To obtain a fertilizer excellent in e.g. plant growth promoting effect at low cost by adding a specified amount of calcium sulfate to the burnt ash derived from sludge produced in e.g. sewage treatment plants to utilize such burnt ash of sludge as a waste.

The burnt ash derived from sludge produced in e.g. sewage treatment plants is incorporated width 20-50wt.% of calcium sulfate followed by, cording as necessary, addition of 10-50wt.% of a conventional fertilizer to produce the objective fertilizer. Thereby, since the burnt ash derived from sludge is rich in such ingredients effective for plant growth as nitrogen and phosphorus and also has been incorporated with an appropriate amount of calcium sulfate, intake of the nitrogen, phosphorus and Ca ingredients each essential for plant growth into plants can be accomplished. Besides, this fertilizer rapidly neutralizes waste materials such as tannic acid involved in the physiological activities of plants or the acidic matter contaminated into the soil due to the external environment such as acid rain, contributing to the activation of plants with proper pH circumstances. Therefore, when this fertilizer is applied to field crops, the state, etc., of the resultant harvests can be improved.