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Compositions of solutions equilibrated with the salts at 25°C yielded the solubility products 7.1 x 10-14 for MgNH4P04 . 6H2O and 2 4 x 10-11 for MgKPO4 . 6H2O. Both salts dissolve incongruently in water with precipitation of trimagnesium phosphate. Conditions are defined for the precipitation of MgNH4P04 .6H2O without contamination with trimagnesium phosphate.Although the precipitation of MgNH4P04 .6Hz0, is a well-known analytical procedure, no measurement of its solubility product has been reported. The value of 2.5 x 10-13 reported by Bube 1 is based upon an estimate that the compound dissolves congruently in water to give a 1 x 10-3 M solution. The preparation of the analogous potassium compound, MgKP04.6H20, was described by Agte et aZ.2 and by Bassett and Bedwell,3 but there are no data on its solubility.Measurements were made of the solubility products of each compound in experiments designed to establish the conditions under which it dissolves congruently.

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Approximately 30 crystalline phosphate compounds, in addition to colloidal precipitates of variable composition, were identified as reaction products following the addition of various fertilizer solutions to soils and soil constitutents. The identifications were made by means of X‐ray, petrographic, and chemical analyses. The findings help to clarify the mechanism of phosphate reactions in soils, which includes the dissolution of fertilizer and soil constituents and the precipitation of new products.

IV. Morphological and Optical Properties of Crystalline Pyrophosphates Refractive Crystal System, Class, and Habit Indices