LABORATORY 1 SOIL TEXTURE I Objectives Determine soil texture by mechanical analysis using the pipette method. Estimate soil texture by the feel method. II Introduction A General Soil texture is the relative proportion of sand, silt, and clay in a soil. Within each of these soil separates there is a continuum of particle sizes. Thus, there is a particle size distribution in soil ranging from the largest sand particles to the smallest clay particles. Texture is considered to be a permanent characteristic of a soil since weathering only very slowly changes particle size. Furthermore, cultivation and other management practices do not alter the sizes of individual soil particles. In some cases, however, erosion or deposition may rapidly alter the particle size distribution. Texture is an important property of soils because particle size determines the surface area of solids per unit volume or mass of soil. Texture also influences the pore size distribution in soil. A sandy soil is dominated by large individual soil particles and, therefore, has a relatively small total surface area and large pore spaces between soil particles. At the other extreme, a clay soil consists of tiny individual particles and has a large total surface area but small pore spaces. However, small soil particles tend to be associated with one another to form aggregates of soil particles. Thus, the pore space in a clay soil consists of small intraaggregate pores and much larger interaggregate pores. In fact, the total volume of pore space per unit volume of soil (porosity) of a sandy soil is actually smaller than the porosity of a clay soil. Many chemical and physical processes in soil occur at the surface of soil particles. For example, the surface of soil particles is the site of a physicochemical reaction known as ion exchange. The negative (and, in some cases, positive) charge carried by certain soil particles is balanced by surface adsorbed cations (or anions). In ion exchange, an ion in solution exchanges for a surface adsorbed ion of like charge. Ions adsorbed in this way serve as a reservoir of nutrients for plant growth. -1Adsorbed ions also complicate the matter of determining soil texture by mechanical analysis. Adsorbed ions, though in very close proximity to the surface of soil particles, are often not really bound to the soil particles. Rather, these exist as a diffuse layer Bhighest concentration nearest the surface and decreasing moving away from the surface. The nature of this diffuse layer is affected by the type of ion and the total concentration of ions in the soil solution. In turn, the interaction of adjacent soil particles is affected by the diffuse layer. For example, small soil particles in a concentrated solution of Ca2+ tend to be closely associated with one another (flocculated), whereas in a dilute Na+ solution, small soil particles tend to be independent of one another (dispersed). It is crucial in a mechanical analysis to have soil particles thoroughly dispersed and independent. B Definition of Soil Separates Separate Diameter (mm) Very coarse sand 2.00 - 1.00 Coarse sand 1.00 - 0.50 Medium sand 0.50 - 0.25 Fine sand 0.25 - 0.10 Very fine sand 0.10 - 0.05 Silt 0.05 - 0.002 Clay Less than 0.002 Particles larger than 2.0 mm in diameter are not considered in determining soil texture. C Principle Underlying a Mechanical Analysis Soil particles suspended in solution settle out at a rate that depends on the size of the particles Bthe larger the particle, the faster it settles. Settling rate is given by Stokes= Law v = (2/9) (DS - DL) g r2 / η where v is settling velocity (cm / s) DS is particle density (g / cm3) DL is water density (g / cm3) g is acceleration due to gravity (cm / s2) r is radius of particle (cm) η is water viscosity (g / cm-s) -2Given that v is distance / time, the time required for all sand-sized and all silt-sized particles to settle a distance of 10 cm in water at room temperature can be calculated. Substituting appropriate values in Stokes= Law gives Very fine sand (0.050 mm) 48 s Silt (0.002 mm) 8 h Thus, if you completely disperse a sample of soil in water and agitate the suspension so that at time zero sand, silt and clay particles were uniformly distributed in the water, an aliquot of the suspension taken at 48 s and above the 10 cm depth will contain only silt and clay particles, no sand. Similarly, an aliquot taken at 8 h will contain only clay particles. From these two aliquots, percentages clay, silt and sand (since percentages clay, silt and sand sum to 100) can be determined. D Use of Textural Triangle to Determine Textural Class Soils are grouped into textural classes on the basis of the percentages of sand, silt, and clay. Soils within each textural class have similar soil properties and the class name reflects the relative influence of each soil separate on the properties of that soil. For a soil to be called a sand, it must contain over 85% sand, but soil must contain only 40% clay in order to be classified as a clay. Loams have properties resulting from about equal influences of sand, silt and clay, but the loams contain more sand and silt than clay. The textural triangle (Figure 1) contains twelve textural classes, each of which can be comprised of various combinations of sand, silt, and clay. Each leg of the triangle represents either sand, silt, or clay and is divided into percentages from 0 to 100% since a soil may contain any percentage of a soil separate as long as the total percentage of the three separates equals 100. Each leg of the triangle then represents the base or 0% value for one of the three soil separates. To determine the textural class of a soil based on a mechanical analysis, draw a line at the appropriate percentage for each separate across the triangle, parallel to the base for that separate (one line parallel to each base). If done correctly, the three lines will intersect at one point within a section of the triangle containing the correct textural class name. -3III Procedures A Mechanical Analysis by the Pipette Method To determine the relative masses of sand, silt and clay in the soil sample (thus, the texture) the combined mass of silt plus clay in the first aliquot and the mass of clay in the second must be determined. Also, the