Huh? I don't even know how much it weighs when dry. ;o)

It probably averages about 20-25% heavier than a peat based soil when dry, and about 10% lighter when at container capacity (Container capacity occurs when the soil is fully saturated and has just stopped draining). The difference lies in the much higher % of macro-pores (from which water drains more freely) in the bark-based soils. Sorry I couldn't be more specific. There's going to be a lot of variance, depending on what the bark is like.

Tell us more about your planters (size, especially) & hanging methods if you need more help.

Al

I'm looking at 36" boxes which are 8" wide and 7.5" deep. If my calcs are right that about 1.25 cubic feet of soil. The brackets that come with the boxes say they'll support 37 lbs. The brackets slip over the deck rails. I'm wondering if one pair/box will be enough.

That's what I come up with too. 1.25 cu ft of moist bark weighs about 25 lbs. With peat & perlite mixed in, that volume will weigh less - about 20 lbs. Some quick calculations based on 65% total porosity, 30% air porosity (have to subtract it from total porosity because the macro-pores will drain) and considering the particulates are already about 50% saturated (they're moist if you remember), the soil should gain about another 8-10 lbs of water when at container capacity.

So, wetted soil will weigh about 30 lbs when fresh, a little more as it ages. Remember to add in the weigh of the growing plant material, which could be considerable. You might easily gain another 10 - 15 lbs over a growing season. Don't get upset, I meant the plants will add the weight. ;o)

If the brackets support 37 lbs/pr, you'll need 1-1/2 pair per. If they're rated at 37 ea, 1 pair will be enough.

Al

I usually gain my 10-15 pounds over the winter season! ;)

Thanks for the input. I'm going to check with the company since I'm not sure if the 37 lbs is per bracket or per pair.

A general rule of thumb to design for is that damp soil weighs between 80-100 lbs/cubic foot. I always use 100 as a worst case scenario and it's usually better to err high than try and calculate a low figure and have a collapse. Therefore a 1.25 cubic foot container would weigh, worst case, 125 lbs. Thus, I would use at least 3 of those brackets, or possibly 4, 2 at the ends and one 12" from each other. The brackets say 37lbs but they probably hold twice that so you may get by with three. I think using two would be kind of risky if there is anything or anyone valuable below your planter can hit if the structure collapses. :-)

I do a lot of balancing planters on the edge of rooftops and always use the 100lb figure in my calculations even though under normal conditions I've found the soil to weigh between 40-50lb/cubic foot.

I'm going to stick with my calculations (besides, I already have the advantage of knowing what it weighs).

Schmily was asking about a well aerated mix. Dry sand weighs from 90-110 lbs/cu ft. If we say it weighs 100 to make it easy and sand is about 15% pourous, even if all the pores were to retain water, it would only gain 15% of 1 cu ft, which equals a shade over 9 lbs. Loam and clay are lighter than sand at about 90 and 75 lbs/cu ft dry, and are less porus than sand, so will gain less weight at field (or container) capacity.

All the above for mineral soils. For the open soils I use, the calculation for weight, based on the heaviest ingredient, pine bark which is already 50% saturated and that weighs in at 40 lbs/ 2 cu ft bag, would look like this:
20 lbs (weight of 1 cu ft damp bark).
If the soil is at 65% total porosity and 30% is air porosity (macro-pores which will drain) it means the soil will gain .35 x .5 cu ft H2O @ 62.5 lbs/cu ft or another 11 lbs of water. Knock off about 5 lbs because the actual soil will have perlite & peat, both lighter than bark, and multiply by 1.25 cu ft & you get about 32.5 lbs/1.25 cu ft.

If you are using sand or garden soil in your soils, I can easily see where they might weigh the 50lbs/ 1.25 cu ft you suggest.

I have weighed a 5 gallon bucket of my bark-based soil after it's mixed & moistened & it weighed about 23.5 lbs, including the bucket.

Al