Study on Micro-scale Variation of Soil pH

Abstract

土壤在物理、化學和生物等性質均具空間變異性，其中植物根系的生長以及對養分的吸收，直接受土壤pH微尺度分佈影響，且作物根系多生長在大孔隙處，因此瞭解土壤pH在微空間之分佈變異情況是重要的。而土壤pH微空間分佈變異受根系分泌物、有機質分解、礦物風化和淋洗作用等因素的綜合作用結果，因此若將土壤樣品混合成土糊，以玻璃電極測得之土壤pH值，並無法得知自然土壤中pH的微空間分佈情形。本研究以台灣中部地區主要土綱為代表，利用ISFET pH微電極測定長期未耕作土壤大、小孔隙之pH分佈情形，及相關重要土壤理化性質，進而探討土壤pH微空間分佈變異成因與變異範圍，並研究分析水田及旱田兩種耕作管理方式對土壤微空間中pH變異之影響。研究結果指出，長期受雨水淋洗的未耕作土壤，其土壤大孔隙周圍陽離子被洗失程度較小孔隙為大，所以土壤大孔隙之pH值較小孔隙低，亦較傳統以1：1水土比法所測得者低；且同一土壤中深度20-30公分處之土壤pH微空間變異大於30-40公分處的變異。因入滲水量之差異，處於平坦地形之土壤pH微空間分佈會較陡峭地形土壤之pH微空間變異大。水田或旱田的土壤pH微空間分佈變異程度均小於長期未耕作土壤，但水田因注入大量富含鹽基離子灌溉水之影響，土壤大孔隙之pH值會高於小孔隙，試驗所採集之極育土(Ultisol)水田土壤大孔隙pH平均值比1：1水土比方式所測得者約高0.5個單位，而氧化土(Oxisol)與弱育土(Inceptisol)亦分別約高0.4與0.2個單位。旱田土壤因少灌溉及施肥之影響，土壤大孔隙之pH值低於小孔隙，極育土(Ultisol)旱田土壤大孔隙pH平均值比1：1水土比方式所測得者約低0.6個單位，而氧化土與弱育土亦分別約低0.5與0.4個單位。本研究中所探討之各土綱土壤的pH微空間分佈變異程度大小依序為：大里系、翁子系和社腳系等極育土 > 吳厝系和陳厝寮系等氧化土 > 二林系、平和系、圳寮系、濁水系、座駕系和詔安系等弱育土。由本研究之結果顯示，淋洗作用是導致土壤pH微空間變異的主要機制，因此土壤中之重要理化性質以總體密度、孔隙度及鹽基飽和度對土壤pH微空間變異的影響較為顯著。在非石灰性土壤，土壤質地層理差異大且土壤斑紋含量多的土壤，其土壤pH微空間分佈變異大。There are spatial variations of physical, chemical, and biological properties in soil environment. The growth and nutrient uptake of plant roots are directly influenced by soil pH. Additionally, growth and development of plant roots are mainly in soil's macro-pores. Root excretions, organic matter decomposition, mineral weathering, and leaching are the four major factors affecting micro-scale soil pH variation. It is, thus, important to understand the micro-scale variations of soil pH. The pH of soil pastes measured by glass membrane electrodes can't represent the micro-scale variation of pH in soil environment. In the present study, the soil pH in macro- and micro-pores were measured by an ISFET pH microelectrode. The associated physical and chemical properties were also determined. Further, the causes of variation, ranges of micro-scale soil pH variation at three major soil orders in central Taiwan were discussed. The effects of upland and paddy field cultivation on these distributions were also investigated. The results indicated that macro-pore soil pHs were significantly lower than those of micro-pores in long-term uncultivated lands as a result of cations leaching from macro-pores were greater than micro-pores. The macro-pore soil pHs were also lower than those measured by soil paste. Moreover, micro-scale variation of soil pH was greater at depth of 20-30 cm than at that of 30-40 cm. Due to the difference in rain infiltration, micro-scale variation of soil pH was greater in flat than in slope topography. The extents of micro-scale variation of soil pH of both upland and paddy fields were smaller than that of long-term uncultivated field. However, the pH of macro-pore was greater than paddy fields. This is due to the large amount cations input by irrigation water. The means of measured macro-pore pH of paddy soils of Ultisol, Oxisol, and Inceptisol were 0.5, 0.4, and 0.2 units higher than 1:1 soil paste, respectively. The pH of macro-pore was lower than upland fields. Due to lesser irrigation and fertilizer application, the means of measured macro-pores pH of dry farmland soils of Ultisol, Oxisol, and Inceptisol were 0.6, 0.5, and 0.4 units lower than 1:1 soil paste, respectively. The order of micro-scale soil pH variation of investigated soils were: TTr, TWz, and TCo series of Ultisol > TWt and CCe series of Oxisol > Eh, Ph, Cl, Co, Ts, and Sn series of Inceptisol. To sum up, leaching is the main mechanism causing micro-scale soil pH variation. Consequently, the related soil factors, such as bulk density, porosity, and base saturation of soils prominently affect micro-scale soil pH variation. Moreover, for non-calcareous soils, the variation of micro-scale soil pH increased as texture stratification and mottles in soil increased.封面 謝誌 摘要 目錄 表次 圖次 緒言 第一章 土壤pH微空間分佈變異假說 土壤pH微空間分佈變異產生之原因 影響pH微空間分佈差異之因子 土壤pH微空間分佈變異假說之推論 土壤大小孔隙判別方法與pH測定技術 參考文獻 第二章 長期未耕作土壤pH微空間分佈差異之探討 前言 材料與方法 結果與討論 結論 參考文獻 第三章 台灣主要農耕土壤pH微空間分佈變異與影響因素 前言 材料與方法 結果與討論 結論 參考文獻 總結 附錄1~26各土壤樣品質地剖面資