Controlled release fertilizers containing water-soluble nutrients such as potassium nitrate which are easily leached from the fertilizer by moisture in the soil have a coating which more efficiently retards the release of the nutrients and also provides citric acid soluble silicon to the soil. Nutrient particles are first coated wtih a colloid formed by the acidification of preconditioned calcium metasilicate with citric acid and then mixed with cement. The calcium metasilicate is preconditioned by heating to a temperature of at least 1300 DEG C. to sinter at least a portion of the silicate. The nutrient particles are then pelletized and cured to form a silica gel matrix containing acicular calcium metasilicate and a cement coating on the pellets. The nutrient pellets may be mixed with cement coated urea prills in a silica gel formed by the acidification of preconditioned calcium metasilicate with nitric acid to further retard the release of nutrients, and to provide additional citric acid soluble silicon as well as nitrogen to the fertilizer. In addition a phosphorus source such as a municipal sludge may be added to the silica gel.
