Effects of continuous cropping of vegetables on soil

Due to the limitations on the amount of arable land, climatic conditions and producers' pursuit of high economic benefits, my country's vegetable production generally has problems such as excessive use of chemical fertilizers, highly intensive production, high multiple cropping index, single vegetable cultivation type, and continuous cropping for many years, which have deteriorated the physical and chemical properties of the soil.

During the growth of vegetables, the roots absorb water and nutrients from the soil, and at the same time secrete root secretions such as protons, ions and organic matter into the soil. Under continuous cropping conditions, the accumulation of toxic substances in the root secretions causes serious damage to the root system. In addition to affecting the number and population of root microorganisms, it also has a certain impact on the metabolism, growth and development of microorganisms, thereby increasing root secretions, which in turn inhibits soil enzyme activity, and ultimately leads to various continuous cropping obstacles in vegetable crops.

1. Continuous cropping of vegetables and changes in soil microorganisms

Bacteria are the largest number of microorganisms in the soil, followed by actinomycetes. The number of fungi is the last. After greenhouse cultivation of vegetables, the relative number of the three major types of soil microorganisms has not changed much, and bacteria are still the largest, followed by actinomycetes, and fungi are the least. The longer the soil is continuously cropped, the higher the total content of microorganisms. However, the level of total microbial content is not positively correlated with the level of soil pathogens. The key to the severity of the disease lies in whether the soil microbial structure is reasonable, not in the number. With the increase of continuous cropping years, the number of soil bacteria in vegetable soil increases, and the types and number of fungi decrease. However, the types and number of harmful fungi increase, and the content of actinomycetes decreases with the increase of continuous cropping years. This change in soil microbial structure also causes changes in vegetable growth, development, and quality. Studies have shown that the root vitality of tomatoes changes significantly with the continuous cropping years. There is no significant difference in the quality of tomatoes cropped for 2 and 4 years, while the quality of tomatoes cropped for 8 years is significantly worse. In particular, the Vc content decreases, the sugar-acid ratio decreases, and the accumulation of nitrates is obvious. This shows that with the increase of continuous cropping years, the quality of vegetables decreases year by year.

2. Continuous cropping of vegetables and changes in soil enzyme activity
Soil enzymes are an important indicator of soil biological activity. They participate in the biological cycle of various elements, the decomposition and transformation of soil organic matter, the formation of humus, etc. Soil enzyme activity is a sign of the intensity of soil metabolism, and the abundance and activity of soil enzymes and microorganisms are closely related. The level of soil enzyme activity can reflect the strength of soil nutrient (especially nitrogen and phosphorus) transformation. The distribution of soil enzyme activity is related to the degree of soil maturity, soil physical and chemical properties, and organic matter, while the degree of soil maturity, soil physical and chemical properties, and organic matter are related to soil continuous cropping.
Soil enzymes include extracellular enzymes and intracellular enzymes. Extracellular enzymes are necessary for the degradation of soil macromolecular organic matter (such as cellulose, hemicellulose and lignin), while intracellular enzymes are necessary for the degradation of small molecules (such as amino acids and sugars).

Soil urease is directly involved in the transformation of organic compounds containing nitrogen in the soil. Moreover, the role of urease is extremely specific, and it can only hydrolyze urea to produce ammonia. Urease activity increases with the increase of urea application. However, excessive urease activity in the soil can easily cause ammonia volatilization loss. Therefore, nitrogen fertilizer should be applied to the soil reasonably to control urease activity, reduce nitrogen loss and environmental pollution; invertase can decompose sucrose that cannot be directly absorbed by plants into glucose and fructose, thereby improving the biological activity of the soil. High invertase activity indicates good soil biological activity and high soil fertility. The soil is in good condition; catalase can destroy hydrogen peroxide generated by biochemical reactions in the soil and reduce harm to plants; with the increase of continuous cropping years, the activities of soil phosphatase, soil protease, soil sucrase, soil cellulase and other enzymes have a tendency to increase, and they are positively correlated with the content of soil organic matter. Therefore. Increasing the application of organic fertilizer in the vegetable garden can increase the activity of various soil enzymes. Increase the sustainability of the vegetable garden soil.

3. Allelopathic autotoxicity of vegetable crops
Allelopathic effect refers to the beneficial or adverse effects of the metabolic activities of plants or microorganisms on other plants or microorganisms in the environment. If the allelopathic substances released by plants inhibit the growth of plants of the same species or family in the same crop or the next crop, this allelopathic effect is also called autotoxicity. It is a growth inhibition effect that occurs within the species. Studies have shown that some autotoxic substances are separated from the root secretions and residues of pea, tomato, cucumber, watermelon and melon plants. These substances affect plant growth through multiple pathways such as affecting cell membrane permeability, enzyme activity, ion absorption and photosynthesis.

The main prevention and control measures for vegetable continuous cropping obstacles:

① By rotating and intercropping different crops, the number of pathogens in the soil can be significantly reduced.

② Apply ecological organic fertilizer to change the composition of soil microorganisms.

③ Use natural rainfall to remove salt and reduce the accumulation of salt on the soil surface. Water thoroughly each time to dilute the salt accumulated on the soil surface and let it flow down for the root system to absorb.

④The use of disease-resistant varieties and grafting techniques can overcome the infection of pathogens.

⑤ Use some beneficial bacteria to produce harmful substances to specific pathogens in the soil or reduce the number of pathogens and root infection through competition for nutrients and space, thereby reducing diseases. ⑥ Certain microbial fertilizers can antagonize certain pathogenic microorganisms and inhibit diseases.

It can be seen that soil continuous cropping problems caused by harmful microorganisms can be effectively alleviated through effective agricultural cultivation systems and cultivation techniques.