12. Pumice Soils

Pumice Soils cover about 16% of North Island, formed mainly in unconsolidated rhyolitic (silica-rich) pumice deposited by the Taupo eruption (AD 232 ± 10) from the huge caldera now occupied by Lake Taupo, and the Kaharoa eruption (AD 1314 ± 12) from Mt Tarawera. Other areas of Pumice Soils are formed in 3500-year-old Waimihia pumice deposits and on fluvially reworked pumice alongside rivers. Pumice Soils comprise sandy to gravelly pumice materials. They are weakly weathered with a low clay content and have A, and weakly developed Bw, horizons within pumice deposits that are at least 25 cm thick. Pumice Soils have low reserves of major nutrients and are likely to be deficient in trace elements including cobalt, copper, molybdenum, boron, iodine, and selenium. For agricultural development, it is necessary to add fertiliser and trace elements, particularly cobalt as its deficiency causes ‘bush sickness’ (vitamin B₁₂ deficiency) which leads to death of sheep and cattle. Pinus radiata trees have been successfully grown, supporting a major plantation forestry industry. However, in recent decades, many plantations have been converted to dairying. Pumice Soils are readily eroded by water, and if not protected by vegetation, severe gully erosion can form in a single rainstorm.