ZEOLITES AND THEIR POTENTIAL USES IN AGRICULTURE

Zeolites are natural crystalline aluminosilicates. They are among the most common minerals present in sedimentary rocks. Zeolites occur in rocks of diverse age, lithology and geologic setting, and represent valuable indicators of the depositional and postdepositional (diagenetic) environments of the host rocks. It was reported that of the 40 naturally occurring zeolites studied by research groups, the most well known ones are clinoptilolite, erionite, chabazite, heulandite, mordenite, stilbite, and philipsite. Structurally zeolites are tectosilicates exhibiting an open three-dimensional structure containing cations needed to balance the electrostatic charge of the framework of silica and alumina tetrahedral units. Pores and voids are the key characteristics of zeolite materials. The pores and interconnected voids are occupied by cations and water molecules. The internal surface area of these channels are reported to reach as much as several hundred square meters per gram of zeolite, making zeolites as extremely effective ion exchangers. The Si/Al ratio is an important characteristic of zeolites. The charge imbalance due to the presence of aluminum in the zeolite framework determines the ion exchange property of zeolites and is expected to induce potential acidic sites. The Si/Al ratio is inversely proportional to the cation content, however, directly proportional to the thermal stability. Cations can be exchanged by ion exchange and water can be removed reversibly by application of heat. The unique physical and chemical properties of zeolites, coupled with their abundance in sedimentary deposits and in rocks derived from volcanic parent materials, have made them useful in many agricultural applications. Most of the initial research on the use of zeolites in agriculture took place in the 196os in Japan. A brief review of the literature has pointed out that Japanese farmers have used zeolite rock over years to control the moisture content and to increase the pH of acidic volcanic soils. Ion-exchange properties of zeolites can be utilized in agriculture because of their large porosity and high cation exchange capacity. They can be used both as carriers of nutrients and as a medium to free nutrients. Zeolites are important materials with very broad applications in agriculture and environmental engineering. Zeolite incorporation in soil was found to increase crop yields and promote nutrient use efficiency. Other possible uses being investigated include applications as a carrier of slow-release fertilizers, insecticides, fungicides and herbicides, and as a trap for heavy metals in soils.