for soils, fertilizes and feed there are different extraction methdos (solvents) suggested in the literature: the background is always to asses the "bioavailable" (either for plants or for animals) part of the total phosphorus of the sample.
Drs. Schnug and Milham rightly answered the question.Dyer,1894 used the citric acid to extract plant- available P in soil.As citric acid is a weak acid it is presumed that the acidity created by this extractant simulates root environment/rhizosphere pH.Whatever amount of P it extracts from soil or fertilizer is plant- available.
Citric acid can be used to extract plant-available P in soil and acts in different ways. The acidity simulates root environment/rhizosphere and slightly solubilises the phosphorous complex; citric acid can make complex with bivalent /trivalent metal that insolubilized phosphor, therefore phosphor can be easily released.
For DCP use as animal feed additive, the analysis could be an idea of bioavailability.