Evolution of heavy metal contents in the soil of protected vegetable fields and its environmental quality evaluation in Yongnian county of Hebei province in China

Purpose This paper aims to investigate the contents of heavy metals Cd, Cr, Ni, Cu, Pb and Zn in the soil of vegetable production bases in the Yongnian County of Hebei Province by random sampling. Design/methodology/approach The environmental quality conditions of these heavy metals were evaluated by both the single quality index and comprehensive quality index methods. Findings It was concluded that the average heavy metal contents in the soil of protected vegetable fields in the Yongnian County are all under the limit values prescribed by the environmental quality evaluation standard for farmland of greenhouse vegetables production (2007), so the soil’s environmental quality is classified as first class regarding cleanliness. It is fit for the plantation of non-polluted vegetables. It is shown that there is a cumulative trend of heavy metal contents in the soil with age after analysis of a correlation coefficient between heavy metal contents and age in Yongnian’s protected fields and excessive application of fertilizers or organic fertilizers, which is the main factor leading to an increase in heavy metal contents in the soil. Originality/value It is suggested that the amount of fertilizer should be properly reduced to prevent the soil’s environmental deterioration, the vegetable planting structure should be improved and the vegetables grown in areas with weaker accumulation of heavy metals should be cultivated.