What are the units of output of soil moisture sensor? I want to use a soil moisture sensor using arduino uno R3. I am using the analog output. The values are in the range of 100 to 1300 as in the readings mention in this question. 

May I know what exactly these numbers mean? I read in a website that:


  Most soil moisture sensors are designed to estimate soil volumetric water content based on the dielectric constant (soil bulk permittivity) of the soil.


So what are the units? Basically I want to base an estimate for the normal amount of soil moisture for typical loamy soil and I don't understand how to base this threshold based on the readings given by the sensor. It would be greater help if someone actually knows what an appropriate soil moisture threshold is for loamy soil. 

I searched on the internet but the closest i could come to answers was this table and this particular entry:


  Table 1.1. Typical field capacity and wilting point values (m^3
   m^(–3)) for different soil textures
  
  Soil texture| Field capacity| Wilting point| Available water 
  
  Loam    |0.27| 0.12| 0.15|


I am so sorry if I seem confused but I am very new to the whole of sensors and electronics 
This is the sensor that I am making use of
Those sensors have no units. If you can find a schematic you could possibly relate the value to the resistance of the soil, but you'd need to know the resistance that the soil is paired with to form the resistive divider.

All you can say from the numbers is "This is wet" or "This is dry". Normally you would use trial and error to determine what the values roughly relate to, and say that "Anything under X should trigger watering. Anything over Y should stop watering."