The circuit in #18 produces a bar graph type display. As the soil becomes more and more moist, more and more LEDs come on. With saturated soil you will see 4 LEDs. If what you want is a dot display, where only one LED is on at any time, then you either add some logic between the comparator outputs and the LED drivers, or change the four comparator circuits from a simple level indicator to what is called a window comparator. Each LED is on only when the input is within a window of two values. This takes two comparators per window. It is a bit more complex, but uses less battery power because only one LED is on at a time.

ak

Or you can use a comparator with push-pull outputs (TLC3704), and wire the LEDs between two outputs, plus one LED from the top comparator to the V+ supply and one from the bottom comparator to the V- supply. That way you can indicated five states in dot mode with only four comparators.

 

a comparator with push-pull outputs

opamp

ak

 

No - a comparator with push-pull outputs. Some op-amps will work, but all those with back-to-back diodes across the inputs won't.
Having said that, it's probably a better use of an LM324 than using it as an op-amp. It pathetic slew-rate will minimise the switching noise on the supply!