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In addition, the urban waste components have also undergone great changes, and various types of wastes such as plastic products, glass flakes and scrap metal, which are difficult or impossible to degrade in the environment, account for a considerable proportion. In some cities in developed countries, the above-mentioned types of waste accounts for about 10%-25%, of which plastic waste accounts for 6%-15%, and such wastes in China are also increasing. If sorting and recycling are not thorough, transporting them into farmland with other ingredients as fertilizer will have an adverse effect on the growth and development of soil and crops.
First, the damage of the film cover and residual film to the soil-plant system
1. Due to the blockage of rainfall leaching, and the temperature in the shed is significantly increased, the soil evaporation intensity is large. When there are more soluble salts in the subsoil and groundwater, the salt accumulates on the surface and causes excessive salt damage. Soils that are threatened by salinization in the north of China and areas with abundant salt sources along the beach should pay attention to this problem.
2. Under low temperature conditions, the microbial activity is weakened, which is not conducive to the conversion of nitrogen into effective nitrogen. If excessive ammonia nitrogen is applied, the crop absorbs excessive ammonia nitrogen, which leads to deeper leaf color and poor development of stems and leaves. If too much organic fertilizer is applied, ammonia and nitrous acid will escape from the soil after conversion to ammonia and nitrous acid. When ammonia nitrogen reaches a certain concentration in the atmosphere of the shed, it will cause damage to the aerial parts of vegetables.
3. Insufficient illumination in the shed, photosynthesis weakened, crop disease resistance decreased, and the soil in the shed was continuously used for the same crop for many years. The change of soil microbial population also led to an increase in soil and crop diseases.
2. Effects of agricultural film fragments on soil physical properties and crop growth after entering farmland
1. Due to the barrier of plastic debris, soil moisture movement is blocked, porosity and permeability are reduced, which is not conducive to the circulation and exchange of soil air, resulting in high carbon dioxide content in the soil, which is not conducive to the normal growth and development of crops.
2. Poor soil physical properties lead to difficulties in crop rooting, and reduced fertilizer and water absorption performance. In the soil with residual film in the plough layer, the root of the crop is difficult to penetrate the plastic film and develops in a curved shape, which causes the root system to become shorter, the nutrient range of the root system to be smaller, and the yield reduction is more obvious. According to the survey, in the continuous use of agricultural film cover without completely removing the residual film, the general production is reduced by about 10%, and some can even reach 26%.
3. Some plastic products (such as polyvinyl chloride plastics) or additives contain harmful components and are toxic. After contact with seeds or young shoots, it will inhibit seed germination or burn buds and seedlings.
In addition, plasticizers (phthalate compounds) in plastic products are toxic to plants, especially to vegetables, and phthalate compounds can enter the environment from various sources. Food, food, vegetables, etc., and have obvious enrichment effects, they enter the food chain to affect human and animal health. Diisobutyl phthalate is mainly volatilized from the film into the air, and then enters the mesophyll cells through the pores and water holes. Plant growth points and young leaves have strong physiological activities and are vulnerable. Its toxic effect is mainly to destroy chlorophyll and hinder the formation of chlorophyll, affect photosynthesis, growth retardation, dwarf and slender plant type, and even death. Typical symptoms of vegetables being compromised by diisobutyl phthalate in the film are chlorosis, yellowing or curling of the leaves. Such as sensitive small rapeseed, cauliflower, cucumber and tomato, etc., the new leaves and young shoots become yellowish white, the edges of the old leaves and cotyledons turn yellow, the leaves are small and thin, the growth is weak, and in severe cases, they gradually die and die. The symptoms of the damage usually appear 6-10 days after the film is covered. The long covering time, high temperature, high humidity, small seedling age and poor ventilation are serious. Diisobutyl phthalate is also hydrophilic, and may contain one ten thousandth to two ten thousandths of water droplets in the inner wall of the film. The water droplets contact the leaves, which can cause harm and form yellow net spots on the leaves. Finally, the cells are necrotic and dry.
Third, prevention measures
1. Strengthen publicity and call on people to minimize the use of plastic products for daily use (such as food bags, garbage bags, etc.).
2. It is necessary to promptly and thoroughly remove the discarded agricultural plastic film and reduce its residual amount in the soil.
3. Optimize and improve the recycling and management of waste plastic products from the price and management system, so that waste plastics can be recycled.
4. Develop new environmentally friendly agricultural films such as controlled photolysis and thermal decomposition (50 °C - 60 °C) to reduce the residual amount of agricultural film. Try to use plasticizers with low molecular weight, low biotoxicity and easy degradation to reduce their damage to food, vegetables and humans.
The influence of waste plastic products on soil and crops and its prevention
In recent years, various types of agricultural plastic film have been widely used as greenhouses and plastic film coverings, and have played an active role in the production of vegetables, peanuts, rice and other crops. However, if the management and recycling are not good, a large amount of residual film fragments will be scattered in the field. Unexpected negative effects. According to the survey, the annual residual film of farmland agricultural film in China is 3.06kg/ha, the residual rate is 1.3%; the mulch film is 10.5kg/ha, the residual rate is 12.3%; even some vegetables and peanuts have a residual rate of 40%. 60%.
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