To provide both a method for irrigation in a protected horticulture designed to activate soil in an agricultural plastic greenhouse, eliminate the replant failure and raise the growth rate of crops and an apparatus therefor.

Ozone obtained by an ozone feeder 1 is fed into water drawn up with a storage pump 2 and sufficiently mixed therewith. The resultant water containing the ozone dissolved therein is then fed into an automatic irrigation pipe 5 and sprinkled. The constitution provided with an ozone mixer 3 for sufficiently mixing the water with the ozone and an air regulator 4 for pulling air out of the water is adopted as the apparatus.