Preparation And Nutrient Release Mechanism Of Chemical Synthetic Slow Release Compound Fertilizer

Chemical synthetic slow-release fertilizer has the characteristics of rich nutrient elements,simple synthesis process and long term slow release function so that it has been widely concerned and applied in modern agricultural production.However,traditional chemical synthetic slow release fertilizers only contain nitrogen nutrients and were difficult to degrade and release nutrients in the short term so that cannot meet the nutritional needs of crops with short growth period,and now most of them focus on the investigation of the solubility of chemical synthetic slow release fertilizer,while ignoring the mechanism of nutrient release.In addition,the traditional preparation process was more complex,resulting in expensive production costs,which would greatly limit the rapid development of chemical synthetic slow release fertilizer and its accurate matching application in agriculture.Based on this background,the orthogonal design was used to explore the optimal synthesis conditions of chemically synthesized slow release compound fertilizer from the preparation process direction of nutrient diversification and energy saving and consumption reduction.The chemical synthetic slow release fertilizers with high,medium and low molecular weights were selected.The changes of morphology,structure and molecular composition of chemical synthetic slow release fertilizers with different molecular weights were studied under the conditions of buffer solution and soil culture,and their nutrient release mechanism was discussed.The nutrient slow release performance and application effect of chemical synthetic slow release fertilizer were comprehensively investigated by culture test and pot experiment of Chinese cabbage.The main results are as follows:（1）A low cost chemical synthetic slow release compound fertilizer（PSRF/KCl）was prepared from urea,formaldehyde and diammonium phosphate by spray drying method,which the contents of nitrogen,phosphorus and potassium reached 25.01%,7.28%and 20.57%,and the release rates within 70 days reached 5.1%,42.3%and 61.73%,respectively.The SEM,FTIR,and TG characteristics of PSRF/KCl showed that it has excellent water solubility,special morphological characteristics of coexistence of micropores and crystal and stable thermal properties,and PSRF/KCl could increase the Chinese cabbage yield by 26.2%.Compared with the preparation process of traditional chemical synthetic slow release fertilizer,the raw materials cost and energy consumption cost of PSRF/KCl were decreased by 17.51%and 38.13%respectively.PSRF/KCl was a chemical synthetic slow release compound fertilizer with low cost and multiple nutrients.That has a broad application prospects in modern sustainable agriculture.（2）The high,medium and low molecular weight chemically synthesized slow release fertilizers（HCSRF-NPK,MCSRF-NPK and LCSRF-NPK）with molecular weights of21009g·mol^-1,16700g·mol^-1 and 13297g·mol^-1 were prepared.After cultured in different acid-base buffers and soil,the changes of morphology,structure and functional groups of CSRF-NPK before and after degradation were analyzed by SEM,FTIR,GCMS,etc.The results showed that CSRF-NPK degraded continuously under the catalysis of acid and alkali and soil to release nutrients.There were obvious differences in nutrient release of CSRF-NPK with different molecular weight.With the decrease of molecular weight,its degradation performance improved continuously.The water soluble,nutrient slow release and nutrient leaching characteristics of CSRF-NPK with different molecular weight show more obvious degradation differences between them.It was more intuitive to verify that CSRF-NPK with low molecular weight can significantly improve the yield of Chinese cabbage.（3）The nutrients release process of chemical synthetic slow-release fertilizer can be divided into two stages:rapid dissolution in solution and release of nutrients to the external environment.The H₂O was directly involved in the nutrient release process of CSRF-NPK.The hydrolysis of CSRF-NPK in different p H solutions is considered to be a water assisted mechanism,which the decomposition rate was affected by H⁺and OH^-environment.The release of nutrients follows different catalytic modes in different environments,namely“acid catalysis”at low p H and“alkali catalysis”at high p H.In addition,there were two mainly chain breaking positions at both ends of the molecular chain,and the middle molecular chain would produce infinite chain breaking positions at both ends until it was decomposed into small molecules.Under the same weight of CSRF-NPK,the number of molecular chains will decrease with the increase of molecular weight.Therefore,in the process of chain breaking,the contact area between effective attack ions and CSRF-NPK would be significantly reduced.