单项选择题

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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 tile 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 kinds of ways such as heavy, 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 simply method called "manual texturing", through determining the feel of a moist sample when rubbed 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 is predominantly 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 through 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 a golf ball.
It can be inferred from the passage that a soil sample with little or no clay in it ______.