Genesis and properties of non-allophanic andisols in Japan

Abstract

This paper is a review of recent studies on the genesis and properties of non-allophanic Andisols in Japan. Non-allophanic Andisols have the udic soil moisture regime and show mostly thick, very dark humus horizons, reflecting repetition of intermittent ash deposition and humus accumulation. It is not possible to differentiate non-allophanic Andisols from allophanic ones in the field but it should be possible to differentiate them in the laboratory. Non-allophanic Andisols show absence or virtual absence of allophane and imogolite and abundance of 2:1 layer silicates in the clay fractions. It is considered that formation of allophane and imogolite is inhibited by supply of a large quantity of organic matter from the vegetation under low pH (about 5 or less) conditions. Al ions released from the parent ash form a stable complex with humus under this weathering condition. Therefore, active Al of non-allophanic Andisols consists largely or wholly of an Al—humus complex. Various hypotheses have been proposed concerning the sources of 2:1 layer silicates. Though the source problem has come to no definite conclusion, it is noted that several distinctive properties of non-allophanic Andisols are attributable to 2:1 layer silicates as described below. Non-allophanic Andisols show many characteristic physical and chemical properties such as low bulk density, high water retention, a large quantity of available water, remarkable accumulation of organic carbon, high CEC dominated by variable charges, low base saturation, high fluoride reactivity, large phosphate retention, etc. These properties are also common to allophanic Andisols. However, there are significant differences between non-allophanic and allophanic Andisols. Non-allophanic Andisols have an Al—humus complex as the main active Al, very strong soil acidity, high KCl exchangeable Al, high Al toxicity, and a critical pH (H 2O) of about 5.