大陆阅读机经二

1讲的是土壤的分层结构，从地表到地下水这段可以分三层。文章对每层有详细的描述。

Topic: Soil Structure

A soil horizon is a layer parallel to thesoil surface, whose physical characteristics differ from the layers above andbeneath. Each soil type has at least one and most have three or four horizons.Horizons are defined in most cases by obvious physical features, chiefly colorand texture. These may be described both in absolute terms (particle sizedistribution for texture, for instance) and in terms relative to thesurrounding material, i.e.‘coarser’or‘sandier’than the horizons above andbelow. Water dissolves and removes nutrients as it passes through the soil.

Many soils, especially temperate soils,conform to a similar general pattern of horizons, often represented as an‘ideal’ soil in diagrams. Each main horizon is denoted by a capital letter,which may then be followed by several alphanumerical modifiers highlightingparticular outstanding features of the horizon. The general classificationsequence of O-A-B-C-R is fairly universal, though some variation exists betweenthe systems used in different parts of the world.

The "O" stands for organicmatter. It is a surface layer, dominated by the presence of large amounts oforganic material in varying stages of decomposition. The O horizon should beconsidered distinct from the layer of leaf litter covering many heavilyvegetated areas, which contains no weathered mineral particles and is not partof the soil itself. O horizons may be pided into O1 and O2 categories,whereby O1 horizons contain decomposed matter whose origin can be spotted onsight (for instance, fragments of rotting leaves), and O2 horizons containingonly well-decomposed organic matter, the origin of which is not readilyvisible.

The A horizon is the top layer of the soilhorizons or 'topsoil'. This layer has a covering of dark decomposed organicmaterials, which is called "humus". The technical definition of an Ahorizon may vary, but it is most commonly described in terms relative to deeperlayers. "A" horizons may be darker in color than deeper layers andcontain more organic material, or they may be lighter but contain less clay.The A is a surface horizon, and as such is also known as the zone in which mostbiological activity occurs. Soil organisms such as earthworms, potworms,insects, nematodes, fungi, and many species of bacteria are concentrated here,often in close association with plant roots. Thus the A horizon may be referredto as the biomantle. However, since biological activity extends far deeper intothe soil, it cannot be used as a chief distinguishing feature of an A horizon.

The B horizon is commonly referred to as"subsoil", and consists of mineral layers which may containconcentrations of clay or minerals such as iron or aluminium oxides or organicmaterial moved there by leaching. Accordingly, this layer is also known as the"illuviated" horizon or the "zone of accumulation". Inaddition it is defined by having a distinctly different structure orconsistency to the A horizon above and the horizons below. They may also havestronger colors (is higher chroma) than the A horizon. Plant roots penetratethrough this layer, but it has very little humus. It is usually brownish or redbecause of the clay and iron oxides washed down from A horizon.

The C horizon (parent rock) is simply namedso it comes after A and B within the soil profile. This layer is littleaffected by soil forming processes (weathering), and the lack of soildevelopment is one of the defining attributes. The C Horizon may contain lumpsor more likely large shelves of unweathered rock, rather than being made upsolely of small fragments as in the solum. "Ghost" rock structure maybe present within these horizons. The C horizon also contains parent material.It forms the framework of the soil. The A and B layers are formed by thislayer. The C horizon forms as bed rock weathers and rock breaks up intoparticles.

R horizons denote the layer of partiallyweathered bedrock at the base of the soil profile. Unlike the above layers, Rhorizons largely comprise continuous masses (as opposed to boulders) of hardrock that cannot be excavated by hand. Soils formed in situ will exhibit strongsimilarities to this bedrock layer.

1. Soil horizon can be defined by thefollowing factor EXCEPT