Texture of Soil
To study the mineral particles of a soil, scientists separate them into groups according to their size. These groups are called soil separates or fractions. The relative proportions of different sized Soil separates are called soil texture.
A number of different classifications have been proposed to determine the particle size distribution. The classification established by U. D. Department of Agriculture (USDA) most commonly followed. According the to this classification particles with diameter ranging 20 to 75 mm are classified as gravel; in between 2.0 to 0.05 mm sand (2.0 – mm 1.0 coarse, 0.5 – 0.25 mm medium, 0.25 – 0.10 mm fine, 0.10 – 0.055 – 0.05 atm very fine); from 0.05 to 0.002 mm silt; and below 0.002 mm clay.
|Name of soil separate|
|Diameter limits (mm) (USDA classification)|
|Clay||less than 0.002 mm|
|Very fine sand||0.05-0.10 mm|
|Fine sand||0.10-025 mm|
|Medium sand||0.25-0.50 mm|
|Coarse sand||0.50-1.00 mm|
|Very coarse sand||1.00 200 mm|
Physical and Chemical Nature of Soil Separates
The physical nature of soil separates is as under:
Gravel: The rock fragments represent the rocks from which the regolith and in turn of the soil have been formed by weathering. They are composed of aggregates of minerals and are usually quite coarse. The size of gravel is in between 2.0 to 0.05 mm.
Sand: The sand grains may be rounded or irregular in shape. These are smaller in size ranging from 0.05 – 2 mm in diameter, but each particle can be seen with naked eye. The sand particles are loose and single grained. These do not stick together and feel dry when rubbed between fingers. Large spaces are present between individual sand particles, therefore water and air pass through these rapidly. Thus, the water holding capacity of soil rich in sand particles is low. The sand particles are often fragments of rocks as well as minerals dominated by quartz (SiO2).
Silt: The silt particles range from 0.05 to 0.002 mm. These irregular in shape. The dominant mineral in silt particles is quartz. The silt particles are powdery when dry and even when wet not sticky. The silt particles contain sufficient quantities of Inorganic and organic nutrients, therefore make the soil fertile.
Clay: The smallest mineral particles are clay fraction of soil that is less than 0.002 mm in diameter. These are electron microscopic and vary in shape from plate-like to rounded. The clay particles form sticky mass when wet, i. e., plastic, therefore can be molded easily. These aggregate into hard clods when dry.
Classification of Soil Texture
Particle size analysis is the basis classification of soil. The soil classes can be determined either by feel method (common field method) or mechanical analysis (laboratory method). The major classes recognize on the basis of soil are: sands, loams, clays and silts. Specific textural classes are recognizing within each group.
The sand group Includes all sods which contain at least 70% sand and 15% clay separates by weight. The properties. of such soils are characteristically those of sand. These are commonly called sandy soils. Two specific textural classes are recognized within sandy soils namely sand and loamy sand generally.
The loam group is a complicated soil class. It is a mixture of sand, silt, and clay particles that exhibit the properties of all the separates contained by it. Loam soils are considered best for agriculture purposes. A loam in which sand is dominant is classified as sandy loam; and in which silt and clay dominate are silt loam and clay loam respectively.
A soil containing not loss than 35% clay particles is designated as clay. Such soils dominated by characteristics of clay separates. The soil that contains more sand than clay is termed as sandy clay, and the soil rich in silt is called silty clay, and the soil rich in clay is known as clayey soil.
The soils with at least 80% silt and 12% or less clay is called silts. Such soils are dominated by the properties of silt.
Importance of Soil Texture to Plants
Determination of soil texture is of utmost importance to plants the following ways:
Relative Resistance to Root Penetration
The soils with high silt and clay contents are less porous, therefore retard root growth resulting in poor branching of the roots, so the roots cannot penetrate the soil deeply. The seedlings growing in such conditions are unable to survive during dry periods as these are unable to draw water from deeper layers. On the other hand, the soils rich in sand are porous, and allow deep root penetration so that the seedlings establish themselves well.
Infiltration of Water
Rain falling on a course-textured soil (rich in sand separate) penetrates immediately and there is no water loss as runoff. But rain water penetration in fine-textured soils (rich in silt and clay separates) is slow resulting in huge water loss as runoff. This water while running over the surface of the soil causes erosion (gully and sheet erosion) making the soil unfit for the growth of plants.
Rate of Water Movement
The rate of water movement through a soil is Inversely proportional to the soil texture, finer the soil separates the slower the rate of water movement. This is because a fine- textured soil has small pore spaces that offer resistance to mass movement of water. The sandy soils are unsuitable for growth of shallow-rooted plants because water moves rapidly and become out of reach of plant roots, However, deep rooted plants can grow in sandy soils, if clay layers are present in a soil, these resist percolations of the water so that the surface soil layers remain saturated after rainy season making the soil poor in aeration and unfit for the growth of plants. The loom soils are most suitable tor growth of plants as water rises due to capillarity in these soils (capillaries are formed by silt particles).
Water Holding Capacity
The water is retained by soils as films which coat the surfaces of particles, as wedges held in the angles between grains and as moisture imbibed by the colloids. A soil has more surfaces, angles and colloidal material, therefore can hold more water per unit volume of soil than coarser soils. Thus, more water is available to plant roots in soils because these absorb more water during rainy season as compared to sandy soils. Although fine-textured soils have high water holding capacity but on the other hand they resist giant root penetration, poorly aerated, most of water is held in upper layers of the soil and constant water supply favors damping off fungi, making the soil unsuitable for pants. Thus, the effect of texture on soil moisture is complicated.
Most of the mineral ions required by the plants as their nutrition are adsorbed on the surfaces of the colloids, therefore it is a general rule that the finer the texture of a soil greater Its fertility. Sandy soils: unfit for the growth of plants as these soils contain few colloids and fewer nutrients therefore, heavy rainfall result in leaching thus making the sandy soils Infertile. The loam soils are high in colloidal contents, therefore more fertile and fit for growth of plants.