Nutritional quality and safety of hydroponic vegetables
Hydroponics is one of the main types of soilless culture. Hydroponics is the cultivation of a relatively closed environment that can store nutrient solution through specific facilities, and keep the water, fertilizer, gas, heat and other conditions required for root growth relatively stable, and all or part of the root system is "immersed" in the nutrient solution. method.
Strictly speaking, hydroponics is pure soilless cultivation. Because there is no intermediate adsorption and buffering medium between the root system and the nutrient solution, it is in direct "immersion" contact. To make plants grow healthily, we must first ensure that the root system grows well, and whether the root system grows well depends on the rhizosphere environment. The temperature of the rhizosphere environment, oxygen, pH, ion concentration and the ratio of different ions should be appropriate. And whether the mineral nutrients in the nutrient solution are complete, whether anion and cation are balanced, whether it contains toxic and harmful substances, whether it contains pathogenic microorganisms, pests, etc., will all be reflected in the appearance of the root system. If the root system is white, firm, elastic, not distorted, discolored, or broken, it proves that the root system is growing healthy. When the root system grows healthy, the plant can absorb water and fertilizer normally, and the growth of the plant's ground part will be normal. Conversely, if the root system exhibits black root tips, yellowing or even decay of the root system, the stems, leaves, flowers and fruits above the ground will also show "abnormal" symptoms. Therefore, hydroponics technology is one of the scientific research methods to test whether the plant nutrient elements are complete and the rhizosphere environment is suitable.
Plants growing in nature are a process of "growth adaptation". In the growth process of plants, in addition to experiencing four seasons of climate change, they also have to face natural disasters such as drought, waterlogging, high temperature, and strong wind. In addition, plants may also face nutrient deficiency, nutrient imbalance, competition between species, pest infestation, biting and other hazards. Compared with traditional agriculture, facility agriculture creates relatively superior growth conditions for plants, and avoids threats such as meteorological disasters, competition between natural species, and biological confrontation factors. Therefore, the growth of crops, fruits and vegetables is better. However, certain diseases and insect damage still exist. Soil is a kind of organism, it is alive, it carries the multiplication of many biological populations, complex biochemical succession, mineral chemical evolution and other processes. Although the soil has become an ideal medium for crop growth through people's improvement, complex "biological groups", such as various fungi, bacteria, actinomycetes, various soil-dwelling insects, molluscs, and pathogenic microorganisms, still have an impact on crop growth. Favorable or harmful effects.
In addition to studying the pros and cons that affect plant growth and development, current scientific experiments are more important to explore how to create conditions that are more conducive to plant growth and development, avoid and reduce the interference of harmful factors, and make target crops grow better, with higher yields, Better quality.
A further cultivation mode than soil cultivation is substrate cultivation. Substrate cultivation is relatively isolated from the interference of soil harmful organisms, and its suitable physical and chemical environment is more suitable for the growth and development of plants. The role of the substrate is to fix the root system, maintain water and fertilizer, and support the absorption of water and fertilizer by the root system. Part of the nutrients in the soilless culture medium is absorbed directly by the root system, and the other part is absorbed indirectly. The preparation target of the substrate should be "optimized soil". If the physical and chemical properties of the prepared substrate are not as good as the local soil, it means that this kind of substrate soilless cultivation is lack of progress. Although compared with soil, substrate has some advantages, for example, substrate cultivation can eliminate the interference of certain soil harmful substances. However, the physical and chemical properties of the substrate will "buffer" the chemical properties and efficacy of the nutrient solution. Therefore, compared with soil and substrate cultivation, the absorption of water and fertilizer in hydroponics is more direct and efficient. However, because small fluctuations in the physical and chemical properties of the nutrient solution will affect or even harm the normal growth of hydroponic crops, the management requirements for hydroponics are also stricter. In order to further clarify the scientific principles of safe and high-quality production of hydroponic vegetables, this article discusses the following aspects:
Mineral nutrition in hydroponic nutrient solution
According to the analysis by researchers of Wei Zhaoye, hydroponic nutrient solution is based on the law of nutrient demand for vegetables under normal growth conditions, and the various nutrient elements that can be provided to vegetables in the soil are supplied in a more pure and efficient form and a more reasonable ratio. Give vegetables. Therefore, the nutrients provided by hydroponics are "the essence of the effective nutrients in the soil", and there is no essential difference from the form (ion state) of the "effective nutrients" in the soil. It is just that most of the organic or insoluble inorganic nutrients in the soil must be decomposed by soil microorganisms, and the roots themselves secrete organic acids to dissolve, chelate, or use other chemical methods to regulate the transformation, in order to form ionic nutrients and be absorbed by vegetables. Therefore, the process of vegetables absorbing nutrients from the soil is "consuming physical energy."
Hydroponics uses artificially prepared nutrient solutions, which are completely dissolved ionic nutrients and can be directly absorbed by vegetables. In addition, the hydroponic nutrient solution can eliminate other non-essential (including human body) minerals and various toxic and harmful substances that are not necessary for vegetable growth in soil cultivation, which is more in line with the nutrient requirements of vegetable growth. The hydroponic nutrient solution can provide nutrients for the growth and development of vegetables more accurately, more rationally and more effectively. Therefore, hydroponic vegetables rarely have problems such as insufficient mineral nutrition or harmful minerals.
With the development of urbanization and industrialization, arable land around cities has appeared to varying degrees of heavy metal and other pollution problems. If vegetables are grown on these contaminated arable land, it will undoubtedly cause excessive levels of heavy metals and other pollutants in vegetable products. It is almost impossible for these cultivated land to grow pollution-free and pollution-free vegetables. The hydroponic production technology can isolate soil and irrigation water pollution from the source, and use treated water for irrigation to produce high-quality pollution-free vegetables.
Some planting methods use organic fertilizer that has not been completely processed, and the organic fertilizer may contain pathogenic microorganisms. If pig manure, chicken manure and other livestock manure are used, there is a risk of infecting humans and animals (poultry) with comorbidities. Hydroponic vegetables avoid this hidden danger. Moreover, the nutrient content of organic fertilizer itself has the problem of imbalance. For example, most organic fertilizers have high nitrogen content and insufficient phosphorus, potassium and other trace elements. This is a stress in itself for normal vegetables. Therefore, the yield, quality, and even taste of vegetables grown with organic fertilizers are not the best. From this point of view, hydroponic vegetables are safer than those grown with organic fertilizer, and the yield and quality of vegetables are higher.
The nutritional value, taste and quality of hydroponic vegetables
Plant mineral nutrition studies have shown that the essential mineral nutrients for plant growth are 17 kinds [silicon (Si) is an essential element for certain plants], except for carbon (C), hydrogen (H), and oxygen (O) which are derived from water and air The remaining 14 minerals: Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulfur (S), Iron (Fe), Boron (B), Manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), chlorine (Cl), and nickel (Ni) are all obtained from the soil or need to be supplemented by artificial fertilization. The mineral nutrient elements in the hydroponic nutrient solution regulate the nutrient supply according to different types of crops and different growth and development stages. Since the nutrient supply during the growth process is more sufficient and timely than that of soil cultivation, the yield and quality of hydroponic vegetables are higher than that of soil cultivation.
The rapid growth of hydroponic vegetables is due to adequate and timely nutrient supply, and when they are grown in greenhouses or greenhouses and other environmentally controllable conditions, the various external environmental conditions for vegetable growth meet or approach their optimal growth requirements. Therefore, the growth rate of hydroponic vegetables is faster than that of soil cultivation. However, hydroponic vegetables will not decrease in quality due to their fast growth. On the contrary, due to its sufficient and timely nutrient supply and good environmental regulation, the photosynthesis efficiency of plants is high, and the amount of nutrient synthesis is more and faster than that of soil cultivation. Therefore, the yield of hydroponic vegetables is high, and the internal quality and taste are better.
Drought, water logging, nutrient imbalance, insect biting, pathogenic microbial infestation and other stress or damage in soil cultivation will inevitably induce plants to secrete many secondary metabolites such as polyphenols, quinones, and sesquiterpenes to resist these Coercion. If things go on like this, more secondary metabolites will accumulate in vegetables, and most of these secondary metabolites are colored or bitter, which will affect the appearance and taste of vegetables. On the contrary, the various conditions for the growth of hydroponic vegetables are more suitable or closer to the ideal state, and the plants will not accumulate too much secondary metabolites. Therefore, the taste of hydroponic vegetables will be better.
Green leafy vegetables produced by hydroponics, such as mustard greens, water spinach, Chinese cabbage, etc., due to sufficient water and nutrient supply, their growth rate is faster, and the accumulation of crude fiber and lignin is faster than the accumulation of other amino acids, protein and other flavor substances The speed is relatively lagging, so the content of crude fiber and lignin is less, while the content of VC and other nutrients is significantly increased. The hydroponic tomatoes, cucumbers, melons and other melons and fruit crops have neat appearance, uniform coloring, suitable taste and higher nutritional value. Experiments have shown that the VC content of hydroponic tomatoes is 154.9 mg/kg, while the VC content of soil-grown tomatoes is 124.2 mg/kg. The VC content of hydroponic tomatoes is 19.8% higher than that of soil-grown tomatoes. The crude fiber content of hydroponic mustard is 2.8%, while that of soil cultivation is 4.6%. The crude fiber content of hydroponic mustard is only 61% of the fiber content of soil cultivation of mustard. Therefore, the taste of hydroponic vegetables is better than that of traditional soil-grown vegetables, and they are more tender and delicious, and the "vegetable flavor" is stronger without bitterness. Therefore, hydroponic vegetables can highlight the inherent qualities of the vegetable varieties themselves.
Nitrite content of hydroponic vegetables
At present, people's concerns about the edible safety of hydroponic vegetables mainly come from whether the content of nitrate and nitrite in hydroponic vegetables exceeds the standard. In fact, nitrate and nitrite in vegetables are two completely different substances. A large number of studies have shown that it is nitrite rather than nitrate that is harmful to the human body. For this reason, when China revised the agricultural industry standards for pollution-free vegetables in 2005, it cancelled the nitrate content in some local standards as one of the indicators for measuring pollution-free vegetables, and the use of nitrite content has a direct impact on human health. As one of the indicators for measuring pollution-free vegetables.
The content of nitrate and nitrite in vegetables is affected by many factors. In nature, nitrate must be reduced to form nitrite through the action of nitrate reductase of nitrifying bacteria. The hydroponic nutrient solution circulates and the dissolved oxygen content in the nutrient solution is always at a high level, that is, the redox potential in the nutrient solution is high, and there is no condition for reducing nitrate to nitrite. Therefore, the content of nitrite in hydroponic vegetables is very low. In soil cultivation, it is easy to produce reducing conditions due to local soil (when the root zone is flooded due to rain or irrigation). In soil cultivation, the possibility of reducing nitrate to nitrite is higher than that in hydroponic cultivation. Therefore, vegetables grown in soil may also contain high nitrate and nitrite. Moreover, hydroponics has better controllability than soil cultivation. The nitrate content in vegetables can be controlled by adjusting the supply and supply time of nitrate in the nutrient solution, which is impossible for soil cultivation. The analysis results of local agricultural product quality and safety inspection and supervision centers also show that the nitrite content of hydroponic vegetables is mostly only a few tenths of the national limit standard. Therefore, hydroponic vegetables do not have the problem of excessive nitrite content.
Utilization rate of water and fertilizer by hydroponics
The average utilization rate of fertilizer applied in traditional soil cultivation is about 30%~50%. Nitrogen fertilizer applied to the soil is easily nitrified and lost with the irrigation water. It will also be lost due to the volatilization of ammonia and denitrification to produce N2O and N2. In the end, only about 50% of nitrogen can be absorbed and utilized by plants. After phosphate fertilizer is applied to the soil, a large part of it will form the precipitation of phosphate such as iron sulfate, aluminum, calcium, etc., which cannot be absorbed and utilized by crops. The utilization rate of phosphate fertilizer is only about 20% to 30%. After potash fertilizer is applied to the soil, part of it is lost due to the influence of irrigation water and surface runoff, and the utilization rate is not high. Hydroponics supplies different nutrients according to different varieties of crops and different growth periods, and all nutrients are water-soluble. About 90%-95% of the nutrients can be absorbed and utilized by crops, so its nutrient utilization efficiency is very high. , And the waste nutrient solution can be recycled after treatment, and will not pollute the environment.
Since hydroponics does not have the problems of water leakage and runoff loss in soil cultivation, its water use efficiency is high. The water consumption of hydroponic crops is only about 1/5~1/10 that of soil cultivation, so hydroponics is suitable for promotion in drought and water shortage areas.
In summary, due to the sufficient and balanced supply of water and fertilizer for hydroponic vegetables, and the environment in such facilities as greenhouses and greenhouses is more suitable for vegetable growth. Therefore, compared with traditional soil cultivation, hydroponic vegetables have higher yield, better quality, better taste, and higher safety.