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Published byClyde Pearson Modified over 9 years ago
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Physical properties – Texture Texture – proportions of sand, silt, and clay Determines water holding capacity, water availability, nutrient supply capacity
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Soil Texture Proportions of sand, silt, and clay Not OM – nonetheless important Not coarse fragments – nonetheless important
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“Big” smaller really small Sand silt clay Relative Size Comparison of Soil Particles
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The small arrows indicate the proper direction in which to draw the lines. Loam: Unequal proportions, Equal properties
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Texture Surface area per unit volume –1 g sand ~ 0.1 m 2 –1 g silt ~ 1 m 2 –1 g clay ~ 10-1000 m 2 lowest highest Large surface area means more charge so greater ability to hold water and nutrients Coarse textured soils larger pores vs. fine textured soils greater total pore space (volume)
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particle surface area pore volume nutrient supply capacity plasticity and cohesion swelling
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pore size infiltration rate drainage rate aeration
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Influence of Texture SandSiltClay Water-holding capacity Aeration Drainage Nutrient retention LowMediumHigh GoodPoor Medium SlowVery slowHigh LowMedium
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Physical properties Density – Porosity – particle density: mass per unit volume (no air) bulk density: mass per unit volume (with air) Both: no water the volume percentage of the total bulk soil NOT occupied by solids
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But soil properties greatly influenced by – Pore size range Particle heterogeneity & Aggregation –Finer pores – water unavailable, poor aeration, little waterflow, –Finest pores – too small for microbes Pore network Aggregation
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Aggregation influenced by Coarse scale – biotic: –Roots, Burrowing animals (mammals, earthworms) –Sticky networks: root hairs, fungi Fine scale – physical/chemical: –Clay properties: Flocculation, bridging (multivalent cations) –Clay/humus/cation complexes –Cementing: Iron oxides (Ultisols & Oxisols) –Volume changes in clays: shrink/swelling, freeze drying
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