Abstract
New factors controlling nature of organic matter in paddy soils appear successively with time during long term utilization of alluvial soils in paddy field. Characteristics of organic matter of immature paddy soils derived from alluvial deposits are more or less dependent on nature and amount of debris of aquatic organisms which are sedimented with mineral particles. In the case of lake deposits, phytoplankton as well as reed are shown to be main sources of organic matter of the soils, which is verified by examining organic matter content, content of chlorophyll like substances, C/N ratio, free iron content and soil texture. Oxidative destruction of the chlorophyll like substances and of mineralizable nitrogen compounds contained in the soils is concurrently caused by lowering ground water table, but the former is more intensive than the latter. Comparison of these two values suggests a possible method to evaluate degree of oxidative condition developed in the paddy soils. Long term farming of the alluvial soil in paddy field with low ground water table is supposed to allow furrow slice to reach a stationary state in respect to organic matter content. This organic matter content of the furrow slice is found to be positively correlated with the capacity of phosphate retention of the furrow slice.
