Soils Lecture 2 Intro cont'd 2024 Study PDF

9/15/2021 Soils Lecture 2 1 Soils are quite variable in the natural and tilled landscapes Need to be able to understand different soil types First we must be able to recogniz...

9/15/2021 Soils Lecture 2 1 Soils are quite variable in the natural and tilled landscapes Need to be able to understand different soil types First we must be able to recognize basic characteristics and describe them 2 Soil Profile Layering – thickness, number of layers Colour – colours represent different layers and help to distinguish their borders Stones – coarser material, depth and frequency 3 1  9/15/2021 4 5 6 2  9/15/2021 Aggregates – compounded lumps of material, smaller soil particles aggregating to form larger clumps Mottles – mottled areas of colour distribution in the soil profile, often yellowish or reddish tones - yellow and red indicate oxidized iron (Fe) or rust - generally associated with a drainage or O2 penetration issue - an uneven distribution of H2O or O2 in the soil 7 Mottles 8 9 3  9/15/2021 Basic Description of Soil 1. Colour 2. Texture 3. Structure 4. Horizons 10 Colour An indicator of soil content Brown – iron (Fe) Red/yellow – extensive weathering (iron oxides) Black – organic matter (also dark brown) Blue/grey – wet, iron and manganese (reduced) Mottles – blotches of red/yellow and blue/grey White – salt accumulation, typically arid regions 11 Colour 3 components Hue – spectral colour Value – light/darkness Chroma purity/greyness We use the Munsell Colour Chart to determine the colour of the soil (lab) 12 4  9/15/2021 Munsell colour chart 13 14 15 5  9/15/2021 Physical Nature of Soils Before we get to the other descriptive characteristics of the soil lets look at the mineral/inorganic components of the soil 16 Sand Soil particles that fall between 2.0mm - 0.05mm in size Rounded or irregular/angular in shape Cannot be moulded Low water holding capacity Large pore spaces between the grains of sand 17 Silt Soil particles that fall between 0.05mm and 0.002mm in size Irregular in shape Usually a clay film covering the particle Some stickiness and absorption qualities 18 6  9/15/2021 Clay Soil particles that are less than 0.002mm in size Very small particles Large surface area by volume Very sticky and plastic 19 20 Soil Texture relative proportions of sand, silt and clay in a soil 21 7  9/15/2021 Soil Texture by feel Sand feels gritty Silt feels floury Clay feels sticky and pliable Source Plaster 2002 22 Texture determination Texture estimation – use the feel test Determination in the lab The relative proportions are determined by a procedure known as particle size analysis. Sand sizes are determined by sieve analysis The silt and clay content are determined by sedimentation - hydrometer analysis of a sample dispersed in a column of water.. 23 Soil texture determination Sieve the coarse fraction 24 8  9/15/2021 Soil textural class A name assigned to a group of soils having similarity in range of sand, silt and clay. The name is obtained by plotting the results of particle size analysis on a textural triangle Source Strahler 1978 25 Clay – fine texture Silt – medium texture Sand – coarse texture 26 Sand Silt Clay Textural class 92.5 5.4 2.1 sand 61.1 28 10.9 sandy loam 40.0 41.2 18.8 loam 19.6 61.3 19.1 silt loam 3.4 89.3 7.3 Silt 28.6 41.9 31.3 clay loam 10.2 31 58.8 clay Textural class determined by plotting results on textural triangle 27 9  9/15/2021 Significance of Soil Texture Many chemical properties, plant nutrition aspects and physical properties may be broadly assessed from particle size. pore size – plant available water water movement – aeration root penetration surface area – nutrient holding capacity stickiness when moist and solid when dry 28 Soils for Category 1 – 5 athletic fields 29 Soil Structure The arrangement of individual soil particles into definable aggregates (or peds) which are classified on the basis of size, shape, and degree of distinctness 30 10  9/15/2021 31 How Structure is Created in Nature Wet/dry Freeze/thaw Biological activity Cultivation Stabilization organic material – adhesive properties between smaller soil particles to create aggregates -decomposition products, sticky/adhesive - secreted by roots Fine roots, root hairs Inorganic compounds – CaCO3, Fe and Al oxides - some salts aren’t desirable, can promote compaction 32 33 11  9/15/2021 Significance of SoilTexture/Structure Porosity Pore Size Aeration Water Movement Root growth Erosion 34 Soil Texture The relative proportions of soil separates (sand, silt and clay) ▪Soil structure The arrangement of individual soil particles into definable aggregates which can be described in terms of size, shape and stability 35 Soil is 3 dimensional Has horizontal layers – understand what these layers are and how they develop – Process of soil formation 36 12