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.
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.
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.