Here is a simple and compact digital soil moisture tester to check whether the soil is dry or wet. It can also be used to check the dryness or wetness of cotton, woolen and woven fabrics.
For indication, the tester (see Fig.1) uses a number of LEDs driven by ubiquitous display driver IC LM3915 (IC1). The display panel shows the relative magnitude of conductance (reciprocal of resistance) between the two test probes when these are inserted in the soil.
The tester measures the dryness or wetness through soil resistance readings (0 to about 5 kilo-ohms) and indicates in dot mode by sequential lighting of LED1 through LED9. The first LED (LED1) lights up when the conductance is high (resistance is almost nil).
Normally, the soil resistivity varies from 0 to about 5 kilo-ohms. So for calibration, connect a 5-kilo-ohm potmeter between the two probes. Using trimpot VR1, set resistance to zero (minimum). LED1 should glow. Similarly, set resistance to 5 kilo-ohms (maximum). Now, LED9 should glow.
Assemble the circuit on a general-purpose PCB. After construction, enclose the tester (including battery) in a small plastic cabinet as shown in Fig. 2. The probes can be constructed from a pair of new injection needles. The needles should be firmly fixed about 2.5 cm apart on a piece of laminated plastic sheet. Use short length of a flat, twin wire-lead flexible cable for interconnection. After wiring the circuit, power the circuit using switch S1 and adjust zero-set trimpot (VR1) slowly such that LED1 lights up when probes are shorted. The tester is best powered by a compact 9V alkaline battery.
EFY note. For proper calibration, use a good soil moisture meter.