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a 741 is an old op-amp (operational amplifyer). Bassicaly it compares voltages at the two inputs (pinw 2 and 3), and gives an output based on the comparison. in your circuit this output can be possitive or negative (i think)
bassically, the more moisture in the soil, the higher the resistance. The resistors form a potential divider, which changes the voltage going into the pin. the output is changed accordingly.
your's is a slightly more complicated version of this, but the principle is the same.