Optimization Of Preparation Technology Of Urea Phosphate Compound Fertilizer And Its Synergistic Mechanism Of Phosphorus In Coastal Saline Soil

The low utilization rate of phosphate fertilizer has always been the limiting factor of crop production in calcareous saline soil due to the strong fixation of phosphorus caused by high p H and free calcium carbonate.Urea phosphate is a kind of strongly acidic fertilizer,which can significantly improve the low utilization rate of phosphate fertilizer in calcareous saline soil.To improve the utilization rate of phosphate fertilizer in saline soil,we discussed the synthesis process optimization of urea phosphate fertilizer,the development of urea phosphate nanocomposite fertilizer and the mechanism of phosphate synergism in saline soil.First,in view of the lack of optimization model of urea phosphate fertilizer synthesis process,the optimum process conditions of urea phosphate fertilizer synthesis were determined by response surface method(RSM),and the urea phosphate fertilizer was characterized by thermogravimetric analysis(TGA),x-ray diffraction(XRD)and scanning electron microscope(SEM),which provided technical support and theoretical basis for the synthesis of urea phosphate fertilizer.Secondly,in view of the high cost of urea phosphate fertilizer,easy moisture absorption and uneven nutrition,which is not conducive to mechanized fertilization,montmorillonite,urea phosphate,urea and potassium chloride are used as raw materials.a new type of high strength spherical urea phosphate-based nanocomposite fertilizer was prepared by agglomeration and granulation.Furthermore,the effects of urea phosphate compound fertilizer on the yield and components of wheat and maize,phosphorus utilization efficiency,soil nutrient status and soil p H were studied through the pot experiment of wheat and maize for two seasons a year and the field positioning experiment of maize for two years,which provided theoretical and technical support for the phosphorus utilization efficiention in saline soil.The main results were as follows:(1)Urea phosphate(UPK)fertilizer was prepared from potassium chloride,urea,and phosphoric acid.The reaction temperature,molar number of potassium chloride,reaction time and phosphoric acid concentration were optimized by four-factor and three-level response surface design(RSM-BBD).The thermal stability,crystal structure and micro-morphology of urea phosphate fertilizer were determined by TGA,XRD and SEM.The process parameters with the highest K₂O content and urea phosphate fertilizer yield are as follows:reaction time60min,molar number of potassium chloride 0.32mol,reaction temperature 78?,phosphoric acid concentration 70%.Under the optimized conditions,the predicted value of K₂O content was 3.51%,and the yield of urea phosphate fertilizer was 69.8%.It was similar to the experimental value that the content of K₂O was 3.42±0.35%and the yield of urea phosphate fertilizer was 67.58±1.25%.The addition of potassium chloride changes the crystal shape of urea phosphate,reduces the aspect ratio of the crystal,and increases the fluidity of the crystal,which is beneficial to the industrial production of urea phosphate fertilizer.(2)A new type of high strength spherical urea phosphate nanocomposite fertilizer was prepared by agglomeration granulation method using montmorillonite,urea phosphate,urea,and potassium chloride as raw materials.Polyphosphate/montmorillonite nanocomposite fertilizer was prepared based on the principle of in-situ polymerization.The polyphosphate formed by heating polymerization of urea phosphate and urea improves the strength of fertilizer particles.The addition of potassium chloride prevented the excessive expansion of montmorillonite and kept the fertilizer particles spherical.The prepared nanocomposite fertilizer was characterized by SEM,TEM,FTIR,XRD,XMT and so on.FTIR analysis showed that there were polyphosphates in fertilizers and hydrogen bonds between polyphosphates and montmorillonite.XRD analysis showed that the interlayer spacing of montmorillonite increased,which confirmed the formation of polyphosphate/montmorillonite nanocomposites.The effects of raw material mass ratio,reaction time and reaction temperature on the strength and fluidity of nanocomposite fertilizer particles were studied by response surface optimization(RSM-CCD).The optimum preparation parameters were determined as follows:reaction temperature103?,reaction time 237 min,mass ratio of material is 1?1?0.93?1(urea phosphate?urea?potassium chloride?montmorillonite).Under these conditions,the fertilizer particles hardness was 64.75±0.48N,and the angle repose was 30.84±0.95°.The nutrient release and application effect of the fertilizer were evaluated by sand column leaching test and pot experiment.The results showed that compared with the traditional fertilizer,the nanocomposite fertilizer could significantly reduce the nutrient release rate,promote the growth of maize,and increase the yield by 24.27%.The preparation of urea phosphate nanocomposite fertilizer with high strength spherical particles provides technical support for its mechanized fertilization and large-scale application in modern agriculture.(3)The pot rotation experiment of wheat-corn was carried out on the urea phosphate compound fertilizer prepared by agglomeration granulation.The results showed that compared with traditional phosphate fertilizer treatment,the yield of wheat and maize treated with urea phosphate compound fertilizer increased significantly by 9.39%-9.89%and 18.14%-19.23%,respectively,and the content of soil available phosphorus increased by 6.26%-35.41%and19.44%-28.39%,respectively,at jointing stage and filling stage of wheat.At maize jointing stage and 12-leaf stage,the phosphorus utilization rate increased by 4.73%-6.22%and 7.95%-9.11%,respectively,and the partial productivity of phosphate fertilizer increased significantly by 9.37%-9.88%and 18.14%-25.90%,respectively.Compared with traditional phosphate fertilizer,the application of urea phosphate compound fertilizer significantly reduced the fixation of phosphorus in soil and promoted the absorption and utilization of phosphorus by wheat and maize.(4)A two-year field positioning experiment of urea phosphate compound fertilizer was conducted in coastal saline soil.The effects of urea phosphate compound fertilizer on soil p H,soil bulk density,phosphorus supply intensity,phosphatase activity,phosphate fertilizer utilization efficiency,maize root growth and yield were studied.The results showed that compared with traditional phosphate fertilizer,the average maize yield and phosphorus utilization efficiency of total phosphate urea compound fertilizer treatment increased significantly by 13.55%-18.89%and 7.09%-7.21%,respectively.There was no significant difference in yield between urea phosphate compound fertilizer with 50%reduction and normal traditional phosphate fertilizer.Compared with the total traditonal phosphate fertilizer treatment,the soil p H of the total phosphate urea compound fertilizer treatment decreased by 0.15-0.28,the soil available phosphorus content significantly increased by 21.25%-58.90%,the soil phosphatase activity significantly increased by 8.21%-10.46%,and the total root length density of maize increased significantly by 26.19%-28.97%.There was no significant difference in the total root length density between urea phosphate compound fertilizer with 50%reduction and normal traditional phosphate fertilizer.Through the analysis of PCA principal components and Pearson correlation coefficient,it is concluded that the application of urea phosphate compound fertilizer mainly has a significant effect on soil p H,soil ACP/ALP and maize root length density.Maize yield was significantly negatively correlated with soil p H,and positively correlated with maize root length density,soil available phosphorus content and soil phosphatase activity.In a word,the application of urea phosphate compound fertilizer in coastal saline soil area is an effective way to improve soil quality,improve crop productivity and achieve stable and high yield.