After an unseasonably warm week (including near-record heat on Monday), it cooled down enough to make working on the second planter bearable.  I started by dumping the soil we purchased last week—20 bags of it—into the planter, along with half of a bale of peat moss.  After stirring it around a bit and leveling it out, I estimated that it filled the planter to a depth of about five inches.  At that rate, we will need another 20 bags of top soil to reach the halfway point, after which we will start adding some compost into the mix.

For kicks, I made some rough estimates of how many bags of soil we will need to fill the planter to the top.  Using a plan area of four feet by 12 feet and a depth of 22 1/2 inches, I extrapolated that we will need 90 bags of soil in total.  To confirm this estimate, I measured a bag of soil; the approximate dimensions are 21” x 15” x 3”.  Using these measurements, the volume of a typical bag of soil is 0.547 cubic feet and dividing this into the volume of the planter—90 cubic feet—yields an estimate of 165 bags.  This is almost twice my other estimate; not the confirmation I was looking for.

Sometimes, approximate measurements are not good enough.  Using my estimate of 90 bags and the planter’s volume of 90 cubic feet, the volume of a 40-pound bag of soil would be one cubic foot.  This is a more believable number both in terms of practicality and marketability (I would be surprised to see a bag labeled as 0.547 cubic feet).

Assuming unit volume and bag dimensions of 21” x 15”, I calculated that the thickness of a bag would be around 5 1/2”; increasing the bag size to 22” x 16”, I got a thickness of about 5”.  This may have been the thickness of the bag when it was first filled.  After being stacked on pallets, shipped, and then stored for who knows how long, the bags got compressed to the 3” thickness I measured.  That’s my hypothesis, anyway.

We made a run to the Plant Depot and picked up another 20 bags of top soil.  After emptying the bags into the planter and leveling it out, the depth of soil was about 10 inches.  We seem to be on track for 90 bags of soil when we’re done.