According to the statistics of the Food and Agriculture Organization of the United Nations in 2017, the global food production was 2.627 bilion tons, of which 618 milion tons were produced in China, accounting for 23.5% of the total global food production during the same period. In order to maintain such high yields, China's agricultural production reguires the consumption of a large amount ot farmland and treshwater resources every year. However, the scarcity of the aforementioned resources in China is extremely evident. According to statistics, the per capita arable land area is less than 0.1 hm2, which is only one-third of the world's per capita quantity and less than one-seventh of that of the United States; China's per capita freshwater resources are less than 2200 m3, only 1/4 of the world average, making it one of the countries with the poorest per capita water resources in the world. Therefore, a large amount of chemical fertilizers must be used in agricultural production in China to ensure overall yield.

However, in China's agricultural production, the utlization rate of fertilizers is not satisfactory. Taking nitrogen fertilizer as an example, in 2017, the total amount of nitrogen fertlizer applied in China reached 22.06 million tons, accounting for 35% of the world's total. However. the comprehensive utilization rate of nitrogen fertilizer in China was less than 35% that year. causing great waste. Therefore, agricultural science and technology workers in China are gradually starting to study high-end water-soluble fertilizers to adapt to the process of water fertilizer integration[1-2l. According to the research conducted by the National Agricultural Technology Promotion Center, there are currently over 30 milion hectares of arable land suitable for the integration of water and fertilizer in China, while the current application proportion in the country is only 3.2%. Therefore, the development potential of water-soluble fetilizer in China is very huge and it is a key focus for future fertilizer development.

Water soluble fertilizer is a multi element compound and quick acting fertilizer that is completely soluble in water. it has the characteristics of good water solubility, no residue, and can be directly absorbed and utilized by the roots and leaves of crops.As an important component of water fertilizer integration, water-soluble fertilizers have obvious advantages. Firstly, it can significantly improve the utilization rate of fertilizers. According to statistics, the utilization rate of conventional fertilizers in China is about 30%, while the utilization rate of water-soluble fertilizers is between 70% and 80%. It can also reduce the total amount of fertilization, which meets the basic requirements of the national dual carbon cyce. Secondly, water-soluble fertilizers have high nutrient content and comprehensive nutrition, which can significantly improve crop yield and quality, making them one of the key development directions for the future fertilizer industry; Finally, the promotion and use of water-soluble fertilizers.accompanied by the integration of water and fertilizer, can save a large amount of freshwater resources and help improve the quality of life of Chinese residents.

At present, there are still many problems to be solved in the production and use of water-soluble fertilizers in China. Poor water solubility and high content of insoluble substances can easily cause scale blockage in pipelines, especially in areas with high calcium and magnesium ion concentrations in irrigation water. At present, the requirement for water insoluble substances insoluble fertilizers in China is 0.5%, while the integrated water fertilizer system is generally fixed or semi fixed, with extremely fine and difficult to clean water outlets, which are easily blocked by water insoluble substances. Part of the salt in the fertilizer will corrode the pipeline. At present, the pipes of the integrated water and fertilizer system are mostly made of carbon steel or plastic, among which the pipes made of carbon steel are prone to corrosion by oxygen, water, acid and alkali, which shortens the service life of the system and increases the cost of use. The main component of water-soluble fertilizers is chemical fertilizers, which can easily cause soil compaction and imbalance of soil microbial communities after long-term use, ultimately leading to soil fertility degradation. Based on the above reasons, with the development of water fertilizer integration, major domestic and foreign chemical companies have successively developed chemicals with scale and corrosion inhibition effects to solve problems in the production and use of water-soluble ferilizers, Among them, polyaspartic acid and its derivatives are the most widely studied substances.

1.1Application of polyaspartic acid in water-soluble fertilizers

Polyaspartic acid (PASP) is an artificially synthesized water-soluble protein that naturally exists in the mucus of marine shellfish such as oysters. lt is an active substance used by marine shellfish to enrich nutrients and create shells. Polyaspartic acid, as a new type of fertilizer synergist, can enhance the absorption of nitrogen, phosphorus, potassium, and trace elements by crops;In addition, polyaspartic acid is non-toxic, harmless, and fully biodegradable, making it a globally recoanized green chemical.Research and application results at home and abroad have shown that polyaspartic acid, as a synergistic agent for water-soluble fertilizers, has main effects in the following aspects.

1.1 Dispersion effect of polyaspartic acid

The main reasons for pipeline blockage during the use of water-soluble fertilizers include precipitation caused by chemical reactions between fertilizers, decreased solubility caused by water pH, and water insoluble substances in fertilizers. These water insoluble substances formed through different pathways gradually adhere to the interior or outlet of the pipeline,especially water insoluble salts such as calcium and magnesium, thereby blocking the entire system.

Polyaspartic acid, as a new type of green dispersant, can prevent and alleviate the formation and aggregation of inorganic salt scale when applied to drip (spray) irrigation systems. lt can disperse the formed scale into small particles suspended in the water system, thereby reducing the blockage problem of water-soluble fertilizers in the system during use. According to research, polyaspartic acid, as a chelating dispersant in water circulation systems, has good chelating and dispersing effects on iron oxides, calcium carbonate, titanium dioxide, zinc hydroxide, magnesium hydroxide, magnesium oxide, manganese dioxide,etc. Koskan et al. believe that polyaspartic acid can prevent the deposition of scale on heat transfer surfaces and water system pipelines.

Meanwhile, research on the effects of polyaspartic acid molecular weight and system temperature on scale inhibition has confirmed that the scale inhibition effect of polyaspartic acid is closely related to its molecular weight, but not to the system temperature. lt is generally believed that the scale inhibition effect of polyaspartic acid synthesized by different methods is closely related to its corresponding scale. For example, polyaspartic acid using aspartic acid as raw material has better scale inhibition effect on CaF2, while polyaspartic acid using maleic anhydride and its derivaives has beter scale inhibition effect on BaS04, SrS04, CaS04, etc. Ross et al. confirmed that the optimal weight average molecular weight range for dispersing polyaspartic acid such as calcium carbonate, calcium sulfate, and barium sulfate is between 10000 and 4000. Quan Zhen hua and others found that when the water temperature is below 60 °c, the temperature change has little effect on the scale inhibition rate of polyaspartic acid; When Ca2+is 800mg/L and the dosage of polyaspartic acid is only 3 mg/L, the scale inhibition rate can still reach over 90%. At 20 °c, polyaspartic acid causes a delay of at least 150 minutes in the nucleation of calcium carbonate. These studies all indicate the universality of polyaspartic acid scale inhilbition performance to temperature.

1.2 Corrosion Inhibition of Polyaspartic Acid

It is generally believed that the polar groups (including N and O groups) in polyaspartic acid are adsorbed on metal pipelines，greatly improving the activation energy of metal ionization process. At the same time, the non-polar groups (alkyl R) are arranged in a directonal manner away from the metal, forming a hydrophobic film, thereby inhibiting the corrosion of metal pipelines by aqueous solutions, effectively protecting drip irrigation systems in the integration of water and fertilizer, extendinc the service life of equipment, and reducing production costs. Polyaspartic acid has corrosion inhilbition effects on various metal materials such as carbon steel, copper, brass, and white copper in various systems [25]; When the concentration of polyaspartic acid is 100 mg/L, the corrosion inhibition rate of carbon steel can reach 93%, and at this concentration,polyaspartic acid can slow down the corrosion rate of carbon steel by 90%, effectively extending the service life of the pipeline.

In relevant studies, researchers have found that polyaspartic acid has a good inhibitory effect on pipeline corrosion in water systems under different pH conditions. Benton's research suggests that using polyaspartic acid and its salts with a molecular weight of 1000 to 5000 and a concentration of 25 mg/L in a corrosive salt solution medium with a pH of 4.0 to 6.6 can effectively inhibit the corrosion of carbon steel by carbon dioxide, When Kalota et al.and Silverman et al. [30l studied the corrosion inhibition performance of polyaspatic acid on iron under different pH, temperature, and moisture conditions, they found that polyaspartic acid has good corrosion inhibition performance when pH is greater than 10. Mansfeld et al. [31 found that good results can also be achieved at pH ranging from 8 to 9. Therefore, polyaspartic acid can solve the corrosion of pipelines during the use of different formulas of water-soluble fertilizers, which is helpful for the applicaion of fixed or semi fixed pipeline systems.

1.3 Synergistic and quality enhancing effects of polyaspartic acid

Polyaspatic acid, as a fertilizer synergist or nutrient enhancer, has been reported in terms of slow release and efficiency enhancement, increased fertilizer utilization, improved crop quality, and increased yield and income. Research has shown that adding polyaspartic acid to water-soluble fertilizers can prolong the effectiveness of the fertlizer, ensure that crops absorb nutrients evenly throughout the entire growth process, and thus ensure the effective utilization of fertilizers. The experiment conducted by Lei Quankui et al. showed that the utilization efficiency of N, P, and K fertilizers in peanuts increased to varying degrees after the application of polyaspartic acid, and the peanut was less prone to nutrient deficiency symptoms throughout the entire growth season. Cao Dan et al. studied the persistence of polyaspartic acid and demonstrated that using polyaspartic acid once a year has a yield increase effect on both crops.

According to reports, polyaspartic acid can effectively activate the essential medium and trace elements for crop growth,improve the absorption efficiency of large amounts of elements, and thus increase fertilizer utilization. After use, it can enhance crop stress resistance, regulate enzyme activity in crops, increase yield, and improve crop quality. Li jiangang et al. found that applying polyaspartic acid to crops such as green vegetables resulted in varying degrees of increase in vitamin C and soluble sugar content, which can effectively improve the quality of vegetables and fruits. jiao Yongkang et al. found through foliar spraying of different types of polyaspartic acid chelates that the use of polyaspartic acid not only increases the yield and quality of Huangguan pear, but also reduces yellowing. The losses caused by the crow's claw disease of the pear tree. Tang Huihui et al. found through their study on the application of polyaspartic nitrogen fertilizer to Northeast spring maize that PASP N was used for maize cultivation under the condition of reducing total nitrogen by 1/3, without reducing maize yield and effectively regulating enzyme activity in maize at different stages, which is beneficial for weight loss and efficiency improvement. Xu Yanwei et al. found that after applying urea containing polyaspartic acid to rice, the effectiveness of the fertilizer was significantly improved, and the fertilizer was not removed during the growth season. Cao Dan et al. found that applying polyaspartic acid to cultivate poplar seedings requires an appropriate reduction in nitrogen usage in order to alleviate high nitrogen stress caused by high nitrogen utilization efficiency.

1.4 Environmental Protection Characteristics of Polyaspartic Acid

Polyaspartic acid is a polymer mainly composed of amino acids, which can be completely degraded by microorganisms in the environment into usable low molecular weight amino acids, water, and carbon dioxide. Someone used the OECD301A methodto study the biodegradability of polyaspartic acid and found that the amount of carbon dioxide released by polyaspartic acid treatment was close to the reference glucose. in addition, Xiong Rongchun and others have also demonstrated that polvaspartic acid is a green chemical with excellent biodearadability.

2 Outlook

With the gradual realization of the goal of "one control, two reductions, and three basic" in China's agriculture, the integration process of water and fertilizer is becoming increasingly rapid, and the demand for water-soluble fertilizers especially high-end water-soluble fertilizers, is increasing. Polyaspartic acid, as a green and environmentally friendly chelating dispersant and fertilizer synergist, can not only effectively prevent the scaling of chemical fertilizers and corrosion of pipelines, but also enhance efficiency and quality, with strong application prospects.

In response to the current application status of polyaspartic acid in water-soluble fertilizers, combined with the characteristics of polyaspartic acid promoting root production, requlating crop enzyme activity, enhancing nutrient absorption, and chelating dispersed metal elements, the author believes that the development of polvaspartic acid basec water-soluble fertilizers should focus on special water-soluble fertilizers and high-end water-soluble fertilizers, especially suitable for potato and other crops to harvest tubers and tubers, And in specialized water-soluble fertilizers for fruits and vegetables with nutrient absorption barriers, such as pear and chicken feet disease.

Source:Thinkdo chemicals