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Hi HikariA, You should still have a few pins available between analog and digital ones. So you should not need to buy more or different material, anyway, if you generally want more sensors or more possibilities to expand the project I highly suggest you to get a different arduino instead of connecting multiple uno's. An arduino leonardo is probably what you are looking for. Also bare in mind that sometimes phmeters comes with a temperature sensor integrated so you might be able to remove the temp and humidity sensor and use a phmeter instead for all the readings needed (temperature, humidity and ph).Hope this turns helpful! Good luck with your project and if you publish it please drop the link here so I can follow!! (Plus dont forget to click the "I made it" button ;)F.