(7) Soil texture

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7.1 What is the soil texture?

                                     45% of the soil is composed of minerals components. Soil texture is based on a weight ratio of the three soil separates (sand, silt, and clay).

                          Therefore we can define as; Soil texture is relative proprtions of sand,silt and clay in a mass of soil.

                              7.2 How the soil texture is formed?

                                       Texture is the result of weathering parent material. Weathering can be occurred in two types these are physical withering and chemical weathering. In physical weathering, rocks are brake down into small pieces. In chemical weathering, chemical composition of minerals is converted to other forms.

                                       Weathering rate is depended on chemical composition and structure of parent material. It causes to from different textures soils. When parent material is shale and easily weather, it formed clay-rich soils. When parent material is graphite and relatively slow weathered, it forms sandy, coarse soils

                       7.3 Classification of soil grains 

                                    Soil particles

Coarse fraction                                                Find fraction

(<2 mm)                                                           (2 mm>)

Gravel                                                             Sand (5 mm- 0.5)mm                                         Coarse                                                             Silt (0.5 mm-0.002 mm)

    7.4 Coarse grained soils

     Coarse soils are considered as single-grained soils. They are visible to naked eye (Without using microscopes). The diameter of coarse soil is more than 2 mm. Gravels, cobbles and boulders are belong to coarse grained soils. Coarse soils are not considered as fine fraction.

     7.5 Fine grained soil

	Sand	Silt	Clay
Diameter	2 mm-0.05 mm	0.05 mm-0.002mm	0.002 mm
Feeling	Gritty	Smooth	Sticky
Surface area	Low		Large
Nutrients for plants	Less	More	Most
Permeability of water	Rapid	Moderate	Slow
Shape	Rounded/angular
Visibility	Era visible	Non-visible without microscope	Non-visible without microscope
Wet	No stickiness	No stickiness	Stickiness
Plastic	Non	Non	Very

7.6 Properties related to texture

      1)   Porosity

                         Although sand particles have relatively larger pores, it doesn’t consist of more pores than other because sandy soils are not well aggregated therefore amount of pores in unit volume is relatively lower. Clay soils are well aggregated therefore it has amount of pore in unit volume is relatively higher. Clay soil has usually small size pores. Therefore clay soils have relatively larger porosity than sandy soil

     1)   Infiltration

                           Water porn through the soil vertically. Sandy soils has relatively larger infiltration capacity than silt and clay soils because sandy soils are composed of large pores that are connected to together very well therefore water can move relatively  more rapidly.

         3) Erodibility

                                  Erodibity is ability to erode of soil particles. Soils have high   percentages of silt and clay, these soils are more tended to erode under some unbeneficial conditions than sandy soils. 

     4) Water holding capacity

                           Clay soils have relatively more water holding capacity than silts and sands because clay soils has macro pores between aggregates and micro pores within the aggregates and these pores are not connected together very well so water doesn’t drained downward in soil therefore clay soils can store relatively more water.

7.7 Importance of soils

                    1) Water flow potential

                    2) Water holding capacity

                    3) Fertility potential

                    4) Suitability for many urban uses like bearing capacity

                    5) Aeration

                    6) CEC (Cation exchange capacity)