The world's population is expected to reach 8 billion by 2025 and most of this growth in population will occur in developing countries. To feed the world with such a marked increase in population, a great improvement in food production must be achieved particularly in these countries. To meet this challenge, present agricultural productivity must be increased on the cultivated land. However, in many developing countries, particularly in Africa, reduced soil fertility caused by continuous cropping with low nutrient input and the resultant nutrient mining of soils is a major threat both to food...