SOIL (Part I)
Stages if soil formation-Infancy, youth maturity and old age:
- Pedology is the study of origin, formation and geographic distribution of soils in nature.
- Edaphology is the study of soil in relation to crop growth.
- A pedon is the smallest volume that can be recognized as soil individual and it is 3-dimensional.
- Typically there are 4 horizons-O,A,B and C
- O horizon is the organic horizon found in the forest soils.
- C-Horizon consists of unconsolidated parent materials.
- Solum=A+B horizon Regolith=A+B+C horizon
- Dokuchaiev, the father of soil science, gave the factors of soil formation, which were late, modified by
- There are various processes of soil formation but two are important: Literisation & Podzolisation.
- Laterite is used form preparing bricks.
Nature and composition of soil:
- Soil is 3-phase, particulate, disperse, porous, open and heterogeneous system.
- Idle soil contains 50% solid matter and 50% pore space.
- International system given by
- Coarse sand 2mm-0.2mm
- Fine sand- 0.2mm-0.02mm
- Clay – less than 0.002mm or less than 2 microns
- India follow International system
Physical properties of Soil:
- Light texture or coarse textured soils are easy to plough whereas heavy textured soils or fine textured soils are difficult to plogh.
- The 12 textural groups from light or coarse textured to heavy or fine textured to heavy or fine textured soils is sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay loam, clay loam, silty clay loam, sandy clay, silty clay and clay.
- Gravel is neither a soil separate nor one of the soli textures.
- Particles greater than 2mm are not considered as soil.
- Textured is determined by International Pipette and Bouycous Hydrometer method.
- Stoke’s law is applied in the determination of soil textures.
- Clay is called the active fraction of the soil. It is made up of alumino-silicates.
- Soil texture is a static property i.e. it cannot be changed.
- Structure is denoted in the order of GRADE-CLASS-TYPE.(GCT)
- Loamy texture and granular or crumb structure is best suited for agriculture crops.
- Bulk density is approximately half that of particle density. So bulk density is always lower than particle density.
- Value of Bulk density is 1.4 to 1.8 Mg/m3 due to dominance of quartz whose density is also of the same valur.
- The particle density of soil is arounsd2.56 mg/m3. Bulk density increases on compaction whereas decreases on addition of organic matter.
- Porosity % in sandy soil=30%, clay soils=50-60% and loamy soil= 40-50%.
- Macropores are greater in coarse textured soil and are occupied by air while micropores are greater in fine textured soils and are occupied by water.
- Micropores size <0.06mm & macropores >0.06
- It can be found out using Munsell Colour chart. Three variables are used to denote soil colour. They are hue-dominant wavelength, Value-relative lightness of the colour and chroma-purity of the colour.
Plasticity and cohesion:
- Plastic soils are cohesive
- Particles smaller than 1 micron are said to exhibit colloidal activity.
- Colloidal properly increases with decrease in diameter.
- Colloids exhibit the property of sacrificial activity such as the capacity to hold solid, gases, salts and ions.
- Soil colloids have high exchange capacity, which increases with silica sesquioxides ratio.
- Water has maximum density at 4 degree celcius
- Soil moisture constants: Hygroscopic water, capillary water and gravitational water.
- Water held at tension of more than 31atm is hygroscopic water. It is not available to the plants.
- Water held below 1/3rd-atm is called gravitational water and it is drained from the soil due to gravity.
- Water held at tensions beyond 15-atm is not available to the plants. 15 atm pressure represents wilting point.
- If water is allowed to drain by gravity after supplying water, some water remains event after drainage due to gravity. It is called field capacity.
- Water field capacity held at 1/3rd
- Water held between 1/3rd and 15 atm is called available water.
- Soil air contains 10 times carbon dioxide as that of air.
- Ideally 2/3rd of soil pores are filled with water and 1/3rd with air.
- Submerged soil contain less oxygen.
- In soil, heat is mainly transfer through conduction.
- Sandy soil absorbs more heat than clayey soils.
- pH measures only active acidity.
- Soil with pH less than 6.5 is acidic 6.5-7.5 is neutral and above 7.5 is basic
- Fertile soils are saturated with Ca++ and Mg++ ions
- If soils saturated with more than 15% exchangeable sodium, than that soil is called Alkali soil. If soil is saturated with H+ ions, Then, the soil is said to be unsaturated or acidic.
- OM on decomposition by humification process gives humus.
- In hilly and altitudes, OM is above 1% and in mangroves it is 10-30%
- CN ratio of OM is 10:1 whereas that of Indian soils is 5:1 to 25:1 with an average of 14:1
- The smell of soils after fresh showers is due to Actinomycetes.
- Symbiotic N fixer- Rhizobium in Legumes, Non-symbiotic or Free living is Azotobacter.
- In paddy algae or Azolla fixes N.
- The amount of N fixed is 50-150kg.
- In plant N is fixed as R-NH2, which is converted to ammonia.
- Optimum condition for nitrification process is field capacity and pH above 5.
- Amorphous clay mineral: Allophane.
- There are two types of soil survey- Reconnaissance soil survey and detailed soil survey.
- Largest area of India is occupied by alluvial soils 75 Mha.
- 2/3rd of TN is of red soil. The parent material for red soils is mostly granite.
- Saline soils are formed mainly in aird and semi-arid regions due to rising of salt level inte water table. Also it is due to secondary salinization due to poor quality irrigation water.
- Saline soils are called white alkali soils and are said to cause boron toxicity. Treated by leaching.
- Alkali soils are called black soils and are reclaimed by applying Gypsum.
- Saline tolerant varieties: rice, wheat, barley, maize, sorghum and millets.
- Sensitive crops; Pulses and oilseeds.
- Acid sulphate soils: pH less than 3.5 due to Hydrogen Sulphide gas production, found in Kerala and Sunderbans. Causes Akiochi disease. Cat clays are associated with these soils.