SCIENCE OF SOIL

1. SCIENCE OF SOIL

2. Why we need to know about soil?

3. Soil A collection of natural bodies developed in the unconsolidated mineral and organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants and has properties due to the effects of climate and living matter acting upon parent material, as conditioned by topography, over a period of time.

4. GENESIS OF SOIL Rocks are chief sources for the parent material over which soils are developed Types of rocks- • Igneous • Sedimentary • Metamorphic Genesis includes –weathering of rocks & formation of soil

5. Primary and Secondary Minerals • Primary Minerals: Minerals that have persisted with little change in composition since they were extruded in molten lava(eg. quartz, mica and feldspars).They are most prominent in sand and silt fractions. • Secondary Minerals: Minerals such as the silicate clays and iron oxides, have been formed by the breakdown and weathering of less resistant minerals as soil formation progressed.

6. Weathering of rocks It is physical and chemical disintegration and decomposition of rocks. Weathering creates the parent material over which the soil formation takes place. Later weathering, soil formation and development proceeds simultaneously.

7. Physical weathering • Temperature • Water • Wind • Plants & animals

8. Chemical weathering • Solution • Hydration • Hydrolysis • Acidification • Oxidation • Reduction

9. Chemical weathering As soon as physical disintegration of rock and mineral begins, chemical decomposition starts. Water and its solution – hydrolysis, hydration, dissolution KAlSi3O8 + H2O ------> HAlSi3O8 + K+ + OH- 2 HAlSi3O8+ 11 H2O ---- Al2O3 + 6 H4SiO4 Al2O3 + 3H2O ----- Al2O3.3H2O Acid solution weathering • CaCO3 + H2CO3 -----> Ca2+ + 2 HCO3-

10. Oxidation 3 MgFeSiO4 + 2 H2O H4Mg3Si2O9 + SiO2 + 3FeO • 4 FeO + O2 + 2 H2O -- 4 FeOOH It is particularly manifest in rocks containing iron

11. Soil formation The mineral weathering combines with the associated physical and chemical phenomena constitute the process of soil formation. It includes- • The addition of organic & mineral materials • The loss of these materials from the soil • Translocation of materials from one point to Another within the soil column • Transformation of minerals & organic substances within the soil

12. Two Approaches: • Pedological • Edaphological

13. Edaphology (edaphos means soil or ground in greek) is the study of soil from the stand point 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 fibre as their ultimate goal. The origin of the soil ,its classification, and its description are examined in pedology (pedon-soil or earth in greek). Pedology is the study of the soil as a natural body and does not focus primarily on the soli’s immediate practical use. A pedologist studies, examines, and classifies soils as they occur in their natural environment.

14. Composition of soil

15. Soil Profile and its Layers(Horizons) • Examination of a vertical section of a soil as seen in a roadside cut or in the walls of a pit dug in the field, reveals the presence of more or less distinct horizontal layers. Such a section is called a profile, and the individual layers are known as horizons

17. Topsoil and Subsoil • When a soil is ploughed and cultivated, the natural state of the upper 12-18 centimeters(5-7 inches) is modified. This manipulated part of the soil is referred to as the surface soil or the topsoil. • The subsoil is comprised of those soils layers underneath the top soil.

18. Mineral (inorganic) and organic soils • Mineral soils: Mineral or inorganic in composition, low in organic matter ranges from 1 -6%. • Organic soils: 50% organic matter by volume (at least 20% by weight).

19. Soil Texture and Soil Structure • Soil Texture: Proportions of different sized particles present in soil. • Soil Structure: The arrangement of the sand silt and clay particles within the soil.

20. Table: General properties of three major inorganic soil particles

21. Soil Air Soil air differs from the atmospheric air in several respects- First ,the composition of soil air is quite dynamic and varies greatly from place to place within a given soil. Second, soil air generally has a higher moisture content than the atmosphere; the relative humidity of soil air approaches 100% when the soil moisture is optimum. Third, carbon dioxide in soil air is often several times higher than the 0.03% commonly found in the atmosphere, Oxygen decreases accordingly and, in extreme cases 5-10%, or even less, as compared to about 20% for normal atmosphere.

22. Composition of soil air

23. Soil Organic Matter • Soil organic matter comprises an accumulation of partially disintegrated and decomposed plant and animal residues and other organic compounds synthesized by the soil microbes as the decay occurs. Such material is continually being broken down and re-synthesized by soil microorganisms. Consequently, organic matter is a rather transitory soil constituent, lasting for a few hours to several hundred years. • Organic matter binds mineral particles into granules that are largely responsible for the loose. easily managed condition of productive soils and increases the number of water a soil can hold. • It is also major soil source of phosphorus and sulfur and the primary source of nitrogen (3 elements essential for plant growth)

24. Organic matter, including plant and animal residues, is the main source of energy for soil organisms. Without it biochemical activity would come to a near standstill. • In addition to the original plant and animal residues and to their partial breakdown products, soil organic matter includes complex compounds that are relatively resistant to decay. These complex materials, along with some that are synthesized by the soil microorganisms, are collectively known as humus. This material is usually black and brown in colour, is very fine(colloidal) in nature.

25. Soil Water • Water is hold in the soil for varying degree of tenacity depending on the amount of water present and the size of the pores. • Together with its soluble constituents, including nutrient elements(eg. Ca, P, N and K), soil water makes up the soil solution, which is the critical medium for supplying nutrients to growing plants. • The movement can be in any direction; downward in response to gravity, upward as water moves to the soil surface to replace that lost by evaporation, and in any direction toward plant roots as they absorb this important liquid. Although some of the soil moisture is removed by the growing plants, some remains in the tiny pores and in thin films around soil particles. The soil solids strongly attract the soil water and consequently compete for it with plant roots.

26. Soil Solution • The soil solution contains small but significant quantities of soluble inorganic and organic compounds, some of which contain elements that are essential for plant growth • Critical property of the soil solution is its acidity or alkalinity. Many chemical and biological reactions are dependent on the levels of hydrogen ions and hydroxide ions in the soil. These levels influence the solubility, and in turn the availability to plants, of several essential nutrient elements such as Fe, Mn, P, Zn and Mo.

27. The concentration of hydrogen(H+) and hydroxide ions(OH-) in the soil solution is commonly ascertained by determining its pH. Technically the pH is the negative logarithm of the concentration of hydrogen ion in the soil solution. Thus each unit change in pH represents a tenfold change in the activity of the H+ and OH- ions. Acidity Alkalinity 3 4

28. Clay and Humus • The attraction of ions such as Ca2+, Mg2+, and K+ on the surfaces of colloidal clay and humus is not as exciting as is the exchange of these ions for other ions in the soil solution. For example, an H+ ion released to the soil solution by a plant root exchange readily with a potassium ion(K+) adsorbed on the colloidal surface .The K+ ion is then available in the soil solution for uptake by the roots of crop plants. A simple example of such cation exchange illustrates this point. K+ + H+(aq) H+ + K+(aq) (adsorbed) (in soil solution) (adsorbed) (in soil solution) colloid colloid

29. -ve charge +ve charge Al Ca Mg Clay Micelle K Na H Ionic double layer

30. pH-dependent charge Common in humus, allophane, Fe & Al hydroxides Negative Charges Al – OH + OH- = Al – O- + H2O -CO-OH + OH- = -CO-O- + H2O No Charge -ve charge These reactions are reversible. If the pH increases, more OH ions are available to force the reaction to the right Positive charge Under moderate to extreme acid soil conditions Al – OH + H+ = Al–OH2+.In some cases, Al-O- + H+ = ALOH + H+ = Al–OH2+ High pH Low pH

32. Essential nutrient element and their sources

33. Soil Degradation • Soil degradation is a concept in which the value of the biophysical environmentis affected by one or more combination of human-induced processes acting upon the land. It is viewed as any change or disturbance to the land perceived to be deleterious or undesirable. Natural hazards are excluded as a cause, however human activities can indirectly affect phenomena such as floods and bushfires. It is estimated that up to 40% of the world's agricultural land is seriously degraded.

34. Causes The major causes include: • Land clearance, such as clear cutting and deforestation • Agricultural depletion of soil nutrients through poor farming practices • Overgrazing • Inappropriate Irrigationand over-drafting • Urban sprawl and commercial development • Land pollution including industrial waste • .

35. Vehicle off-roading • Quarrying of stone, sand, ore and minerals Overcutting of vegetation • Overgrazing • shifting cultivation without adequate fallow periods, absence of soil conservation measures, • Population pressure

36. Effects • The major stresses on vulnerable land include: • Accelerated soil erosion by wind and water • Soil acidification and the formation of acid sulfate soil resulting in barren soil • Soil alkalinisation owing to irrigation with water containing sodium bicarbonate leading to poor soil structure and reduced crop yields • Soil salinization in irrigated land requiring soil salinity control to reclaim the land • Waterlogging in irrigated land which calls for some form of subsurface land drainage to remediate the negative effects • Destruction of soil structure including loss of organic matter • Ultimately results into low vegetation cover, extensive soil erosion which leads towards desertification

37. Every year 84 billion tonnes of productive top soil are lost world wide through degradation. • Degradation has already affected 1900 m ha of land globally (De Man et. al. 2007). • Additionally each year over 14 million acres of productive lands are oversalted because of improper water management.

38. Soil Erosion • Soil erosion is the process of detachment of soil particles from the parent body and transportation of the detached soil particles by wind or water. Mechanism of Water Erosion: • Detachment • Transportation Causes: • Natural • Anthropogenic

39. Forms of Water Erosion • Sheet Erosion: uniform removal of top soil in thin layer from the field, least conspicuous. • Rill Erosion: channelization begins ,no longer uniform. • Gully Erosion: unchecked rills result in increased channelization of runoff. • Ravines: manifestation of prolonged process of gully erosion. Deepening & Widening of gullies used to form ravines. • Landslides: occur in mountain slopes when the slope exceeds 20 per cent and width 6 m. • Stream-bank Erosion: Seasonal streams or rivulets often change their course from season to season due to blockage of their previous course by transported rocks, clods of soil & vegetation grown during lean periods.

41. Ravine erosion Gully erosion

43. Forms of wind erosion • Suspension- Most spectacular method of transporting soil particles is by suspension. Dust particles of fine sand ( less than 0.1 mm dia) are moved parallel to ground surface and upward. About 5-15 % of wind erosion afftected soil is transported by this process. • Saltation- Particles in the range 0.1-0.5 mm diameter are lifted by the wind, then fall back to the ground, so they move in a hopping or bouncing fashion. These particles cause abrasion of the soil surface and as they hit other particles they break into smaller particles, a process called attrition. Depending on conditions, this process may account for 50-70% of the total movement of soil. • Surface creep- Rolling and sliding of larger particles (more than 0.5 mm dia) along the surface. Surface creep account to 5-25% of total movement due to action of wind.

45. Soil Conservation Definition Soil conservation is using and managing land based on the capabilities of the land itself.

46. Soil Conservation Measures Agronomic Measures • Contour Cultivation – By ploughing and sowing across the slope, each ridge of plough furrow and each row of the crop act as an obstruction to runoff, providing more opportune time for water to enter into the soil and reduce soil loss. • Tillage – Tillage alters soil physical characters like porosity, bulk density, surface roughness and hardness of pans. Conventional tillage includes ploughing twice or thrice followed by some secondary operations like harrowing and planking that smoothen and pack the soil in seed-bed and/or control weeds. • Mulching – Mulches are any material such as straw, plant residues, leaves, loose soil or plastic film placed on the soil surface to reduce evaporation, erosion or to protect plant roots from extremely low or high temperature.

47. Mechanical Measures • Contour Bunding – Runoff from any given surface is along the line of greatest slope and the velocity of runoff increases with the vertical distance through which it is moved. The contour bund being on the same elevation, assures that the depth of water against the bund is uniform throughout its length. It ensures uniform distribution of water above the bunds and therefore, better cultivation possibilities than any other type of bund. As the bunds are at regular intervals, they intercept the runoff from attaining erosive velocity and causing erosion. The velocity of flowing water is slowed down and water thus held on the field for a longer time, soaks into the soils. • Broad Base Terrace - A terrace is a combination of ridge and channel built across the slope. These terraces have wide base and low height of ridge and usually formed with machinery. BBTs are constructed in soils with high clay content which develop deep cracks in summer (e.g. Black soil).

48. Bench Terracing - Bench terracing consists of transforming relatively steep land into a series of level strips or platforms across the slope of the land. It reduces the slope length and consequently erosion. The field is made into a series of benches by excavating the soil from upper part of the terrace and filling in the lower part. On steeply sloping and undulated land, farming practices is possible only with bench terracing. It is usually practiced on slopes ranging from 16 to 33%. Trenching –Contour trenches are made in non-agricultural land for providing adequate moisture conditions in order to raise trees or grass species. The trenches are usually 60 cm × 48 cm in size. The spacing varies from 10 to 30 m. Vegetative Barriers – these are closely spaced plantations-usually a few rows of grasses or shrubs --- grown along contours . They act as barrier to check the velocity of overland flow and entrapment of silt load behind them. Khus (Vettiveria zelanica) is the most suitable plant for this purpose.

49. Grassed Waterways – These are drainage channel either developed by shaping the existing drainage ways or constructed separately. Suitable perennial grasses that are not edible by cattle, deep rooted and spreading type are established subsequently for the stability of the waterway (e.g Panicum repens, Brachiara mutica, Cynodon dactylon, Paspalum notatum). The objectives are- 1. to provide drainage, 2. to convert gullies or unstable channels into stable channels by providing grass cover, and 3. for leading water at non-erosive velocity into a water body. Gully Control – The basic approach to gully control involves reduction of peak flow rates through the gully and provision of stable channel for the flow that has to be handled. Temporary and permanent structures such as check dams, drop-spill ways are constructed.

50. Agrostological Measures • Grasses prevent soil erosion by intercepting rainfall, by binding the soil particles and by improving soil structure. A grass-legume association is ideal for soil conservation. E.gPennisetumpupureum, Cenchrusciliaris, Setariasphacelata. Forestry Measure • Afforestation and re-forestation in wastelands