Lots of folks say the device is a scam.
The device appears to measure conductivity and do some transform of that to give the numbers.
Given a carefully controlled experimental system where you hold moisture, N, P, etc all constant, you could measure conductivity as you vary K concentration, plot a graph and get a y_K=A_k*x+b_k calibration. But if moisture changes, or any of the other variables in the system change, or you sprinkle in a little salt to increase conductivity and change x, you can't tell whether the change you see in the results of the y_k is a result of the conductivity X being changed by changes in moisture, N, P, K, salinity, etc..
It is as if a person's height was correlated with their age, weight, sex, nutrition, country, and socio-economic status, and you measured someone's height as 1.600m. Could you reliably predict their age, sex, weight, sex, nutrition country, and wealth all from the one number?
Since they are all Y = AX + B calibrations, where X is conductivity, you could just read conductivity and calibrate your own As and Bs inside the Arduino.