Soil Order

Granular Soils [N]

These highly productive soils have been used continuously for horticulture for up to 40 years at Pukekohe, despite some erosion.

Granular Soils are clayey soils formed from material derived by strong weathering of volcanic rocks or ash. Dry or moist soil samples may be easily parted into small hard fragments. When wetted and rubbed between the fingers the clay becomes sticky and may be easily remoulded with little cracking.

Granular soil, Pahiatua Saddle.
Granular soil, Pahiatua Saddle.
Patumahoe silt loam. Credit - The Living Mantle
Patumahoe silt loam. Credit - The Living Mantle

Occurrence

Granular Soils occur only in the northern North Island, particularly in the lowlands of the Waikato and South Auckland regions. Parent materials are usually strongly weathered tephras, mostly older than 50 000 years. They cover 1% of New Zealand.

View map View map of Granular soils.

Physical properties

Polyhedral structure is usually well developed. The soils are slowly permeable and have limited rooting depth.  Topsoils have limited workability when wet.

Chemical properties

Soils are strongly weathered with low nutrient reserves, and low phosphorus status and sulphate in B horizons. They are usually dominated by kaolin-group clay minerals, with some vermiculite.

Soil groups

Soil orders are divided into soil groups based on variation in factors such as drainage status, parent material, chemical and physical properties

[NP] Perch-Gley Granular

Periodic wetness caused by a perched water table

[NE] Melanic Granular

Less acid and higher natural fertility than other soil groups

[NX] Oxidic Granular

Transitional to Oxidic Soils

[NO] Orthic Granular

Other Granular Soils