Is my math correct for fertilizer?

Unfortunately, compost is prohibitively expensive for my new raised bed (12x3 feet) but I was able to find some very cheap cow manure compost with an NPK of .2-.05-.2. In total I will be using 15 cubic feet of compost which equates to roughly 375 lbs. The rest of the soil will be inert materials that do not add nutrition such as peat moss, vermiculite etc.

My concern is the low phosphorus level of .05. To make up for this i will be adding bone meal (2-17-0) to try and make a roughly .2-.2-.2 mix aka I need to add in .15% aka .0015 (in decimal) of phosphorus. Given that Im using 375 lbs of compost * .0015 = .5625 lbs of phosphorus

Given that the bone meal is 17% phos by weight aka .17, then i would need .5625/.17 = 3.31 pounds of bone meal to make my mix a .2-.2-.2

Thanks for sticking around for my fun math problem, any insight on potential pitfalls in anything I said or calculated is appreciated! One concern I have is that bone meal is also quite high in calcium (approximately 15%), is this of any concern?

Have you considered hugelkultur for filling your bed? In addition to significant cost savings, it is a permaculture approach to developing raised bed soil. There are several good videos on the same.

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thank you for the suggestion, that is actually with hugelkultur. Its a very tall, 30 inch bed and the bottom 18 inches are logs and woodchips. I want the top 12 inches to be actual soil. Given that its 12x3, in order to fill the top 12 inches of the bed requires 36 cubic feet of soil.

Great thanks. It sounds awesome.

Do you have any horse stables near you? That would be preferable to cow, which usually is laced with salt, not good for most gardening purposes. Or if you have a source of good quality leaves such as maple? Or nearby wooded area where you can borrow some quality topsoil, or a stream bed where the sand would be useful? Usually being creative can lead you to some better ideas. The bagged cow manure would be my last choice, better to buy topsoil. Having extra calcium is a plus if you want to grow tomatoes. I guess I’m lucky, I have a great source of river sand that’s full of trace minerals as well as a horse barn. I use them annually. And my two local cities collect and stick pile their park leaves so it’s easily to use them to crest rich humus soil.
Just thoughts
Kent, wa

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Very good to know, thank you for the input! What do you think about mushroom compost?

It’s ideal for most purposes but I would not want it at a 100%, mixing with other soil should do well. A 2-3” mulch of mushroom compost would be ideal if you have the lower 9-10” of garden soil