A Review of Properties , Nutrient Supply, Cultivation and Management of Volcanic Soils, \\1th particular reference to Papua New Guinea.

Abstract

Soils derived from volcanic ash are called andisols. Andisols have good physical properties and are able to resist erosion because of the stable nature of dominant clay minerals such as allophane in them. However, they have a poor ability to supply nutrients. The dominance of allophane, allophane - organic matter complexes, and dependency of cation and anion exchange mechanism on soil pH contribute to the poor chemical fertility of volcanic ash soils. Such features give peculiar characteristics to andisols and make their management difficult. Intensive weathering of the ash on volcanic soils has resulted in losses of Si and bases. Si may combine with Al or Fe to form allophane, imogolite, gibbsite or halloysite, depending on the stage of weathering and prevailing climates. These clay minerals form complex associations with organic matter, and together they influence the charge characteristics of andisols. The review of research on this soil type indicates that the principle constraints to agricultural production are likely to be imposed by chemical properties. The specific chemical problems of weathered andisols are high P-fixation, low pH, low available K, Mg, Ca, low BS(%), low ECEC, deficient B, Mo, Zn and suboptimal levels of Mn, Cu and Fe. The evidence is overwhelming that the management of organic matter is the key to sustaining agriculture on this soil type. It results in increasing the magnitude of the negative charged sites, enhances the ECEC, supplies a wide range of macro and micro nutrients, as well as maintaining the soil physical and chemical fertility. The review identifies pitfalls in the management of andisols and discusses practical and cost effective ways of tackling them.