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window boxes: estimate weight of wet potting soil

New York, NY

I know the approximate weight of my window box (15 lbs) filled with soil (20lb/cu foot*1.91 cu ft=38lbs). Does anyone know a calculation to estimate how much the soil will be when wet? I know water is 62 lb/cu foot but what is the ratio of soil to water in a big rain assuming its draining properly?

I saw one calculation on another site that said 120 lb/cu foot for wet soil but my nursery man says that's way too high for potting soil. I realize that it depends on density of soil and drainage but I thought there might be a guideline out there somewhere.


Everett, WA(Zone 8a)

>> what is the ratio of soil to water

I think that "drainage" makes a huge difference - like a factor of two or three or more. It depends a lot on how "open" and also how dense the potting mix is. I've seen heavy junk I wouldn't add to my garden, and fluffy gritty fast-drianing stuff ... and fluffy fine never-draining stuff!

Say a potting mix was mostly peat moss. It seems to me that peat moss holds a lot of water forever and lets hardly any air in. The density might approach 62 lb/cu ft, say 50-60 at a wild guess.

Say it had a fair amount of clayey soil in it. Now the density might EXCEED 62 lb/cu ft, since clay is even heavier than water, and you ain't gonna have NO air in that sucker!

Say it was a fast-draining mix of coarse pine bark chunks and perlite. Say you waited for the excess water to d5rain out, and air filled all the voids. It might be so light that if you bent over to lift it, the ratio between the weight you expected and the actual weight would make you throw it right over your head.

So, "it depends". My beleief at this time is that, for any container and almost any plant, you wnat something coarse that drains fast, and holds only enough wtaer to keep the plant from wilting between visits from you.


Powder Springs, GA(Zone 7b)

Go for the worst case - water only and no drainage. Water logged soil is not going to weigh more than water itself (unless the soil is granite!) hence the 120 lb/CF is full of holes.

I assume this has nothing to do with health of the plants or fast draining soil but rather how much weight can you expect on the side of your house or railing or whatever has to support it.

Everett, WA(Zone 8a)

I don't know myself, but I found websites giving the specific gravity of many rocks as 2-3, and "wet soil" as "2".

Since the densitty of water is 62 lbs/cu ft, an S.G. of "2" means 124 lbs/cu ft.

One rule of thumb for usable soil is "1/3 mineral, 1/3 water and 1/3 air". I think that would knock 124 down to 83 lb/cu ft for "wet but usable mineral soil".

Any decent potting soil had BETTER drain at least that well! And typically it would have more organic components than mineral components (and some expanded minerals like vermiculite and perlite).

But if I were attaching a window box to soemthing, and expected to water it until water ran out the holes, I would allow for 100% water-logged, even if that were only a safety margin.

But I would also want a safety-factor of 2 or 3.



Powder Springs, GA(Zone 7b)

If you have a box full of soil and it weighs X lbs and then pour water in weighing Y lbs to the top that pushes all the air out then you have X + Y lbs. In other words the soil doesn't weigh any more than it did before you put the water in but the box weighs more due to the weight of water poured in to fill the air pockets. If you empty the box and put rocks in that weigh Z lbs and then add water weighing Y2 lbs then the total weight is Z + Y2 lbs. Same principle, just different materials.

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