Humults of Windward Oahu, Hawaii, and the Potential Ratings of These Soils For Selected Uses

Abstract

An investigation of the Humults (Ultisols) of Windward Oahu, Hawaii, was made with the objective of determining and classifying the soil and landscape parameters for selected agricultural and nonagricultural uses, characterizing and verifying the classification of the soils of the study area, showing the geomorphic relationships of the Ultisols with other soils, proposing the potential rating of the soils for the production of banana and sweet potato and for dwellings and for construction of local roads or streets, and identifying the areas of further research required for efficient interpretation of soil and landscape data for different uses. The soil order Ultisols dominate the sloping eroded landscape of Windward Oahu. Most of these soils are highly leached, contain appreciable amounts of extractable aluminum, and fix large amounts of phosphorus which can affect plant growth. Because of the appreciable amounts of organic carbon in the upper portion of the Bt or argillic horizon, these soils are further classified in the suborder Humults. The four important soil series, covering slightly over 10,000 hectares, are the Waikane, Lolekaa, Alaeloa, and Paumalu soils. They occur in areas with an annual rainfall ranging from 900 to 2,300 mm and with an isohyperthermic temperature regime. Pedons or profiles of these Humults were collected and laboratory data were obtained to determine the range in the characteristics of the chemical, physical, and mineralogical properties. These soils are very strongly acidic to strongly acid, with the pH values ranging from 4.5 to 5.4. The cation exchange capacity ranges from 13 to 25 meq/100 g of soil, while the base saturation of the surface horizon ranges from 22 to 45 percent. The base saturation decreases with depth. Because the subsoils have high amounts of extractable aluminum ( as much as 12 meq/100 g) and consequently high aluminum saturation ( as much as 88 percent), crop production on these soils is limited to plants that can tolerate appreciable amounts of soil aluminum or crop production can be maintained only with proper management. Banana and sweet potato are two crops which can tolerate such a condition. Chemical, physical, and mineralogical properties were determined to verify the classification of the Humults. The laboratory data suggest a need to study further the Humults of Windward Oahu. It is likely that some of the Humults may need to be reclassified at the lower categories. In general, the Humults of Windward Oahu are Tropohumults with some of them tending to have properties of Palehumults. They all are of clayey family, generally with varying mineralogy. The Humults of Windward Oahu occur on a sloping landscape and in association with lnceptisols, Entisols, Mollisols, and Vertisols. Association with Oxisols are of limited extent. Exposure of Windward Oahu to the Northeast Tradewinds and the subsequent effects of erosion is attributed to the occurrence of the existing landscape and soil associations and distribution. A system of determining the soil potentials of the Humults of Windward Oahu for selected agricultural and nonagricultural uses was proposed. Potential ratings for the production of banana and sweet potato were used to illustrate the system for agricultural uses and ratings for dwellings and for the construction of local roads or streets were used to illustrate the system for nonagricultural uses. Certain soil and landscape parameters which cannot be altered easily by man (noncontrollable parameters) have strong influence or weightings than other parameters which can be altered by man (controllable parameters). Various criteria pertinent to the specific uses were rated and evaluated. The results of the ratings suggest that much of the Humults, especially those with steeper slopes, have poor or fair ratings for the production of banana and sweet potato in Windward Oahu. Humults with less steep slopes, however, have better ratings. The latter soils are limited in extent. The predominance of Humults with poor and fair ratings are to be expected because these soils are by definition highly leached soils with low inherent fertility and occur on sloping landscapes. The value of the potential rating system appears to be then m deciding which areas of Humults should be considered as “first” choice for a specific use. Similar results of the ratings for dwellings and local roads or streets suggest that Humults in areas of steeper slopes have poor or fair ratings while those of less steep slopes have better ratings. These findings suggest further that in general soils that are good or fair for banana and sweet potato are also good or fair for dwellings and local roads or streets. Although the final decision to use a particular site for a specific use will depend on other factors, such as socio-economic conditions, the concept of soil potentials can be used as an initial step in guiding the land users, land planners, and others in evaluating a soil, a parcel of land, or an area for a specific use. Areas of further research include the systematic compilation of yield data of specific crops and refinement of the soil potentials approach. There is also a need to display the potential ratings for selected uses especially by means of a computerized display system. Several systems are in existence but they should easily be accessible and updated and be subject to modification so that the information may be useful not only for general planning, but also for specific planning.