The soil species of paddy soil are generally classified on the basis of the patterns of solum structure and the development degree of the diagnostic horizon.Therefore,the basic horizons of paddy soil should be established and the criteria for division of the horizons in soil solum should be defined in advance.Owing to the different patterns of solum structure,the developmant degree of the diagnostic horizon in subsoil is different.For example,in addition to the differences in morphological features,the periodically waterlogged paddy soil with moderately developed percogenic horizon is rich in crystalline iron and higher in ratio of crystalline iron to the amorphous iron in the horizon;and the soil with weakly developed percogenie horizon is commonly less in crystalline iron a.nd lower in the ratio of crystalline iron to amorphous iron in the horizon;while the soil with well developed percogenic horizon is rich in amorphous iron in the horizon.