Soil pollution in vegetable greenhouses
With the continuous development of industrial production and the increasing urbanization, a large amount of toxic organic matter and inorganic matter caused by wastes such as industrial “three wastesâ€, municipal solid waste, and sewage, enter the soil through migration, irrigation, precipitation, etc. Contamination. With the growing demand for vegetables from the majority of residents, the growing area of ​​vegetable greenhouses in various regions has been expanding, and the pursuit of yield and benefits by vegetable farmers has become more and more urgent. In order to increase the output of vegetables, vegetable farmers make extensive use of production materials such as fertilizers, pesticides, and agricultural films, and may bring certain heavy metals, pathogenic microorganisms, and organic pollutants into the soil, causing pollution to greenhouse soils. From the analysis of agricultural pollution sources, the main causes of soil pollution in vegetable greenhouses are as follows.
1.1 Excessive use of chemical fertilizers
Fertilizer is the "grain" of vegetables and plays an important role in increasing the yield of vegetables. However, because chemical fertilizers often contain many harmful substances, such as heavy metals, fluorine, and toxic organic compounds, the repeated use and excessive use for many years will pollute the soil. At the same time, the frequent application of nitrogen fertilizer affects the content of nitrate in soil, and the accumulation of soil nitrate increases with the increase of total nitrogen application rate. Excessive application of nitrogen fertilizer will increase the nitrate content in vegetables. Accumulation is not harmful to the plant itself, but it jeopardizes feeding animals and humans. In addition, the long-term use of a single type of fertilizer, especially physiological and acidic fertilizers, will lead to acidification of the soil. Acidification of the soil will lead to the release of toxic substances, or increase the toxicity of toxic substances, resulting in adverse effects on vegetables.
1.2 Organic fertilizers are not harmlessly treated
Organic fertilizers made from harmlessly treated human and animal wastes, municipal solid waste, and plant residues carrying pathogenic bacteria are applied to the soil. Some pathogenic bacteria will multiply in the soil and cause biological contamination of the soil, especially in greenhouses. In a high-temperature, high-humidity microclimate environment, it is more suitable for the growth of pathogenic bacteria. Pathogens can enter the vegetables through the soil, causing the vegetables to produce lesions, reduce the yield and quality of vegetables, and thus endanger human health. According to the survey, the detection rate of coliforms in vegetables in Shanghai suburbs was 13.7%, the detection rate of parasite eggs was 11.9%, and the vegetables were seriously contaminated with pathogenic bacteria. According to research by Sun Xijuan, the total number of bacteria in human and animal feces is as high as 108-109/g, and the total coliform bacteria are 107-108/g. In order to avoid the pollution of soil pests caused by the application of organic fertilizers, the national regulations stipulate that the index of the mortality rate of animal manure eggs and the number of fecal coliforms shall comply with the requirements of NY884, that is, the mortality of maggot eggs should be ≥95% and the fecal coliform group count. ≤ 100 cells/g (mL).
1.3 Pesticide Residue With the seed dressing, soaking, spraying and drug irrigation, pesticides enter the soil will be adsorbed by the soil rubber particles and organic matter, part of the degradation by the soil, the absorption and transformation by the vegetables, and some form a residue. The pollution caused by the improper use and excessive use of vegetable farmers is particularly serious. In order to reduce costs, some vegetable farmers tend to use low-cost, high-toxicity, high-residue pesticides. The use of pesticide residues, especially unqualified pesticides, is also an important factor causing soil contamination in vegetable greenhouses.
1.4 Sewage Irrigation
The use of uncontaminated sewage or incompletely treated sewage irrigation of vegetables causes secondary pollution of the soil. Studies have shown that Hg, As, Cd, Pb, Cr and other heavy metals in wastewater are trapped and fixed by soil in different ways. Ninety-five percent of Hg is quickly absorbed by soil mineral colloids and organic matter, and generally accumulates on the soil surface, top-down. Decreasing. The pathogenic microorganisms and parasites in the sewage cause soil pathogen contamination. From the aspect of soil micro-morphology, soil pores of sewage irrigation are fine, air permeability and water permeability are poor, and it is easy to cause mildew, rot or even necrosis of vegetable roots, which is not conducive to the growth of vegetables.
1.5 Plastic film residue
Plastic film is widely used in greenhouse vegetable cultivation, especially the popularization of full-membrane technology, which results in a substantial increase in residue. The use of ultra-thin plastic film has brought serious harm to soil health. The relevant departments in China stipulate that the thickness of polyethylene film should not be less than 0.008 mm, but individual companies and vegetable farmers often produce and use ultra-thin films at low prices in order to reduce costs. The thinner the film, the worse the strength and tensile strength, and it is easy to age. It is difficult to pick up after crushing, and it is easier to form residues.
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