Soil texture and structure

PROPERTIES OF SOILS Soil depth: Deep soils have greater capacity to store more water and nutrients than shallow soils and hence they are highly productive. Soil Texture: The soil size groups are called as soil separates- sand (coarsest) silt and clay (the smallest) Texture - important - determines water intake rate (infiltration), water storage, ease of tilling the soil, the amount of aeration and fertility of the soil

FractionSoil SeparateDiameter range (mm) Sand Very coarse sand Coarse sand Medium sand Fine sand Very fine sand Silt Clay Less than Particle size of sand is higher than 2.00 mm. in dia. are excluded from soil texture Stones and gravels make little or no contribution to the basic soil properties especially in water holding capacity and release of plant nutrients

NAMING OF SOIL TEXTURE The words sand, silt, clay and loam are used to name soil texture. Loam refers to a mixture of sand, silt and clay and exhibits the properties of each fraction equally Thus, 60% sand, 25% silt and 15% clay = Sandy loam 25% sand, 45% silt and 30% clay = clay loam 25% sand, 54% silt and 18% clay = silt loam And 10% clay, 20% silt and 70% sand = coarse sandy loan

PARTIAL SIZE ANALYSIS The determination of the percentage of the soil separates in the sample is called partial size analysis The words sand, silt, clay and loams followed by sand separates (sieving) and the silt and clay by rate of settlement of the particles in the water Texture GroupSandSiltClay Sand Sandy Loam Loam Silt Loam Sandy Clay Loam Silty Clay Loam Clay Loam Sandy Clay Silty Clay Clay

SOIL TEXTURE IN POND SOILS Soil material composed of a mixture of different sized particles The soil containing at least 30% of clay is ideal for fish pond construction Heavy clay soils are sticky and difficult to dry and till

SOIL STRUCTURE Natural aggregates (units of soil) are called peds and they vary in their water stability Peds are natural groups of primary particles that occur and persist within the soil and broken into any shape by plowing Peds are absent in soils have low content of clay Soils are described by three characteristics Type (shape) Class (size) Grade (strength of cohesion).

TYPES OF STRUCTURE There are four types of soil structure. Plate like Prism like Block like, Sphere like

Prismatic peds are taller than they are wide Occasionally found in A- horizon Common in B- horizons of well developed soils Some very strongly developed soils have rounded tops called Columnar structure, results from either old age or a high percentage of sodium in soil solution.

Blocky Like: Blocky peds are approximately equal in their vertical and horizontal dimensions Angular blocky peds with sharp and round corners in sub angular blocky type Angular peds are common in B-horizon and sub angular in either A or B- horizons

Sphere Like: Are generally round or sphere like. There are two types of peds – Granular and Crumby: less pours and crumby type is especially pours Are characteristic of many surface soils Soil parent material is structure less and may either massive or single grain structure Developed as the weathering agents convert the parent material into soil. – Wetting and drying cause swelling and shrinkage. – Freezing and thawing. – Animal activity may loosen the soil. – Cementing agents produced by plants, animals and microbes.

SOIL POROSITY: Pore space - an empty space as an error Portion of the soil volume not occupied by solids, either mineral or organics The volume percentage of total pore space in a soil is known as soil porosity

Pores in the soil - result of irregular shapes of primary particles and their aggregation Contains water and living things without pore space Sandy soils - less pore space Clay loam and clay soils - high total pore space; hold large amount of water

MEASUREMENT OF SOIL POROSITY Total porosity is calculated by the data on bulk density and partial density The relationship between partial density (Dp), bulk density (Db) and pore size (p) is given by D p -D b x 100 % Pore space = Dp Clay soil may have 50-60% pore space whereas sandy soils have low pore space to be about 30% Clay soils posses’ greater total porosity than the sand

Types of Pores Macropores – Allow air and water movements easily Micropores – Ex: Sand (Sand Soils – readily) capillary pores in which movement of air and water is restricted to some extent. E.g: Clay and Clayey soils

BULK DENSITY OF SOIL Bulk density - the mass (weight) per unit volume of a dry soil including pore space It is also expressed in mega gram per cubic meter (Mg/m 3 ) in metric system and gm/cc. in CGS system The bulk density (g/cm 3 ) of soil is always smaller than its particle density Particle density is the unit volume of the solid portion of soil is called partial density Normal soils will have the particle density of 2.6 mg/m 3

Loose soils and porous soils have low weights per unit volume and compact soils have high values The bulk density of sand dominated soils mg/m 3; organic peat soils mg/m 3 Bulk density normally decreases as mineral soils become finer in texture Generally, soils having lower bulk density they have favorable physical conditions

The percentage of pore space in soils is calculated from the data in bulk density and particle density Bulk density x 100 % Solid = and Particle density % Pore space = % solid bulk density % pore space = x 100 Particle density Or% Pore space = {1 – bulk density} x / cm 3

A soil with a bulk density of 1.12 gm / cm 3 and a particle density of 2.65 g/cm 3 As soil with a bulk density of 1.20 g / cm 3 and a particle density of 2.65 g / m x = 45% of solid 2.65 and{ x} X = 55% pore space 2.65

SOIL COLOR Indicates soil features Change in soil color - indicates a difference in the soils mineral origin (parent material) or in soil development White colors are common when salts or carbonate (lime) deposits present in the soil Dark soil absorbs more heat than do light colored ones; hold more water The color of the soil is a result of the light reflected from the soil

Soil color reflection is divided into 3 parts Hue : It denotes the dominant spectral color (red, yellow, blue and green) Value : It devotes the lightness or darkness of color (The amount of reflected light) Chroma: It represents the purity of the color and strength of the color

WATER HOLDING CAPACITY Water in the soil - contains nutrients in solution - for plant growth and their physiological process The amount of water that a soil can store in the form that is available for plant use is known as the available water-holding capacity of the soil Most soil profiles can store between 2 and 10m (5to25 cm) available water Fine textured soils - high water holding capacity as small particles have a large total surface and large volume spaces

Loose sandy soils have large pore spaces; hold much air and little water Clay soils influences water holding capacity of soils Organic matter can hold a weight of water in excess of its own weight 100 to 150 % Loamy soil can hold water as it contains about 50% pore space by volume

SOIL TYPES AND THEIR DISTRIBUTION Soils are classified based on individual soil components, chemical characteristics and their distribution for better understanding and management practices Soil classification refers to soil taxonomy. Soil taxonomy is based on examination of specific soil properties -field observations and soil properties- surface horizons, moisture, temperature, color, rains etc. and laboratory analysis

Categories of soil- In U. S soil classification based on: Land surface, soil maps and location of individual soil. Importance of soil classification Soil classification helps in planning aquaculture projects, preparation of soil maps to choose suitable areas and pond management operations

DIAGNOSTIC HORIZONS AND CLIMATIC CONDITIONS A)Horizons – i) Surface horizons ii) Sub Surface horizons I. SURFACE HORIZONS: (≡ EPIPEDONS) Mollic horizons Soil in grass lands, when dry soil are soft and not massive, mollic epipedons – 18 cm. thick and dark, due to organic carbon (> 1%) base saturated ( > 50%) and pH is more than 7.0

Umbric horizons Soil in humid forests,quite similar to mollic epipedons but highly leached than mollic due to high rainfall, base saturation is < 50% and pH is less than 7.0 Orchric horizons Soil in forests where rainfall is less, thinner soil layer, Low in organic carbon and lighter in color

Histic horizons Soil has high organic matter (0 – horizons), thick layer - several meter, it is classified as fibric, humic and sapric,histic soil develop in wetlands and saturated with water at least 30 days/y Melanic horizons Soil develops due to lava or volcanic ash and has anodic soil properties, soil is athick and black, OC concentration 6-25%,contain more mineral matter and soil development is not in wetlands

Anthropogenic horizons Color, OC and soil structure similar to mollic soil and low in phosphorus. Plaggen horizon Similar to mollic horizon, long term application of manure and plowing gives this plaggen horizon

II.SUBSURFACE HORIZONS Diagnostic substance horizons are E and B horizon- Eight horizons Cansbic horizons Weakly developed B horizons, Less weathered and sands finer particles Agrillic horizons More clay in B – horizons soil, canbic soils can give rise to agrillic

Natic horizons Accumulation of dissolved or leached salts from the upper zone Hence, it is called translocated B horizons, contains 15% or more exchangeable sodium Kandic horizons It is like agrillic – H, but clay has low cat ion exchange capacity (< 16 meq/100gm) Present in many tropical and subtropical soils

Spodic horizons Formed in areas of high precipitation and sand textural parent material Iron and aluminum oxides leached from surface soils accumulate in B – horizon to form spodic horizons. Iron and aluminum chelated by organic matter hence chelated compounds Albic horizons White colored horizon developed between A and B – horizon due to leaching of clay or iron and aluminum oxide. They found in E – horizon

Oxic horizons Soil in high rain fall climate condition highly leached soil. B – horizon comprised of iron and aluminum oxides quarts sand This soil is final stage, weathering low in cot ion exchange capacity contains low concentration of plant nutrients Calcic, gypsic and salic horizons Horizon formed by the accumulation of calcium carbonate or calcium-magnesium carbonates, gypsm or salts of high soluble than gypsm in the B – horizon

SOIL MOISTURE RESUMES - It is refers to 10 – 90 cm. depth soil layer Wet soils – saturated with water Moist soils – water content lower than saturation but greater than permanent wilting percentage Dry soils – below the permanent wilting percentage

SOIL TEMPERATURE REGION Soil temperature in roots zone between 5 and 100 cm Pergelic - < 0 0 c Frigid c Mesic c Thermic c Hyper thermic - > 22 0 c

Histosoils (ist) – Histic epipedons with high organic matter Andisoils (and) – formed from weathering of volcanic materials

Spodosoils (od) – spodic orign aluminum, iron and humus have been transported into the B – horizons Oxisoils (ox)- (oxic horizon) - common in tropics, old, deep red and contains more iron

Vertisoils (ert) – 2:1 type clay, soil swell when wetted and shrunk when dried Aridisoils (id) – aridic soil moisture regime, lack of moisture for plant growth, bases accumulation is A – horizons

Ultisoils (ult) – argillic horizon, found in warm, humid climate,base saturation is<30% Altisoils (alt) – argillic horizon, base saturation is > 30%.

Mollisois (oll) – mollic horizon, found in grass land Inceptisol (ept) – cambric horizon, they are recent soils, which begin to develop

Entisoil (ent) – they are recent soils, common in alluvial areas or on steep slopes

SOIL SURVEY REPORTS Soil contains - soil profiles, engineering properties scale, physical properties, chemical properties etc

SOIL GROUPS IN INDIA Soil map of India has divided into 20 major areas, among which 8 most common in India Red soils, Laterite soil,Black soil, Alluvial soil,Forest and hill soils, Desert soils, Saline and alkaline soils and Peats and marshy soils Forest and Hill soils It occurs at high elevations, as well as lower elevation in India, good rainfall, it covers about 17% of the Indian soil Too shallow, too steep, too stony or too infertile to be used for the production of crops

Desert soil It is mostly sandy, occurring in low rainfall tracts of Gujarat, Rajasthan and Punjab Soils have more soluble salts and low in organic matter Desert soils are often quite productive when irrigated Without irrigation the soils are often brown

Saline and Alkaline soils They occur in areas having slightly more rainfall than desert soils Soil in semi-arid areas of Bihar, UP, Gujarat, Punjab and Rajasthan Soil are characterized by salty and alkaline Soils can be used for crops by having proper irrigation and drainage system to remove salts

Peaty and Marshy soils Soils formed by wet lands, lowering areas bordering the Arabian Sea and the Bay of Bengal The soil, when properly drained and fertilized, soil can produce good crop of rice

Red soil Red soils are characterized by sandy loam or sandy clay in texture, low in lime and red in color Soil deficient in N & P, lime and organic matter Responsive to soil management practices irrigation, manure and fertilizers It is in states of Madras, south Karnataka, S-E Maharashtra, Central AP, South MP and West Orissa

Laterite soil It is common in western AP, Karnataka, Kerala, South Maharashtra, MP, Orissa and Assam The soils are characterized by open, porous and rock like It is red color, low in N,P,K and lime. Soil has low response to management practices

Black soil Soil is common in Maharashtra, North Karnataka, MP, West AP and South TN Soil is mostly clay, farm deep cracks Lime present at different depths, soil responds well to the application of N & P fertilizers and manures Black soil is also called as “Black cotton soils

Alluvial soil Alluvial soil occurs along river Soils are variable in composition and more productive Soil deficient in nitrogen, soil responds well to phosphatic fertilizers Rice, sugar cane and wheat are main crops