Leaching phosphates from rock

[asylumdown]

So I recently completed 2.5 months worth of lab work for my masters. I was doing soil analysis stuff and one of the tests I was *going* to do (I ended up not doing it because it all took 10 times longer than I thought it would) was to test my soil samples for 'plant available' phosphorous, i.e. the phosphate component of the soil that could theoretically become available for biological activity.

Now, I'm by no means an expert on this stuff, but I had to do a tremendous amount of research in to this to figure out what 'plant available' phosphorous actually was, and how you'd go about testing for it. Turns out, 'labile' (the stuff that can enter biological cycles) phosphorous is an INCREDIBLY complex topic, and there's not actually a single way to completely measure it because what is 'plant available' is as much a function of time and chemical process in otherwise inert parent material as it is a measure of the water soluble phosphate that you can detect moment to moment.

Anyway, asides from the interesting implications that has for people who have seemingly mysterious algae problems when all their nutrient parameters test undetectable, the most commonly used testing protocol got me thinking:

If you're an analytical soil scientist who's interested in doing a quick and dirty test of the amount of phosphate that is 'plant available' in a soil sample at any one moment, the way you'd go about doing it would be to do an aqueous extraction of phosphate, filter the extracting solution, and then test the solution for the amount of dissolved phosphate using either spectrophotometry, or more direct measurements of ions.

The interesting bit, is that the extracting solution a soil scientist would use to get this 'plant available' phosphorous out of the soil is a simple bicarbonate solution.

The logic comes from a paper published by Olsen et al. in 1958, who proposed (this next bit is quoted from the Natural Resources Conservation Service at http://soils.usda.gov/technical/methods/): "... introduced 0.5 M (molar) sodium bicarbonate (NaHCO3) solution at a pH of 8.5 to extract P from calcareous, alkaline, and neutral soils. This extractant decreases calcium in solution (through precipitation of calcium carbonate), and this decrease enhances the dissolution of Ca-phosphates. Moreover, this extracting solution removes dissolved and adsorbed P on calcium carbonate and Fe-oxide surfaces."

When I read that for the first time it was like a lightbulb went off. I thought "wow, if you were using Marco rock, or any other form of 'dead' base rock for your cycle, or you were using rock that you'd just bleached or treated with muriatic acid, you could really enhance the health of your system by leaching phosphates from them using a week long baking soda bath!"

so my questions are thus:

1. Will this principle work with calcareous rock?

2. Is this something that everyone already knows about and it's commonly used and recommended but because I live under a rock I've just never heard about it?