## 4 Discussion  
Researchers have developed many composite materials to prepare water-retaining slow-release fertilizers. [PERSON] et al. used compost-based substrates modified with polyacrylamide, guar gum or bentonite to improve the water-holding capacity and plant growth [39]. The nano-composite PAAm/methylcellulose/montmorillonite hydrogel prepared by [PERSON] et al. was used to study the synergistic effect of fertilizer slow release [40]. [PERSON] et al. used carboxymethyl cellulose-based nanocomposites to prepare NPK slow-release fertilizer with water retention in soil [41]. [PERSON] et al. prepared and characterized a kind of slow-release and water retention fertilizer based on starch and halloysite [42].  
Some previous studies have reported the influence of several factors in encapsulation efficiency and water absorption. The results of this study are consistent with the factors affecting the encapsulation efficiency of alginate capsules studied by [PERSON] et al. [43]. A study by [PERSON] et al. also revealed the importance of alginate properties and calcium ion concentration in the gelling process [44]. In this study, it was found that, when the concentration of urea was increased, the encapsulation efficiency increased, which is in agreement with the results in microalgae composite hydrogels for slow-release urea fertilizers [45].  
The influence of CaCl\({}_{2}\) concentration is clear with regard to encapsulation efficiency, possibly due to the effect of Ca\({}^{2+}\) concentration on the hardness of sodium alginate microspheres [46]. Different from the results, [PERSON] et al. found that sodium alginate, rather than CaCl\({}_{2}\), had a significant effect on the EE in terms of encapsulating astaxanthin oleoresin [47]. This may be related to whether the embedded material is hydrophilic or lipophilic.  
The optimization results of WA show that the water absorption is not only influenced by single factor, but also by the interaction between influencing factors, which was consist with the study of [PERSON] et al. [48]. The interactions between the concentration of sodium alginate and carboxymethyl starch sodium, as well as the concentration of carboxymethyl starch sodium and CaCl\({}_{2}\), have significant effects on the model. The sodium alginate composite network, with carboxymethyl starch sodium, plays an important role in improving the water absorption of fertilizer. These results are similar to those of [PERSON] et al. and [PERSON] et al. [49; 50]. The water absorption property of SCPU was attributed to the presence of -OH and C-O-C groups in sodium alginate and carboxymethyl starch sodium, and -NH\({}_{2}\) groups in polydopamine [51; 52]. The formation of hydrogen bonds in the polymeric chains and hydrophilic groups promoted the adsorption of water into the complex, resulting in the high water adsorption property of slow-release fertilizers [53; 54].  
Chemical fertilizer, especially urea, is used in large amounts every year. The global market of urea is expected to reach $72.07 billion in 2024. The market prospect of water-retaining slow-release fertilizer is quite broad and is in short supply. The price of SRFs is a very important factor, one which should be considered in application. According to the costs of material, usage and labor, SCPU achieved a higher price ($1959 \(\text{hm}^{-1}\)) than urea ($1197 \(\text{hm}^{-1}\)). In spite of this, the price of SCPU is lower than that of other similar products, reported in a previous study ($4973 \(\text{hm}^{-1}\)) [55]. Although SCPU is more expensive than urea, its application can reduce environmental pollution and improve the drought resistance of crops. Therefore, research into related products is very meaningful. We will focus on expanding the fertilizer to commercial scale production as soon as possible after continuously optimizing the preparation equipment and conditions.