Hydroponics, also known as "water culture", is a key type of soilless cultivation. It uses specific facilities to build a relatively closed environment that can store nutrient solution, keeping the water, fertilizer, air, heat and other conditions required for root growth stable, and submerging all or part of the roots in the nutrient solution. In a strict sense, it is pure soilless cultivation, with roots in direct "submersion" contact with the nutrient solution, without intermediate adsorption and buffering media, precisely simulating the ideal growth environment for plants and laying a solid foundation for plant growth.
The success of hydroponics hinges on the growth of roots, which depends on the rhizosphere environment. The temperature, oxygen, pH, ion concentration and ion ratio of the rhizosphere environment need to match the plant's needs. Meanwhile, the quality of the nutrient solution is crucial. Its mineral nutrients must be complete, anions and cations balanced, and free from toxic and harmful substances, pathogenic microorganisms and pests. A healthy root system is white, firm, elastic, without distortion, discoloration or breakage, ensuring normal water and fertilizer absorption and above-ground growth of plants; if the roots turn black, yellow or rot, the stems, leaves, flowers and fruits will show abnormalities. This also makes hydroponics a scientific research method to test the nutrition of plants and the suitability of the rhizosphere environment.
In nature, plant growth is a "growth adaptation" process. Plants have to go through seasonal climate changes and withstand natural disasters such as drought, waterlogging, high temperature and strong wind. They also face threats such as nutrient deficiency, nutrient imbalance, species competition, and pest infestation and biting. The complex and harsh living environment limits the growth potential of plants and affects their yield and quality.
Compared with traditional agriculture, protected agriculture creates relatively superior conditions for plants, avoiding some meteorological disasters, species competition and biological antagonism threats, making the growth of crops, fruits and vegetables better. However, soil, as a quasi-organism, has a complex "biological community", such as fungi, bacteria, actinomycetes, soil-dwelling insects, mollusks and pathogenic microorganisms, which still have positive and negative impacts on crop growth. The limitations of soil cultivation prompt the continuous innovation of cultivation models.
Substrate cultivation relatively isolates plants from the interference of harmful soil organisms. Its physical and chemical environment is suitable for plant growth, fixing roots, retaining water and fertilizer, and helping root absorption. The nutrients in the soilless cultivation substrate are partially absorbed directly by the roots and partially absorbed indirectly. The preparation goal is to create an "optimized soil". If its physical and chemical properties are not as good as the local soil, it lacks progressive significance. However, the physical and chemical properties of the substrate will "buffer" the effect of the nutrient solution, to a certain extent, affecting the water and fertilizer absorption efficiency.
Compared with soil and substrate cultivation, hydroponics has more direct and efficient water and fertilizer absorption. The nutrient solution in hydroponics does not need complex transformations such as microbial decomposition and organic acid dissolution and chelation by roots like soil nutrients. It is a fully dissolved ionic nutrient that can be directly absorbed by vegetables. It can also exclude useless and harmful components in the soil, precisely meeting the growth needs of vegetables and reducing problems such as insufficient mineral nutrition and harmful substances. However, because small fluctuations in the physical and chemical properties of the nutrient solution will affect the growth of hydroponic crops, its management requirements are stricter, also promoting growers to improve their technical control.
The hydroponic nutrient solution extracts the essence of effective nutrients in the soil according to the nutrient demand law of normal vegetable growth, supplying nutrients in a purer, more efficient form and more reasonable ratio. It is essentially the same as the "effective nutrients" (ionic state) in the soil. However, most organic or poorly soluble inorganic nutrients in the soil need to be converted into ionic states through microbial decomposition, organic acid action by roots and other processes to be absorbed. Vegetables "consume energy" to absorb soil nutrients; while the hydroponic nutrient solution directly provides ionic nutrients, simplifying the absorption process and improving resource utilization efficiency.
The development of urbanization and industrialization has caused heavy metal and other pollution in the cultivated land around cities. Growing vegetables on such polluted land is likely to lead to excessive pollutants in the products, making it difficult to produce pollution-free vegetables. Hydroponics technology isolates soil and irrigation water pollution from the source, using treated water for irrigation, providing a guarantee for the production of high-quality pollution-free vegetables, meeting the current consumers' demand for food safety, and helping agriculture cope with the dilemma of soil pollution.
Some cultivation methods use incompletely treated organic fertilizers, which have the risk of pathogenic microorganisms. For example, pig manure and chicken manure may cause zoonotic (avian-human) diseases. Moreover, the nutrient imbalance of organic fertilizers, with most having high nitrogen content and insufficient phosphorus, potassium and trace elements, is a stress on vegetables, affecting yield, quality and taste. Hydroponic vegetables avoid these hidden dangers, ensuring production safety and product quality, and highlighting their advantages in food safety control.
Plants need 17 kinds of mineral nutrient elements for growth. Carbon, hydrogen and oxygen are obtained from water and air, and the remaining 14 kinds are obtained from the soil or artificial fertilization. The hydroponic nutrient solution regulates nutrient supply according to the crop type and growth period, which is more sufficient and timely than soil cultivation, promoting the improvement of vegetable yield and quality. For example, hydroponic leafy vegetables (such as mustard, water spinach, Chinese cabbage, etc.) have sufficient water and nutrients, fast growth, lagging accumulation of crude fiber and lignin, low content, and significantly increased VC and other nutrients; fruit and melon crops (such as tomatoes, cucumbers, thick-skinned melons, etc.) have neat appearance, uniform coloring, suitable taste and higher nutritional value. Experiments show that the VC content of hydroponic tomatoes is 19.8% higher than that of soil-cultivated tomatoes, and the crude fiber content of hydroponic mustard is only 61% of that of soil-cultivated mustard.
In soil cultivation, stresses such as drought and waterlogging can induce plants to secrete secondary metabolites (such as polyphenols, quinones, sesquiterpenes, etc.). Accumulation of these metabolites affects the appearance and taste of vegetables (with color and bitter taste). Hydroponic vegetables grow in suitable conditions, with less accumulation of secondary metabolites and better taste. Hydroponic vegetables taste tender and delicious, with a rich "vegetable flavor" without bitterness, highlighting the inherent quality of varieties and meeting consumers' pursuit of delicious food ingredients.
People worry about excessive nitrate and nitrite in hydroponic vegetables. In fact, they are different, and nitrite is harmful to the human body. In 2005, China revised the pollution-free vegetable standards, using nitrite content as a measurement index, highlighting its direct impact on human health and providing a scientific basis for defining food safety.
In nature, nitrate is converted into nitrite under reductive conditions by the action of nitrifying bacteria. The hydroponic nutrient solution circulates, with high dissolved oxygen content and high redox potential, without the conditions for nitrate to be reduced to nitrite. Therefore, the nitrite content in hydroponic vegetables is low. In soil cultivation, reductive conditions are likely to occur due to root zone flooding, etc., and the possibility of nitrate being reduced to nitrite is high, so vegetables are likely to contain high nitrate and nitrite. Moreover, hydroponics can control the nitrate content in vegetables by adjusting the supply amount and time of nitrate in the nutrient solution, which is difficult to achieve in soil cultivation. Inspection results in various places show that the nitrite content of hydroponic vegetables is mostly several tenths of the national limit standard, proving its food safety advantage.
The average utilization rate of fertilizers in traditional soil cultivation is 30%–50%. Nitrogen fertilizer is easily nitrified and lost, volatilized as ammonia, and lost due to denitrification, with only about 50% of nitrogen absorbed; phosphate fertilizer is easily precipitated, with a utilization rate of 20%–30%; potash fertilizer is also lost due to irrigation and runoff, with a low utilization rate. Hydroponics supplies nutrients according to crop varieties and growth periods, with good water solubility of nutrients, and about 90%–95% can be absorbed, with high resource utilization efficiency, reducing resource waste and meeting the needs of sustainable agriculture.
Hydroponics does not have water leakage and runoff losses in soil cultivation, and the water consumption is only 1/5–1/10 of that in soil cultivation. It has great promotion potential in arid and water-deficient areas, can alleviate the contradiction between water resource shortage and agricultural water use, help agriculture develop stably under water resource constraints, and provide solutions for vegetable production in water-deficient areas, which is an important embodiment of sustainable agriculture.
The waste nutrient solution of hydroponics can be recycled after treatment without polluting the environment, building a resource circulation system. The loss of fertilizers in traditional soil cultivation is likely to cause non-point source pollution. Hydroponics has significant advantages in environmental protection and sustainable agriculture, conforming to the development direction of green agriculture and promoting the transformation of agriculture to an environment-friendly type.
Currently, agriculture faces challenges such as soil pollution, water resource shortage, and ecological environmental protection pressure. With advantages such as efficient resource utilization (high water and fertilizer utilization rate, water saving), avoiding soil pollution, and producing safe and high-quality vegetables, hydroponics has become an effective path to address these challenges, providing technical support for the sustainable agriculture and helping agriculture achieve green transformation under resource constraints.
Consumers' demand for food safety, safe, high-quality and delicious vegetables is growing. Hydroponic vegetables have high yield, good quality, good taste and high food safety, which can meet the needs of high-end markets, boutique fruit and vegetable markets, and consumer groups pursuing food quality, expanding the agricultural market space, improving agricultural economic benefits, and promoting the upgrading of the agricultural industry under the concept of sustainable agriculture.
Hydroponics technology involves multiple fields such as facility design, nutrient solution formula research and development, precise environmental regulation, and crop growth monitoring. Its development promotes agricultural innovation. From intelligent facilities to create a stable rhizosphere environment, to precise nutrient solution formulas to adapt to different crops, and then to digital monitoring and management to ensure crop growth, new technologies and methods are constantly emerging, improving the level of agricultural modernization, leading agriculture towards precision, intelligence and high efficiency, and becoming a key driving force for sustainable agriculture.
