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  3. Soil quality is one of the most basic and perhaps least understood indicators of land health. Soil supports plant growth and rep

Soil quality is one of the most basic and perhaps least understood indicators of land health. Soil supports plant growth and rep

admin2010-05-25  77
问题      Soil quality is one of the most basic and perhaps least understood indicators of land health. Soil supports plant growth and represents the living reservoir that buffers the flows of water, nutrients, and energy through an ecosystem. Soil quality refers to the capacity of a soil to perform these beneficial functions and is determined by its texture, structure, water-holding capacity, porosity, organic matter content, and depth, among other properties. Because soils naturally vary in their capacity to perform these functions, we must tie our understanding of soil quality to landscapes and land use. We must understand soil quality for two important reasons: First, we must match our use and management of land to soil capability. Second, we must establish baseline understanding about soil quality so we can recognize ongoing trends. If soil quality is stable or improving, we have a good indicator that the ecosystem is sustainable. If soil quality is deteriorating, the larger ecosystem will almost certainly decline with it.
     People describe soil types in all kind of ways such as beavy, light, sandy, clay, loam, poor or good. Soil scientists describe soil types by how much sand, silt and clay is present. This is called texture. It is possible to change the texture by adding different things to the soil. You can roughly estimate the texture of a soil by a simple method called" manual texturing", through determining the feel of a moist sample when robbed between the thumb and forefinger. If the soil sample is predominantly sand, it will feel very gritty, If it is predominantly silt, it will feel smooth or slippery to the touch. And if it’s predominant clay, it will feel sticky.
     Particle size has a lot to do with a soil’s drainage and nutrient holding capacity. Sand is the largest particle and doesn’t hold many nutrients. Silt is a soil particle whose size is between sand and clay. When wet, silt feels smooth, but not sticky. Clay is smooth when dry and sticky when wet. Soils high in clay content are called heavy soils. Clay also can hold a lot of nutrients, but doesn’t let air and water throuhgh it well. To better understand how big these three soil particles are, think of them like this. If a particle of sand were the size of a basketball, then silt would be the size of a baseball, and clay would be the size of golf ball.
What does the passage mainly discuss?
选项 A、Characteristics of soil quality.
B、Particles typically found in soil.
C、The importance of soil texture.
D、How to determine soil texture.
答案A
解析 主旨题。根据文章对土质相关因素的阐述，即土质的分类、土质中分子的大小，可知本文主要讨论了土质的特点，故选A
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