Phosphogypsum, fly ash and sewage sludge were mixed in various proportions to prep. an allowable fertilizer. Humidity and pH of the components were used as their crucial properties. The mixt. compn. was optimized statistically by single and multicriterial anal. The optimum fertilizer contained 39% phosphogypsum, 41% sewage sludge and 20% fly ash (pH 8.5, humidity 38.5%).