The Vertisols

Vertisols do not occur extensively in Hawaii or the world, but knowing something about their behavior and performance is helpful because that knowledge applies to a large group of related soils. The Vertisols have many of the characteristics of soils from the temperate regions so that what we learn from standard textbooks on soil science applies better to our Vertisols than to the Andisols or Oxisols we have just described.

A good example of a Vertisol is the Lualualei soil from the Waianae area of Oahu. It is dark and nearly black in color and forms deep shrinkage cracks during the dry summer months. Loose surface material falls into the cracks and is trapped in the subsoil when the soil swells with water from winter rains. This annual recycling of material from top to bottom eventually inverts the soil. Thus the name Vertisols.

The creeping soils that wreck homes, warp sidewalks and rupture sewage and water mains are Vertisols or their close relatives. On Oahu they create havoc in Kalihi, Manoa, Palolo, Aina Haina, and Kuliouou Valleys. The problem areas are localized, and almost always occur on the talus slope half-way into the valley. Landslides are rare deep in the valleys because the soils there behave like Oxisols, and are also rare near the valley entrances because rainfall is low and talus slopes are short and shallow or absent.

Vertisols are also fertile soils. Their dark color is often mistakenly attributed to organic matter, but our Vertisols have lower organic matter contents than most other soils in the state. When they occur in large, level tracts of land as in Lualualei Valley on Oahu, they make excellent agricultural land. We can learn a great deal about the Lualualei soil from its family name, which is, very fine, montmorillonitic, isohyperthermic Typic Chromustert. Starting from right to left, we learn the Lualualei soil is a Vertisol from the last three letters of Chromustert. We were introduced to the next three letters "ust" in Ustox and Ustand, so we know that an Ustert is a Vertisol that occurs in places with a pronounced dry season. The Black Cotton soils of India and the Tropical Black Earths of Australia are Usterts. Chromusterts are readily recognized by their dark colors. A Typic Chromustert is one that represents the typical Chromusterts. We have seen the word isohyperthermic in other soils and the term simply describes the temperature regime associated with the soil. Farmers wishing to grow leafy vegetables in an isohyperthermic temperature regime normally make provisions to protect their crops from sun and heat, which some do with frequent sprinkler irrigations.

Another distinguishing characteristic of the Lualualei soil is its low requirement for phosphorus. This characteristic is associated with its high silicon content. Unlike the low silica Oxisols of Kauai that benefit from calcium silicate application, the Lualualei soil is naturally rich in silica. Silicon and phosphorus are closely related elements so that a given amount of phosphorus will go a long way to keep a soil well-supplied with this nutrient in soils high in soluble silicon. The high silicon soils are, as one would expect, found in valley bottoms where silicon, calcium, magnesium, potassium and sodium dissolved from the surrounding rocks accumulate. This concentration of riches creates a chemical environment which favors the syntheses of very fine, montmorillonitic soils indicated in the family name. Very fine refers to the high clay content of the Lualualei soil, and montmorillonitic to a mineral that imparts the shrink- swell characteristics to it. Montmorillonitic soils have high cation and water retention capacities. They are stone-hard when dry and soft as warm butter when wet so that the water content range over which very fine, montmorillonitic soils can be cultivated is very narrow. You can contrast this with the very fine, kaolinitic Wahiawa soil that is able to support heavy traffic a few hours after a heavy rain or the medial Kula and Waimea soils that remain loose and friable even when baked dry in the sun. The intent of Soil Taxonomy's authors was to enable its users to visualize a soil's appearance and behavior from its name.