Southon has returned to Bath Lane this year, dropping down four divisions after playing for Burridge in the Southern Premier League Premier Division.
After taking six wickets in his first five County League fixtures, he doubled his seasonal tally with a 6-22 burst off 9.5 overs against St Cross 3rds.
He dismissed openers Jack Good and Arthur Hurren and St Cross were 37-3 when Southon removed Murad Hayat (6).
In his second spell, Southon continued to run through the home order as St Cross lost their last five wickets for 17 runs to be dismissed for 131.
It was the first time Southon had ever collected a league six-fer - his previous bests being 5-20 (v South Wilts, 2018) and 5-35 (v Alton, 2021) in the top flight of the SPL for Burridge.
Having inflicted damage with the ball, Southon then shared an unbroken match-clinching stand of 73 with wicket-keeper Jeremy Bulled.
Bulled followed up his previous week’s 83 with 49 not out, while Southon was undefeated on 40.
Overseas all-rounder Keegan Fortune starred as Portsmouth 2nds suffered a sixth loss in seven league games.
The South African top scored with 84 as Hythe & Dibden posted 259-7 (Carlin Joy 3-50) after electing to bat first.
Keegan, the division’s leading wicket-taker, then took his seasonal tally to 20 - at an average of just 10.15 - with 3-18 as cellar dwellers Portsmouth were bowled out for 206.
Sujeeth Daini (39) top scored for the visitors, with Thomas Jones (33) and tailender Matthew Walton (27 not out) next highest.
James Hughes hit a league best 86 as Burridge 2nds defeated their Bournemouth counterparts.
Chasing 230, Hughes and Srujith Wickramasinghe (54) put on 89 for the first wicket.
There was later a 48-run stand for the fourth with Nick Damley-Jones (31) and, though Hughes finally fell, Burridge won by five wickets.
South African-born teen James Van Gool (73) and Robert Morey (62) had top scored for Bournemouth as they posted 229-8 at Chapel Gate.