It is the key to the development of green agriculture to fully understand the role of chemical fertilizers and change the fertilization methods.

As the "grain" of grain, chemical fertilizer is an efficient nutrient brought to us by modern science and technology. Since the production of ammonium sulfate in Europe in 18, after more than 2 years of development, chemical fertilizers have formed a complete system of production, supply, sale and use, which can completely provide necessary mineral elements such as nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, copper, zinc, boron, molybdenum, chlorine and silicon for agricultural products such as grain, cotton, oil, fruits and vegetables. Since the application of chemical fertilizer for more than 2 years, the population has doubled continuously, while the nutrition level has been greatly improved, and the progress of human civilization has shown exponential growth, which has completely broken through the bottleneck that traditional agriculture relies on the natural recovery of soil fertility.

1. Chemical fertilizer is the technological achievement of industrial revolution and the material support of modern agriculture

Chemical fertilizer originated in Europe and is the product of industrial revolution. In 18, Britain took the lead in recovering ammonium sulfate as fertilizer from industrial coking, but it was not until 198 that Germany invented modern synthetic ammonia process that sufficient supply of fertilizer was realized. The application of chemical fertilizers has made Europe's living standards improve rapidly and become the world economic center. In view of the great contribution of chemical fertilizer to human civilization, German inventors of synthetic ammonia technology FritzHaber(1918) and CarlBosch(1931) won the Nobel Prize in chemistry successively.

(I) Characteristics and historical achievements of chemical fertilizers

Chemical fertilizers come from nature and have high supply efficiency. The main raw material of nitrogen fertilizer comes from the atmosphere, and other fertilizer raw materials are mainly minerals. The mechanism of nitrogen fertilizer production is similar to that of biological nitrogen fixation. Inert N2 in the atmosphere is changed into active nitrogen (ammonium salt, nitrate) which can be used by crops through high temperature, high pressure and catalyst. A synthetic ammonia plant built on a 1-hectare land can produce 3, tons of N every day, which can meet the needs of 1 million mu of farmland to maintain the yield of 8-1, kg per mu a year, which is about 1 million times higher than the traditional biological nitrogen fixation efficiency. Chemical fertilizer makes farmland change from a long period of fertilization-production to a short period of continuous production, which greatly improves the output efficiency of farmland.

Chemical fertilizer has high nutrient concentration and strength, which reduces labor intensity. The nutrient content in chemical fertilizer is generally more than 4%, which is more than 1 times that of traditional organic fertilizer. Urea contains 46% nitrogen, and only about 25 kilograms of urea is needed to meet the nitrogen supply of 1 kilograms of an acre of farmland. A labor force can complete transportation and application by hand for half a day. However, it takes many people months to collect, compost, transport and apply organic fertilizer in traditional agriculture. Chemical fertilizer liberates farmers from heavy labor such as fertilizer collection and composting, which greatly improves farmers' labor production efficiency.

chemical fertilizer is efficient, which is beneficial for crops to absorb in time. The nutrients in chemical fertilizer are mainly inorganic, and do not need to be decomposed by microorganisms. After being applied to soil, they will be quickly absorbed by crop roots. For example, chemical nitrogen fertilizer will be completely released in 3 ~ 15 days after it is applied to soil, which can quickly meet the needs of crops at the stage of vigorous plant growth. Fertilizer can also be applied by irrigation or even foliar spraying, which greatly improves the nutrient absorption efficiency of crops.

chemical fertilizer itself is harmless. Fertilizer has high nutrient content and low impurities. For example, urea contains 46% nitrogen, which is the nutrient element needed by crops, and the rest is mainly CO2, which will be released back into the atmosphere after being applied to the soil, which is harmless. In addition, it also contains about 1% water and 1% biuret, and biuret is harmless to crops when it is strictly controlled below 1.5%, and it will be decomposed in soil and used by crops and microorganisms. Other phosphate fertilizers, potash fertilizers and trace elements are all extracted from minerals, and their basic components are harmless.

(II) Fertilizer in China is hard to come by

At present, China is the largest producer and consumer of fertilizer in the world. As a developing country, it is a great thing to meet its own fertilizer supply and export it abroad. The growth of chemical fertilizer industry has provided a solid foundation for the sustained and rapid development of China's agriculture and national economy.

ensuring the supply of chemical fertilizers is a basic national strategy. Since the founding of New China, chemical fertilizer has been the key guarantee of strategic resources, and even in difficult times, the growth of chemical fertilizer consumption has not stopped. The state has taken a series of measures to ensure the supply of chemical fertilizers, such as the policy of rewarding and selling, the policy of purchasing and selling without profit, production subsidies, application subsidies and so on. In 213, the subsidy amounted to more than 1 billion yuan. The special policy for chemical fertilizers has a long duration, a wide range and a great strength that no other commodity has.

establish a chemical fertilizer industry system at all costs. Fertilizer production is an industrial system highly dependent on resources, which is extremely difficult for developing countries with weak industrial base. Before 198s, 4% of China's chemical industry investment, 5% of high-quality smokeless lump coal, 3% of imported natural gas and more than 6% of imported sulfur resources were used for fertilizer production. In order to improve the storage and transportation capacity of chemical fertilizers, the state has also built special railway lines, transmission lines, railways and dock warehouses for large and medium-sized chemical fertilizer plants.

the road to technological innovation is very difficult. In the early days of the People's Republic of China, it was extremely difficult to import fertilizer production equipment and materials. Adhering to the principle of "self-reliance and self-sufficiency", China has embarked on the arduous road of fertilizer research and development and production. Hou Debang and other scientists have been working hard for eight years since 195s to develop a fertilizer technology with China characteristics-"carbonation method" to produce ammonium bicarbonate, and built a modern nitrogen fertilizer industrial system with independent innovation. Phosphate fertilizer has been explored from superphosphate-calcium magnesium phosphate-nitrophosphate-ammonium phosphate-compound fertilizer for half a century. The potash fertilizer industry began to look for minerals in Qarhan Salt Lake in Qinghai in 1956, and it was not until the successful development of the "reverse flotation cold crystallization" process at the beginning of this century that large-scale production began.

(3) Chemical fertilizer is an important guarantee for satiety, eating well and eating healthily

The first green revolution in the 196s was an important milestone in the history of human development, which improved the supply of agricultural products through high-yield varieties, irrigation, mechanization and large-scale application of chemical fertilizers. According to FAO statistics, from 196s to 198s, developing countries increased the yield of grain crops by 55% ~ 57% through fertilization, and chemical fertilizer is more significant for China.

half of China's grain output comes from chemical fertilizers. Before the founding of New China, China has been using traditional agricultural production methods, that is, using crop straws, human and animal manure, green manure and other ways to improve soil fertility, and the grain output has been at a low level for a long time. From the Qin and Han Dynasties to the Qing Dynasty, the yield of wheat and rice per mu in China only increased from 16 kg and 8 kg to 195 kg and 291 kg. In the more than 7 years since the founding of New China, the average yield of wheat in China has reached 7-8 kg, and that in high-yield areas has reached 1,5 kg. Among them, the application of chemical fertilizer played a key role. Scientists' research has proved that the yield difference between crops without chemical fertilizer and those with chemical fertilizer is 55% ~ 65%.

chemical fertilizers have significantly improved the nutritional level of Chinese people. In recent years, the per capita supply of vegetables and fruits in China has continued to grow, enriching recipes and improving the nutritional level of residents. The per capita animal protein supply increased from 1.4 kg in 1961 to 15.5 kg in 214. The increase in fruit and vegetable production is mainly through modern production methods (greenhouse, irrigation, chemical fertilizer and pesticide). The growth of meat products and dairy products comes from the increase of feed supply, and feed production also depends on the application of chemical fertilizers. Chemical fertilizer greatly enriches the nutrient supply in agricultural production system, and provides a basis for producing more protein, energy and minerals needed by human beings.

chemical fertilizer has improved soil fertility. Cultivated land quality is the basic guarantee of food security. In traditional agriculture, the nutrient content of cultivated land is mainly determined by pedogenic minerals, and most soils are deficient in nutrients to varying degrees. For example, the content of soil available phosphorus in China is relatively low. According to the data of the second soil survey conducted in the 198s, the average content is only 7.4mg/kg (for example, the optimum content of corn should not be less than 8mg/kg). Through the application of phosphate fertilizer, the content of available phosphorus in soil in China has increased to 23mg/kg in recent 3 years. Fertilizer application can also increase crop biomass, improve surface coverage and reduce soil erosion. Soil itself is also a carbon sink, which can store greenhouse gases produced by human activities and reduce the negative impact of industrialization. In addition, the application of chemical fertilizer can improve the yield of crops, which provides a broad land space for urban construction, transportation, industrial and commercial development.

2. Scientific understanding of several problems in the utilization of chemical fertilizers

Now, the application of chemical fertilizers has brought some problems, but there are many misunderstandings about it, leading to some negative effects being over-amplified. In fact, it is easy for everyone to understand that fertilizer is compared to food. A series of health problems such as hypertension, hyperlipidemia and hyperglycemia caused by unreasonable diet and overnutrition are the problems of food intake, not the food itself. Like diet, the wrong ways such as excessive application of chemical fertilizers, unreasonable nutrient collocation and extensive application methods will also have negative effects, but they need scientific analysis, correct understanding and rational treatment.

(1) the relationship between chemical fertilizer application and non-point source pollution.

At present, water pollution has been more prominent, but there are three major sources of water pollutants: agricultural non-point source discharge, industrial enterprises and rural and urban residents' sewage discharge, and atmospheric dry and wet deposition related to fossil energy discharge. According to the 214 China State of the Environment Bulletin, the total amount of ammonia nitrogen discharged from wastewater in China is 2.385 million tons, including 1.381 million tons from domestic sources, 755, tons from agricultural sources, 232, tons from industrial sources and 17, tons from centralized sources. It can be seen that agricultural sources are lower than domestic sources. Agricultural non-point source pollution includes fertilizer loss, nitrogen and phosphorus nutrient loss caused by livestock and poultry breeding and aquaculture. According to the research, the contribution of fertilizer nutrient loss to agricultural source nitrogen and phosphorus emissions is 11.2% and 25.7% respectively, which is generally low. In fact, most of the phosphorus and potassium elements in chemical fertilizer that are absorbed by crops in the current season will remain in the soil and be used by crops in the next season.

(2) the relationship between chemical fertilizer application and air pollution.

air pollution, especially smog, has had a great impact on our lives. Generally speaking, if the nitrogen fertilizers applied in agricultural production, such as urea, ammonium bicarbonate and diammonium phosphate, enter the soil and are not absorbed and utilized by crops, some nitrogen will be discharged into the atmosphere in the form of ammonia and nitrogen oxides, causing air pollution. These losses are very small if deep application of soil covering, application by stages and selection of reasonable products are adopted. The research shows that at present, nitrogen fertilizer contributes about 5% to the total emission of nitrogen oxides in China. With the change of fertilization methods, this proportion will gradually decrease.

(3) the relationship between chemical fertilizer application and soil quality.

In recent years, China's soil health problems have aroused widespread concern. Farmers intuitively feel that the soil is hardened and polluted, and it simply boils down to the role of chemical fertilizers. In fact, soil hardening is not the function of chemical fertilizer. Soil hardening is mainly caused by flood irrigation, flooded irrigation and unreasonable farming. Rational use of chemical fertilizer, especially combined with organic fertilizer, can improve soil structure. In addition, chemical fertilizers have little impact on soil heavy metal pollution. Only ammonium phosphate in chemical fertilizers will bring in a certain amount of heavy metals, and the cadmium content in phosphate rocks in China is very low. According to the current fertilization amount (5 kg/mu, calculated by the average cadmium content of 1mg/kg), the cadmium brought in is only .5 g/mu every year, while the cadmium brought in by industrial and mining exploitation and sewage irrigation is much higher than that of fertilizers.

(4) the relationship between the application of chemical fertilizers and the quality of agricultural products.

The appearance, nutrition, contents and storage characteristics of agricultural products are directly related to the application of chemical fertilizers. People often say that "the melon is not fragrant and the fruit is not sweet after using chemical fertilizer", which is the result of unreasonable application of chemical fertilizer. Some fruit farmers blindly pursue big fruit and super-high yield, invest a lot of nitrogen fertilizer and ignore the cooperation of other elements, resulting in large fruit and a lot of water, but the soluble solids and sugar content can not keep up, which reduces the flavor. In fact, the quality of crops is related to the proportion of nutrient absorption. The nutrient structure and application method of chemical fertilizer are reasonable, and the healthy growth of melons and fruits makes the fruits more fragrant and sweeter.

Third, promoting fertilizer reduction and increasing efficiency is the key measure to adjust the structure and mode of agriculture and realize green development.

General Secretary Xi Jinping pointed out, "We should unswervingly accelerate the transformation of agricultural development mode, and shift to intensive development with equal emphasis on quantity, quality and efficiency, improving competitiveness, agricultural technological innovation and sustainable development as soon as possible, and take the road of modern agricultural development with high output, product safety, resource conservation and environmental friendliness". At present, China's agricultural production has encountered the double squeeze of price "ceiling" and cost "floor". Only by changing the mode of production and reducing the already excessive cost of agricultural materials can we improve agricultural quality and increase efficiency, save costs and increase efficiency.

In agricultural production, chemical fertilizers and pesticides are the main material inputs. Generally speaking, wheat, corn and rice account for 45% of the material input. Horticultural crops are even higher, and the input of fertilizers and pesticides in apple production accounts for 2/3 of the total material cost. The price of agricultural products can't go up mainly because the quality is not too hard, and chemical fertilizer is the decisive factor of quality. The problem of resources and environment has gradually become a rigid constraint factor affecting agricultural production. The deterioration of the environment will not only directly increase the cost of agricultural production, but also indirectly increase the cost of agricultural production. Therefore, the importance of chemical fertilizer in transforming agricultural production mode is self-evident. Based on the national conditions, in accordance with the requirements of "increasing production and fertilization, economic fertilization and environmental protection fertilization", we should carry out the action of zero growth of fertilizer use, implement the four-word policy of "precision, adjustment, reform and substitution", and gradually correct the face of excessive and unreasonable fertilization.

precision means promoting precise fertilization.

according to the soil conditions, crop yield potential and comprehensive nutrient management requirements in different regions, the fertilization limit standards for each region and crop unit area should be formulated reasonably. Hundreds of thousands of experiments of soil testing and formula fertilization have proved that precise fertilization can achieve the effect of reducing weight by 5 kg, increasing yield by 5-8% and increasing 1 yuan per mu of grain crops, while cash crops such as fruit, vegetable and tea can reduce weight by 2-9 kg, increase yield by 1-2% and increase income by more than 2, yuan. However, due to the diversity of soil types and the complex planting system in China, there are more than 7 million farmland, so it is a long-term work to realize accurate fertilization in each land, which requires comprehensive protection of technology, policy and system.

adjustment means adjusting the structure of fertilizer use.

China's farmers are too scattered, so in the past, general chemical fertilizer products have been the main products, which can meet the basic production requirements, but they are not optimal. First of all, it is necessary to optimize the ratio of nitrogen, phosphorus and potassium, enhance the coordination and synergy between large elements and medium and trace elements, and make soil crops more nutritious. Secondly, according to the different soil conditions and crop needs in China, we should develop suitable high-efficiency fertilizer products and ensure that these products can be used in the field. This requires the fertilizer industry to meet the needs of agriculture to upgrade products, the fertilizer marketing system to truly serve users, and in-depth innovation and localization technology in the agricultural field.

change, that is, improve fertilization methods.

At present, due to the shortage of labor and agricultural machinery, unreasonable fertilization phenomena such as surface application, spreading application and "one-bombardment" are more common. It is necessary to speed up the research and development of suitable fertilization equipment and promote the transformation of fertilization methods. For example, the volatilization of nutrients in the surface application of nitrogen fertilizer will exceed 2%, while the deep application of covering soil can be reduced to less than 5%. Protected vegetables and some field fertilizers are applied with water, which can be gradually changed into water and fertilizer integration and foliar spraying. The change of fertilization method needs the close cooperation of fertilizer products, agricultural machinery, agronomy and facilities, which is a systematic project and needs to be intensified and promoted for a long time.

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