Total and Extractable Micronutrients in Tropical Acid Soils of Lampung, Indonesia

The study aimed to determine the total and available amounts of iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu) in the humid tropical acid soils of Lampung, Indonesia, and to identify the key factors affecting them. Topsoil samples were taken from 10 agricultural fields and 6 natural fields in 16 sub-districts of Lampung, Indonesia. The average total contents of Fe (26.36 g kg−1), Cu (8.97 mg kg−1), and Zn (53.24 mg kg−1) are generally below the world-soil average contents and those in other acid soils, which may be due to eluviation of Fe, Cu, and Zn into subsoil induced by high rainfall and the low natural contents of especially Cu and Zn in the parent rocks (acid volcanic rocks and sandstone). In contrast, the average total Mn content (1.20 g kg−1) is 2 to 8 times higher than that commonly found in acid soils and the world-soil average content, indicating that the parent materials may contain high Mn-bearing minerals. The total Fe, Mn, and Zn are positively correlated to the clay content, exchangeable-Ca and -Mg, and exchangeable-Mg, respectively, whereas the total Cu has no correlation to any soil properties. The amounts of available Fe, Mn, Cu, and Zn account for on average 0.38%, 6.20%, 21.50%, and 4.40% of the total contents, respectively. Around 6%, 12.5%, and 56% of the soil samples exhibit Mn, Cu, and Zn deficiency, respectively, and 25% face Fe toxicity. Total Fe and Mn are the main factors determining available Fe and Mn, respectively, whereas exchangeable-Mg and organic-C content mainly control available Cu and Zn, respectively.