Toxicity Prevalence

Manganese toxicity is a major problem worldwide and occurs mainly in poorly drained, acid soils owing to the interactions mentioned previously. However, not all poorly drained soils are sources of manganese toxicity as reported by Beckwith and co-workers (99), who noted that flooding often increased the pH, thus reducing the availability of manganese. Tropical, subtropical, and temperate soils have all been reported to be sources of manganese at concentrations high enough to produce visible symptoms of toxicity. In the tropics, toxicity has been reported in tropical grasses grown in the Catalina (basalt) and the Fajardo (moderately permeable) clayey soils of Puerto Rico (100), and in ryegrass (Lolium spp. L.) grown on red–brown clayey loam and granite–mica schists in Uganda, Africa (101). Among the subtropical regions, toxicity has been reported in subtropical United States in poorly drained soils and soils on limestone (102) and on ultisols. However, the impermeability of soils does not seem essential for manganese toxicity (103). In southeastern Australia, manganese toxicity has been reported in fruit trees grown in neutral-pH duplex soils (104), in French beans (Phaseolus vulgaris L.) grown in manganese-rich basaltic soil (105), and in pasture legumes (106). There is very little information available on manganese toxicity in temperate regions, though one report found toxicity on soils characterized by low pH and high concentrations of readily exchangeable manganese (107).