Im a bit confused what readings does the serial monitor provide when the soil moisture sensor detects presence of soil moisture. Does it have computations or unit of measure? 
Without knowing which sensor and what code you used its much harder to say.
The usual one just measures resistance of the soil so will most likely give a reading between 0 and 1023, not really calibrated to any measurement standard. Readings can also be affected by changes in temperature.
I used the df robot soil moisture sensor
Assume you mean this one Moisture_Sensor__SKU_SEN0114_-DFRobot with the example sketch from the website the numbers you are seeing are digitized voltages 0=0V, 1023=5V. These readings cant be calibrated accurately because they are affected by lots of factors like soil type, water salinity and temperature.
Cheap sensors like this are good to monitor your pot-plant but if you want calibrated repeatable output to water your commercial orchard or greenhouse you need to invest a lot more money see http://www.soilsensor.com/soilsensors.aspx for a summary of commercial sensors.