ULTISOL, a weathered, frequently reddish, acidic soil type of humid areas in the middle to low latitudes. The characteristic profile consists of a moderately to strongly acidic clay horizon, a dark surface horizon rich in humus, and a leached horizon. Not as intensely weathered as the oxisols, with which they are geographically associated, ultisols still contain appreciable amounts of such primary minerals as feldspars and micas, as well as a mixture of clay types.

The addition of lime raises the pH of these soils, thereby reducing aluminum toxicity and increasing the availability of nutrients provided by application of organic fertilizers. Where this treatment has been used, in areas such as the southeastern United States and Southeast Asia, which have a long year-round growing season and ample rainfall, the ultisols rank among the world's most productive soils.

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