How long does your calibration last?
Well, it depends according to our culture facility, and how homogeneity is our irrigation water, because of that, sometimes there is no recalibration required, let me explain; if we are watering a field with lawn or grass, like in a typical residential house, using regular water, the conductivity will keep quite homogeneous during all the time, having a reasonable proper measurement. But, you are right, if we are adding nutrients to the water, first we have to calibrate the system with these conditions, and if we change again the proportion of the fertilizers, the system will need a new recalibration to have a proper behavior. But imagine that our irrigation water comes from a tank, where we add the fertilizers there, in that case, It could be relatively easy to add a sensor node to the tank, measuring the conductivity and recalibrating the soil moisture automatically.
About the probe corrosion, well, you are right, copper probes are not a good choice, but the fact to be used for our initial prototypes, taught us to increase their longevity, just adding a short term electric pulse for sampling.
#define SM_POWER_PIN 2 // Soil Moisture and Field Capacity POWER PIN (ELECTRODES).
https://github.com/Artesanos/SAC-Sistemas-Automatas-de-Cultivo/blob/master/Electronica/1-Canal%20%28Riego%29/Version%20AC/Rev_1.4/Codigo/SAC_1S_AC_1_4/Sensors.h (https://github.com/Artesanos/SAC-Sistemas-Automatas-de-Cultivo/blob/master/Electronica/1-Canal%20%28Riego%29/Version%20AC/Rev_1.4/Codigo/SAC_1S_AC_1_4/Sensors.h)
But in any case now we are using stainless steel (316 A4) probe.
Anyhow, have a look at https://lowpowerlab.com/forum/index.php/topic,1317.msg9060.html#msg9060 As a serious user, it would be great to hear your thoughts on that thread.
Sure, let me have a look first :)