北西部九州における赤色土および水田土壌の微細形態に関する2,3の知見(20周年記念号)

抄録

The present paper deals with micromorphological properties of the Red soils in northwestern Kyushu, changes in soil structure of paddy soils and young polder soils by farm mechanization, and morphological, physical and micromorphological properties of some paddy soils under the different conditions of rice culture in the Chikugo, the Saga and the Isahaya areas of northern Kyushu. The results obtained are summarized as follows: 1. The A_0 horizons in Red soils are characterized by a silicate moder form and by a predominance of large pores. There are three kinds of fecal pellets in the insects or mite-bitten cavities of plant remains or among partially decomposed plant fragments. One is relatively small (0.01-0.2 mm in size), pale yellow, yellowish brown or reddish brown and dense, egg-like in shape and interspersed with comminuted plant remains in the A_0 horizons of the soils. The second one present among partially-decomposed plant leaves in the A_0 horizon is small (0.05-0.8mm in size), yellowish brown, reddish brown and brown, and irregulary massive in shape and dominated by fine mineral grains. The last one is the fecal pellets and microaggregates (>4 mm and 4-1 mm in size) of earthworms among decomposed plant leaves in the A_0 horizons. The fecal pellets of earthworms in the A_0 horizon of the Setaka soil have higher pH value (about 6), and higher nitrogen contents (about 0.6) than those of the A horizon of the soil (pH value is 4.8 and total nitrogen is 0.1 per cent). CEC, contents of exchangeable bases (calcium, magnesium, potassium and sodium) and base saturation degree in the fecal pellets of eathworms are 47.8 me, 3.4 me, 1.9 me and 60 per cent, respectively. The occurrence of these fecal pellets indicates that litter-feeding soil faunas such as mites, springtails, macroarthropodes and earthworms play an important role in the mechanical comminution and the chemical decomposition of leaf litter at early stage of humification and in biological circulation of soil elements. 2. All the A horizons are characterized by a mull-type humus and porphyroskelic fabrics and are dominated by relatively large pores. The B horizons have a weakly-developed spongy and moderately- and strongly-developed porphyroskelic fabrics and are more compact than the A horizons. Plasmic fabrics of the B horizons are silasepic, skel-silasepic, insepic and mosepic. The B horizons of the Red soils examined contain abundant oriented clays than do the A horizons. Layered, moderately- and strongly-developed oriented clays were mainly found on pore walls. These oriented clays resemble the illuviation cutans. 3. As a result of the land preparation of the paddy soil by using bulldozer, the past plowed horizon (the buried IIA_<11>gG horizon) has become more compact than the upper and the lower horizons. The past plowed horizon (the buried IIA_<11>gG horizon) has a strongly-developed porphyroskelic fabric and a fluidal structure. Low permeability of paddy soil is due to the occurrence of the compacted IIA_<11>gG horizon formed by using bulldozer. 4. According to a classification system of rice soils devised by I. Kanno, paddy soils in the Chikugo, the Saga and the Isahaya areas of northern Kyushu have been classified as "Intermediate Gley-like Rice soils" and "Surface-water Gley-like Rice soils" at the level of genetic soil type, respectively. Those are subdivided into the following subtypes, that is, "Imperfectly-drained gley-like rice soils", "Drained gley-like rice sois" and "Well-drained gley-like rice soils". Micromorphological features of the soils are summarized as follows: (a) All the uppermost horizons have moderately-developed porphyroskelic and weakly-developed spongy fabrics and are dominated by relatively large pores, and nodules and mottles that are composed of iron and manganese. The plowsoles of the seven soils have

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