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I have no idea what math needs to be done to figure this out, but let me set the stage for you.

I'm building a BIG planter box in my yard. 27ft X 54ft X 5ft high. We want to utilize our whole yard to grow in, and as it stands, we just have way too much standing water/flooding to do it in the ground without raising it up.

I've gone through a few different ideas of the best way to do it, but I've settled on this: I'm using reclaimed 2x4s from pallets to build wooden walls, which I then will screw into posts around the perimeter of the box. Trying to save as much money as possible, since I simply can't afford all the supplies if I were to buy them new.

That bears the question, just how many posts do I need? I was planning on buying 6X6 posts, but using 4X4s would be cheaper if I could get away with it. I know I need at least a post on each corner, and probably some unknown amount of posts in between the corners. Factoring in concrete needed to support the post, each 6X6 post would cost about $70, and each 4X4 would cost about $50. I appreciate help figuring this out, more so if I can see some sources on how to calculate this myself. Thanks in advance!

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    You're going to be containing 270 cubic yards of dirt in this box. I'm pretty sure you've figured out that one post at each corner isn't going to cut it. What you're really doing is building 4 retaining walls that happen to connect in a rectangle instead of being linear. Think like that and you'll be on the right track. I'm reasonably certain that you're not going to hold all that dirt back with a 2"x4"x54' wooden wall unless you're putting a post every 4-6 feet at a maximum.
    – FreeMan
    Sep 26 at 14:44
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    According to WikiHow an average cubic yard of dirt is roughly 2100 pounds and up to 3000 pounds when wet (which yours will be since you're gardening in it). You're looking at 810,000 pounds (405 tons) of weight to contain. Don't under sell this. If you're going to splash out the cash for this much dirt, build a proper retaining wall to hold it back.
    – FreeMan
    Sep 26 at 14:48
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    That's not a planter box, it's a major land grading exercise, It probably needs planning and building permission (you say you are raising your entire property 5 feet?), which I think and hope you would not get. It's impractical -- how will you tend a 5-foot high garden? A ramp would need to be about 50 feet long. Why does it need to be so high? You obviously don't have 5 feet of water in your yard.
    – jay613
    Sep 26 at 15:08
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    If you're going to be doing this much dirt works, consider putting the efforts into improving drainage from your yard instead of building this massive earthworks on it. By improving the drainage of the yard, you'll also be reducing the standing water that's attempting to enter your house and destroy it, and that will save you considerable money and heartache in the long run.
    – FreeMan
    Sep 26 at 15:18
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    Suggestion: Plan differently. Few things need 3 feet of dirt. Don't grow those. Some things grow in pots. The pots can sit on wood tables you make from pallets. Most things will grow in a foot of soil. Look at pictures of a typical public allotment garden. They are often located in wet/mushy areas. Build several boxes, 4 feet wide and 2 feet high so you can stand or kneel next to them and reach at least half way across. Put enough gravel/sand at the bottom to get out of the water, and soil above that. One box can be a little higher to accommodate plants that need more soil.
    – jay613
    Sep 26 at 15:20

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Instead of a giant earthworks project, consider making raised beds instead. Raised bed gardening becomes more and more attractive with the passing years because there's less bending and stooping required when you till, cultivate, weed and harvest. It also lets you plant in good soil that you bring in if the local soil is sub-optimal, such as hardpan, sand, rocky soil or too wet as in your case. If you get occasional standing water, you can make a low boardwalk between the raised beds to keep you out of the water and mud.

Backwoods Home Magazine has run numerous articles on raised bed gardening. (No personal connection except that I'm a subscriber.) Here is one such article that uses pallets to construct the raised beds. Photo by Joe Mooney.

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  • From all I've read now from the comments, I think this will be the way to go. I was concerned about soil depth at first, but it sounds like my initial plan is just too much to handle.
    – bob152637485
    Sep 26 at 18:57
  • @bob152637485 Should use google to find how much area you do need. It is different from a big family living totally off the land to a family that just wants to grow a few things to have fresh. Also remember everything you grow and not eat right away, needs storage space somewhere, pantry, freezer/s, shelves for canned goods. Storing food the right way is almost as hard/important as growing the food.
    – crip659
    Sep 26 at 19:09
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To answer the actual question: you need as many posts as you need. Your pallet is, what, 4’ long, so your posts are a little less than 4’ apart. Typically, posts with ground contact are made of pressure treated wood, but if it was me, I’d consider rot-resistant untreated alternatives.

Lots of valuable side information in the comments, especially regarding fixing the yard drainage first. The money you spend on drainage will probably be massively offset by the savings in topsoil you don’t have to buy.

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  • I agree on the useful information in the comments, and all in all, I think it's back to the drawing board to rethink my plans!
    – bob152637485
    Sep 26 at 15:49

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