Four crucial errors in defence were the difference between St Kilda winning and losing to Port Adelaide, according to coach Ross Lyon.

The Saints lost for just the second time this year, going down to Port Adelaide by seven points in the round seven clash at Marvel Stadium.

"It was a really high-pressure game, they applied a lot of heat. I thought the scoreboard wasn't an indicator of the first quarter, what was really going on out there. But we made them pay," Lyon said.

"The second quarter was the reverse, we dominated entries. I thought we had some shallow entries, so I asked them to put it in (deep), and I need to be careful what I say, because they're so coachable. We didn't get bang for our buck, and they scored quite easily. 

"We're disappointed, because we thought in the end we could have won, but there's some execution – everyone wants to complicate it, but if I really simply look at it, outside their forward 50 possession gains where we turned it over and they kicked four goals, they basically almost didn't score off possession. 

"We defended the ground really well. They just got a bit of clean air, scored out of the middle and a few stoppages, but nothing dramatic. To me, we turned it over four times in our D50. If I took them out, and I think a little bit of efficiency, I'm not unhappy. A five-day break, we worked really hard, we gave ourselves an opportunity to win."

One of those four turnovers in defensive 50 came after Josh Battle was called to play on upon taking a half-step. Charlie Dixon smothered the kick, and Sam Powell-Pepper kicked the resultant goal, a crucial major in a tight game.

"I had a discussion with the umpires during the week about that stepping off the line. It didn't even look like he stepped off it. I rang Dan Richardson during the week, because there was one on Benny Paton. It's really interesting, that half a step," Lyon said.

"Not that I'm blueing, we just need some clarity, because it feels like it's been really tightened up. The rule is a bit old, you're meant to go line of goals from wherever you are on the ground, so I'd like that clarified, because that hurt us."

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One aspect of the gameplan Lyon will be working on ahead of next week's clash against North Melbourne is getting bang for buck when going forward.

"I mean you can complicate it, but our efficiency all year, even though we've been scoring quite well, we might have a quarter where we're really efficient, it's been a bit of an issue," he said.

"Even though we've been in the top eight for attack, we've had lots of opportunities, but we're down at the bottom end for inside 50s, for efficiency. Ultimately, that probably hurt us overall. I thought our last quarter was really good, we fought on. But to their credit, they brought a lot of heat, they're a really good team."