土壤质地文献译文.docx

SOIL TEXTURESoil texture is a term commonly used to designate the proportionate distribution of the different sizes of mineral particles in a soil. It does not include any organic matter. These mineral particles vary in size from those easily seen with the unaided eye to those below the range of a high-powered microscope. According to their size, these mineral particles are grouped into separates.A soil separate is a group of mineral particles that fit within definite size limits expressed as diameter in millimeters. Sizes of the separates used in the USDA system of nomenclature for soil texture are shown in Table 1 .Since various sizes of particle have quite different physical characteristics, the nature of mineral soils is determined to a remarkable degree by the particular separate that is present in larger amounts. Thus, a soil possessing a large amount of clay has quite different physical properties from one made up mostly of sand and/or silt. The analytical procedure by which the percentages of the various soil separates are obtained is called a mechanical analysis. Mineral soils (that is, those soils consisting mainly of rock and mineral fragments, rather than plant remains and other accumulated organic materials) are a mixture of soil separates, and it is on the basis of the proportion of these various separates that the textural class names of soils are determined. There are twelve major textural classes. Their compositions are defined by the USDA textural triangle ( Figure 1 ).Determination of Soil Texture in the FieldIn the field, the percentages of sand, silt, and clay particles in a soil are estimated by feel. The soil is rubbed between the fingers and the thumb and an estimate of the amount of the various separates present is made based on the degree to which the characteristic properties of each are expressed. This process of estimation requires skill and experience, but accuracy can be improved by frequent checks of such estimates against the finding