The present disclosure, in various embodiments, discloses hydrothermal methods, hydrothermally modified materials and dried hydrothermally modified materials. Certain dried hydrothermally modified materials can readily releases ionic species such as alkali metal ions (K+, Na+), silicate salts, and alkaline earth metal ions (Mg2+, Ca2+). Some dried hydrothermally modified materials can readily release aluminum ions and/or silicon, such as in the form of soluble silicates. Such processes and materials are useful, for example in economically preparing potassium releasing fertilizers.