I think it's more complicated than that. I have heard that bone meal only works to provide phosphorus to plants if the soil is acidic. That may also be the case with rock phosphate. That pertains to solubility. Also, I believe a smaller fraction of phosphorous in bone meal is actually plant-available compared to rock phosphate. So the P in at least bone meal doesn't really correspond to plant-usable phosphorus that well.

Seems to me if your tests come back with a medium level of phosphorus, you're good to go. No?

Well I would question adding anything if you have a medium ranking - did they give a PPM rating? That tells a great deal more.

But if you decide to add something then you really can't compare bone meal to rock phosphate. Bone meal is VERY slow acting with minimal, if any, immediate results, only minimal results 6 months from now and well into next year. But it is long lasting.

Rock phos is much faster acting with results this season but not nearly as long lasting. So the numbers game doesn't really apply to either.

If you are working with much water run-off, a good grade that encourages water run-off, or have nearby water sources that could be contaminated by the excess phos then stick with the bone meal.

JMO

Dave

Dan and I were typing at the same time.

Appreciate the responses.

The lab measured phosphorus, potassium, calcium and magnesium in lbs. per acre. Only the micronutrients were measured in parts per million

It reported that soils that measured 12-35 lbs. per acre of phosphorus were considered moderate or medium and advised the addition of bone meal or rock phosphate in the amounts I previously mentioned. (All of my beds fell into this range.)

If using inorganic fertilizer, it was recommended that I apply 4 lbs. of 5-10-5 per 100 square feet.

Much depends on the crop that you are growing. If you were planting tulip bulbs then either of those products, although expensive P sources, might be suitable. But for most seeds for which you want rapid germination it is well known that germinating seedlings benefit from having "Readily available" P in close proximity to the seed. So any reliable recommendation will include a seedrow P application even if soil test results indicate moderate P levels. This element tends to adsorb (cling to) soil particles so even if it's present it probably isn't readily available.

Thanks Barrie. FWIW, nearly all the tests on the various beds showed medium levels of phosphorus, high levels of potassium and very high levels of calcium and magnesium. The micronutrients were measured as sufficient or insufficient. (All the beds were sufficient.)

Barrie makes a good point about readily available phos so do keep in mind that there are other quick-fix sources you can use like bat guano and steamed fish bone meal that can be row-applied if necessary.

Dave