Research On Dynamic Ionic Control Of Nutrient Solution

Soilless cultivation is one of the key technologies for improving the level of water and fertilizer management in greenhouse crop production.Reasonable water and fertilizer supply according to crop growth requirements can not only improve the yield and quality of greenhouse crops,but also improve water and fertilizer utilization efficiency and reduce environmental pollution.Due to the problems of delayed development of ion-selective electrodes such as poor accuracy,short life cycle,vulnerability to interference.and high prices.automatic irrigation systems of nutrient solution are usually operated by feedback control of EC and pH.It is difficult to achieve feedback control of ionic concentration for the nutrient solution.In this thesis,the effects of potassium supply on tomato plant growth,fruit development and quality formation in substrate culture were studied with potassium contents in various organs for suitable potassium ion supply in nutrient solution.Regression models based on specific formulas for calculating EC and pH of nutrient solution by using ionic activity were established and a concept of ionic EC contribution percentage was proposed for inverse calculation of ionic concentration combined with EC practical measurement.Through the algorism of ionic EC contribution percentage,the dynamic ionic control system of nutrient solution was improved from the EC control level to the ionic concentration control level.The main results and conclusions in this thesis are summarized as follows:(1)The plant growth,fruit development and quality formation of tomato cultivated in substrate were analyzed under nutrient solution supply with potassium ionic concentration at 1,4,8,12,and 16 mmol/L.The results showed that the plant height,stem diameter,flower number,biomass accumulation,fruit weight,yield and quality of the tomato plants were improved at higher potassium supply in nutrient solution.However,no significant differences were found under nutrient solution supply with potassium ionic concentration at 12 and 16 mmol/L.Although increasing potassium supply in nutrient solution could significantly promote chlorophyll synthesis and photosynthesis,but extravagant absorption of potassium by tomato plants is failure to bring benefits for fruit development and quality formation in the later stage.The responses of potassium contents in various organs of tomato plants to potassium supply levels in nutrient solution were studied in different growth stages.In order to ensure the fruit development and quality formation of tomato cultivated in substrate,it is suggested that the potassium supply in nutrient solution should be 8 to 10 mmol/L in seedling stage,10 to 12 mmol/L in flowering stage,12 to 13 mmol/L in fruit setting stage,and 13 to 15 mmol/L in fruit maturity stage.Therefore,Japanese horticultural experimental formula is suitable for greenhouse tomato cultivation,but it is necessary to regulate the potassium ionic concentration dynamically in different growth stages.(2)There were significant linear and quadratic correlations between ionic activity and EC and pH of nutrient solution and the ionic EC contribution percentage of each ion in the solution with different relative concentrations did not change.In the mixed nutrient solution with specific salt added,the ionic EC contribution percentage of the anions and cations from the added salt increased significantly,but that of other ions decreased correspondingly.The variations of ionic EC contribution percentage of potassium ions were similar with the addition of different salts at the same potassium ionic concentration,but those of the different anions were different.The relative errors of the EC and pH estimated by the regression models based on ionic activity compared to the measurements by commercial instrument for Japanese horticultural experimental solution and Yamasaki tomato solution were 1.33%and 1.84%in EC.0.22%and 0.09%in pH,respectively.Even after adding specific salts,the relative errors of the eatimation for the two solutions were less than 0.4%in EC and 0.5%in pH,respectively.Therefore.the EC and pH of nutrient solutions can be accurately estimated based on ionic activity,and the concept of ionic EC contribution percentage proposed in this research can be used for inverse calculation of ionic concentration combined with EC practical measurement.(3)The dynamic ionic control system of nutrient solution consisted of nutrient solution controller,information collection device,solution configuration and irrigation device,and other auxiliary devices.The EC and pH information collector and duty ratio control of high-speed valve for adding mather solution were focused as the key components and techniques for automatic and intelligent functions of the dynamic ionic control system of nutrient solution.The EC and pH collector was designed to achieve in higher accuracy,longer life,and lower cost in EC and pH measurements.The self-learning function of the duty ratio control of high-speed valve for adding mother solution can not only improve EC control level of nutrient solution,but also provide precise addition of mother solution or salts.