| In this study,controlled-release urea and controlled-release compound fertilizer with different release dates and different release periods were used as research objects to study the differences in the release characteristics of controlled release fertilizers between chemical industry standards for controlled-release fertilizers at 25°C,the agricultural industry standards for slow-release fertilizers-registration of fertilizer requirements and European standards for controlled-release fertilizers;under the temperatures of 5°C,15°C,and 25°C,chemical industry standards,european standards and soil incubation method on release characteristics of controlled-release fertilizers;During the corn planting period,the effect of field fertilizer buried bottle buried bag to nutrient release characteristics of film-coated controlled-release fertilizers.It aims to link the organic fields of natural soil,soil incubation,and water extraction to provide method guidance and theoretical basis for the objective evaluation of controlled-release fertilizers.At the same time,in order to quickly detect the nitrogen concentration of the extract,the Kjeldahl method,p-dimethylaminobenzaldehyde colorimetric method,and refractive index method were used to determine the nutrient release rate of the controlled-coated/slow-release urea,and the morphological changes of nitrogen after urea was placed under different temperature and hydrostatic conditions were determined to determine the applicable conditions of p-dimethylaminobenzaldehyde colorimetric method and refractive index method in the above standards.In order to improve the accuracy of the release rate of slow/controlled release urea,scientific and rational use of standards to provide technical guidance and theoretical basis,for controlled-release compound fertilizers,six controlled-release compound fertilizers that are popular in the market were used as research objects,and nutrient extracts were prepared using the 25°C still water extraction method specified in the controlled-release fertilizer industry standard(HG/T 4215-2011).The correlations between the amounts of nitrogen,phosphorus and potassium measured by the Kjeldahl method,molybdenum-antimony anti-colorimetric method,and flame photometry were compared with the conductivity of the extract.A corresponding fitting relationship equation was established to investigate the reliability of the conductivity method for the rapid detection of nutrient release characteristics of controlled-release compound fertilizers.In addition,conductivity method was used as a control,and the release characteristics of controlled release KCl were measured by the refractive index method,which provided a theoretical and technical basis for the rapid evaluation of controlled release KCl.The main results are as follows:(1)The release rate of the tested fertilizers in water at 25°C was the European Standard for slow-release fertilizer>chemical industry standard>agricultural industry standard,and the trend of cumulative release rate of fertilizers for the three standards is consistent,among them the average release rate of fertilizers tested of European standards was 1.39 times that of chemical industry standards;(2)Under the conditions of 5°C,15°C and 25°C,the general standards for the release rate of slow release fertilizer in European standards,the chemical industry standards,and the soil release rate for soil incubation method were all increased as the temperature rises,and the fitted equations for the first two methods are good.At room temperature 25°C,the release rate of slow-releasing fertilizers in European standards is faster than that in controlled-release fertilizers chemical industry,and both release rates are higher than in soil incubation method.However,this rule changes at 5°C and 15°C.The release rate of soil incubation method is faster than that of other methods.There is no significant difference between the European standard test release rate and the controlled release fertilizer chemical industry standard.As the temperature decreases,the type of fertilizer release also changes,with S-type and L-type,respectively,becoming uniform release;(3)Comparing the cultivating method and buried bag method with soil incubation method under 5°C,15°C,and 25°C temperature conditions,it was found that there is a large difference between 5°C and 15°C and it is similar to 25°C.B.For buried bottles and buried bags in soils,the cumulative release profile of several fertilizers is generally consistent with the comparison between the extraction of solid fertilizers in static water and their release in soil at the same temperature.(4)The cumulative release rates measured by the controlled release urea PCU3 and PCU6 refractive index methods were 1.9%and 6.8%higher than the Kjeldahl method,respectively;the data measured by spectrophotometry were 30.9%lower and 16.6%lower than the Kjeldahl method.With the increase of temperature and time,urea gradually decomposes,the conductivity value of urea solution gradually increases,and the content of ammonia nitrogen gradually increases.The relationship of electrical conductivity measured at three temperatures of 60°C and ammonia nitrogen is y=0.4758x-20.425,R2=0.9706.The trend of conductivity is consistent with the trend of ammonium nitrogen in solution,indicating that the main product of decomposition of urea solution is ammonium nitrogen.And it was found that the conductivity of urea solution was above 50 uS/cm,and the urea content in the determination of refractive index was higher than the actual content.According to the relationship between conductivity and refractometry measured urea deviation from actual concentration,conductivity was used as the dependent variable Y,the urea deviation from the actual concentration measured by the refractive index method is the independent variable X.The equation y=539.14x=136.52,R~2=-0.8619 is established,and the refractive index method data is corrected.The cumulative release rate of the fertilizer CRU6-1 measured by conductivity-corrected refractometry was reduced from 9.4%to 1.3%after correction for the Kjeldahl method.Fertilizer CRU6-2,corrected error decreased from 8.4%to 4.7%.(5)Under the same nitrogen content,the linearity equations between the conductivity and the refractive index of ammonium chloride,ammonium nitrate,and ammonium sulfate can be expressed by a single equation,and the correlation coefficient R~2 is above 0.999.And the refractive index values of the three ammonium salts are all higher than those of urea,which shows that there is a very significant correlation between the inorganic ion conductivity and the refractive index.At the same time,the nitrogen of the ion form has a higher refractive index than that of the organic molecules.Therefore,when urea is decomposed,the ion content in the solution increases,and the data measured with the refractive index will be higher than the actual data.(6)The refractive index method was linearly related to the data obtained by the conductivity method.The binary equation y=1.0535x-0.8914,r=0.9988,n=24 was obtained.Under the experimental conditions,the relative standard deviation(RSD)of the data measured by the refractive index method is 0.00 to 0.40%,and the RSD of the conductivity method is 0.00 to 1.09%. |
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