SOIL PROPERTIES The various properties of soil include-1. Color2. Texture3. Humus4. Soil structures
COLOUR Colour is the foremost physical property that can be seen with naked eye and is useful in interpretation of valuable insight into the soil environment, thus it can be very important in assessment and classification of soil.
DIFFERENT SOIL COLOURS The most influential colours in a well drained soil are white, red, brown and black. White indicates the predominance of silica (quartz), or the presence of salts. Red indicates the accumulation of iron oxides. Brown and black indicate the level and type of organic matter. A colour triangle can be used to show the names and relationships between the influential colours.
FACTORS DETERMININGSOILCOLOUR Four main factors influence the colour of a soil:1. Mineral matter derived from the constituents of the parent material2. Organic matter3. The nature and abundance of iron4. Moisture content
INTERPRETATION USINGSOIL COLOUR If we understand what determines soil colour, then we can use colour to make some inferences about, history, chemistry and hydrology. For example, an obvious change in colour between horizons is a real indication of changes in soil properties which are the result of biological activity, water movement and weathering.
DETERMINING SOILCOLOUR Soil colour should be determined on moist surfaces of freshly broken (not sliced) soil samples. Like any other soil property, colour must always be observed throughout soil profile, paying special attention to the differences between soil horizons. Colour characteristics such as mottle size, percentage and contrast should be observed and recorded. A system that uses specially printed colour charts (Munsell Soil Colour Charts) gives an international standard. It divides colour into wavelength, lightness, and colour saturation. Where a Munsell Chart is not available, simple names as listed in the triangle can be used.
Munsell Color ChartsHue = dominant spectral or “rainbow” color Red, Yellow, Blue, Green Number increases and the color is more brilliant as grayness decreases 8
Color Determination in the field ALWAYS USE MOIST SOIL Munsell’s color charts values:-1. Absolute black = 02. Absolute white = 103. Light soils hue value = 7 or more4. Medium soils hue value = 5 – 65. Dark soils hue value = 4 or less 9
MOTTLES Spots of different colors in the soil Generally indicate that the soil has periods of inadequate aeration each year Usually rust colored Bluish, grayish, & greenish subsoils – with or w/o mottles = indicate longer periods each year of waterlogged conditions & inadequate aeration.
HUMUS The term was coined in1790–1800; (< Latin: earth, ground) Dark brown or black organic substance made up of decayed plant or animal organic matter, that provides nutrients for plants and increases ability of soil to retain water
•In soil science, humus refers to any organic matter that hasreached a point of stability, where it will break down nofurther and might, if conditions do not change, remain as it isfor centuries. Humus significantly improves the structure ofsoil and contributes to moisture and nutrient retention.•In agriculture, humus is sometimes also used to describemature compost, or natural compost extracted from a forestor other spontaneous source for use to amend soil. It is alsoused to describe a topsoil horizon that contains organicmatter
Humus also controlsthe colour of soilhigher the humuscontent darker is thesoil colour.
TEXTURE Refers to the size of particles. Three types are…. Sand (Large) Silt (Medium) Clay (Small) TEXTURE = % of sand , silt and clay.
SOIL STRUCTURE The soil structures commonly seen are-1. Blocky2. Platy3. Massive4. Prismatic5. Granular
1. BLOCKY Particles cling together in angular aggregates. Typical of soils with high clay content. Typical of B horizons. beds are large about 5 to 50 millimeters.
3. MASSIVE Soil has no visible structure. Hard to break apart & appears in very large clods.
4. PRISMATIC Prismatic structure are bounded by flat to rounded vertical faces. Units are longer vertically and top of the prisms are normally flat. It is commonly found in B horizons.
5. GRANULAR Is the best for most plants. Particles cling together to form rounded aggregates. It is commonly found in A horizons. Beds are small usually between 1 to 10 millimeters.
SOILCLASSIFICATION Soil can be classified according to the grain size as-1. Fine grained- Clay2. Medium grained- Silt3. Coarse grained- Sand
CLAY < 0.002 mm Flat plates or tiny flakes Small clay particles are colloids If suspended in water will not settle
Wet clay is very sticky and is plastic or it can be molded readily into a shape or rod. Easily formed into long ribbons
Pores spaces are very small and convoluted Movement of water and air very slow Water holding capacity Tremendous capacity to adsorb water- not all available for plants. Chemical adsorption is large
SILT < 0.05 mm to > 0.002 mm Not visible without microscope Quartz often dominant mineral in silt since other minerals have weathered away.
Does not feel grittyFloury feel –smooth like silly putty
Smaller particles – retains more water for plants and have slower drainage than sand. Easilywashed away by flowing water – highly erosive. Holds more plant nutrients than sand.
SAND Feels gritty Considered non-cohesive – does not stick together in a mass unless it is very wet.
Sand has less nutrients for plants than smaller particles Voids between sand particles promote free drainage and entry of air Holds little water and prone to drought
On the basis oftexture soil can beclassified as :-1. Sandy soil2. Loamy soil3. Clayey soil