Measuring the moist of the soild

For plants it is important to measure when water is needed, especially when a automatic watering system is designed.

Resistive measurement

Different methods are widely available. The most common is the resistive measurement between two poles. Simple PCBAs can be ordered on internet, which can be sticked in the soil.

Figure of a default set that can be ordered rather cheap

The circuit board can be used either as analog interface (giving a analog value indicating of the moisture of the ground) or a digital pulse when it is below/above a certain value.

The circuit board can be replaced by a single resistor if we are interested in the analog value, which was not done during this experiment.

But after 1 - 2 months having it turned on (with a default +3.3V on one of two sides, it did not work anymore.

A simple visual inspection showed that the PCBA inserted in the soil was "eaten" away.

It also shows that both sides (left and right) have detoriated differently.

Figure showing the resulting PCBA that was inserted in the soil.

This can be the result of having the biasing continue on the board, and always in the same direction.

 Solutions:

• Only apply a bias voltage when measuring the soil
• Apply a AC voltage or switch the measurement 'direction'
• Add a protective skin over the PCBA and do a capacitive measurement
• Buy a more expensive version that can withstand it for a longer time

Next step in this experiment will be using a soil measurement sensor consisting of two metals, and not a PCBA.

Based on an old soil-sensor bought second hand.

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