Soil profile and soil physical properties anand corrected

Soil Profile and Physical Properties

1. By Dr. AB Jadhav , SSAC, AC, Pune

2. Soil Profile     

3. O Horizon The letter ‘O’ stands for organic, which implies that this horizon is rich in humus, i.e., the organic matter of plant and animal origin. This organic matter, which is usually in various stages of decomposition, gives this horizon its characteristic dark color. In order to be identified as the O horizon, the layer has to have more than 20-30 percent organic matter. Furthermore, depending on whether the origin of the decomposed matter is visible to the naked eye or not, the layer is divided into two zones – O1 and O2. A Horizon (Topsoil) The A horizon is the topmost layer of the mineral soil. As it lies just below the O horizon, this layer also has some amount of humus in it and therefore, it is darker than the layers below. Similarly, the soil particles in this zone are smaller and finer compared to the lower horizons of the soil. It is this layer where seeds germinate and roots grow. Even the soil organisms, like earthworms, fungi, and bacteria, are mainly concentrated in this layer. As most of the biological activities take place in this layer, it is often referred to as the ‘biomantle’ .

4. E Horizon The E horizon, which lies between the A and B horizons, is known by its characteristic light color, and its sand and silt content. It is poor in mineral and clay content as these are lost to the lower layers in the process of leaching and therefore, it is also called the layer of eluviation (leaching). The soil particles of this layer are larger in size than those in the A horizon, but smaller than those in the underlying B horizon. The E horizon in rarely seen in a cultivated area where it is mixed with the A horizon because of plowing activity. B Horizon (Subsoil) The B horizon is rich in clay and minerals like iron or aluminum. It is usually reddish or brown in color, which can be attributed to the presence of the iron oxide and clay. Though this layer has a high mineral content as compared to the topsoil, the chances of some organic matter reaching this layer as a result of the leaching process cannot be ruled out. Plant roots may reach this layer, but the chances of this are pretty less, and therefore, its humus content is very low.

5. C Horizon The C horizon is mainly made of large rocks or lumps of partially broken bedrock. It is considered the transition layer between soil and parent material. This layer is least affected by weathering as it lies deep within the soil and is inaccessible to the soil-forming agents. That explains why the rocks in this layer haven’t changed much since their origin. As plant roots do not reach this layer, the C horizon is typically devoid of organic matter. R Horizon (Bedrock) The R horizon is basically the deepest soil horizon in the soil profile. Unlike the horizon above, this horizon does not comprise rocks or boulders, but instead is made of continuous mass of bedrock. It is very difficult to dig through this layer. In some regions, the bedrock is located just a few inches below the surface, and in some, it is found several feet below.

6. Soil Texture :- Soil texture refers to the relative proportion of sand, silt, clay. The soil separates are expressed in terms of diameter in mm of the particles. The size of particles in soil is not subject to ready change. Thus it is considered as a permanent feature and a basic property of soil. Mechanical analysis of soil separates i.e. the proportion of sand, silt, clay is done by Bouyoucos hydrometric method.

7. CLASSIFICATION ACCORDING TO ISSS SYSTEM SR. NO. SOIL SEPARATES DIAMETER (mm) 1 Coarse sand 2.0-0.2 2 Fine sand 0.2-0.02 3 Silt 0.02-0.002 4 Clay <0.002 Reference :- text book of fundamental of soil science. ( Mali & Patil )

8. CLASSIFICATION ACCORDING TO USDA SYSTEM SR. NO. SOIL SEPARATES DIAMETER (mm) 1 Very coarse sand 2.0-1.0 2 coarse sand 1.0-0.5 3 Medium sand 0.5-0.25 4 Fine sand 0.25-0.10 5 Very fine sand 0.10-0.05 6 silt 0.05-0.002 7 Clay < 0.002

9. Reference :- Text book of fundamental of soil science ( Mali & Patil ) Textural triangle diagram according to ISSS system of classification of soil particles

10. Importance of soil texture :- To asses value of land. Suitability of land to raise different crop Governs nutrient holding and nutrient supplying ability of soil. Evaluate the amount of amendment needed for soil reclamation. Determine the pore spaces and distribution of micro and macro pores.

11. Soil Structure The arrangement and organization of primary and secondary particles in a soil known as soil structure. Types of structure :- i . Plate like: The aggregates are arranged in horizontal plates. Horizontal axis is more develop than vertical. When the plates are thick – platy, thin – laminar.

12. ii. Prism like : The vertical axis is more developed than horizontal giving a pillar shape. Length 1-10 cm. When the tops are rounded - columnar. When flat/plane and clear cut - prismatic.

13. iii. Block like: All the dimensions are about the same size. when the faces are flat and distinct and the edges are sharp angular - angular blocky. when the faces and the edges are mainly rounded -sub angular blocky.

14. iv. Spheroidal:- All rounded aggregates are placed in this category. The aggregates of this group are usually termed as granular which are relatively less porous. When the granules are very porous – crumb.

15. Factors Affecting soil structure Climate Organic matter Tillage Plant roots and residues Animals and microbes Fertilizers Wetting and Drying Water Inorganic cements- CaCo 3 Clay

16. Particle Density The weight per unit volume of solid portion of soil is called Particle Density. It is also called as True density. The particle density of normal soil is 2.65 g/cc . Particle density is higher if larger amount of heavy minerals are present in soil. With increase in inorganic matter the soil particle density increases. It is determined by Pycnometer method.

17. Bulk Density The oven dry weight of soil per unit of its bulk volume is called Bulk density. It is also called as False Density and it is dynamic property of soil. The bulk density of the normal soil is 1.25 to 1.33 g/cc . Bulk density is always smaller than particle density. Addition of bulky organic manures decreases Bulk Density thereby increases porosity, air exchange, microbial activity which ultimately improves health of soil. Clay soil has lower bulk density while sandy soil has higher. It is determined by Clod Coating method.

18. Bulk density Particle density B.D. = Wt. of oven dry soil/bulk volume (Solid + Pore space) P.D.= Wt of oven dry soil/ volume of solid Also called apparent density Also called true density BD is smaller than particle density Higher Particle size increases, bulk density also increase e.g. Clay -1.1 g cm 3 while sandy – 1.6 g cm 3 Particle size increases…particle density decrease Sand- 2.6 and clay-2.83 g cm 3 It is dynamic property It is stable property It is determined by clod coating method It is determined by pycnometer It is varies from 1.1 to 1.7 g C 3 Pore space will not affect the bulk density Pore space will not affect the particle density

19. Soil porosity (Refers to percentage of pore space) Soil porosity refers to that part of soil volume that is occupied by air not and by soil particles . The amount of pore space in soil determined by arrangement of particles of sand, silt and clay . In sandy soil the pore space is high and in clay soil pore space is low. Macro pores are high in sandy while Microspores in clay Presence of organic matter increases the pore space. With increase in particle density, the porosity is decreased.

20. Porosity = (1 – BD/PD) X 100 BD- Bulk density of soil PD-Particle density For Example BD is 1.25 g/cm 3 and PD is 2.65 g/cm 3 (1-1.25/2.65) X100 (1-0.47 ) X 100= 0.53 X 100 = 53 % is porosity

21. Soil colour Soil colour is due to either mineral matter or organic matter and mostly both. White colour indicates the dominance of silica or presence of soluble salts . Red colour indicates the accumulation of iron oxide. Brown and Black indicates the level and type of organic matter. Soil colour directly influences soil temperature.

22. Measurement of soil colour Soil colour is measured using Munsell colour chart and expressed in terms of Hue, value and chroma. Hue – Dominant spectral colour dominant spectral colour . (0 (less red) to 10 (more yellow)) Value - Intensity of colour . Relative light of colour as compared to absolute white (Black – 0 and white 10) Chroma - Purity of colour . Relative purity or strength of the spectral colour . e,g , Dark grey or black brown

23. Black colour of soil Organic matter Dark gray colour Due to Na, Ca bass and organic matter Red colour Due to iron oxide White colour Due to silica and lime Brown colour Organic matter and iron oxide 5YR 5/6 5YR= hue 5 = Value 6 = Chroa

24. Factors influencing soil colour Parent material. Alternate wetting and drying condition Soil moisture. Organic matter.

25. Soil consistency It refers to the resistance of soil materials to deformation or rupture . It depends upon the degree and kind of forces which attract one molecule to another . It varies with size, shape and arrangement of soil particles and colloids.

26. Soil Temperature Soil temperature is measured by using Soil thermometer at various depth like 10cm, 15cm. Soil temperature affect other soil properties as well the plant growth. It is influenced by the soil colour . Temperature Affects nutrient and water movement.

27. Effect of Soil Temperature on Soil and Plant Growth Microbes activity- Lowest activity when soil temp. is below 5 C and above 54 C . Optimum temp. for microbe activity is 25-35 C . Absorption of soil water Physical properties of Soil Plant Disease Nutrient availability Seed germination and plant growth Soil formation

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