Magnesium in New Zealand soils

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Abstract
Results are presented from a survey of exchangeable magnesium (Mge) and “reserve” magnesium (Mgr) in New Zealand soils. Reserve magnesium was estimated by boiling the soil for 15 min in Im HCl at a soil : extradant ratio of 1 : 40. Among soils of the main zonal sequence, exchangeable magnesium levels appear to be generally adequate, with the exception of some yellow-brown earths, and podzols. Topsoil exchangeable magnesium values, on average, typically exceed those for subsoils, with the exception of the brown-grey earths and the central yellow-grey earths, soil groups that are characterised by heavy-textured subsoils (pan horizons) in which quite high levels may occur. Among azonal soil groups, low levels of exchangeable magnesium are found in certain jellow-brown sands, particularly in the northern region. Among intrazonal groups, low exchangeable magnesium levels occur in many of the soils derived from rhyolitic volcanic ash, such as the yellow-brown pumice soils, and also within a small area of southern yellow-brown loams derived from coarse-textured parent materials of mixed glacial origin. In the soil groups derived from intermediate to basic volcanic parent materials, low levels of exchangeable magnesium are uncommon and occur only among the oldest and most strongly weathered (mature) members of the groups, the younger soils being well supplied. As determined by the acid extraction procedure, reserves of magnesium in zonal soil groups are adequate to abundant in regions of weak to moderate weathering, i.e., in soil groups of the sequence brown-grey earths to central yellow-brown earths. In regions of strong weathering and/or strong leaching (i.e., northern yellow-brown earths to podzols) magnesium reserves are generally low. The difference between these two categories of zonal soils would appear to be mainly one of abundance of 2:1 type clay minerals (with magnesium present as a substituent in octahedral sites). Among intrazonal soil groups, reserves of magnesium appear to be related to content of ferromagnesian minerals. Weakly weathered soils from intermediate to ultrabasic parent materials have high reserves; strongly weathered soils from andesitic or basaltic rocks and soils from rhyolitic volcanic ash, both of which are low in ferromagnesian minerals, have low reserves. Based on the levels of exchangeable and reserve magnesium found in the present work, and taking into account weathering and leaching factors where relevant, soil groups showing evidence of present or likely future magnesium deficiency are indicated. It is suggested that for such soils particular attention should be paid to trends in levels of exchangeable magnesium with time.