Soil Texture: Definition, Determination and Importance

After reading this article you will learn about:- 1. Definition of Soil Texture 2. Determination of Soil Texture 3. Importance.

Definition of Soil Texture:

Texture means size of the particles. Soil texture is the average size of the soil particle which depends on the relative proportion of sand, silt and clay in the soil. If the proportion of sand in the soil is increased, the average size or the soil particles increases and the resultant soil becomes coarser in texture.

On the other hand if the proportion of clay in the soil is increased, the average size of the soil particles decreases and the resultant soil becomes finer in texture.

Determination of Soil Texture:

1. Determination of Soil Texture in the Laboratory:

Soil texture is determined with the help of the triangular texture diagram as shown in Fig 4.1, drawn as follows:

First draw an equilateral triangle of sides 10 cm each, divide them into 10 equal parts and mark them 10, 20, 30, 40, 100. The left right and bottom of the triangles are named clay, silt and sand side respectively.

Locate the point 55 on the clay line and the point 45 on the sand line, and draw lines through them but do not complete as shown in Fig. 4.1.

Locate the point 60 on the silt side and the point 40 on the clay side and draw a line through them till they intersect the first drawn 55-45 line.

Similarly locate the point 73 on the silt side and the point 27 on the clay side and draw a line through them till they intersect the first draw 55-45 line at the point as shown in Fig. 4.1.

Again locate the point 28 on the silt side and the point 72 on the sand side and draw a line through them which will pass through the point (Fig. 4.1).

Draw another line through the point 20 on the clay side and the point 80 on the silt side. It intersects the previously drawn 28- 72 line at b (Fig. 4.1) Again locate the point 48 on the clay side and the point 52 on the sand side and draw a line through them which passes through the point b. Again draw another line through the point 7 on the clay side and the point 93 on the silt side. It intersects the 48-52 line at C as shown in (Fig 4.1).

Draw a line parallel to the clay side, through the point 50 on the sand side till it intersects the previously drawn 73-27 line (Fig. 4.1).

Draw a line parallel to the clay side through the point 40 on the silt side till it intersects the previously drawn 60- 40 line at D as shown in Fig 4.1, through which draw another line is drawn parallel to the silt side till it intersects the previously drawn 73-27 line at e (Fig 4.1.)

Draw a line parallel to clay side through the point 20 on the sand side and another line parallel to the sand side through the point 88 on the silt side till they intersect one another. Join the points 70 on the sand side and 15 on the clay side. Join 85 on the sand side and 10 on the clay side. Erase the unnecessary portions of different lines. Then the diagram looks like the Fig. 4.1. Name the different compartments as shown in Fig. 4.1.

Use of the Triangular Textural Diagram:

Locate the percentages of silt and clay on the silt and clay side lines respectively of the triangular textural diagram. Then draw lines through these points, inwards, parallel to the clay side in the first case and to the sand side in the second case.

The textural name of the compartment in which they intersect is the textural name of the soil concerned. If they intersect just on the line between the two compartments, then the textural name of the finer compart­ment is to be taken.

Example 1:

Find out the texture of the soil of the following composition

Solution:

Locate the points 28 and 20 on the silt and clay lines respectively of the triangular textural diagram, then draw lines through these points inwards, parallel to the clay line in the first case and parallel to the sand line in the second case. They intersect just on the line between the compartments called sandy clay loam, and loam, as shown in Fig 4.1.

Since sandy clay loam soil contains 20 to 35 per cent clay whereas loam soils contain 7 to 27 per cent clay as seen in Fig 4.1, the average size of the soil particles of sandy clay loam is less than that of the soil particles of loam. Hence sandy clay loam is of finer texture than loan, so the texture of the give soil is sandy clay loam.

Example 2:

Find out the texture of the soil of the following composition

Solution:

Percentage of gravel in the soil = 25 x 100/105 = 23.8 weight of the soil excluding gravel = 105 – 25 = 80 gms

Percentage of sand in the soil excluding gravel = 50 x 100/80 = 62.5

Percentage of silt in the soil excluding gravel = 20 x100/80 = 25.0

Percentage of clay in the soil excluding grave; = 10 x 100/80 = 12.5

Locate the point 25 on the silt line and the point 12.5 on the clay line. Then draw lines through 25 and 12.5 inwards, parallel to the clay line in the first case and parallel to the sand-line in the second case (Fig. 4.2). They intersect in the sandy loam compartment.

As the soil contains more than 20 per cent gravel, the word “Gravelly” is to be written before the determined textural class-sandy loam. Hence the texture t)f the given soil is gravelly sandy loam. The word “Gravelly” is not to be written before the determined textural class if the soil contains 20 per cent or less gravel.

Textural Classification of Soils:

Textural classification of soil means classification of soils on the basis of their texture into different groups or classes called, clay, silty, clay sandy clay, clay loam, sandy clay loam, loam, silty loam, silt, sandy loam, loamy sand and sand as shown in the triangular textural diagram.

2. Determination of Soil Texture in the Field:

Sand particles are gritty, which means that if you rub them between your fingers, you feel that you are rubbing some minute coarse grains between your fingers. This feeling of coarseness is called grittiness. Similarly, silt particles have the feel of talcum powder.

They are extremely smooth. Moist clay can be rolled between the hands to form a long thin wire. A ball can be made from moist clayey soil. The ball becomes firmer when the percentage of clay in the soil increases. Grittiness decreases and ball formation, stickiness and wire forma­tion increase as the soil texture becomes finer as shown in Table 4.1.

Importance of Soil Texture:

The capacity of soils to store nutrients and water increases when their clay percentages increase. Clays can adsorb i.e. hold on their surfaces, large amounts of nutrient ions and water molecules, because they have a large surface area per unit volume, are negatively charged and therefore have positive ions and dipolar water molecules, which can be absorbed by the roots whenever required. Hence fertilizer and water may be added to clayey soils at longer intervals than to the sandy soil.

Wet clayey soil sticks to implements. Ploughing a clayey land soil produces very hard clods. Therefore a clayey land soil should be ploughed to an intermediate soil moisture content, when it is dry enough to loss its sickness, yet moist enough not to produce hard clods.

More power is required to plough a clayey land soil than a sandy land soil. So clayey soils are called heavy soils, and sandy soils are called light soils.

Clayey soils are usually poorly drained and aerated, whereas sandy soils are excessively drained and aerated, but cannot retain enough water and nutrients for the growth of crops.

Loam soils are therefore usually best for growing most crops because they are reasonably well drained and aerated and can retain enough water for growth of must crops.