction of chemical phosphorus fertilizer without yield loss; (2) Management combining Chinese milk vetch and phosphate rock powder is more effective in cadmium fixation than single measures, with an evaluation of its economic feasibility. The field trials involved five treatments: Winter fallow without phosphate rock powder and conventional phosphorus fertilizer application during the rice season (WF-P); winter fallow with phosphate rock powder and no phosphorus fertilizer during the rice season (WF-PR); Chinese milk vetch season without phosphate rock powder and conventional phosphorus fertilizer during the rice season (MV-P); Chinese milk vetch season with phosphate rock powder and no phosphorus fertilizer during the rice season (MV-PR); and the Chinese milk vetch season with phosphate rock powder and 50% reduced phosphorus fertilizer during the rice season (MV-PR-P1/2). Additionally, soil cadmium speciation, calcium release, soluble organic matter analysis, material characterization, and hydroponic experiments were conducted to systematically elucidate the multiple mechanisms of phosphorus supply and cadmium reduction through Chinese milk vetch-activated phosphate rock powder.

Field trials indicated that Chinese milk vetch can enhance the utilization rate of both phosphorus fertilizer and phosphate rock powder and biologically activate low-grade phosphate rock powder. The combination of Chinese milk vetch and phosphate rock powder with a 50% reduction in chemical phosphorus fertilizer (MV-PR-P1/2) achieved higher rice yield and net income compared to the conventional phosphorus application (WF-P), while reducing cadmium content in brown rice and total phosphorus runoff losses. Changes in soil available cadmium, acid-soluble cadmium, residual cadmium, calcium, soluble organic matter, and pH were consistent with phosphate precipitation, calcium-mediated ion exchanges, DOM complexation, and pH-dependent reactions — indicating that these processes collectively contributed to cadmium fixation. Hydroponic experiments further confirmed that this treatment reduces cadmium accumulation in seedlings and enhances cadmium retention in roots. The conclusion was that using Chinese milk vetch to activate phosphate rock powder as a partial substitute for chemical phosphorus fertilizer is a feasible option for managing the risk of cadmium in rice paddies. Given the regional and interannual differences in trial responses and the variability in phosphate rock composition, the stability of this approach is said to require long-term multi-site validation. It is believed that future studies should focus on whether long-term applications lead to the accumulation of fluoride and other associated pollutants.

Figure 1: Research framework and field comprehensive benefits of Chinese milk vetch activating low-grade phosphate rock

Xie Jiancheng, a doctoral student, and Chang Danna, an assistant researcher at the Institute of Agricultural Resources and Regional Planning, are the co-first authors of this paper. Associate Professor Gao Songjuan from Nanjing Agricultural University and Researcher Cao Weidong from the Institute of Agricultural Resources and Regional Planning are the co-corresponding authors. The research was supported by the National Key Laboratory of Efficient Utilization of Dryland and Semi-arid Arable Land, the National Key R&D Program (2021YFD1700200) during the 14th Five-Year Plan (2021-25), and the National Green Manure Industry Technology System (CARS-22). Special thanks go to the teams of researcher Xu Changxu from Jiangxi Academy of Agricultural Sciences and researcher Wu Ji from the Anhui Academy of Agricultural Sciences for their strong support of this study.

Citation:

Xie J, Chang D, Li X, Liang T, Chen X, Liu J, Xu C, Bu R, Wang H, Wu J, Gao S, Cao W, 2026. Milk vetch activates low-grade phosphate rock for phosphorus supply and cadmium mitigation in paddy fields. Journal of Hazardous Materials 515, 143004.

Link: https://doi.org/10.1016/j.jhazmat.2026.143004