Design and testing of a nutrient mixing machine for greenhouse fertigation

To improve the efficiency and accuracy of water-nutrient mixing, a multi-channel fertigation machine was designed; the hardware includes a programmable logic controller (PLC), a touch system and multi-channel sensors. A sectional forecast control algorithm based on the nutrient dilution model was proposed to control the fertigation machine. When the 100-fold concentrated nutrient solution was diluted by a factor of 20-200, the results showed that the standard deviations of the electrical conductivity (Ec) and the pH for four repeat measurements were 0-0.4 mS cm−1 and 0.057-0.12, respectively; therefore, the performance of the nutrient solution was stable. When the target Ec values were 1.0, 1.5, 2.0, 2.5, 3.0 and 4.0 mS·cm−1, the variation coefficients relative to the target Ec values for three repeat measurements were 2%, 0.94%, 0.87%, 0.63%, 2.37% and 1.8%, respectively. The multi-channel water-nutrient mix resulting from the sectional forecast control algorithm was precise; the error of the Ec was less than 0.05 mS·cm−1, which satisfies the requirements of fertigation for soilless cultivation in greenhouses.