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New Zealand Journal of Forestry (1967) 12(2): 189–199
©New Zealand Institute of Forestry

Research article
Pumice Soils as a Medium for Tree Growth. I. Moisture Storage Capacity

G.M. Will and E.L. Stone

Tree roots penetrate to a depth of at least 9 ft in Kaingaroa silty sand, the soil type of the central region of Kaingaroa Forest. The eight major pumice ash layers making up the profile to this depth were sampled to determine moisture storage capacity available to deep-rooted plants. Soil textures of these layers range from silty sand to gravel, but the porous nature of primary particles results in low bulk densities (0.57 to 0.96) and high moisture capacities (up to 51% by weight). All layers except one can store over 30% moisture by volume. Drainage of excess water is slow in its later stages and two to three weeks, rather than days, are required to reach field capacity. After recharge, these soils contain more than 30 inches of plant-available water within the upper nine feet.
This exceptionally high storage allows sustained growth of Pinus radiata throughout the long growing season. Although moisture is available at all times, it is possible that growth may be restricted by inadequate nutrient uptake when the surface layers are near wilting point.

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