Preliminary Discussion on Research Progress and Prospect of Slow Released Fertilizers

Xun CHEN，Hongjuan WANG，Xianjin YANG，Youping WANG，Changyan XlANG

1.Institute of Fruit and Tea，Hubei Academy of Agriculture Sciences/Fruit and Tea Division，Hubei Innovation Center of Agricultural Science and Technology/Hubei Tea Engineering and Technology Research Center，Wuhan 430209，China；

2.Wuhan Lvyin Chemical Co.Ltd，Wuhan 430011，China；

3.Institute of Plant Protection and Soil Science，Hubei Academy of Agricultural Sciences，Wuhan 430209，China；

4.Department of Civil Engineering，China Three Gorges University，Yichang 443000，China

Preliminary Discussion on Research Progress and Prospect of Slow Released Fertilizers

Xun CHEN1，Hongjuan WANG1，Xianjin YANG2，Youping WANG3*，Changyan XlANG4

1.Institute of Fruit and Tea，Hubei Academy of Agriculture Sciences/Fruit and Tea Division，Hubei Innovation Center of Agricultural Science and Technology/Hubei Tea Engineering and Technology Research Center，Wuhan 430209，China；

2.Wuhan Lvyin Chemical Co.Ltd，Wuhan 430011，China；

3.Institute of Plant Protection and Soil Science，Hubei Academy of Agricultural Sciences，Wuhan 430209，China；

4.Department of Civil Engineering，China Three Gorges University，Yichang 443000，China

Slow/controlled release fertilizers （SRFs/CRFs）have been paid more attentions by the researchersin recent years.In this paper，the application effects and methods，types，current problem and development prospect of SRFs/CRFsboth at home and abroad were reviewed.The production principles and processes of ureaformaldehyde slow release fertilizers were introduced；and It is suggested that the urea-formaldehyde slow release fertilizers show great development to ease energy and environment pressure.

Slow release fertilizers；Urea-formaldehyde slow release fertilizer；Application prospect

S low release fertilizers（SRFs for short）had a release rate of nutrients effective for plants after application into soil far lower than that of instant fertilizers，i.e.they could release nutrients slowly.They havea slow effect or long-term effect on crops，but they could only slow the release rate of fertilizers rather than control the release rate completely.The higher form of slow release fertilizers was controlled release fertilizers，which release nutrients in a rate in accordance with the nutrient quantity required by crops，resulting in the highest utilization rate of fertilizers，and in a broad way，controlled release fertilizers include slow release fertilizers.In a real sense，controlled release fertilizers could regulate the supply intensity and capacity of nutrients including N，P and K through physical，chemical and biological means according to nutritive peculiarity of crops including staged and consecutive nutrient demand，and realize long-acting high-efficiency plant nutrient complex capable of supplying nutrients in a desired rate.

Currently，scholars both at home and abroad deemed that the slow/controlled release fertilizer is an effective way for realizing high yield，high quality，high stress tolerance and low pollution，thereby reducing wasting of resources in the 21stcentury.In the 1980s，developed countries including America，Japan，Europe and Israel focused on the research of slow/controlled release fertilizers，hoping to improve the utilization rate of fertilizers by changing the characteristics of fertilizers［1-4］.For this purpose，our country gives priority to the development of slow/controlled release fertilizers，and published related supportive polices，and in the“No.1 document”of central government in 2007 and “the Outline of the National Program for Long-and Medium-Term Scientific and Technological Development（2006-2020）”，emphasis was put on the development of slow/controlled release fertilizers. Slow release fertilizers have the effects of effectively controlling the re-lease rate of nutrients，prolongingeffective period of fertilizers，maximizingthe utilization rate of fertilizer，reducing nutrient loss，reducing environment pollution and reducing consumption of social resource，and is thus of great importance to agricultural sustainable development.

Research Progress in Foreign Countries

America performed research firstly in the world on slow/controlled release fertilizers，mainly on sulphur coated urea （SCU），as well as on sulphur coated potassium chloride（SCK），sulphur coateddiammonium phosphate（SCP），etc.Improved SCU has a layer of olefin polymer coated on its surface，and is designated as Polys，which has a selling price lower than those of polymer-coated fertilizers，and is widely used in American market.Osmocote is still the most influential coated fertilizer in the world.Most controlled release fertilizers were used in the form of mixtures with instant fertilizers in America，and in order to avoid fracture of coatings during mixing，wear-resistant controlled release fertilizers were developed；and in order to reduce the pollution of polymer to environment，controlled release fertilizers coated with biodegradable films were developed in succession.

Japan is more advanced in researching and applying controlled release fertilizers，mainly macromolecular film-coated fertilizers.Sulphur-coated fertilizers were developed in 1975，and applied to rice，corn and watermelon with good fertilizer efficiency.In early 1980s，Chissocorporation developed a thermoplasticpolyolefin resincoated fertilizer，Nutricote，which is aninternational famous brand as well as Osmocote.Multiple companies in Japan developed in succession thermoplastic and thermosetting resincoated fertilizers with their own national characteristics，the nutrient release of which has the dual functions of accurately controlled release and slow release，and these productswere widely applied to rice，flowers and vegetables.

European countries put particular emphasis on the research of slightlysoluble nitrogen compound-containing slow release fertilizers.Germany focuses on the production of coated fertilizers using polymers as coatings，which could control the release of nutrients in a controlled rate or at proper time，and thus could better satisfy the demand of crops to nutrients.A patent of controlled release fertilizer in England adds K，Ca and Mg into phosphate glass to form glassy-state controlled release fertilizer into whichN is added in the form of CaCN.Such fertilizer has an induction period during which no nutrient is released，and therefore，it is suitable for growth of seedlings.The slow release fertilizer in France usesmetal pernitride coated bysodium tripolyphosphate or sodium hexametaphosphate as a soil additive firstly，and combines polymer-coated fertilizer and bacteria secondly.Spain prepares a series of fertilizers by coating urea with pine lignin pulp waste liquid.Holland developed a biodegradable coated fertilizer by kneading inulin，glycerol，potato starch and fertilizer.The patent of former Soviet Union prepares a coated fertilizer by coating urea with multiple layers ofureaformaldehyde（UF），phosphoric acid，aluminum powder and butadiene latex or with polyvinyl acetate and phosphoric acid，which improvesutilization rate of the fertilizer by 15%.Czechoslovakia coats urea with UF resin，which could regulate the release rate of nutrients by changing the granularity of the coating agent and the thickness of coating layer.

Domestic Research Progress

A urea-formaldehyde（UF）material was developed in our country in 1971，andammonium bicarbonate coated by calcium magnesium phosphate was developed in 1973，which not only could control the release of nutrients，but also could inhibit the volatilization of ammonia，achieving good fertilizer efficiency and remarkably-increased yield；in the 1980s，Institute of Soil Science in Nanjing and Zhengzhou University of Technology developed phosphate-coated urea，realizing a type of compound fertilizercoated slow release fertilizer；in the 1990s，on the basis of coated compound fertilizer，Zhengzhou University of Technology developed slowly-available multi-nutrient sulphur-coated urea，which is produced industrially under the trade nameof Luxecote with an effective period of 90-120 d.Under the condition of reducing fertilizer application amount by 1/3 of conventional amount，yield of rice could be kept constant substantially.By applying to wheat，a nitrogen utilization rate of 57.2%could be achieved.Shenyang Institute of Applied Ecology of Chinese Academy of Sciences developed a multi-nutrient slow releasenitrogen fertilizer added with humic acid，dicyandiamide and phosphorus，potassium and microelements and a slow release fertilizer special for flowers which do not cause environment pollution.By the late 1990s，Han Xiaori from Shenyang Agricultural University presided“the research on novel coated controlled-efficiency special fertilizer”supported by the Foundation for the Returned Overseas Chinese Scholars，Ministry of Education of China，and “the research on novel slow release fertilizer special for rice and corn”supported by the Science Foundationof Liaoning province，and completed the coating agentadopting polyvinyl alcohol（PVA）and starch and the mixed coating technique using coatings and nitrogen fertilizer synergists，slow-release formulations，water-retaining agents and soil conditioners.With these achievement，coating could be realized at room temperature，good slow release effect could be obtained，coating material proportion and thickness could be regulated according to crop types and soil conditions，the demand from crops for nutrients in whole vegetative period could be satisfied by performing fertilization once，and in addition，the used coatings could be degraded in soil without toxic and side effects，satisfying the requirements of environment-friendly fertilizer.

Types of Slow Release Fertilizers

Slow release and controlled release fertilizers could be divided into 3 types according to their solubility andrelease ways:（1）water-soluble fertilizers controlled by physical barrier factors，such as coated granulated fertilizers and matrix-based compound fertilizers，in which coated granulated fertilizers could be further divided into organic polymer-coated fertilizers（thermoplastic and resin types）and inorganic coated fertilizers （such as sulphur and mineral coated fertilizers）；（2）slightly-soluble organic nitrogen compounds，which could be divided into biodegradable slightly-soluble organic nitrogen compounds（such as UF and otherUF condensationcompound）and compound mainly degraded chemically （such as IBDU，idobutylidenediurea）；and （3）slightly-soluble inorganic compounds，including metal ammoniumphosphate，partially acidified phosphate，etc.Fan and Singh divided slow release and controlled release fertilizers into diffuse type，corrosion or chemical reaction type，expansive type and permeating type according to release control manners. Addition of biological inhibitors（such as urease inhibitor and nitrification inhibitor）into nitrogen fertilizers has been consideredas realizing nitrogen slow release to some degree，especially in the soil with higher cation exchange capacity（CEC）.However，after applying them into soil，only the hydrolysis of urea into ammonium or the conversion of ammonium ions into NO-2and NO-3by nitrifying bacteria were delayed，and the fertilizers are called as stabilized fertilizers rather than slow release or controlled release fertilizers.

Developing Direction of Slow Release Fertilizers

Currently，a series of problems including low fertilizer utilization rate and environment pollution caused by excessive application and unreasonable nutrient element matching exist in the application of chemical fertilizers mainly［5-8］.The most significant disadvantages of ordinary single-nutrient type chemical fertilizer and multi-nutrient type compound fertilizers lies in the release rate of nutrients in fertilizers which is out of control，and the loss of nutrient elements caused by water and soil loss and escaping into atmosphere thus is severe.Coated controlled release fertilizers could better control the release of nutrient elements，but the high price restricts the large-scale application thereof.These defects all are because of the characteristics of various fertilizers，and could not be overcome fundamentally.

Coated controlled release fertilizers have the effect of significantly improving fertilizer utilization rate.However，the coating materials for most slow release fertilizers sold in the market adopt chemosynthetic nondegradablehigh-molecular polymers，which are likely to leave residues in agricultural soil，resulting in soil degradation and severe pollution.UF fertilizers do not need coating materials，and greatly reduced environment risk while reducing production cost to certain degree.Achieving slow release effect gradually by the action of microorganisms in soil on the chemical structures of fertilizers has become the main development direction of slow release fertilizer industry.

Production Process and Action Mechanism of Ureaformaldehyde Slow Release Fertilizers

Urea-formaldehyde（UF）belongs to synthesized organic slightly-soluble slow release nitrogen fertilizer［9］，and is prepared from condensation of urea and formaldehyde under certain condition，including condensation compounds such asMDU，DMTU，TMTU，TMPU and PMHU，in which nitrogen is released via soil microorganisms.The effective period depends on the size of the molecular chain of the component，the condensation compound with a long molecular chain has a long effective period，and the effective period could be controlled artificially by controlled reaction conditions.

The method of condensing urea and aldehydes （especially for formaldehyde）is a common way for preparing slow release fertilizer.UF is the most common organic compound serving as slow release nitrogen fertilizer，and it is still one of the most leading slow release and controlled release fertilizers currently，with very long research，application and production histories.UF is prepared from the reaction of formaldehyde and excessive urea by controlling pH value，temperature，the ratio of urea to formaldehyde and reaction time.The product is a mixture of bipolymers and oligomers includingmonomethylolurea，emethylolurea and methylene-urea.Base-catalyzed condensation mainly gives water-soluble products （such as methylolurea and monomethylene-urea）. Adding acids into these mixtures gives rise to oligomers with longer condensation chains and low water solubility，while longer chains result in slower nitrogen release.Typical UF product contain 30%-40%of N.In order to provide the standard of N release rate for UF products，the compound was divided into 3 constituents:（1）cold water-soluble constituentN （CWSN，25℃），mainly including urea，bipolymers and short-chain polymers，in which N is released at a rapid rate to a slow rate；（2）hot water-soluble constituent N （HWSN，100℃），including methylene-urea and medium-chain polymer，in which N is released into soil at a slow rate；and （3）hot waterinsoluble constituent（HWIN），including methylene-urea and long-chain polymer，which is degraded in soil at a very slow rate，N of which is inactive in soil. The activity index of such compound could be determined according to above definition:AI=（CWIN-HWIN）/（CWIN×100）.AI provides the estimated value of nitrogen released from the hot water-soluble constituent in soil（about 6 months）.The decomposition of UF fertilizers is mainly due to biological action，and the release of nitrogen intensely depends on the changes in soil property（such as biological activity，clay content，pH）and external conditions（such as water content，humidity and temperature）.In many cases，the proportion of the nitrogen provided by the low molecular weight UF part exceeds the amount required by crops in early growth stage，while the low molecular weight part provides nitrogen at a too-low rate，which might be the main reason for the graduallydecreasing demand for organic nitrogen compound type slow release fertilizers and the steadily-increasing demand for coated fertilizers.

Prospect for Application of Urea-formaldehyde

UF slow release fertilizers，due to their good slow release feature and physical property，could be prepared into granular and powder products to be formed into mixed fertilizers，granular fertilizers and liquid fertilizers for application，and are capable of prolonging nutrient release period，enhancing soilmicroorganism activity and improving fertilizer physical character.They are a kind of slow release fertilizer with good potential，and of profound significance in promoting upgrading of nitrogen fertilizer industry and relieving energy source and environment stress.

Owing to the high nutrient utilization rate and environment protection effect of such products，the demand and production quantity of them gradually increases year by year，industrial production has been conducted in America，Western Europe and Japan，and the products have been sold to international market.Domestic UF and compound fertilizers containing UFhave an annual production capacity above a million tons，and are used for golf lawns，gardening，vegetables and farmer land.They have been gradually applied to industrial crops in recent years，especially forthe industrial crops mainly using nitrogen fertilizers，such as tea and tobacco.However，the production process for fertilizers shouldbeimproved bycombining physical，chemical and biological methods，to develop the slow release compound fertilizers suitable for different crops，respectively，andthe nutrient release mechanism，environment effect and fertilizer efficiency of UF slow release fertilizers should be further researched，to develop environmentfriendly more-economic UF slow release formula fertilizerswith self-owned intellectual property right；and form transformation and whereabouts of UF slow release fertilizers in soil and fertilizer efficiency on crops should be systematically strictly evaluated，to provided scientific basisfor satisfying the nutrient demand by different crops under different agroecological environments and to develop slow release fertilizers with good social，economic and ecological benefits.

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Responsible editor：Yingzhi GUANG

Responsible proofreader：Xiaoyan WU

浅论缓释肥的研究进展

陈勋1，王红娟1，杨先进2，王友平3*，向长
（1.湖北省农业科学院果树茶叶研究所/湖北省农业科技创新中心果茶分中心/湖北省茶叶工程技术研究中心，湖北武汉430209；2.武汉绿茵化肥有限公司，湖北武汉430011；3.湖北省农业科学院植保土肥研究所，湖北武汉430209；4.三峡大学土木工程系，湖北宜昌443000）

该文就国内外缓/控释肥料研究进展、缓释肥种类、存在的问题和发展方向进行了综述。重点介绍了脲甲醛缓释肥料的生产原理、生产工艺，说明脲甲醛缓释肥料极具发展潜力，脲甲醛缓释肥料有利于缓解能源、环境压力。缓释肥料能有效地控制养分释放速度，延长肥效期，最大限度地提高肥料利用率，减少养分流失，降低环境污染、减少社会资源消耗，对农业可持续发展具有重要意义。

缓释肥；脲甲醛缓释肥；应用前景

国家茶叶产业技术体系建设专项资金（CARS-23）；湖北省农业科技创新中心资助项目（2011-620-005-003-04）。

陈勋（1979-），男，湖北武穴人，副研究员，硕士，主要从事于茶树栽培育种和资源高效利用研究，E-mail：cx2006202@163.com。*通讯作者。

2015-07-05

Supported by the Special Fund for Construction of National Tea Industry Technology System （CARS-23）；Funding Project of Hubei Agricultural Science and Technology Innovation Center（2011-620-005-003-04）.

*Corresponding author.

Received:July 5，2015 Accepted:November 9，2015

修回日期 2015-11-09