Manifestation and Control measures of continuous cropping obstacles of vegetables

Manifestation and Control measures of continuous cropping obstacles of vegetables

1 definition of continuous cropping obstacle

Continuous cropping obstacle means that after the same crop or near-source crops (such as pepper, eggplant, tomato and other Solanaceae crops) have been planted year after year, even under normal management, there will be some phenomena, such as poor growth, reduced yield, poor quality, serious diseases and insect pests and so on.

2 manifestation of continuous cropping disorder

2.1 the growth potential of vegetables weakens and the yield decreases

Wei Hui et al found that with the increase of continuous cropping years, the plant height, diameter and length of pseudostem, leaf area increased at first and then decreased, and the number of bulbs decreased.

2.2 serious diseases and insect pests and poor quality

It was found that with the increase of continuous cropping years, the VC content, crude fiber content, soluble sugar content and chlorophyll content of leek decreased, the plant height, leaf area and daily net photosynthetic rate of potato decreased year by year [2], and the occurrence degree of main diseases and insect pests of cucumber increased year by year.

3 causes of continuous cropping obstacles

3.1 allelopathy

Crops (donors) release certain chemicals to the environment, affecting the growth and development of other organisms (receptors), which is called allelopathy, including both promotion and inhibition [4]. Yang Yuhua et al found that different vegetable aqueous extracts had different effects on seed germination, seedling growth and root development of four kinds of vegetables, and the degree of allelopathy of different vegetable crops was different. The order from strong to weak is cabbage > cucumber > pepper > tomato [5].

Toxins from root secretion and decomposition of branch and leaf residues are important factors affecting vegetable continuous cropping obstacles. Crops release chemicals into the environment through leaching, residue decomposition and root secretion, which will cause direct or indirect damage to the crop itself. This phenomenon is called autotoxicity [6]. Autotoxicity is a special allelopathy in which the recipient and the donor belong to the same plant.

Among the root exudates, phenolic acids are mainly toxic to crops, such as phenylacrylic acid, p-hydroxybenzoic acid and so on. Root exudates can inhibit the process of soil nitrification, affect the transformation of nitrogen forms, inhibit the absorption of soil nutrients by roots, inhibit the activities of catalase and peroxidase, destroy the integrity of cells, and reduce photosynthate and chlorophyll content. LV Weiguang et al found that benzene acrylic acid has strong toxicity, 50mg/kg benzene acrylic acid can significantly reduce the activity of dehydrogenase and ATP in cucumber roots, and significantly inhibit the activity of soil microorganisms, thus affecting the growth of cucumber and the absorption of nutrients by roots.

3.2 soil nutrient imbalance

The root distribution range and depth of the same kind of vegetables are the same, and the absorption proportion of various nutrients in the soil is also the same. after continuous cropping, it is easy to cause the lack of some nutrients in the soil, and some nutrient elements are excess, the soil nutrients will be out of balance. For example, nitrogen is easy to cause magnesium deficiency, cucumber mesophyll chlorosis after magnesium deficiency, pepper magnesium deficiency leaf tip yellowing; phosphorus mostly induces boron deficiency, mesophyll darkens, leaf veins turn yellow; potassium deficiency causes chlorosis between leaf veins and affects photosynthesis; potassium deficiency fruits are unevenly colored.

Fengshenghai and other studies found that peanut could fix nitrogen by itself, but continuous cropping broke the original soil nutrient balance, significantly reduced the content of phosphorus, potassium, copper, zinc, iron and other nutrients needed by peanut, resulting in poor development of rhizome and reduction of flowering and fruiting number. and then affect the yield [8].

3.3 soil salinization and acidification

In recent years, the cultivation of various facilities (plastic film, large and medium-sized greenhouse, intelligent greenhouse, etc.) has increased, and the soil salt content is generally 2-13 times higher than that of open-field soil, and salinization tends to increase with the increase of greenhouse age. There is no Rain Water leaching in the soil in the greenhouse, the temperature is high, the soil water evaporation is large, and the fertilizer and other salts in the lower soil rise along the soil capillaries, forming a thin layer of white salt on the soil surface, that is, soil secondary salinization.

The amount of fertilizer applied to vegetables is greater than that of other staple crops, especially the heavy application of chemical fertilizer itself will aggravate soil salinization [9]. The accumulation of salt in soil resulted in the increase of soil solution concentration and osmotic potential, which led to the weakening of water and fertilizer absorption capacity of vegetable roots. With the increase of the concentration of soil solution, the antagonism between nutrient elements will be prominent, which will affect the absorption of some elements by crops, which will lead to element deficiency, hinder growth and decrease yield and quality.

3.4 changes of root microorganisms

Japanese scholar Hiroshima believes that the change of soil microorganisms is most closely related to continuous cropping obstacles. All biochemical reactions in soil can only be completed by the action of microorganisms and enzymes. Continuous cropping of the same crop, but the enrichment of some specific microorganisms in the soil, especially plant pathogenic fungi, will promote the occurrence of root diseases and reduce the yield year by year [10]. Liu Suhui et al found that after continuous cropping of garlic for 10 years, the number of ammonifying bacteria, nitrifying bacteria and aromatic compound-degrading bacteria in soil decreased sharply year by year, while the number of fungi increased year by year [11].

4 prevention methods of continuous cropping obstacles

4.1 reasonable selection of cultivated soil

There are three kinds of soil texture: clayey soil, loam soil and sandy soil. Under the same water and fertilizer management conditions, the aeration and permeability of clayey soil are poor, and the accumulation of allelochemicals in the soil will strongly inhibit the growth of crops. The toxic substances in sandy soil are easy to be lost, but the fertility is low. Therefore, vegetable farming is more recommended to choose sandy loam with high fertility and good drainage.

4.2 soil improvement

For soil improvement under protected cultivation, we can choose the method of soil exchange, that is, to replace the soil layer of 30~40cm in the greenhouse with high quality and fertile soil in the field, or to choose a suitable substrate for soilless cultivation, which can effectively avoid continuous cropping obstacles and soil-borne diseases, reduce pesticide consumption, save water, fertilizer and labor costs, and produce vegetables on non-arable land.

When changing crops, some areas adopt the method of irrigation and washing salt to improve soil salinization and acidification. Irrigate and keep the water layer of 5~10cm for more than 5 days, and change the water once or twice during the period. After drainage, the soil moisture is suitable for ploughing and soil preparation. There are also areas where the greenhouse film is removed during the empty stubble period and the salt is leached by natural rainfall. However, irrigation and washing salt is easy to cause the loss of soil nutrients, and the salt dominated by nitrate nitrogen is easy to enter the groundwater with water, resulting in pollution.

The suitable pH value for vegetable growth is neutral or slightly acidic. For soils with pH less than 5.5, quicklime should be used to neutralize acidity and reduce the content of nitrate nitrogen in soil. For soils with pH values of 5.5-6.0, alkaline fertilizers such as plant ash should be applied. For soils with pH greater than 7.5, acid fertilizers should be applied appropriately.

In order to achieve the purpose of soil disinfection, drug disinfection, high temperature greenhouse and winter frozen shed can be used to prevent soil-borne diseases and underground insect pests.

4.3 selection of resistant varieties or application of grafting techniques

Breeding varieties with resistance to continuous cropping is an important way to solve the problem of continuous cropping, and the seed quality should meet the requirements of GB/16725.3-2010. Watermelon varieties with resistance to continuous cropping, such as Sumi 5 and Xinong 8, were introduced and popularized in northern Jiangsu, and the yield increase effect reached 15% to 20% [13].

Grafting can improve the absorption characteristics of plant roots, change the content of endogenous hormones, enhance plant photosynthetic capacity and increase the activity of protective enzymes, so as to achieve the purpose of disease resistance and yield increase. Disease-resistant rootstocks of melons and eggplant vegetables are used more and more in production. In Japan, cucumber cultivation all uses grafting technology, and the proportion of tomato grafting cultivation is about 50%. In the application of grafting technology in northern Jiangsu, Japanese red eggplant was selected as the rootstock, and the yield was increased by 10% to 15% [Yoshama Huayao].

4.4 reasonable rotation

Rotation is not a simple variety replacement. Following a certain law of rotation in the process of protected vegetable cultivation is helpful to speed up the recovery of soil fertility, reduce diseases and insect pests, increase yield and improve quality.

Leafy vegetables need more nitrogen fertilizer, eggplant fruits need more phosphate fertilizer, and rhizomes need more potassium fertilizer; eggplant fruits and melons have deeper roots, while leafy vegetables have shallower roots. Rotation of these different varieties can make full use of various nutrients in different layers of soil.

The pH value of the soil will change after some vegetables are planted, such as cabbage will increase the acidity of the soil, pumpkin can neutralize the acidity, onion (acid-sensitive) will increase the yield as the next crop of pumpkin, and the yield will be reduced as the next crop of cabbage.

4.5 rational fertilization

Long-term simple application of chemical fertilizer will make the soil hardening, air permeability, water permeability and fertilizer supply capacity become worse. In particular, the excessive amount of nitrogen fertilizer will increase the content of soluble salt and nitrate in soil and promote the occurrence of diseases and insect pests. Zhang Aijun et al think that rational fertilization can obviously alleviate the obstacle of continuous cropping [13].

At present, many bio-organic fertilizers popularized in production have the characteristics of low cost, safety and effectiveness, which can reduce and prevent soil salt surface accumulation, improve soil physical structure, enhance the activity of microorganisms in soil, and maintain soil fertility. to a certain extent, reduce pesticide residues and heavy metal pollution, increase the yield of vegetables and improve the quality of vegetables.