Comment on Stallion Responsive WordPress SEO Theme by SEO Dave.
I checked your cache and looks like you were running Stallion Responsive 8.2.1 (released January 2015) before the update. With the 8.2.2 (released April 15th 2015) the only SEO change between 8.2.1 and 8.2.2 is in the Stallion SEO Posts Widget (makes your recent/popular posts widget) which can now use tags as the silo SEO sorting option (before April 2015 it was no silo SEO or silo SEO via categories).
Looks like before/after the update you weren’t using the SILO SEO options built into the Stallion SEO Posts widget, so the April update per se has made minimal to no SEO changes to your site.
Changing the layout from 2 sidebars to 1 sidebar has no direct HTML/SEO changes to the site. The layout changes are CSS based which aren’t an SEO factor per se.
That being said Google has started to look at what your site looks like : see the PageSpeed Insights Tool Results, so Google does ‘see’ the output of a webpage like we do in a browser, so changing layout via CSS changes only (like Stallion does) could have a small SEO impact (I don’t know how important this is, if it is).
We know Google ranks content higher in the HTML code as more important, changing Stallion layout has no HTML code changes (the changes are small CSS code changes), so the question is does Google link actual output (what is seen in a browser) to ranking and that appears to be maybe (if it isn’t a factor today, I think it will be one day).
You would assume content loaded above the fold will be considered more important SEO wise, does changing the layout from 2 sidebars to 1 sidebar push the important content higher/lower in what you see in a browser?
When I look at the difference between a Stallion 2 sidebar layout vs a 1 sidebar layout the main content positioning doesn’t change that much, if anything the 1 sidebar layouts tend to push the important content higher in the code, so if anything switching to a 1 sidebar layout should improve rankings (if Google uses it as a ranking factor).
I see your site at the top of page 4 for the ‘south facing greenhouse’ SERP. With a SERP you were page 2 (not a great ranking) that sort of fluctuation isn’t unusual, I’d look at your backlink profile for if you are loosing important links over the past 6 months. Everyday new sites go online, more competition means we have to keep working on SEO to maintain rankings, if you aren’t regularly building new links your competition will pass you.
It’s also important if you regularly add new content to generate new backlinks to maintain overall link juice, every time you add a new webpage it costs link juice to index and rank it, if you only add new content and not build new links your older SERPs will drop because your older content is getting less link juice. Basically more content you add, more links you’ll need, if your content doesn’t generate natural backlinks you’ll have to find them or accept adding new content could loose you traffic long term.