An alkali metal ion source with a moderate rate of release of the ion (e.g. potassium) is formed by a method that includes: 1) combining an particulate ore that contains at least one of an alkali metal ion-bearing framework silicate (e.g. syenite ore) with at least one of an oxide and hydroxide of at least one of an alkali metal and alkaline earth metal such as calcium hydroxide; 2) milling the mixture of these two components optionally, with water, optionally, milling the dry components separately and blended thereafter, optionally, with water; 3) forming a mixture by adding water to the solid mixture after milling, if water was not added before milling; 4) exposing the mixture to an elevated temperature and pressure to form a gel that includes silica and the alkali metal of the framework silicate.