De Ridder, making his first Saints start after signing from Dutch club De Graafschap last month, struck home low past Torquay goalkeeper Robert Olejnik in the 16th minute to give his new side the lead.
Lee Mansell equalised less than a minute later for the battling visitors but Saints secured their place in the second round through a free-kick from Richard Lambert and late goals from Richard Chaplow and Jonathan Forte.
The home side started the game without Alex Oxlade-Chamberlain, who on Monday signed for Arsenal, the injured Radhi Jaidi while striker Lee Barnard failed to shrug off an ankle injury and was also forced to sit out.
Saints piled on the early pressure with Torquay defender Chris Robertson having to be alert to make a couple of early clearances before a first chance for Morgan Schneiderlin drifted high and wide.
Torquay, looking to snap a run of eight years without a Carling Cup victory, hit back with a chance for Taiwo Atieno before Chaplow came close with a long-range shot at the other end.
De Ridder's opener looked to have broken the visitors' game resistance but the Gulls hit back almost immediately when Lee Mansell headed home an Ian Morris cross inside the box.
Saints went back in front 10 minutes later when Lambert fired his free-kick into the bottom left corner of the net after Mansell had given away a free-kick for a foul on Schneiderlin.
The visitors had their moments in the second half with Lathaniel Rowe-Turner bringing a save out of Bartosz Bialkowski then Lee Holmes blocking an effort from Mansell after a goalmouth scramble.
Bialkowski then saved from both Eunan O'Kane and Danny Stevens before Saints finally made the game safe when Chaplow nudged home from inside the six-yard box and substitute Forte wrapped it up in injury time.