3. The role of seaweed fertilizer in modern agricultural production

3.1 Promote plant growth

Seaweed fertilizer is rich in natural plant growth regulators, which can promote seed germination and plant growth. Seaweed liquid extract not only improves the germination rate and vigor of tomato seeds, but also promotes early germination, increases plant height, number of leaf branches and yield. Seaweed fermentation liquid 300 times diluted for foliar spraying can significantly improve cherry tomato fruit shape index, leaf area, yield per plant. Concentration of 20% seaweed liquid fertilizer on pepper seed germination rate, biochemical indicators and pepper pigment characteristics increase the best effect. In winter wheat, foliar spraying of seaweed bio-organic fertilizers during the period of nodulation and maturity can promote plant growth, increase the spike weight and capacity of a single plant, and thicken the stalks so as to play a role in the resistance to collapse.Chaikina et al. demonstrated that the water-soluble extracts from red, brown and green algae had a growth-promoting effect on the soybean root system and were effective in promoting the growth of soybean roots in both low (10-4g/mL) and ultra-low (10-4g/mL) concentrations. mL) and ultra-low concentrations (10-10 to 10-12 g/mL) were stimulatory. Seaweed extract has obvious growth-promoting effect on both early and late rice, especially during the tillering period, which increased the yield by 9.83% and 8.43% respectively. After 2 years of field trials confirmed that 1% of the bubble algae extract to increase grape yield 26%, 35% of the number of fruit and skin anthocyanin content of 21%. Foliar spraying of new algae bio-fertilizer 1000 ~ 1500 times, dragon fruit single fruit weight, single fruit volume, diameter and longitudinal diameter are significantly higher than the control. Seaweed fertilizer can also significantly increase the bud density and 100 bud weight of tea trees, is the best foliar fertilizer to promote the growth and development of tea trees in low-yield renovation tea plantations. From the above research, different forms of seaweed fertilizer in a specific concentration of fruits and vegetables, crops and so on have good growth-promoting function.

3.2 Improve product quality

Seaweed fertilizer can improve the taste of fruits and vegetables, improve the nutritional and functional components of fruits and vegetables. Application of seaweed fertilizer (1200-1600 times) is conducive to increasing the anthocyanin, soluble protein, vitamin C content of purple rape. Seaweed fertilizer and 2‰ agar oligosaccharides can be used together to significantly increase the content of flavonoids, fructose, pectin, free amino acids, soluble sugar and vitamin C in okra fruits. In 1500 times of seaweed bio-fertilizer treatment, strawberry's soluble solids content, sugar-acid ratio, reducing sugar content and hardness increased significantly compared with the control, of which the VC content increased by 50.2%; in 2500 times of seaweed bio-fertilizer treatment, the content of anthocyanins and flavonoids increased by 42.0% and 30.6% compared with the control, respectively, and the high concentration (1000-500 times) inhibited the accumulation of anthocyanins and flavonoids. The accumulation of anthocyanins and flavonoids was inhibited at high concentrations (1000-500 times). Spraying seaweed foliar fertilizer increased the amino acid content of grape plants, and 0.25% and 0.50% of brown algae extract had a significant effect on the quality of grapes, of which the 0.25% dose in the dry season significantly increased the amino acid content of grapes, while the 0.50% dose in the rainy season was more effective. Foliar seaweed fertilizer can balance the potential content of date palm, improve the fruit shape index, reduce the total acidity of date palm, and increase the content of soluble solids.

Seaweed fertilizer can also improve the essential oil content and composition of vanilla plants. By spraying kelp extract, significantly increased the essential oil content in mint and basil, mint l-menthol ketone oil and l-menthol oil content reached 32.4% and 32.6%, respectively, basil chavicol methyl ether, linalool and cineol reached 38.7%, 29.1% and 9.1%, respectively. The potential toxicity of pulegone and methoxyfuran in peppermint oil was also reduced, while showing higher antimicrobial potential than the control. The spraying of seaweed fertilizer significantly increased the volatile oil content of rosemary, and increased the content of monoterpenes and sesquiterpenes and other components.

3.3 Enhancement of resistance

Application of seaweed fertilizer can improve the adaptability of plants under drought, low temperature, salinity and other environmental stresses. Under drought stress conditions, alginic acid, brown algae polyphenols, mannitol and betaine can significantly increase the relative water content and chlorophyll content of vegetable heart leaves, and improve leaf SOD and POD activity, of which alginic acid has the best effect. Seaweed extract increased the activity of antioxidant enzymes in corn plants under normal and water deficit conditions, and increased the length of cob and the number of grains per cob. Seaweed extract or seaweed fertilizer enhanced antioxidant enzyme activity, increased chlorophyll a, chlorophyll b, total chlorophyll, and increased leaf and fruit fresh and dry weight in pepper or tomato plants under salt stress.

Seaweed fertilizer can also enhance the disease resistance of crops. Seaweed fertilizer applied alone or mixed with biochar, etc. can reduce the incidence of tobacco green blight and disease index. Seaweed liquid as a carrier for the preparation of composite biological seaweed fertilizer can inhibit the reproduction of tomato gray mold pathogen, disease index than the control reduction of more than 38.86%. Foliar application of brown algae polyphenol eckol (10-6 mol / L) not only significantly increased the length of cabbage stems and roots, as well as its fresh and dry weight, leaf area and number of leaves, and under the premise that all control plants are seriously infected with aphids, eckol treatment group did not see infection.

3.4 Efficiency enhancement

Seaweed fertilizer and inorganic salt, other organic fertilizers or bacterial fertilizer mixed application, can effectively improve the overall fertilizer efficiency. Alginic acid (0.8% ~ 1.2%) can significantly increase the utilization rate of potassium fertilizer, promote the root growth of non-balling cabbage, significantly increase cabbage yield. Alginate oligosaccharide urea and organic fertilizer can significantly increase the number of nodes of soybean main stem, the total number of pods and rhizomes, and improve the total dry matter accumulation and yield up to 22.5% and 9.7%. Seaweed fertilizer and Trichoderma bulbiflorum ND35 bacterial fertilizer mixing treatment can improve the efficiency of photosynthesis and water utilization of cucumber, are more than 21.9% higher than the control yield; seaweed fertilizer and bacterial fertilizer mixing treatment to improve the quality of cucumber effect is obvious, soluble sugar, soluble protein and vitamin C content were increased by 7.9%, 173.0% and 310.2% than the control, and the content of nitrate was reduced by 15.6%. Seaweed fertilizer mixed with bacterial fertilizer can promote the cucumber on the soil N, P, K absorption and utilization, especially for fast-acting potassium absorption effect is obvious. Concentration of 5% kelp extract (KSWE) can improve sugarcane yield more than 8%, in addition to 6.25% KSWE + 50% fertilizer treatment, compared with the control (water + 100% fertilizer), sugarcane yield is equal, indicating that KSWE has the effect of improving fertilizer efficiency.

4. Algae fertilizer action mechanism

Algae extracts have the property of promoting plant growth, but the mechanism of influencing plant growth is not fully understood. The following mechanisms may be involved for seaweed fertilizer to work well in agriculture:

4.1 Improve soil physicochemical properties and establish a good micro-ecological environment

Seaweed fertilizer can improve soil granular structure, internal pores, improve soil organic matter, total nitrogen, nitrate nitrogen, ammonium nitrogen, effective potassium and effective phosphorus content, enhance the function of fertilizer storage, reduce the effective content of soil heavy metals.

Organic matter, active substances and trace elements contained in seaweed fertilizer can activate all kinds of beneficial bacteria in the soil, significantly increase the number of bacteria, fungi and bacteria in the soil, and increase the proportion of beneficial bacterial groups, while the relative multiplicity of the main taxa of soil bacterial community and the change of bacterial α polymorphism may be related to the enzyme activity in the soil.

4.2 Promote cell division, improve plant photosynthesis efficiency and enhance nutrient uptake

Seaweed is rich in growth regulator components that can break seed dormancy, promote germination, promote cell division, cytosol enlargement, but also can lift the apical dominance, promote the growth of side shoots.

Spraying seaweed fertilizer can effectively increase the plant net photosynthetic rate, stomatal conductance and transpiration rate, and reduce the leaf intercellular carbon dioxide concentration. Polysaccharides, betaine and other substances can regulate cellular osmosis and promote nutrient transport in the plant.

4.3 Inducing the synthesis of protective proteases in plants

Seaweed fertilizer can significantly increase the activity of antioxidant enzymes in plants, reduce the content of malondialdehyde and reactive oxygen species, and may resist the oxidative damage caused by abiotic stress by participating in the regulation of plant carbohydrate metabolism, growth, hormone signaling, and antioxidant-related genes, and reducing oxidative stress-induced cell death and chloroplast degradation of lipids and other mechanisms.

5. Analysis of bottlenecks in the application of seaweed fertilizer and prospects

5.1 Product quality control

The quality of seaweed fertilizer is affected by raw materials, processes, dosage forms and other aspects. At present, the raw materials used by seaweed fertilizer producers are mainly brown algae, including macroalgae, bubble algae, kombu, kelp and sargassum, etc., and other algae such as seaweed, Murraya and Ulva and so on. Although China's National Standards for Alginate Fertilizers and Urea Containing Alginate were formally implemented on April 1, 2017, the quality of seaweed fertilizer products still varies due to different production raw materials. Biological preparation methods are highly respected for their green, efficient and sustainable advantages, however, the current microbial fermentation methods are less stable and it is difficult to form a unified production standard. Therefore, enterprises need to strengthen product quality control to ensure the actual use of the product; strengthen the research on biological process, screening efficient fermentation strains, clear fermentation process and effective products, to provide strong support for the standardization of enterprises, large-scale production of seaweed fertilizer.

5.2 Low market share

From the query of effective fertilizer registration and release office of Plantation Management Department of Ministry of Agriculture and Rural Development of the People's Republic of China, fertilizers with seaweed as raw material account for a relatively small percentage, which still belongs to new niche products. It is well known that the market share depends on the degree of acceptance of the product by end users. Although seaweed fertilizers have shown good results in both plot trials and field applications, and the demand for new fertilizers with higher content of scientific and technological components from growers in the new era is higher than in the past, scientific and technological promotion still needs to be further strengthened in order to enhance the awareness of users in general. In addition, cost reduction is also an urgent problem to be solved in the promotion and application of seaweed bio-fertilizer.

5.3 Prospect

With the development of society and the continuous progress of science and technology, people pay more and more attention to the quality and safety of agricultural products and the sustainable development of agriculture. Organic agriculture is an important development direction and trend of sustainable agriculture, and the requirements for fertilizers under the organic agricultural system are strict, and the globally recognized organic standards all list seaweed fertilizers as permitted substances. Therefore, the economic value of seaweed fertilizer in agriculture is huge.

The large-scale industrialized production and wide application of seaweed fertilizer is conducive to the in-depth development and utilization of seaweed resources in China, which can promote the production of green organic agriculture in China, improve the quality and safety of agricultural products, and push forward the healthy development of plantation industry. Therefore, the research and development and application of seaweed fertilizer has significant social benefits, ecological benefits and economic benefits, and has a broad development prospect. With the application and continuous research and development of seaweed bio-fertilizer, seaweed bio-fertilizer will play an important role in China's modern agriculture.