England ended the day 145 runs ahead with nine second innings wickets remaining following a second successive batting collapse.
After England lost five wickets for 38 late on day one before declaring on 9-395 South Africa lost their last seven for 44 on Monday.
That left Proteas dismissed for 281, after which England closed on 1-31 with first innings centurion Maia Bouchier dismissed for a 19-ball duck.
Resuming on 0-17 overnight South Africa lost Anneke Bosch in the second over caught behind off Bell.
Captain Laura Wolvaardt then put on a 92 with Annerie Dercksen (41) before Lauren Filer's shorter delivery to the latter was chipped up to the slip cordon. Heather Knight failed to hold on but palmed the ball up for Sophie Ecclestone to take an excellent one-handed catch.
Wolvaardt went on to make 65 before being dismissed by Ecclestone.
Sune Luus (56) and Marizanne Kapp (57) proved to a difficult duo to break as a dominant afternoon session saw the pair add 99, but debutant Ryana MacDonald-Gay (2-50) found the breakthrough to take her first Test scalp by bowling Kapp.
The seamer struck again after a delay caused by lightning in the area to dismiss Nadine de Klerk and Bell (4-49) hastened South Africa's demise with three quick wickets, including two in two balls.
South Africa earned a valuable wicket six overs into England's second innings when Bouchier feathered Ayanda Hlubi's delivery to Sinalo Jafta but Tammy Beaumont and Knight steered England to the close.
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