Some Diagnostic Subsurface Horizons in the Proposed Chinese Soil Classification System

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88 SOIL SURVEY HORIZONS Tucker, R.E., H.V. Alminas, and R.T. Hopkins. 1984. Geochemical evidence for mineralization on St. Thomas and St. John, U.S. Virgin Islands. USDI-GS Open-File Rep.

1 88 SOIL SURVEY HORIZONS Tucker, R.E., H.V. Alminas, and R.T. Hopkins Geochemical evidence for mineralization on St. Thomas and St. John, U.S. Virgin Islands. USDI-GS Open-File Rep. 84 (preliminary report). U.S. Virgin Islands Office of Planning and Research U.S. Virgin Islands Economic Indicators, V.1. Government. Some Diagnostic Subsurface Horizons in the Proposed Chinese Soil Classification System Xueming Yang, W.R. Effland, and M.L. Thompson 1 A revised system of Chinese soil classification has been proposed by a group of soil scientists from the Nanjing Institute of Soil Science, Academia Sinica (Soil Classification Group, 1985). This system contains two parts: (i) descriptions of diagnostic epipedons and subsurface horizons based on quantitative morphological and physicochemical characteristics; and (ii) a framework of known soil types organized by diagnostic epipedons, horizons, and other soil characteristics. Table 1 shows the comparisons of diagnostic subsurface horizons used in the U.S. Soil Taxonomy (Soil Survey Staff, 1975) and in the proposed Chinese system. The objective of this article is to describe some diagnostic horizons in the proposed Chinese system that may be related to the cambic horizon of Soil Taxonomy. The proposed Chinese system, in effect, replaces the cambic horizon with gleyed, phosphic, silica-alumina, and paddy agric diagnostic subsurface horizons. A companion article (Yang et al., 1988) compares the diagnostic epipedons of Soil Taxonomy and the proposed Chinese system. The number and types of diagnostic subsurface horizons proposed for the Chinese system vary from those in Soil Taxonomy. These differences occur because of the diversity in both distribution and occurrence of soils and varying experiences in soil classification. In the proposed Chinese system, some horizons may be more than one type of diagnostic horizon. For example, the paddy agric horizon may also meet the criteria of the illuvial argillic horizon. However, some diagnostic horizons take precedence over others. Thus, a soil with both a paddy agric horizon and an argillic horizon would be classified based on the paddy agric horizon's characteristics. The definitions of diagnostic subsurface horizons as well as the system itself are first approximations in need of testing and further revisions. IXueming Yang was a visiting scientist appointed as an associate lecturer, Agronomy Dep., Iowa State Univ. He is from Changchum, People's Republic of China where he teaches in the Soil Science and Agrochemistry Dep., Jilin Agric. Univ. At Iowa State Univ., Mr. Yang worked with the soil morphology and genesis group and studied the U.S. system of soil classification based on Soil Taxonomy. W.R. Effland is a research associate and M.L. Thompson is an assistant professor, Agronomy Dep., Iowa State Univ., Ames, IA

2 FALL Table 1. Comparison of diagnostic subsurface horizons in Soil Taxonomy and the proposed Chinese system. t Diagnostic horizon Albic Natric Spodic Agric Calcic Gypsic Salic Sulfuric Argillic IIIuvial Argillic Degraded Argillic Fragipan Argillic Pan Oxic Sesquioxide Carnbic Silica-Alumina G1eyed Paddyagric Phosphic Sajang Salic Pan Silica-Sesquioxide Placic Sombric Petrocalcic Petrogypsic Duripan t Occur in the system, - Chinese USA Not used in the system. Gleyed Horizon Notes Same names and similar meanings. Occur in Chinese system but not in U.S. Soil Taxonomy. Occur in U.S. Soil Taxonomy but not in Chinese system. A gleyed horizon is a subsurface horizon with strongly reducing conditions formed by reduction and movement of Fe and Mn during long periods of water saturation in the presence of organic materials. 1. Hue is loy or bluer, matrix chroma is :s 1, and color changes quickly if exposed to air. 2. Redox potential :s250 my; reduced Fe 2 [extracted with Al 2 (S04)3] is ~ 3070 of total Fe; ratio of crystalline Fe to total Fe is :s Without or rarely with mottles, Fe and Mn nodules, or pipestems. 4. Massive. 5. Organic matter content is >0.5%. The gleyed horizon is a diagnostic subsurface horizon for certain wet soils (a category at the order level of the proposed Chinese system). This kind of soil is located mainly in the northeastern plain and valley plain in

3 90 SOIL SURVEY HORIZONS Table 2. Properties of a meadow soil with a gleyed horizon (Nanjing Institute of Soil Science, 1975, p. 210). Hori- Struc- Mottles and Organic Clay zon Depth Color ture nodules ph matter <0.001 mm cm in A brown m few AB dark grey 1 gr both common Bg grey 1 gr few G grey and white m none the Nei Mongolia autonomous region and in the northeastern part of China. Parent materials are variable, with most being river deposits and some colluvial and lacustrine sediments. Climate is variable by area, but mean annual soil temperature ranges between 0 and 10 C (32 and 50 F). The period of freezing lasts 5 to 7 months, and depth of freezing is 1 to 3 m (3-10 ft). Table 2 presents some morphological properties of a meadow soil with gleyed horizons. The wet soils with a gleyed horizon might be classified by Soil Taxonomy in several aquic suborders, such as Aquolls, Aqualfs, Aquents, Aquepts, Aquults, and others. Phosphic Horizon A phosphic horizon is a phosphorus-accumulating horizon formed by decomposition of the fecal material of birds and the combination of phosphate with Ca in the soil. 1. Phosphate (P ) ~ and calcium carbonate (CaC0 3) equivalent ~ ~ 20 cm (8 in.) thick. 3. May occur as A or B or AB horizons. The soils with this horizon are distributed in islands in the South China Sea in tropical areas. The climate is maritime, with mean annual temperature > 26 C (79 OF); the difference of maximum and minimum temperatures in a year is only 4 to 6 C (7-11 OF). Annual precipitation is > 1500 mm (59 in.), and annual average evaporation is about 2500 mm (98 in.). Distribution of precipitation is monsoonal. Parent material is weathered rock coral. Aerial weathering of rock coral is relatively recent, and it is estimated that the age of rock that is 12 m (40 ft) above sea level is about 10,000 yr (Nanjing Institute of Soil Science, 1980). Weathering of rock coral is rapid because of its loose structure and relatively high surface area. The chemical composition of coral is dominated by CaC0 3, with the CaC0 3 content possibly exceeding 95%. Phosphic horizons have a ph between 8.0 and 9.0. They have as much as 12% organic matter in the surface and a value as high as 4% at a 50-cm (20-in.) depth. The accumulation of bird fecal material is the main source of organic material

FALL 1988 91 Table 3. Chemical properties of a phospho-calcic soil in a coral island in the Xisha Islands (Nanjing Institute of Soil Science, 1980, p. 629).

4 FALL Table 3. Chemical properties of a phospho-calcic soil in a coral island in the Xisha Islands (Nanjing Institute of Soil Science, 1980, p. 629). Organic Depth ph matter Total N P20S K20 CaO em in on coral limestone soils. Table 3 shows the chemical composition of a typical soil with a phosphic horizon. The phosphic horizon extends from 2 to 50 cm (1-20 in.). The primary soil-forming processes are (i) accumulation of organic material, (ii) dissolution of CaC0 3, and (iii) accumulation of P in calcium phosphate compounds. Soils with phosphic horizons may be classified as Psamments and Rendolls by Soil Taxonomy. Silica-alumina Horizon A silica-alumina horizon is a "B" or illuvial horizon that is dominated by 2: 1 clay minerals. This horizon formed mainly in northeastern China and the area north of the Yangtze River and east of the loess plateau. 1. Various brown colors, < rock fragments. 2. The 2: 1 and 2: 1: 1 clays dominant. 3. Silica/alumina ratio (Si0 2 / A1 20 3) of clay fraction> Cation exchange capacity (CEq >24 meq/loo g of clay. 5. Iron and Mn nodules or cutans may occur in humid soil moisture regimes. 6. Evidence of clay accumulation may occur but does not reach the minimum of the argillic horizon. 7. May be subdivided by base saturation (B.S.) into a. High B.S. silica-alumina horizon (> 50% B.S.). b. Low B.S. silica-alumina horizon «50% B.S.). This horizon is a diagnostic horizon for silica-alumina soils (order level). Silica-alumina soils cover a large area in China. Soils in the area have several soil moisture and temperature regimes, such as udic, ustic, and perudic moisture regimes and cryic, frigid, mesic, and thermic temperature regimes. Similar to a cambic horizon, this horizon may occur over an illuvial-argillic horizon, a degraded argillic horizon, or a calcic horizon (Soil Classification Group, 1985). Table 4 reports some of the chemical properties of a silicaalumina soil. The silica-alumina horizon (B*) occurs between 20 and 35 cm (8 and 14 in.) for this profile. The soils in the silica-alumina order have been classified according to Soil Taxonomy as Eutroboralfs, Hapludalfs, Paleudalfs, Haplustolls, Eutrochrepts, Hapludolls, and others.

5 92 SOIL SURVEY HORIZONS Table 4. Chemical properties of a silica-alumina soil. Cation Si0 2 Si0 2/ A120 3 Hori- ph Organic Base exchange Clay in clay for clay zon Depth (H 20) matter saturation capacity <0.001 mm fraction fraction A A2 B* Btl Bt2 Bt3 BC cm in OJo meq/ OJo 100 g soil Paddy Agric Horizon A paddy agric horizon is a special diagnostic horizon for paddy soils. 1. Below a plowpan or above a gleyed horizon if they exist. 2. Generally, the amounts of clay and silt are higher than those of the overlying horizon(s). 3. Macropores are commonly filled with irrigation water, while intrapedal pores tend to be air-filled. 4. Ped surfaces are covered with Fe and Mn cutans of brown, yellowish brown, and reddish brown; mottles are ~ 400/0 of matrix; Fe and Mn nodules often occur; the ratio of crystalline Fe to total Fe has the highest value in the profile. 5. Base saturation is higher than in the overlying horizons. More than 25 million ha (62 million acres) of paddy soils, accounting for 25% of the total cultivated land in China, are distributed over all of China. One-fourth of the world's paddy soils is located in China (Gong Zitong, 1986). The profiles of paddy soils result from the processes of eluviation during reducing conditions and illuviation during oxidizing conditions under longterm cultivation, fertilization, and irrigation. A characteristic sequence of horizons may occur in the profile of a paddy soil (Gong Zitong, 1986). The horizons are: 1. Plowed horizon: 12 to 18 cm (5-7 in.) thick, a zone where root systems of crops are concentrated. 2. Plow pan: formed by the compression of plowing, 8 to 10 cm (3-4 in.) thick, weak platy structure, high water retention capacity, and bulk density about 1.5 g/cm 3 3. Paddy agric horizon: see criteria above. 4. Gleyed horizon: mayor may not be present. Gong Zitong (1986) postulates that the processes of paddy soil formation include the following: (i) mechanical leaching; (ii) dissolution of salts; (iii) reductive eluviation of Fe, Mn, Co, and Ni; (iv) chelation; and (v) ferrolysis (Brinkman, 1979).

6 Table 5. Properties of a paddy soil (Xu Qi et al., 1980, p. 15). Hori- Mottles and zon Depth Color nodules cm in. A Y 6/1 few Fe-humic coatings and rust streaks P loy 5/1 few W Y 711 common distinct Bg Y 7/4 common Particle-size distribution (mm) ph <0.001 (H2O) 0, Organic Total matter P 0, Cation exchange capacity meq/l00 g 'Tl > t'"' t'"' -\ \0 w

7 94 SOIL SURVEY HORIZONS Soils with paddy agric horizons (paddy soils) could be classified in a wide range of aquic suborders in Soil Taxonomy. Table 5 presents some properties of a paddy soil with a paddy agric horizon. The horizon designated W* (38-65 cm, in.) might be identified as a paddy agric horizon in the proposed system. References Brinkman, R Ferrolysis, a soil-forming process in hydromorphic conditions. Agric. Res. Rep PUDOC, Wageningen, Netherlands. Gong Zitong Origin, evolution and classification of paddy soils in China. p In B.A. Stewart (ed.) Advances in soil science, Vol. 5. Springer-Verlag, New York, New York. Nanjing Institute of Soil Science, Academia Sinica Soil geography of Nei Mongolia and the west part of the northeast. (In Chinese.) Science Press, Beijing, People's Republic of China Soils of China. 2nd ed. Science Press, Beijing, People's Republic of China Soil resources in Heilongjiang Province and the northeast of Nei Mongolia. (In Chinese.) Science Press, Beijing, People's Republic of China. Northeastern Institute of Forest Soil Science, Academia Sinica Soils of the Northeast China. Science Press, Beijing, People's Republic of China. Soil Classification Group of the Nanjing Institute of Soil Science Preliminary draft of Chinese soil taxonomy system. Soil Sci. (In Chinese.) 6: Soil Survey Staff Soil taxonomy: A basic system for making and interpreting soil surveys. USDA-SCS Agric. Handb U.S. Gov. Print. Office, Washington, DC. Xu Qi. Lu Yan-chun, Liu Yuan-chang, and Zhu Hong-guan The paddy soil of Tai-Hu Region in China. Shanghai Scientific and Technical Publ., Shanghai, China. (In Chinese and English.) Yang, X.M., W.R. Effland, and T.E. Fenton Some new diagnostic epipedons for soil classification in China. Soil Surv. Horiz. 29; Hydric or Nonhydric? That is the Question! Michael D. Sweeney 1 A hydric soil is defined as a soil that is saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions in the upper part (National Technical Committee for Hydric Soils, 1987). One criterion for hydric soils is that they be in Aquic suborders or subgroups. The Aquic moisture regime implies a reducing regime that is virtually free of dissolved oxygen because the soil is saturated by groundwater or by water of the capillary fringe (Soil Survey Staff, 1987). For differentiation in the highest categories of soils that have an Aquic regime, the whole soil must be saturated. In the subgroups, only the lower horizons are saturated. The requirements of the swampbuster provision of the 1985 farm bill have led scientists to take a critical look at the criteria for defining wetlands. (Associate professor; Soil Science Dep.; North Dakota State Univ., Fargo, ND

Temperature and Precipitation. Next stage will depend on climate and parent materials

Temperature and Precipitation. Next stage will depend on climate and parent materials Next stage will depend on climate and parent materials Rainfall and temperature will determine nature of vegetation Resistance of parent material to weathering will determine rate and type of clay formation,