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Taro Facts: Did you know that...

Taro-growing Soils

Both upland and lowland taro-growing soils have isothermic or warmer soil temperature regimes. An isothermic temperature regime is one with a mean annual soil temperature at 50 cm depth higher than 15 oC and the difference between mean summer and mean winter soil temperatures is smaller than 5 oC. Isothermic or warmer regimes are found in the lowland tropics within a twenty degrees latitudinal belt from the equator or in the equator within a 1,000 m altitudinal belt.
Lowland Taro-growing Soils

Lowland taro-growing soils have some common features such as (i) an aquic moisture regime, which is a regime virtually free of dissolved O2 because the soil is saturated by groundwater or by water of the capillary fringe, (ii) mottles with chroma of two or less in the soil profile, which is an indication of the soil being saturated with water at some period of the year unless the soil is artifically drained, or (iii) an extragrade Aquic in their taxonomic names. To differentiate from soils with an aquic moisture regime, soils with an extragrade Aquic only have their lower horizons saturated with water. Soils with an aquic moisture regime are classified as Aquents, Aquepts, Aquolls, Aqualfs, Aquods, Aquox, and Aquents.
Upland Taro-growing Soils

Upland taro-growing soils are generally located in high rainfall areas and usually have an udic or wetter moisture regime. An udic moisture regime is defined as one where the soil moisture control section is not dry in any part for as long as 90 consecutive days. Soils with an udic moisture regime are classified as Udalfs, Udolls, Udults, Udox, Uderts, Orthox, and so on.
Taro-growing Soils in the Pacific Basin

Although taro is grown worldwide, it is in the Pacific Basin where cultivation is widespread. The Pacific Basin comprises a conglomerate of islands spreading along the southern and northern portions of the Pacific ocean. The islands of the Hawaiian archipielago account for most of the lands in the North Pacific while the Cook Islands, the Salomon Islands, Western Samoa, Fiji, and the Loyalty Islands make up the bulk of the lands in the South Pacific.
Several soils abound in the Hawaiian Islands. But Oxisols are the most extensive of all. Oxisols are found in isothermic or warmer temperature regimes and in all but aridic moisture regimes. Four suborders are recognized, Torrox, Ustox, Orthox, and Humox. If irrigated, Torrox and Ustox are highly suitable for taro production while Orthox and Humox, being more acid or infertile, require soil amendments for taro production. Oxisols are also found in the South Pacific in relatively high rainfall regimes. These soils are classified as Orthox in the Salomon Islands, Western Samoa, and the Loyalty Islands, and Orthox and Ustox in Fiji. Ultisols are also found in the Hawaiian Islands in close association with Ferralsols. Ultisols generally occur on steeper slopes than Oxisols but both Oxisols and Ultisols have essentially identical chemical and mineralogical properties.
Andisols comprise the bulk of agricultural lands in many islands of the Pacific Basin. In Hawaii, extensive areas of highly suitable Andosols for upland taro and vegetables are found in the Islands of Maui and Hawaii. Recent volcanic activity on the Island of Hawaii has produced extensive deposits of volcanic materials on which Andosols of varied characteristics have developed in climates ranging from warm and dry to warm and humid. In the South Pacific, Andosols occur in New Zealand, Fiji, Western Samoa, Vanuatu, and Papua New Guinea).
Soil Properties
Soil Depth. Taro requires deep planting for optimum corm set. Therefore, shallow soils provide with a restrictive environment for corm and root development. Shallow soils are customarily defined as those having rock or hard root-restricting layers within the upper 50 cm of the soil profile. Hard root-restricting layers include (i) clayey (more than 35% of expandable clay), salic, natric, calcic, petrocalcic, gypsic, and petrogypsic horizons, (ii) duripan and plinthite, or (iii) the presence of lithic or paralithic contact.
Water Retention Capacity. Taro has specific water needs which are met either from water stored in the soil or from rainfall and/or irrigation. In upland taro, the ability of a soil to store water becomes an important issue due to the high variability and unpredictability of rainfall. As a general rule, highly weathered, coarse-textured soils have low water retention capacity. Some clayey soils, particularly those having kaolinitic, halloysitic, ferritic, gibbsitic, or oxidic mineralogies, also exhibit low water retention capacity.
Soil Texture. Except in extremely clayey soils (more than 35% of expandable clay minerals), soil texture alone does not restrict root development. Physical impedance to root growth arises when soils are subjected to compaction. Because taro generally requires good soil physical conditions for optimum root and corm growth, agronomic practices such as land preparation, planting, spraying, and harvesting increase the likelihood of soil compaction. In general, clay content is not of primary concern in taro production because plant response to the prevailing substrate depends more upon the type of clay mineral dominating the exchange complex than the amount per se. Vertisols (more than 35% of expandable clay minerals), for example, are highly suitable for lowland taro but are less suitable for upland taro due to the adverse effects of swelling and shrinking on root and corm growth. Clayey soils of the tropics (well aggregated kaolinitic-oxidic soils with more than 60% clay), on the other hand, can be as suitable for taro or tanier production as any other soil providing that nutrients and water supply are kept at optimum.
Nitrogen Availability

It is probably realistic to state that taro growth is limited more often by N deficiency than any other nutrient. This is because the crop requires large quantities of N for plant growth and because most tropical soils are N deficient. Organic matter is the major source of N for taro in the tropics, particularly in the South Pacific where cropping systems are built around shifting cultivation. The organic matter content of tropical soils under native vegetation is generally comparable to that of temperate zones. The C/N ratios of tropical soils generally fluctuate between 10 to 15 which indicates a great deal of N is potentially available for plant growth. If brought to cultivation, crop production in these soils is feasible on a sustained basis providing that fertilizers and soil amendments are applied. The problem is that most soils are intensively cropped in subsistence systems with virtually no N fertilization and little organic matter restoration. Initially, the mineralization of the organic matter is usually sufficient to provide enough N for one or two crops. But as organic matter declines, the ability of the soil to supply N for plant growth also declines.
Overall View
Taro production in the tropics proceeds in a wide range of soils and environments. Lowland taro cultivation takes place on deep, somewhat fertile, poorly drained soils located on nearly level surfaces and exposed to seasonal flooding. Upland taro cultivation, in turn, is carried out in a wide spectrum of soils having two major characteristics, an udic or wetter soil moisture regime, and an isothermic or warmer soil temperature regime. Upland taro-growing soils range from the highly weathered, infertile soils of the lowlands to the volcanic-ash, well drained, organic-rich soils of the highlands.

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Manrique International Agrotech
P.O. Box 61145
Honolulu, HI 96839
Phone: (808) 732-4986
Phone (Mobile): (808) 285-3128
E-mail: manrique@lava.net

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