The ions of the compound potassium magnesium phosphate are required nutrients for all green plants. Since the compound is not very soluble, even an overdose of it to the neutral to slightly acidic soil will not cause a toxic concentration of these nutrients. In spite of such low saturation concentrations, however, finely divided potassium magnesium phosphate has a large specific surface that allows it to dissolve rapidly enough to almost immediately replace any nutrients the plants extract from the soil. This compound accordingly makes it possible to provide the soil with a stored fertilizer that will release nutrients as needed.