We are pleased to announce that work will begin the week of Feb 8 to install a regulation dirt infield in time for the 2016 regular season
— Toronto Blue Jays (@BlueJays) February 3, 2016
So this is interesting. In an attempt to look and play like the other infields around the majors, the Toronto Blue Jays will have a new dirt infield for 2016. Instead of just the cutouts around the bases, the field will now have dirt extending all around the basepaths, with the interior remaining turf. (Unlike some fields in Japan, where the entire infield is dirt.)
Will there be a larger affect? Will this lead to more or fewer hits? I would assume turf is faster than dirt, so presumably it would lead to a lower batting average on groundballs. In 2015, the American League hit .242 on groundballs, ranging from the Tigers' .263 mark to the White Sox's .228 average. The Blue Jays hit .232 on groundballs. To see the potential influence of a home park, let's look at each team's difference between home batting average on groundballs and road batting average:
Red Sox: +.023
White Sox: -.003
Blue Jays: -.023
As you can see, nearly every team hits better on groundballs at home. Collectively, the AL hit .252 at home on grounders and .233 on the road. Yes, the road numbers include pitchers hitting in National League parks, but this influence is almost zero. Blue Jays pitchers had just 21 at-bats in 2015 and seven of those were strikeouts.
So what causes this the home-field advantage? I'm not quite sure. In general, all players hit better at home: They're more rested, more familiar with the environment, and so on. But maybe they run a little harder at home? Maybe managers are more apt to play a bench player -- and thus a lesser hitter or inferior fielder -- on the road?
The Blue Jays, however, were the worst team in the league in groundballs at home, 23 points lower than on the road (.220 versus .243). Their pitchers, meanwhile, allowed a .196 average on grounders at home (second-lowest in the AL behind the Astros) and .245 on the road. So maybe the turf at the Rogers Centre isn't concrete but is actually slow.
It's also possible that 2015 was just a weird aberration. In 2014, the Blue Jays hit .277 at home on grounders and .232 on the road. The year before it was .239 at home and .219 on the road. Here's the five-year data since 2011:
Blue Jays hitters on groundballs
Home: .245 (.243 league average)
Away: .225 (.231 league average)
Blue Jays pitchers on groundballs
Home: .220 (.232 league average)
Away: .236 (.242 league average)
Conclusion: I'm not sure the turf had any sizable impact. Blue Jays hitters were a little better at home but a little worse on the road. Blue Jays pitchers were 12 points better at home compared to six points better on the road. Maybe the turf was slightly slower than a traditional dirt infield. If so, we could expect the new all-dirt infield to generate a few more hits. Last year the Blue Jays hit 933 groundballs at home; a five-point increase in batting average -- from .220 to .225 -- would lead to 18 additional hits at home over the season. Of course, keep in mind that the Blue Jays' batting average at home last year was low; they were likely to hit for a higher average on groundballs no matter the infield configuration.
Anyway, we know this: The all-dirt infield will look better and Troy Tulowitzki's knees are probably happy as a result.