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The Andisols

The example we will use is the soil family with the name, medial, isothermic, Typic Eutrustand. In Hawaii, members of this soil family and their close relatives constitute the best vegetable growing soils. We should not be surprised if members of this soil family also turn out to be well suited for growing vegetables, elsewhere in the world.

Two local soils that belong to this family are the Kula soil series on Maui and the Waimea soil series on the Big Island. Like siblings, these soils were derived from similar parent materials (volcanic ash) and developed in places where the temperature is even (iso-) throughout the year and cool (-thermic). A highly prized attribute of these soils which vegetable farmers probably take for granted is their loose, easy-to-work characteristics implied by the term medial. These soils are also non-acid (eutr-) but may not be able to supply fast-growing vegetables with nutrients such as phosphorus and boron. The application rates required to correct deficiencies for phosphorus and boron are higher than one would expect for non-acid soils. There are many other soil characteristics relevant to vegetable production that are not explicitly given in the family name. In fact there are so many soil factors that can affect soil performance that it is not possible to accommodate them all in a soil name. Soil attributes that are important for a particular land use, but not included in the family name should still be inferable from it. These inferred characteristics are called accessory characteristics. The unexpectedly high phosphorus and boron requirements of soils in this family are accessory features, which can be inferred from the last three letters "and" in Eutrustand. A Chilean, Costa Rican, Indonesian, Filipino, New Zealander or Japanese farmer familiar with this classification system will know how to extract a large amount of information about a soil just from the letters "and." The "and" in Andisol, derived from the Japanese word "ando", refers to the dark, nearly black, volcanic ash soils of that country. In fact the accessory characteristics of ando soils are so pronounced and unique that volcanic ash soils were recently placed in an order of their own.

Ustands are Andisols that occur in areas with a pronounced dry season (to burn as in combust). Anyone who has been to Kula or Waimea knows that vegetable production there requires irrigation. Having read this, the reader may wonder how the system accommodates volcanic ash soils that occur in wetter or drier regions. On Maui and Hawaii, the ustands turn into udands (humid) in the wetter zones. Logic would suggest that "id" should be chosen over "ud" to represent humid, but it turns out that there is already a soil order that uses "id" in its name. That order is Aridisol and represents soils of the arid regions.

Most of the soils that occur in deserts are classified as Aridisols. Water is so important in determining the character of a soil that its absence or presence is given priority in the classification scheme. In Aridisols, water is considered to be the most important factor in its development, behavior and performance. How would one classify a soil derived from volcanic ash in a desert? What is more important, water or parent material? In Soil Taxonomy parent material wins out in the case of volcanic ash soils. A desert Andisol is named Torrand. The prefix "Torr" is taken from torrid, meaning hot and dry. In Hawaii we have Andisols that occur in the rainforest, open grassland, and near deserts. Their names from the wettest to the driest are Udands, Ustands, and Torrands. We say that these soils occur in "udic", "ustic", and "torric" moisture regimes. One would expect their fertility to decrease with increasing rainfall and wetness. One would also expect soil acidity to increase with rainfall. Nature produces acid, infertile soils in wet zones and neutral, fertile soils in dry areas. In the original Soil Taxonomy, the inherent fertility of soils was often designated at the great group (3rd category) level. The Kula and Waimea soils were named Eutrandept in the original classification and Eutrustand in the revision. We said that eutric implied a non-acid condition, but it also suggest enrichment as in eutrophication of rivers and lakes. As we move beyond the wetter limits of the Kula and Waimea soils, we come to soils like the Maile series of Maui and Hawaii. The Maile series was once known as a Dystrandept (dystrophy = faulty nutrition) and is now called Hapludand (Hapl = simple). When one enters the rainforest on the slope of Haleakala or Maunakea, the name Dystrandept could not do justice to the soils there. They were called Hydandepts, and are now called Hydrudands. These are some of the most unusual soils in the world. In their natural state, as implied by their name, they consist mainly of water. With the exception of the surface of 10 inches, the underlying soil is 60-70% water by volume. And yet these soils have supported a sugar industry for over a century. While these soils are infertile by almost any criteria, their productivity is constrained more by excess rain and cloud cover than by lack of nutrients. This is because soil infertility can be easily corrected by fertilizers.

We can see from Table1 how calcium, magnesium, potassium, and sodium levels vary as we go from the dry zone to the wet. The dry zone is characterized by an abundance of nutrients, but in the high rainfall areas nutrient level reach near undetectable levels. Most soil scientists trained in Europe or North America would expect the Hilo soil (Hydrudand) to be very acid. A soil so depleted of bases should show pH values between 4.0 and 5.0. What we see are values approaching 6.0., Should such a soil be limed? This question needs a qualified answer. A soil with a pH of 6.0 but having only detectable amounts of calcium requires lime not as an amendment but as a nutrient. The distinction is important because application rates for amending soil pH is an order of magnitude higher than for correcting calcium or magnesium deficiency. And because calcium and magnesium added as lime are relatively immobile, better results can often be obtained by applying the sulphate salts of calcium and magnesium which are more soluble and mobile.

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