A method of measuring amorphous ferric phosphate in the presence of crystalline ferric phosphate is described. This procedure is important because there appears to be a big difference in availability between the amorphous and crystalline forms of ferric phosphate. This difference has been proven for two situations. In the first situation, when amorphous ferric phosphate is used as the source of iron for gypsy moths, growth is normal, but if the crystalline form is used, results are identical to the case when no iron is used. In the second situation, several plant species have been reported to grow much better when amorphous ferric phosphate is used instead of crystalline ferric phosphate as the source of phosphate in fertilizer. Differentiation of the amorphous from the crystalline uses citrate solutions that extract the amorphous form but not the crystalline form. The procedure was optimized for three different sample forms: agar based artificial diet, Wesson salt† (a salt mixture containing all recommended minerals for insect diets), and pure ferric phosphate. A method for overcoming a problem with turbidity that occurs when analyzing some prepared diets is also described.