In the research of water-saving agriculture, engineering measures and biological measures have been fully carried out, and great results have been achieved. However, water-saving agriculture is a complex system engineering involving many aspects. With the deepening of agricultural research in arid and semi-arid regions Various high-efficiency cultivation measures for early-cropping agriculture have emerged one after another, and the application of a small amount of chemical control agents for agricultural drought prevention and drought resistance has increasingly attracted the attention of domestic and foreign experts. The application of water-retaining agent is an important water-saving and drought-resistant technology. The application of water-retaining agent in water-saving agriculture can play the role of water retention, fertilizer retention, moisture improvement and reduction of evaporation.
1 The nature and type of water retaining agent
water-retaining agent, also known as soil water-retaining agent, super-absorbent, super-absorbent resin, and high-molecular water-absorbing agent, is a high-molecular polymer with super-high water-absorbing and water-retaining properties made of strong water-absorbing resin. The physical and chemical properties of the water retaining agent are mainly manifested in: excellent water absorption performance, its water absorption rate is generally tens, hundreds or even thousands of times its own weight, and it absorbs water and moisture quickly; it has good water retention and becomes a hydrogel after water absorption. , The pressure is not easy to fold, the evaporation is slow; the thermal stability is good, the application range is wide; the continuity is strong, and it can repeatedly absorb water; the ammonia absorption capacity is strong, and the nitrogen fertilizer utilization rate in the soil can be increased by about 10%; it is safe and has no effect on humans and animals. It will not change the pH of the soil. In addition, the water-retaining agent also has the characteristics of selective liquid absorption, cold resistance, water permeability, slow release, swelling and so on .
At present, water retention agents at home and abroad are divided into two categories, one is acrylamide-acrylate cross-linked copolymer (polyacrylamide), and the other is starch grafted acrylate cross-linked copolymer (sodium polyacrylate, Potassium polyacrylate, polyacrylamide, starch grafted acrylate, etc.) .
(1) Polyacrylamide type. Polyacrylamide is white granular crystals and its main components are: acrylamide, potassium acrylate, water, and crosslinking agent. Internationally, most of the water-retaining agents produced in France, Germany, Belgium, Japan, the United States and other countries are products of this type of composition. The characteristics of the product are: long service life and long life, the ability to store water and moisture in the soil can be maintained for about 4 years, but its water absorption capacity will gradually decrease and the cost is higher.
(2) Starch graft type. In polyacrylate, if the polymerized monomer is sodium type, long-term use will cause the increase of the sodium ion content in the soil, which will make the soil harden, which is not good for plants. Therefore, most manufacturers of water-retaining agents for forestry and agriculture switch to the production of potassium polyacrylate or acrylamide. This type of product is white or light yellow granular crystals, the main ingredients are: starch, acrylate, water, crosslinking agent. When this product is used for water storage and moisture conservation in afforestation, the service life can only be maintained for more than 1 year, but the water absorption rate and water absorption speed are excellent, and the cost is low .
2 Water absorption mechanism of water retaining agent
The water absorption mechanism of water-retaining agent is related to its molecular structure. It is the result of molecular expansion caused by the ion repulsion of polymer electrolyte and the network structure hindering the interaction of molecular expansion. The molecular chains of this polymer compound are connected infinitely, and the molecules present a complex three-dimensional network structure, which makes it have a certain degree of cross-linking. There are many hydrophilic groups such as hydroxyl groups and carboxyl groups on the cross-linked network structure. When contacted with water, the hydrophilic groups on the molecular surface ionize and combine with water molecules to form hydrogen bonds. In this way, a large amount of Of moisture. In the process of water absorption, the electrolyte on the network chain creates a difference in penetration potential between the solution inside the network and the external water. Under the action of this osmotic potential difference, external water molecules continue to enter the interior of the molecules. The ions on the network are electrolyzed with water, and the positive ions are in a free state, while the negative ion groups are still fixed on the network chain. The adjacent negative ions generate repulsive force, which causes the expansion of the polymer network structure and enters a large number of water molecules in the molecular network structure. The aggregate state of the polymer has both a body type and a linear structure. The body structure can maintain a certain strength and cannot be stretched without limit; while the linear structure is lightly cross-linked between chains, and the linear part can be free Retractable. Therefore, the water retaining agent only swells in water to form a gel without dissolving. When the water molecules in the gel are released, as long as the molecular chain is not broken, its water absorption capacity can still be restored .
3 Application effect of water retaining agent in agriculture
water retention agent shows better water retention performance in the process of agricultural and forestry applications. Relevant experimental studies have shown  that under the same water and fertilizer conditions, the use of water-retaining agents can significantly increase crop yields by 10%-30%, saving water and fertilizers by 5%-20%; on flowers and other cash crops, using water-retaining The agent can significantly extend the watering interval (3-7 days), the flower buds are large, the duration is long, and the economic benefits are high. Huang Fengqiu et al.  found in the research that the use of water-retaining agent to dress seeds under drought conditions (with a concentration of 1:10 is the best) can increase the emergence rate of seeds by 5.0% to 7.2%; at the same time, the experiment also pointed out that the water-retaining agent can increase the chlorophyll content. To enhance photosynthesis, the combination of water retaining agent and seed coating agent can increase crop yield. Apply the water retaining agent to the orchard soil (with irrigation conditions), until the third year, the soil water content can be increased by more than 4% compared to the control; at the same time, it is found that there is a soil moisture prevention device near the 45cm soil layer. Waterproof layer, where the soil moisture content is highest.
The promoting effect of water-retaining agents on crops is mainly achieved by improving soil moisture conditions . The application of water-retaining agents can not only significantly improve the physical properties of sand, but also increase the effective soil water content . Water-retaining agents can be used for soil water retention and moisture conservation. , To improve the water retention performance of the soil . Studies have shown that with 0.5% water-absorbing resin, the soil retention time can be extended by nearly 40 days .
4 Problems in the application of water retaining agent
Water retention agent is known as the most promising chemical for large-scale application in agriculture after fertilizers, pesticides, and mulch film. However, the popularity of water retention agents in my country is still at a relatively low level. The main reasons are: first, the product price is high, second, the performance of the product is far from the actual demand, the third is the failure to recognize a reasonable method of use, and the fourth is the water retention agent in climatic conditions, soil texture, soil moisture conditions, and soil salinity. And the influence of various factors such as ion type, irrigation water quality and irrigation water volume has not been systematically studied.
With the aggravation of drought and water shortage in our country and the implementation of the western development strategy, the need for developing water-saving agriculture, restoring and building the ecological environment has become increasingly urgent. In recent years, water-retaining agents have developed rapidly and have become an important chemical water-saving technology in the fields of high-efficiency and water-saving agriculture, afforestation, grass planting, and vegetable and fruit cultivation. I believe that in the near future, with science and technology as the forerunner and enterprise as the leading factor, water retention agents will play an important role in the field of agriculture and forestry in our country.