Soil - An Introduction PDF

CHAPTER 1: SOIL – AN INTRODUCTION 1.1 Definition and concept of soil The noun soil is derived through old French from the Latin solum, which means floors or ground. What, a soil scientist calls soil, a geologist may calls fragmented rock, an engineer may calls earth, and an economist may call lan...

CHAPTER 1: SOIL – AN INTRODUCTION 1.1 Definition and concept of soil The noun soil is derived through old French from the Latin solum, which means floors or ground. What, a soil scientist calls soil, a geologist may calls fragmented rock, an engineer may calls earth, and an economist may call land. In general, soil is defined as the more or less loose and crumby part of the outer earth crust. Since time immemorial soils have been christened as a medium of plant growth to meet basic food, fuel and fiber needs of humans. Soil health care was central to ancient farmers, amply depicted in a quotation from Sanskrit believed to have been made in 1500 BC “Upon this handful of soil our survival depends. Husband it and it will grow our food, our fuel and our shelter and surround us with beauty. Abuse it and the soil will collapse and die taking man with it”. Some of the famous ancient and contemporary proverbs listedbelow highlight the importance of soil resource and its management: “We know more about the movement of celestial bodies than about the soil underfoot” – Leonardo da Vinci “A nation that destroys its soils destroys itself” – Franklin D. Roosevelt “To forget how to dig the earth and to tend the soil is to forget ourselves” – Mahatma Gandhi “Soil is a rock on its way to ocean” – W. L. Lindsay The concept of soil has been changing with the accumulation of more knowledge on soils. Whitney (1892) developed the concept of soil as a nutrient bin i.e all soils were considered to be naturally high in nutrients for satisfactory plant growth. Geologists considered soil to be the product of rock obtained through weathering. According to them soil is a superficial, un consolidated material of the earth’s crust. For Engineer’s, soil is regolith or any unconsolidated material removed in excavation and used for filling or providing foundation for structures. According to Hilgard (1892), Soil is more or less a loose and friable material in which plants, by means of their roots find a foothold for nourishment as well as for other conditions of growth. Soil, according to Pedologists, a) is a natural body b) both spatial and temporal c) forms at the surface, d) is a result of complex biogeochemical and physical processes, e) is capable of supporting life, and f) can be mapped at an appropriate scale. Soil microbiologists define soil as a polis (society or community) that is governed by soil organisms where fungi are the governing organisms in forest soils whereas other microbes are the governors or soil managers in other ecosystems. Dokuchaiev (1900) viewed soil as a natural body composed of mineral and organic constituents, having a definite genesis and a distinct nature of its own. According to Jeny (1941), soil is a naturally occurring body that has been evolved owing to combined influence of climate and organisms, acting on parent material, as conditioned by relief over a period of time. Soil is a natural body developed by natural forces acting on natural materials. It is usually differentiated into horizons from mineral and organic constituents of variable depth which differ from the parent material below in morphology, physical properties and constituents, chemical properties and composition and biological characteristics (Joffe and Marbut) Ruffin and Simonson (1968) : Soil is a mixture to earth’s uppermost mantle of weathered rock and O.M. W.E.H. Blum: Soils not only serve for agriculture and forestry, but also for filtering, buffering and 1 transformation activities between the atmosphere and the ground water, protecting the food chain and drinking water against pollution and biodiversity. According to FAO, ‘soil is a natural body consisting of layers (Soil horizons) that are composed of weathered minerals, organic matter, air and water; it is a natural medium for the growth of plants’. Soil can be compared to various systems of Animals Digestive System : Organic matter decomposition Respiratory System : Air circulation & exchange of gases Circulatory System : Water movement within the soil Excretory System : Leaching out of excess salts Brain : Soil clay Colour : Soil Colour Height : Soil depth According to the Glossary of Soil Science Terms (soil science society of America, 1970) “Soil is i) the unconsolidated mineral material on the immediate surface of the earth that serves as a natural medium for the growth of land plants, ii) the unconsolidated mineral matter on the surface of the earth that has been subjected to and influenced by genetic and environmental factors of parent material, climate (including moisture and temperature effects), macro and micro organisms and topography, all acting over a period of time and producing a product that is soil, that differs from the material from which it is derived in many physical, chemical, biological and morphological properties and characteristics”. As soil provides nutrients, water, air and anchorage and supports life on earth, it can be called as Soul of Infinite Life (SOIL). Soil is considered as a geo-membrane of the earth, protective filter, buffer, mediator of energy, water, and biogeochemical compounds; sustainer of productive life, ultimate source of elements, and the habitat for most biota; foundation that supports humans; and the dust to which humans would finally return. Generally, soil refers to the loose surface of the earth as identified from the original rocks and minerals from which it is derived through weathering process. The widely accepted definition according to Buckman and Brady (1969) is:“Soil is a dynamic natural body on the surface of the earth in which plants grow, composed of mineral and organic materials and living forms”. Soil as a three dimensional entity Soil is a three dimensional body having length, breadth and depth. They form a continuation over the land surface and differ in properties from place to place. Its upper boundary is air or water and lower boundary is the rock lithosphere. “Soil is a three-dimensional, a natural and dynamic material on earth’s surface that has important function of serving as a substratum of plant, animal and human life and acts as a reservoir of nutrients and water” For a farmer, soil refers to the cultivated top layer (surface soil) only, that is, up to 15-18 cm of the plough depth. Soils widely vary in their characteristics and properties. 2 Important functions of soil  Medium for plant growth – Supplies all essential plant nutrients  Recycling system for nutrients  Recycle of raw materials – assimilates great quantities of organic waste, and turning it into beneficial humus  Natural habitat for so many organisms –Home to organisms  Foundation for engineering structures  System for water supply and purification  Modifier of the atmosphere – major source of water vapour in the atmosphere 1.2 Soil Science – definition and approaches Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to the use and management of soils. Sometimes, terms which refer to branches of soil science, such as pedology (formation, chemistry, morphology and classification of soil) and edaphology (influence of soil on organisms, especially plants), are used as synonymous with soil science. One treats soil as a natural body, weathered and synthesized product in nature (Pedology) while other treats soil as a medium for plant growth (Edaphology). The two basic approaches –Pedological and Edaphologicalare used in studying the soils. (i) Pedological approach: The origin of the soil, its classification, and its description are examined in pedology (from the Greek word pedon, means soil or earth). Pedology is the study of the soil as a natural body and does not focus primarily on the soil‟s immediate practical utilization. A pedologist studies, examines, and classifies soils as they occur in their natural environment. In this approach study of rocks and rock forming minerals, pedogenic factors and process involved in soil genesis, soil survey, soil description and soil classification is made. The information is useful to engineers and planners for decision making. (ii) Edaphological approach: Edaphology (from the Greek word edaphos, which means soil or ground) is the study of soil from the standpoint of higher plants. Edaphologists consider the various properties of soils in relation to plant production. They are practical and have the production of food and fiber as their ultimate goal. To achieve that goal, edaphologists must be a scientist to determine the reasons for variation in the productivity of soils and find means of conserving and improving productivity. This is an applied branch of soil science which is important for Agricultural production and involves study on properties of the Soil which affect plant growth, nutrition and yield of crops. This helps to suggest appropriate soil management measures for crop growth. Soil Science is inter – related with other Agricultural and other allied subjects such as Agronomy, crop physiology, soil microbiology, plant breeding, entomology, pathology and Agricultural economics. 1. Agronomy:- It is the branch of Agriculture dealing with principles and practices of crop production. Crop production is specific to soil type Eg: Cotton grows well in black soils. 2. Crop physiology:- It covers areas related to nutritional deficiencies, Soil, water and plant relationship. 3. Plant breeding and genetics:- The high yielding verifier in various crops requires fertile soil 3  to show its full potential. Also varieties which are salt tolerant needs SAS. 4. Soil microbiology: - Beneficial and harmful micro organisms, bio fertilizers are to be studied in different soils. 5. Plant Pathology: - Soil borne diseases spread through soil form one season to another. In case of green house or pot studies sterilized soil is used. 6. Entomology: - Soil provides medium for the insects to lay the eggs and it acts as breeding hotspots. 7. Agricultural Economics: - The high quality produce fetches more price which is possible if the soil fertile. Crop Physiology Agronomy Agricultural Plant Pathology Microbiology Soil Science Agril. Entomology Plant breeding & Agri. Econ Genetics Soil Science has major six well defined and developed disciplines: 1. Soil Fertility : Deals with the nutrient supplying capacity of soils. 2. Soil Chemistry : Studies of soil chemical properties and the chemical reactions. It is the study of chemical composition of soil in relation to crop needs. 3. Soil Physics : Studies of soil physical properties. 4. Soil biology : Deals with the soil organisms and their roles in soil fertility and crop production. 5. Soil Conservation: Deals with protect and conserve the soils. 6. Soil Pedology : The science dealing with the genesis, survey and classification and the laws of geographic distribution of soils as a body in nature. Pedology: It assumes the status of petrology as a science, and encompasses pedography (description) and pedogenesis (origin of soils or soil genesis) Soil Genesis: It is the mode of origin of the soil with special reference to the processes and soil forming factors responsible for the development of the solum or true soil from the unconsolidated parent material. Soil Survey: Soil survey comprises morphological examination, description, classification and mapping of soils in their natural environment for interpretation and correlation. Soil Classification: It is a process of logical grouping on the properties and characteristics of representative units of natural bodies. Evolution of Science of Pedology A team of Soil scientists from Russia, led by Dokuchaev (1870) reported unique horizontal layers in soils which were associated with different combinations of climate, vegetation and under lying soil material. The same sequences of layers were found in widely separated geographical areas that had similar climate and vegetation. Thus, they distinguished the soil from the undifferentiated weathered rocks or parent material below. This was the beginning of science of pedology. With 4  the translation of book ‘Great Soil Groups by the World’ from German to English by C.F.Marbut in 1927, the science of pedology was known to the world. Pedological Research in India Pedological research in India had made a modest beginning in the later part of 20th century which was mainly focused on soil genesis, classification and mineralogy. In 1950’s soil survey work was initiated as a pre-requisite for many developmental programmes in Agriculture. With the establishment of NBSS & LUP in the year 1976, pedological research work gained momentum. During the past four decades, significant contribution had made in the field of soil resource inventory at different scales, soil correlation, soil mineralogy, paleo - pedology and related research in Edaphology and pedology. International Union of Soil Sciences highlighted following key facts of soils:  It can take more than 1000 years to form a centimeter of topsoil.  There are over 10000 different types of soil exist worldwide.  A handful of soil can contain billions of soil microorganisms. There are more individual organisms in a handful of fertile soil than world‟s human population.  Soil is one of the most complicated biological materials on our planet.  Soil stores 10% of world‟s CO2 emissions. Soil carbon is the largest global carbon pool. 5  Major divisions of the earth’s sphere 1.3 The Earth’s crust There are three divisions of earth‟s sphere corresponding to the solid, liquid and gas, which constitutes the Earth. The solid zone is called Lithosphere. The incomplete covering of water bodies, such as seas and oceans known as Hydrosphere and the gaseous envelope around the earth‟s surface is called Atmosphere. The Earth‟s crust is only about 3-5 miles (8 km) thick under the oceans (oceanic crust) and about 25 miles (32 km) thick under the continents (continental crust). The first layer consists of approximately 16 km of rocks and loose materials, we call them the ‘Earth crust’. Underneath the continents, the crust is almost three time as thick as it is under the ocean (Figure 2.1). Figure 2.1: The Earth‟s crust layers Atmosphere The envelop of air that covers both the lithosphere and hydrosphere is called Atmosphere. An atmosphere (from Greek atmos, meaning „vapour‟, and sphaira, meaning „sphere‟) is a layerof gases surrounding a planet. It contains water molecules and dust, which may act as nuclei for the condensation of water vapors to form clouds or fog. The atmosphere is composed of nitrogen (~78%), oxygen (~21%), Argon (~0.9%) with carbon dioxide and other gases such as neon, helium, krypton and xenon in trace amounts. Oxygen is used by most organisms in respiration process, nitrogen is fixed into the soil by bacteria and lightning to produce ammonia, that is used in the construction of nucleotides and amino acids of microorganisms, and carbon dioxide is used by plants, algae and cyanobacteria in the photosynthesis process. The atmosphere helps protect living organisms from the solar ultraviolet radiation, solar wind and cosmic rays. Hydrosphere It is a sphere of water surrounding the earth that contains absorbed air and carries particles of rock as sediments. Water covers almost three - fourth of the earth’s surface. Ocean occupies most of it with surface water bodies like rivers, ponds, lakes and ground water to some extent. It is the combined mass of water found on, under, and above the surface of the Earth. It is the layer of waterbodies such as seas, rivers, oceans, etc., surrounding the lithosphere. It covers 70%of the earth. It has been estimated that there are 1386 million cubic kilometers of water on the Earth. Saltwater accounts for 97.5% of this amount. Fresh water accounts for only 2.5%. Of this fresh 6  water, 68.9% is in the form of ice and permanent snow cover in the Arctic, the Antarctic, and mountain glaciers. 30.8% is in the form of fresh groundwater. Only 0.3% of the fresh water on Earth is in easily accessible lakes, reservoirs and river systems. Lithosphere It is the solid surface (Continents, ocean basins, plains, plateaus and mountains, valleys, sand dunes) and interior of the earth which consists of rocks and minerals. It amounts to 93.06% of the earth’s mass. It is the rigid, outmost shell of the planet earth. That visible portion of the lithosphere, which rises above the seawater, is known as land. The land is only about one-fourth of the total surface of the earth. Most land is situated in the northern hemisphere. The lithosphere consists of two portions, viz., (i) the upper or outer cool solid surface, and (ii) the inner hot and molten mass. In terms of density, it is the heaviest amongst the three spheres, having density of 5.5 (water density is 1.0). The outer crust has density of ~2.5 to 3.0, while the inside core, consists of much heavier materials. The outer solid layer, called as the earth’s crust is estimated to be ~16 to 32km thick. It consists various rocks together with thin mantle of soil enveloping them. On this crust that life, both animal and plant sustains. The inner mass, which forms the interior of the earth, is in molten condition. In short, the Earth's lithosphere includes the crust and the uppermost mantle, which constitutes the hard and rigid outer layer of the Earth. The uppermost part of the lithosphere that chemically reacts to the atmosphere, hydrosphere and biosphere through the soil forming process, is called the pedosphere. Biosphere The biospheres (from Greek bíos„life‟) which can also be termed as life on Earth, all the living things in or on the planet are categorized as biosphere. The entire ecological communities within the physical surrounding of the earth are within the umbrella of living things (biosphere). These ecological communities interacts with the hydrosphere, lithosphere, and the atmosphere. Interior of the Earth. - The earth ball consists of 3 – concentric rings i.e Crust, Mantle and Core (inner and outer). - Crust: 5-56 km thick, consists of rocks ē density of 2.6-3.0. the skin (5-11 km in oceans and 35-56 km in the continents) is very important from soil science studies point of view. - Mantle: 2,900 km in thickness, comprises of mixed metals and silicates and ultra basic rocks with density of 3 to 4.5. - Core: It is 3,500 km in thickness, comprises of metals, such as nickel and Iron with average density of 9-12. 1.4 The Earth crust – elemental composition The earth‟s crust is principally composed of mineral matter. Altogether, There are 118 elements that have been identified, of which the first 94 occur naturally on the Earth and rest 24 are synthetic elements. These elements which are known to us are found in the earth crust in various combinations to form compounds. The oxygen is most abundant element, but it usually occurs in combination with other elements, which reflects the importance of silicate (SiO2-based) minerals. About 98.5% of the crust is comprised of just 8 elements: In decreasing concentration order of: O >Si >Al >Fe >Ca >Na >K >Mg >others (Table 2.1). Table 2.1: Elemental composition of the Earth’s crust Element O Si Al Fe Ca Mg Na K Others Content (%) 46 27 8 5 4 2 3 3 2 7  The two elements occurring in greatest abundance are non-metallic (O and Si) and comprises 75% of the total composition of the crust. In the process of cooling, the elements began to combine into compounds. The compounds so formed aggregated into large masses giving rise to a variety of minerals. These mineral masses got cemented or pressed into the rocks, as they are now present in the earth‟s crust. These rocks were then exposed to the action of weathering processes over thousands of years. The disintegrated rocks gradually evolved into the soils. Therefore, knowledge of soil forming rocks and minerals and the weathering processes isessential in the studying of soils. The materials of the earth‟s crust fall into two principal categories: Minerals and Rocks. Mineral is a solid, inorganic, naturally formed substance that has a crystalline structure and specific chemical composition. Rock is a solid, inorganic, naturally formed substance without a particular structure or chemical composition. Rock is a solid aggregate of one or more minerals. Most common minerals consist of silica (SiO2) mixed in varying proportions with other elements such as Fe, Mg, Al, Ca, K, and Na. Elemental composition of earth’s crust (% by weight) (after Mason, 1960) Non metallic Oxygen (O2-) 46.60% 74.32% Silicon (Si4+) 27.72% ¾th of total Metallic Aluminum (Al3+) 8.13% Iron (Fe2+) 5.00% Calcium (Ca2+) 3.63% Sodium (Na+) 2.83% 25.68% Potassium (Na+) 2.59% ¼th of total Magnesium (Mg2+) 2.09% Others 1.41% Relative abundance of rocks in Earth’s crust The composition of the upper 5 km to the earth’s crust is as follows: - Sedimentary rocks. Shale 52% Sandstone 15% 74% Limestone & dolomite 7% - Igneous rocks Granite 15% 18% Basalt 3% - Others 8% Total 100% Five kinds of rocks occupy more than 90% of the total continental area. However, the composition of Earth’s crust as a whole differs significantly from the one described above and comprises of the following. - Igneous rocks 95% - Sedimentary rocks 5% Shale - 4% Sandstone - 0.75% Limestone - 0.025% 8  Although sedimentary rocks form only 5% of the total earth crust, yet they are important as they occur to the extent of 74% at or near the surface of the earth (upper 5 k). Igneous are dominant in deeper layers. Composition of Soil / components of soil Soil is composed of solid phase, liquid phase and gaseous phase in varying proportions. In an Ideal soil these components are observed as Sand Primary minerals Inorganic (quartz Mica, Solid (50%) Feldspar Silt Soil Organic-Humus Clay Liquid (25%) Secondary mineral (clay Gaseous (25%) minerals, hydrous oxides Liquid Gaseous 25% 25% Solid 50% Surface soil It is the immediate uppermost loose layer of the earth consisting of organic matter and soil organisms suitable for plant growth. It is called furrow slice (0-15cm) Sub-soil It is the compact soil below the furrow slice soil layer which cannot be cultivated by tillage operation. Difference between soil v/s sub-soil Surface soil Sub-soil 1. It is completely weathered. 1. It is partially weathered. 2. Sub-soil is dominated by quartz 2. Surface soil is dominated by finer particles & other coarse fragments of particles like silicate clays. minerals. 3. Surface soil is porous and friable. 3. Sub-soil is more massive & compact. 4. Due to good aeration, exchange of 4. Due to poor aeration, exchange of gases between atmosphere and soil air gases is limited. is better. 5. The microbial population and their 5. The microbial population and their activity is very high. activity is very low. 6. Organic matter content is high due to 6. O.M content is low due to lack of presence of higher biomass on the soil plant and animal residues. surface. 9  7. Due to presence of O.M, the colour of 7. The colour of sub-soil is light or surface soil is deep brown or dark. yellowish. 8. Surface soil has no hard pan. 8. Sub-soil sometimes has hard pan. 9. It is fertile. Most of the essential plant 9. It is less fertile, very few essential nutrients are present. plant nutrients are present. 10. Cation exchange capacity is very high. 10. Cation exchange capacity is low. List of International Soil Scientists. 1. V.V.Dokuchaiev (1846-1903) 2. J.V.Liebig (1803-1873) 3. C.F.Marbut (1927) 4. J.B.Laws & J.H.Gilbert (1855) 5. Aman & Stout (1939) – Essentiality of nutrients 6. Parker (1951) – Nutrient Index 7. Buol (1975) – Fertility capability class 8. Bray (1909) – Nutrient mobility concepts 9. Mitscherlich (1909) - Law of Diminishing Return, Nutrient diagnosis by pot culture method, Law of physiological relationship Indian Soil Scientists & their contribution 1. Ramamurthy (1968) & Vellayudham - Inductive approach * STCR approach 2. Subbaiah and Asija – Alkaline potassium permanganate method 3. Rathanlal – Climate change & organic carbon 4. S.Govindarajan and R.S.Murthy – Soil survey 5. C.N.Acharya – Bangalore method of compositing 10