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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

  • They measure units of moistosity. That is from dry to wet. The numbers themselves have no intrinsic meaning. At least not without more details about the "soil moisture sensor" that you're using. – Majenko Apr 5 at 14:06
  • @Majenko I have added the senor details that I have used in case it gives something more to work with :) – bobthebuilder Apr 5 at 14:12
  • That device has no units. It doesn't measure anything absolute. It only gives you a voltage that is in proportion to the resistance of the soil compared to some other resistance that isn't specified. So it's just a number from dry to wet. Or maybe wet to dry. Even that isn't clear. – Majenko Apr 5 at 14:24
  • Also that kind of soil moisture sensor is the worst kind of moisture sensing you can possibly do. Fine short term for a pot plant I suppose, but it will corrode in no time unless you take special measures, and the results are, as you see, pretty meaningless. A proper soil moisture sensor costs a lot more and looks like this: davisinstruments.com/product/… – Majenko Apr 5 at 14:30
  • Oh. Makes sense. But this was for the sake of experimentation. So. – bobthebuilder Apr 5 at 14:35
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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."

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  • So what exactly are the outputs obtained in the question I had linked? Were they resistances? – bobthebuilder Apr 5 at 14:49
  • They are the ADC values that equate to the voltage that is read from the sensor which is related to the resistance of the soil and the resistance of the (unknown) resistor in the sensor circuit. – Majenko Apr 5 at 14:54
  • You could calibrate the sensors by measuring the output value in soil of known moisture but it probably wouldn't be worth it because those sensors corrode. There are some very inexpensive capacitance sensors available which would be better – haresfur Apr 9 at 3:19

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