NITISOL

one of the 30 soil groups in the classification system of the Food and Agriculture Organization (FAO). Occupying 1.6 percent of the total land surface on Earth, Nitisols are found mainly in eastern Africa at higher altitudes, coastal India, Central America, and tropical islands (Cuba, Java, and the Philippines). They are perhaps the most inherently fertile of the tropical soils because of their high nutrient content and deep, permeable structure. They are exploited widely for plantation agriculture. Nitisols are technically defined by a significant accumulation of clay (30 percent or more by mass and extending as much as 150 cm below the surface) and by a blocky aggregate structure. Iron oxides and high water content are believed to play important roles in creating the soil structure. Nitisols are also strongly influenced by biological activity, resulting in a homogenization of the upper portion of the soil profile. These soils are related to the Alfisol and Inceptisol orders of the U.S. Soil Taxonomy. Related FAO soil groups originating in tropical climates and also containing layers with clay accumulations are Acrisols and Lixisols.