Corrosion of sensors is something to consider when deploying a project in the field. Solutions range from high-tech, such as specialized gold plating, to low-tech, such as coating the sensors cheaply using paint.
The main tradeoffs are cost vs. precision. Often high-end, expensive sensors are housed in the same low-grade stainless steel as much cheaper sensors tend to use. The only way to really tell is trial and error. A cheap temperature probe coated in paint and properly calibrated can often provide the same output as a sensor costing many times as much. In this scenario, you may be better off with going the low-tech route and routinely maintaining and swapping out degrading sensors.
For soil sensor monitors, a different technique is required since coating the sensors would impede the conductivity that is measured by the sensor. One key measure to extend the life of the sensor is to provide power to the sensor only when measuring the conductivity. Depending on how frequently the moisture is measured, this should increase the lifetime of the sensor much more than constantly providing power.
Another tip is when wiring the sensor to your device, use ones that attach via a 3 pin terminal to allow you to switch them out easily when needed during a maintenance cycle. Going a more expensive route may not be the best solution in this case and just as your plants need regular maintenance, so too will your sensors in a corrosive environment.