Comparative investigations into the content of available molybdenum in important soils of the arid and humid tropics.

The determination of the content of available molybdenum in 196 soil samples from different geographical (Yemen, Sudan, Tanzania, Ghana, Cuba, and Sri Lanka) and climatic (annual precipitation 30-3000 mm) regions gave these results: 1. Arid and subarid soils of dry regions as well as rendzina-type soils contained the highest amount of available molybdenum (0.216 ppm respectively). The lowest content was determined in the ferrallitic soils of the humid tropics (0.045 ppm). These differences are mainly attributed to high pH values and CaCO3 content in aridisols and rendzina-type soils and to a strongly acid soil reaction in ferrallitic soils. 2. The content of available molybdenum was significantly positively correlated with the pH, percentage base saturation, CEC and exchangeable Ca; it was negatively correlated with the clay content. 3. Deficiency of molybdenum is hardly to be expected in arid and subarid soils (24 per cent of all samples deficient). The proportion of samples with a low Mo content and the probability of Mo deficiency increases in this order: vertisols (40%) leads to fersiallitic soils (64%) leads to ferallitic soils (89%).