A PIC is a type of microcontroller with various input/output lines (both digital and analog); digital meaning an on/off, HIGH/LOW signal, analog meaning a varying voltage signal (usually); the PIC supports both. A simple soil moisture meter simply measures the resistance of the soil at the depth in which it is placed by reporting a varying voltage (more complex ones I imagine actually have their own built-in processor that you communicate with using some serial interface and protocol). In short, yes, it is possible to do what you are trying to do using a PIC. However, it will require programming to perform this task; you have not indicated what your level of experience is regarding programming. PICs are generally coded using assembler, C, or a version of BASIC. Do you have experience in any of these languages?