Texture refers to the relative proportions of particles of various sizes such as sand, silt and clay in the soil. The proportions of the separates in classes commonly used in describing soils are given in the textural triangle shown in Fig. 3. In using the diagram, the points corresponding to the percentages of silt and clay present in the soil under consideration are located on the silt and clay lines respectively. Lines are then projected inward, parallel in the first case to the clay side of the triangle and in the second case parallel to the sand side. The name of the compartment in which the two lines intersect is the class name of the soil in question. For examples soil containing 15% clay, 20% silt and 65% sand is sandy loam and a soil containing equal amounts of sand, silt and clay is clay loam.
The percentages of sand, silt and clay in a soil could be determined in a soil laboratory by two standard methods - hydrometer method and pipette method (Black et al., 1965a). Both methods depend on the fact that at any given depth in a settling suspension the concentration of the particles varies with time, as the coarser fractions settle at a faster rate than the finer (Fig.4).
In the field, soil texture could be estimated by the following methods:
(i) Feel method: In this method, the soil is moistened with water and rubbed between the thumb and fingers. The way the wet soil “slicks out”
gives a good idea of the clay content. The sand particles are gritty, the silt has a floury or talcum - powder fell when dry and is only moderately plastic and sticky when wet. Accuracy of this method depends largely on experience.
Volume and weight composition of a soil (Percentage of air and water varies according to moisture saturation of soil)
Mineralogical composition of soil (area within the figure denotes the relative abundance of minerals)
Fig. 3: Soil Textural Triangle
Fig. 4: The Settling of Particles in a Soil Suspension
(ii) Ball and ribbon method:
The procedure of this method as described by Coche and Laughlin (1985) is as follows: Take a handful of soil and wet it so that it begins to stick together without sticking to the hand. A ball of about 3 cm diameter is made and put down. If it falls apart it is sand. If it sticks together roll the ball into a sausage shape 6 – 7 cm long. If it does not remain in this form it is loamy sand. If it remains in this shape, continue to roll until it reaches 15 –16 cm long. If it does not remain in this form, it is sandy loam. If it remains in this shape, try to bend the sausage into a half circle and if it doesn't, it is a loam. If it does, bend the sausage to form a full circle and if it doesn't it is heavy loam. If it does with slight cracks in the sausage, it is light clay. If it does without any cracks, it is a clay.
(iii) Ball throwing method:
The texture of the soil can be inferred by the way a ball of soil acts when it is thrown at a hard surface such as a wall or a tree (Fig.5). The steps to be followed in this method as described by Coche and Laughlin (1985) is as follows: Throw a ball of soil to a tree or wall 3 m away. If the soil is good only for splatter shots when either wet or dry, it has a coarse texture (loamy sand). If there is a “shot gun” pattern when dry and it holds its shape against medium range target when wet, it has a moderately coarse texture (sandy loam). If the ball shatters on impact when dry and clings together when moist but does not stick to the target it has a medium texture (loam, sandy clay loam, silty clay loam). If the ball holds its shape for long - range shots when wet and sticks to the target but is fairly easy to remove it has a moderately fine texture (clay loam). If the ball sticks well to the target when wet and becomes a very hard missile when dry, it has a fine texture (clay).
The usual mechanical analysis of soils in the laboratory gives the percentages of the three size fractions, sand, silt and clay. For special uses, the same methods of laboratory analyses (pipette method or hydrometer method) can provide a much more detailed analysis giving further breakdown of the relative amounts of soil particles for more size classes in the form of a table or graph. The data in the graphical form is given as a particle - size frequency curve (PSF curve). PSF curves for selected soils are shown in Fig. 1.9. The vertical axis represents the cumulative percentage of occurrence of the various particles sizes and the horizontal axis represents the logarithms of the particle size. The vertical axis in the left hand side relate to the percentages of particles passing through sieves of a particular size and the vertical axis in the
right hand side relate to the percentages of particles not passing through sieves of a particular size.
The more vertical the PSF curve or part of the curve, the more uniform the particle size; a vertical line represents a perfect uniform particle size.
The more inclined the curve or part of it, the greater the difference between the particle sizes (i.e. smaller porosity and higher compaction).
The inflexion point of the curve shows the most frequent particle size by weight. Fine textured soils have their curves towards the right hand side of the graph and the coarse textured soils to the left hand side. From the PSF curves, the percentages of silt, sand and clay can be calculated and using the textural triangular diagram the texture could be determined.
Soil texture is an important soil parameter determining the suitability of a site for aquaculture. A clayey soil stabilises pond bottom besides the fact that it absorbs large quantity of nutrients and release them slowly over a long period to the overlying water. The clayey soil normally holds higher amounts of organic matter than light textured soils and thereby increase the productivity of the pond. It should be noted that too clay a soil (very sticky clay) may not be very satisfactory as it may give rise to fixation of phosphorus and create other physico-chemical biological problems. Such soils may give rise to cracks on draining the ponds, thereby increase seepage losses.