Soil is a non-renewable resource upon which mankind depends for survival. Historically, the rise of great civilizations has been linked to the quality of soil and the availability of water. Equally, the demise of such civilizations is often attributed to mismanagement of soil and land in its broadest sense. Crop productivity and soil fertility are thus synonymous. In today's overcrowded world, the challenge to feed and clothe the burgeoning populations of developing countries is a daunting task. Yields have to be increased from existing land areas; adding fertility to the soil to satisfy the demands of higheryielding crops is essential. Soils vary greatly in their capacity to grow crops without fertilizer; even the richest soils experience declining yields without man's intervention.