mineral particles – AEAS精读 (Y10-12)

The mineral particles found in soil range in size from microscopic clay particles to large boulders. The most abundant particles — sand, silt, and clay — are the focus of examination in studies of soil texture. Texture is the term used to describe the composite sizes of particles in a soil sample, typically several representative handfuls. To measure soil texture, the sand, silt, and clay particles are sorted out by size and weight. The weights of each size are then expressed as a percentage of the sample weight. In the field, soil texture can be estimated by extracting a handful of soil and squeezing the damp soil into three basic shapes; (1) cast, a lump formed by squeezing a sample in a clenched fist; (2) thread, a pencil shape formed by rolling soil between the palms; and (3) ribbon, a flatfish shape formed by squeezing a small sample between the thumb and index finger. The behavioral characteristics of the soil when molded into each of these shapes, if they can be formed at all, provide the basis for a general textural classification. The behavior of the soil in the hand test is determined by the amount of clay in the sample. Clay particles are highly cohesive, and when dampened, behave as a plastic. Therefore the higher the clay content in a sample, the more refined and durable the shapes into which it can be molded. Another method of determining soil texture involves the use of devices called sediment sieves, screens built with a specified mesh size. When the soil is filtered through a group of sieves, each with a different mesh size, the particles become grouped in corresponding size categories. Each category can be weighed to make a textural determination. Although sieves work well for silt, sand, and larger particles, they are not appropriate for clay particles. Clay is far too small to sieve accurately; therefore, in soils with a high proportion of clay, the fine particles are measured on the basis of their settling velocity when suspended in water. Since clays settle so slowly, they are easily segregated from sand and silt. The water can be drawn off and evaporated, leaving a residue of clay, which can be weighed.

 

在土壤中发现的矿物颗粒大小不等,从微小的粘土颗粒到大石块。最丰富的颗粒—沙子、泥沙和粘土—是土壤结构研究的重点。纹理是用来描述土壤样品中颗粒的复合大小的术语,通常是几个有代表性的颗粒。

为了测量土壤质地,将沙子、泥沙和粘土颗粒按大小和重量进行分类。然后,每种大小的权重都表示为样本权重的百分比。在田间,提取少量土壤,将湿土挤压成三种基本形状,即可估算出土壤质地;(1)铸型,握紧拳头挤压样品而形成的肿块;(2)线,一种铅笔形状,由滚动的土壤在掌心之间形成;(3)缎带,一种比目鱼形状,由拇指和食指挤压一小块样本形成。当土壤被塑造成每一种形状时,如果它们可以被塑造,那么它们的行为特征就为一般的纹理分类提供了基础。手测土壤的行为是由样品中粘土的量决定的。粘土颗粒粘性很强,受潮时表现为塑料。因此,样品中的粘土含量越高,其成型的形状就越精致和耐用。

另一种确定土壤质地的方法涉及到使用一种叫做沉淀物筛的设备,这种筛是用指定的网格大小建造的。当土壤通过一组筛子过滤时,每个筛子都有不同的筛孔尺寸,这些颗粒就会按相应的尺寸分类。每个类别都可以通过称重来确定纹理。虽然滤网能很好地过滤泥沙、沙子和较大的颗粒,但不适用于粘土颗粒。粘土太小,无法准确筛出;因此,在粘土含量较高的土壤中,细颗粒悬浮在水中时的沉降速度是测量其沉降速度的基础。由于粘土沉降很慢,所以很容易与泥沙分离。水可以被抽走并蒸发,剩下的粘土残渣可以称重。