Soil Texture Lecture 5 PDF

Lecture 5  Soil Texture The physical properties of soils are dependent mainly on the particle-sizes So , study of size of soil particles is very important , this tacks place by mechanical analysis of soil.There are many methods of soil mechanical analysis ,...

Lecture 5  Soil Texture The physical properties of soils are dependent mainly on the particle-sizes So , study of size of soil particles is very important , this tacks place by mechanical analysis of soil.There are many methods of soil mechanical analysis , one of these methods is the sieve method Sieve method In this method : - Set of sieves are arranged in descending arrangement - The soil is put in the upper sieve 3- Shake the set sieves for an hour 4- Determine the weight of soil particles that present in each sieve (gravels – sand – silt – clay - By this method, the soil particles are separated into groups according to the soil particle sizes.  The main groups are gravels , sand , Silt , Clay Determining the % percantge of each group Not that :- A )- If weight of (gravels and sand more than weight of ( silt and clay) this soil is sandy soil B) - If weight of ( silt and clay) more than weight of ( gravels and sand ) this soil is clay soil The sieve with the widest holes is at the top vice versa Soil Particles Diameter (mm) Coarse gravel more than 2.00 ** Fine gravel - 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 **  The soil divided into 3 main types of soils (according to Soil Texture) types of soils Sandy soil Mud (clay) Loamy soil soil Small space Large space (pore) Most particles are large Large particles and Most particles are small Low ratio of particles are small particles are Low ratio of particles small equal approximately are large Soil particles range between coarse gravel of large size, and fine particles which are difficult to be shown by microscopes. Note that :- - gravel and sand are called large particles or coarse particles - gravel and sand have no colloidal properties. Therefore, they are not active in absorption of water or soluble nutrients - Silt and clay are called small particles or fine particles - Silt and clay have colloidal properties. Therefore, they are active in absorption of water or soluble nutrients  Effect of Soil Texture on its Features 1.Water holding capacity In soil with high ratio of fine particles ( colloidal particles) can absorb high amount of water (around them as fin film or soluble nutrients by strong surface force , and vice versa In soils with high ratio of large particles these particles have no colloidal properties so the water around these particles leaching down in case of sandy soil. lomy soil , is intermediate between sandy soil and mud soil. 2- Infiltration of water Rain water represents the main source of water in soil. The ability of soil to capture water is dependent on texture. Thus most of rain water penetrates sandy soil (light soil) and infiltration of water is relatively Rapid and easy whereas in mud soil (heavy soil ) infiltration of water is relatively difficult and slow causing great loss of water by run-off and evaporation from surface area. In lomy soil , infiltration of water is intermediate between sandy soil and mud soil 3- Fertility Soil containing relatively large amounts of fine particles (clay specialy) is more fertile than sandy soil with coarse (large ) particles. This is because the ions required for plant growth and development are adsorbed at the surfaces of fine particles. This explains why the fertility of soil is increased with increase in the percentage of clay particles 4-Mechanical resistance to root penetration Heavy soil consisted of high content of fine and clay particles is most resistant to penetration by roots of plants. Heavy soil contains narrow pores and cementing matter.(fine particles become attached with each other so the pores may becomes more narrow from the abve) This explains the retarded penetration and branching of root systems of plants in heavy soil. light soil is, on the other hand, less resistant. So , root system is more longer branched in sandy soil and vice versa in mud soil. 5- Soil aeration: Aeration is increased with increase in large particles or in soils where fine particles aggregate in large particles. This is due to the fact that large particles include large spaces which facilitate the release of CO2 resulting from respiration of roots and microorganisms and its substitution with (replacing by ) fresh air containing oxygen. So , sandy soil is good aerated soil. The reverse is true , the heavy soil is called bad aerated soil.  6- Rate of water movement in soil: Movement of water in soil is no doubt dependent upon the texture of soil. It is logic that the rate of water movement reaches in maximum in sandy soil. This explains the point that successful plants on sandy soils are those with long roots reaching the deeply accumulated water Determination of water Infiltration time in soil. - In sandy soil , rate of water movement is the highest rate and time of water movement is the shortest time. (hours) - In mud soil , rate of water movement is the lowest rate and time of water movement is the longest time.( days ) - In loamy soil ; rate and time of water movement are intermediate between sandy soil and mud soil - In loamy soil ; rate and time of water movement is moderate  Soil Profile A vertical section in the soil to show the different layers in the soil different. Such section is termed "Soil Profile"; whereas the different layers are called horizons Soil consists of particles of different sizes, different chemical structure and solubility These properties of soil particles may be changed by other factors which are mainly climatic and biotic. For exampl in rainy countries calcium carbonate is dissociated and soluble salts are leached down with rainwater from upper to lower layers of soil. Relatively fine clay particles may be brought also to deeper layers by rainwater. Therefore,. The horizons can be differentiated as follows:  A. Zone of extraction (horizon A) It is the 'upper, surface layer characterized by: 1- Particles with large sizes relatively. This is due to Relatively fine clay particles may be brought to deeper layers by rain water 2- Poor in soluble substances(salts). This is due to the soluble salts are leached down with rain water from upper to lower layers of soil. 3- Root penetration to this layer is easy. Because of most Particles with large sizes - Its particles usually aggregate in the form of compound particles. This is due to presence of disintegrated organic matter al this layer which results from the decay of the remains of plants and animals. Such organic matter is colloidal in nature causing aggregation of soil particles. For this reason the roots of plants can easily inter this layer  B.Zone of concentration (horizon B)It is the following lower layer to horizon (A). In this layer fine clay particles, and soluble salts, leached down with rain water ,are accumulated. Therefore , soil particles are more compact duo to presence of colloidal substances So this layer resists penetration and branching of roots as compared with root growth at horizon (A). However, it must be noted that horizons (A) and (B) represent the soil layers occupied by plant roots. C. Zone of weathered parent material (horizon C) This layer is consisted of the parent rock material which is usually disintegrated or decomposed by weathering factors. D.Bed-rock zone (horizon D) This layer is consisted from the original rock  - Most particles are large A.Zone of extraction Low ratio are small -Poor in soluble salts -Root penetration is easy B.Zone of concentration - Most particles are small Low ratio are large -Rich in soluble salts -Root penetration is difficult v C. Zone of weathered parent material Mixture of large and small particles D.Bed-rock zone (horizon D) Original rook Soil Profile