| 土壤是流域中的基本資源，除非得經過很長的一段時間，它是無法恢復的自然資源。可能需要花超過一個世紀的時間，土壤厚度才能增加1公釐，更可能需要數千年的成土作用，才能產生出足夠的土壤來供養高收益、高質量的森林、草地或農作物。要保持住土壤的功用和生產力，就必須細心地管理和保護好土壤。因為土壤中含有營養物質滋養植物，且保留水分，所以土壤是確保流域興旺的一個關鍵因素。
土壤是地殼薄薄的表層，它由各種礦物顆粒和有機物所組成。土壤的形成是以下各種作用的產物：風和水的風化作用，在重力作用下岩石從山丘上往下崩落，夏季的熱脹以及隨後的冷縮作用，地衣和其他植物所促生的化學作用等。
土壤按照其形成方式，可以分成兩種。
▲ 碎屑沈積土壤是由下面的"母"岩構成物所形成的，這種構成物會碎裂並受到侵蝕，和表面的植物層混合在一起。它們為該地表面的植物層提供養分，通常位於流域海拔較高的地方。
▲ 被"搬運"來的土壤就是指因重力、風或水的作用而被搬運到另一個地方的土壤。這樣的土壤通常位於谷地和洪水沖積而成的平原。這種土壤是流域中種植作物的最佳土壤，因?裏面通常含有更多的分解了的有機物。
氣候對土壤的形成有很大的影響。降雨導致了溢散流失作用－即溶解的微粒通過水滲入土壤。由於流水具有侵蝕力，降雨也會輸送土壤。在決定流域內種植那些植物來建立保護性的植被層一事上，土壤有重要作用。
植物還可以改善土壤、滋養土壤。植物的根部形成土壤空隙，這增加了 土壤的貯水能力。植物腐爛的葉片增加了土壤裏的有機物質，也增加了貯水能力。植物的殘餘物還減緩了表層土的流失，防止土壤表面遭受降雨的侵蝕。陡峭斜坡的土壤深度和貯水能力通常比較低，而且植物的生長率也往往比較慢。
版權歸屬 國際河網 環境信託基金會(陶俊 譯，吳國玢、陳維立審校)
中英對照全文：http://news.ngo.org.tw/issue/water/issue-water-irn00103001.htm
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Soil is a basic catchment resource that, except over a long period of time, is non-renewable. It may take more than a century to produce a single centimeter of soil and thousands of years to produce enough soil to support a high-yield, high-quality forest, grassland or agricultural crop. Careful management and protection of soil is necessary to preserve its function and productivity. Because soil carries plant nutrients and holds water, it is a key element in a catchment's health.
Soil is a thin layer of the earth's crust composed of mineral particles and organic matter. It occurs as the result of wind and water erosion, gravity rockfall from hills and mountains, rock minerals in rain water, heating expansion and cooling contraction in summer, and the chemical action of lichens and other plants.
Soils can be divided into two types based on how they are formed.
▲ Residual soils are formed from underlying 'parent' rock formations which break up, erode and are mixed in with surface plant cover. They support the local surface plant cover and are usually found at higher elevations in the catchment.
▲ Transported soils are soils which are moved by gravity, wind or water to a different location. These soils are usually found on the valley floor and floodplain. Transported soils make the best soil for farming in the catchment, as they are usually higher in decayed organic matter.
Climate strongly affects soil formation. Rainfall causes leaching - the movement of dissolved particles through soil by the water. Rainfall also transports soils, through the erosive power of runoff. Soil plays a major role in determining which plants will establish a protective vegetative cover in the catchment.
Plants also modify and develop the soil. Plant roots create soil spaces, which help increase a soil's ability to store water. Rotting leaves from plants add organic matter to soil, which also increases water-holding capacity. Plant debris also slows surface runoff and protects the soil surface from rainfall-caused erosion. Soil depths and moisture-holding capacities are usually lower on steep slopes, and plant growth rates are often slower.
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