int soilMoistureSensorPin = A0;
will work fine.
A0 is just a variable that contains the actual pin number. For example:
#define PIN_A0 (14)
#define PIN_A1 (15)
#define PIN_A2 (16)
#define PIN_A3 (17)
#define PIN_A4 (18)
#define PIN_A5 (19)
#define PIN_A6 (20)
#define PIN_A7 (21)
static const uint8_t A0 = PIN_A0;
static const uint8_t A1 = PIN_A1;
static const uint8_t A2 = PIN_A2;
static const uint8_t A3 = PIN_A3;
static const uint8_t A4 = PIN_A4;
static const uint8_t A5 = PIN_A5;
static const uint8_t A6 = PIN_A6;
static const uint8_t A7 = PIN_A7;