A process for preparing a lightweight fertilizer containing ammonium polyphosphate and optionally other plant nutrients by heating a mixture of ammonium polyphosphate with a leavening and hardening agent selected from calcium carbonate, calcium oxide, magnesium carbonate, sodium carbonate, sodium bicarbonate, dolomite or manganese carbonate at a temperature of about 250 DEG F to 375 DEG F for a time sufficient to foam the mixture and subsequently cooling the foamed mixture. Preferably, various plant nutrients such as sources for potash are added to the mixture prior to foaming.
