I have pulled someone's unknown-to-me hand loads - might have made perfect sense to whomever did it - there was no notes or labels - but was two visually different kinds of powders and three different bullet weights - I salvaged the bullets and cases, as you intend to do. Getting primers was not as big an issue, then, and I did not know what brand or type they were - so I placed about a 30" x 12" metal plate on metal saw horses and lit my propane plumber's torch - perhaps a cup full of mixed powder at a time - from various hand loads that I had pulled, or spill clean-up from my own klutziness - spread out on that plate - it burns off in perhaps a second or two. The primers had been pressed out of the cases with a Universal de-priming tool in a re-loading press, and had soaked in a salmon can of motor oil perhaps a year - under the torch, all "popped" and various pieces went flying - is some anvils still "missing in action", but I think I found all the cups. I had read that smokeless powder granules are still visible in dirt a year later - so might contain elements for good fertilizer, but not sure that modern smokeless powder actually breaks down in soil, very easily - I have never disposed of it that way - so I have no personal experience with that - I have always burned it.
Typical fertilizer with be identified with three numbers - like X - Y - Z. So first number is % of nitrogen, second number is % of phosphate (phosphorus) and third number is % of Potassium - so the product that is shipped from many potash mines is 0-0-60 (at least one mine in Saskatchewan ships 0-0-62) - so is no nitrogen, no phosphate and 60% (or 62%) Potassium - the rest is likely oxygen, chlorine or other contaminants. Then, can blend that with perhaps 18 - 46 - 0, to make a "custom" product to spread in a field or orchard. The green colour in crop leaves, the root strength and maturation process of the plant all need the different elements at one time or other - some fertilizers also contain trace elements, like sulphur - which some crops want or need. So, if gunpowder contains a lot of nitrogen, and if it can be released and made available to a plant, that will be only a portion of the feed / nutrients that the crop will be needing.
