If you're gardening in raised beds, whether your garden currently looks like this...
...before you get around to spring planting, you'll want to fill or topdress your beds with an extra layer of compost. How much? My general rule is to add as much as it takes to re-fill the frame. Depending on how long it's been and how deep your raised beds are that measure can vary quite a lot.
But in general, how do you figure out how much compost you need? It's a simple length x width x height = quantity. The complexity is that you've got to get your units all on the same page for your math to work. Multiplying inches by feet by yards will produce a number that's of no use whatsoever.
Let's work an example: We're going to top-dress a 12ft x 4ft raised bed with 2inches of compost. First off, we need all the units (ft and in) to be the same; let's use feet because that will prove the most useful on down the road. The 2 inches is the weirdo, the non-feet measurement; thus, our question is how many feet is 2 inches? Well, clearly less than one foot, so we'll use that are our "Did we mess up badly?" check. What we've got to do is convert inches to feet like this:
(because there are 12in in 1 ft): 2in x 1ft/12in
(the inches cancel each other out): 2ft/12 =
1/6ft or .167ft
(Does it pass our "Did we mess up?" check? Yes, less than 1ft).
Okay, so now we can do length x width x height:
4ftx12x.167ft = 8cubic feet.
So, to top dress a 12ft x 4ft raised bed with 2 inches of compost-mix, it will take 8 cubic feet of material. But what's that mean?
Bagged soils tend to come in 1cubic ft bags, so you'd need 8 of them. In the store, mushroom compost typically runs $5-$7/bag, so $40 to $56. And, of course, you'd have some shopping and hauling to do.
Or, if you purchase bulk mushroom compost (from Local Sources), you'd have to purchase 1 cubic yard (i.e. 3ft x 3ft x 3 ft = 27 cubit feet), which you can pick up with a truck or have delivered. After topdressing your 12ftx4ft you'd have more than 2/3rds of your yard of compost leftover, which you could use for extra gardens or alkaline-loving shrubs. If you've got your own truck, this option is great. If not, the delivery charge (typically about $40) + the chore of shoveling compost will likely make you think: there's got to be an easier way to top dress my raised beds.
Voila: if you don't want to deal with calculations or hauling or shoveling compost, give me a shout, and we'll take care of it.
PS- For in-ground gardens, one or two inches of compost topdressing will mostly do the trick. Same length x width x height calculations. However, given our area's potassium-deficient soils, I'd also add 5-10pounds of green sand from Natural Matters per 100square feet.