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I found 2 kilogram (4.4 lbs) sealed plastic bag labelled "CIRP Christmas Island Rock Phosphate 34.5% P2O5". It contains very fine light brown powder.

CIRP

I assume that it is phosphate fertilizer since it mentions Phosphorus pentoxide (P2O5). What exactly is Christmas Island Rock Phosphate (CIRP)? How do I use it to fertilize plants that are phosphorus deficient? There are no usage instructions on the bag.

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Rock Phosphate mined on Christmas Island, at a guess.

Indeed:

EXPORTS The top exports of Christmas Island are Calcium Phosphates ($5.03M), Phosphatic Fertilizers ($2.71M), Chemical Analysis Instruments ($1.65M), Electrical Transformers ($1.26M), and Gas and Liquid Flow Measuring Instruments ($854k), exporting mostly to New Zealand ($3.78M), Indonesia ($2.72M), Singapore ($1.85M), Australia ($1.3M), and Ireland ($1.24M).

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Rock phosphate is a standard type of phosphate fertilizer. It has comparatively little "available" phosphate (first year to crops) but quite a lot that will be released slowly over time (following years.)

UMN Extension lists it (generically) as 34% total, 3% available. Evidently your specific example has an extra 0.5% per analysis. Unlike other common phosphate fertilizers that are entirely "available" the first year (and gone the next) it will keep working for many years. But it's 7-15 times less "available" in the first year than typical processed phosphate fertilizers, so if a plant needs a shock treatment, it's probably not the right choice.

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