Lecture 3

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Particle and bulk density• Particle and bulk density measurements are useful for estimating the type of soilminerals present and the degree of soil compaction, respectively.• Particle density is the mass of a particle per volume (Mg/m3 or g/cm3); the volume ofpore space and weight of water are not included in particle density measurements.• Common soil minerals (quartz, feldspars, micas, and clay minerals) have particledensities between 2.60 and 2.75 Mg/m3, and a value often used to represent theaverage soil particle density is 2.65 Mg/m3.• Bulk density is a measure of the mass per volume (Mg/m3) of a soil. Undisturbedsoils are usually used for bulk density measurements so that a true representation ofthe amount of solid present in a particular soil volume can be calculated; withdisturbed soils, natural soil pore spaces are destroyed.• Bulk density is calculated on an oven-dry weight basis and does not take intoconsideration the amount of water present in the soil at the time of sampling. Mineralsoils, unless developed in volcanic ash, generally have bulk densities greater than 1.0Mg/m3 with a common bulk density of a soil with a loam texture being 1.3 Mg/m3.• Soils with high densities (e.g., 2.0 Mg/m3) will likely have slow water infiltration andpermeability, which can result in ponding or surface runoff, and cause a reduction inplant root growth and limited gas flux. Organic soils typically have bulk densities lessthan 1.0 Mg/m3, whereas mineral soils derived from volcanic ash often have bulkdensities in the range of 0.3 to 0.85 Mg/m3.
Soil Solids• Soil minerals are classified as primary and secondary mineralsaccording to their origin.• Primary minerals are those that formed during the cooling of moltenrock and are predominately silicate minerals (Table 3.2). Igneousrocks are composed entirely of primary minerals; metamorphic andsedimentary rocks can contain various amounts of primary andsecondary minerals.• Secondary minerals are formed in soils from soluble productsderived from the weathering of rocks. Clay minerals, among themost important soil secondary minerals because of their largesurface area and reactivity with ionic and dissolved inorganic andorganic compounds, discussed in more detail in the followingsections.• Carbonates and sulfates accumulate in subsoil horizons and areoften the dominant secondary minerals present in soils in arid andsemiarid regions.
Page 3 and 4: The many functions of soil can be g
Page 6 and 7: The soil is home to a wide variety
Page 8 and 9: The pedosphere, where the worlds of
Page 10 and 11: This road cut in central Africa rev
Page 12 and 13: Information important to different
Page 14 and 15: The light- and dark-colored soil bo
Page 16 and 17: Abundant organic matter, including
Page 18 and 19: Diagrammatic representation of acid
Page 20 and 21: Nutrient elements exist in soils in
Page 22 and 23: Elements Essential for Plant Growth
Page 24 and 25: Quantities of Six Essential Element
Page 26 and 27: Definition• Soil is a natural thr
Page 28 and 29: Outline: Soil PhysicalProperties•
Page 31: Particle size and Soil texturalclas
Page 36 and 37: Soil Organic and InorganicCARBON•
Page 38 and 39: Outline: Physical Processes• Soil
Page 41 and 42: Soil Water Movement,Hydrologic Cycl
Page 43 and 44: Soil Erosion• Predicting soil ero
Page 45 and 46: Clay Minerals• Clay minerals and
Page 47 and 48: Clay minerals• Clay minerals of m
Page 49 and 50: Benefits of Organic Matter• Organ
Page 51 and 52: Soil Acidity• The pH of a soil is
Page 53 and 54: Soil Chemical Processes• Mineral
Page 55 and 56: Soil Microorganisms• Bacteria are
Page 57 and 58: Soil Microorganisms• Actinomycete
Page 59 and 60: Soil Microorganisms• Algae, like

Lecture 3

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