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The Oxisols

A very important group of soils on Molokai, Lanai, Oahu and Kauai fall in the Oxisol order. Most of the saddle area between the Koolau and Waianae Range is occupied by Oxisols. If we begin at the intersection of Kunia Road and H-1 Freeway and drive toward Schofield Barracks, we first encounter the Molokai soil near the HSPA field station. As we rise in elevation and head toward Wahiawa, we enter a narrow strip occupied by the Lahaina soil series. This is followed by a complex intermingling of the Kunia, Kolekole and Wahiawa series. To the untrained eye, all the soils look alike. But if we look westward towards the Waianae Range we see large alluvial fans emerging from the mouths of ancient gullies. These fans extend eastward towards Kunia Road and beyond covering soils developed from lavas of the Koolau Mountains.

The Kolekole and Kunia soils are believed to have formed on the eastern edges of the fan. Both soils are classified at the Great Group (3rd) level as Humitropept. "Humi" refers to the comparatively high humus content, "trop" to the tropical climate and "ept" to the Inceptisol order to which they belong. Inceptisol are soils that possess characteristics of youth which is implied by the term inception or beginning. The soils buried by the fan should be older than the soil developed on the fan, and the older Molokai, Lahaina and Wahiawa soils all show signs of old age. This sign of old age is evident in the subsoil and is characterized by a low capacity of the soil material to retain nutrients such as calcium, magnesium, potassium and sodium. Soil scientists refer to this characteristic as low cation retention capacity. There are many coarse textured soils with low cation retention capacity, but what makes the Oxisols unique is that they are that way even though they are heavy textured (high clay) soils. In old age they have lost their capacity to retain bases such as calcium, magnesium, potassium and sodium. But in losing cation retention capacity, they have gained capacity to retain anions such as phosphate, sulfate, and even nitrate. The 10- 30-10 fertilizers sold in local stores reflect this high capacity of many of our soils to retain and immobilize phosphorus, and to render much of it inaccessible to plants. The often heard statement "high phosphate fixing soils" refers to this characteristic.

In Oxisols phosphate fixation is attributable to their high content of iron and aluminum oxides. The red color of the soils of the Wahiawa plateau is primarily due to the iron oxide hematite. As you move away from the central plateau into the flanks of the Koolaus, the bright red colors fade and turn brownish. This tells you that another iron oxide of a different color is replacing hematite. The brown iron oxide is a hydrated iron oxide called goethite. The content of hydrated iron oxide increases as rainfall increases as is the case as you move upslope on the Koolaus.

Oxisols are old soils, but the Lahaina and Wahiawa soils are Oxisols that still retain youthful features. They may be past their prime but still show considerable vigor as evidenced by their performance as agricultural soils. This youthful quality is indicated in the 4th (subgroup) level of their original name, before it was revised, very fine kaolinitic, isohyperthermic, Tropeptic Eutrustox. The syllable "eptic" in Tropeptic indicates that the Lahaina and Wahiawa soils have Inseptisol-like qualities. Inceptisols, if you recall, are young soils. The features that make the Lahaina and Wahiawa soils Tropeptic or young-looking is the way the soil particles are clumped into well-defined aggregates. The aggregates in our Oxisols are especially stable because of the cementing action of iron oxides.

Stable soil aggregates allow heavy, wheeled-vehicles to transport cane or pineapple on unpaved "dirt" roads shortly after a heavy rain. Stable aggregates make our soil less erosive by enabling rain water to seep into rather than flow over the soil surface as run-off.

In the revised classification, the Wahiawa series is now a Rhodic (red) Eustrutox, and the Lahaina a Typic Haplustox. Typic indicates the norm and Haplu means simple which on the Lahaina soil corresponds to its lower fertility. (The Lahaina series is classified as a Haplustox on the basis of limited laboratory data. There is no good reason to believe that more laboratory data will show it to be an Eutrustox.)

If the youngest soils are found on the youngest island and the middle- aged soils in the intermediate islands, we should expect to find the oldest soils on Kauai. On Kauai we have soils so rich in iron and aluminum oxide, several aluminum companies looked into the feasibility of mining the soils for aluminum ore.

The oldest soils of Kauai are not only chronologically old, they are also genetically old as well. These are the prematurely old soils associated with lavas of Mt. Kilohana. By Kauai standards, Mt. Kilohana is young just as Diamond Head and Mt. Tantalus are young by Oahu standards. Under warm and humid conditions, the lavas of Mt. Kilohana and Mt. Tantalus weather comparatively quickly into clay and oxides. The rock, already low in silicon is stripped of this element through the leaching action of rainwater. What remains after a geologically brief period is the insoluable residue of iron and aluminum oxide. In some cases the silicon content falls below one percent, an unheard amount in an agricultural soil. These are the soils that benefit greatly from application of lime in the form of calcium silicate. Calcium silicate rejuvenates these soils by raising soil pH, supplying calcium and by reversing the desilication process associated with aging. Resilication with calcium silicate restores cation retention capacity, reduces phosphate fixation and provides a source of silicon for plants. Silicon is not considered an essential plant nutrients, but sugarcane agronomists consider this element to be "agronomically" essential. Most plants, especially grasses including sugar cane, need silicon for strength, resilience and protection against pests. The Kapaa and Pooku soils of Kauai are prime examples of the type we have just described. Both are classified as very fine, ferruginous isohyperthermic Typic Acrudox. The name tells us that they are high in clay, iron oxide-rich and occur in regions with warm and even temperatures throughout the year. The name also tells us that members of this Oxisol family occur in humid (udic) environments.

The prefix "Acru" carries a special meaning. In Greek "acro" refers to extremity. In soil science "acro" implies extreme weathering. The Kapaa and Pooku soils are as extremely weathered as any soil one can find in the world. We now know that these soils, however old, can be rejuvinated through good management. The soil family name helps us to diagnose their deficiencies and prescribe treatment for improvement.

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