In most humid tropical and subtropical regions where acid soils prevail, warm and moist conditions result in weathered soil types. These tropical soils are depleted in available phosphorus, and usually the total phosphorous level is low, ranging from

0. 01 to 0.1% (Chen and Ma 2001). Much of the phosphorus is bound to aluminum and iron complexes during pedogenesis (Walker and Syers 1976). Through adsorp­tion and precipitation mechanisms aluminum forms insoluble and stable comple­xes with inorganic and organic phosphates, forming highly insoluble phosphorous compounds; therefore, their solubilization is a prerequisite for phosphorous uptake by plants. Soils suffering from aluminum toxicity are generally associated with phosphorous deficiency. The phosphorous-use efficiency in such soils is around 10-15% in the best situations (Verma et al. 2005). The low phosphorous status of these soils is of great concern because large amounts of phosphorous need to be applied to raise the concentrations of available soil phosphorous to an adequate level (Sanchez and Uehara 1980). Low phosphorous availability is considered to be one of the main limiting factors to plant growth in acid soils, in addition to human wealth in tropical areas (Barber 1995; Sanchez 2002). For temperate soils, ash amendments have been shown to alleviate phosphorous deficiencies (see Chap. 2, Schiemenz et al. 2011).