Chemical and Physical Composition of Soils

Fractional volumes of solids and pores varies with location and depth. Thus, bulk density (ρ B = m S / V T ) varies depending on compaction / loosening. Particle density (ρ S = m S / V S ) varies with relative proportions of different solids. Together, ρ B and ρ S determine the porosity (V P / V T ). V P = V T – V S V S = m S / ρ B and m S = ρ B V T V P / V T = 1 – ρ B / ρ S

. ρ S does not usually vary much among mineral soils since the densities of common soil minerals does not vary much. However, packing of discrete particles and their organization into larger secondary bodies (aggregates) greatly affects ρ B, porosity and distribution of pore sizes. Pore-size distribution greatly affects fluid (air and water) flow. Consequently, it greatly affects biological and chemical processes. Sand2.000 – mm Silt0.050 – mm Claysmaller

Very approximately and ignoring the effect of aggregation on pore size, pore-size matches particle size. Sand = big pores Clay = tiny pores Furthermore, mineral particle surface area is inversely related to particle size. Example, for cube V = S 3 andA = 6S 2 A / V = 6 / S This situation is exaggerated for sheet-like clay minerals. V = λS 2 A = 2S 2 A / V = 2 / λwhere λ << S and S is very small

Many of the most important reactions in soils are surface reactions. So the capacity of sandy soils for these is very limited compared to finer-texture soils, particularly fine clay soils. Furthermore, since flow velocity depends on pore size, residence time in coarse soils is short, affecting approach to equilibrium (kinetics).

Shear stress, ____________________ τ S = η dv / dy____________________ Upper plate moving Arrow indicates velocity in fluid Shear force is A τ S For fluid movement in a cylindrical pipe, tube or pore, at equilibrium pressure force = shear force. Pressure force, ΔP (π y 2 ) = -2πyL (η dv / dy)for origin at center, i.e., dv / dy < 0 So, v = (ΔP / 4ηL) (R 2 – y 2 )at any point y Q = ΔP πR 4 / (8ηL)Poiseulle’s Law v AVG = Q / A = ΔP R 2 / (8ηL)

Although one may thoroughly characterize a certain volume of a soil and know texture, density, porosity and pore-size distribution, a nearby volume at the same depth may very well be different. A volume of soil above or below it will very likely be appreciably different. In part, this is due to overburden compaction. In part, this is due to the pedogenic history of the soil which has led to the development of a sequence of vertically oriented zones in the soil that are biologically, chemically and physically different –the profile.

Mineralogy Primary or secondary Primary less stable than secondary and the latter predominate in soils that have been subject to weathering.

Primary Secondary layer aluminosilicatesAl and Fe oxides

Minerals commonly have local irregularities in structure / composition. In many minerals this is so spatially frequent as to contribute to the average composition, e.g., isomorphic substitution in layer alumniosilicates. Al 3+ exists where Si 4+ would otherwise be or Mg 2+ instead of Al 3+. In other cases, the co-precipitation phenomenon manifests as: Inclusion (separate mineral) Surface adsorbed and occluded species Solid solution

Only ~ 2 % of soils are predominantly organic. Although the rest are called mineral, this terminology diminishes the importance of soil organic matter in affecting biological, chemical and physical processes. The importance of organic matter is especially big in coarse soils. Soil organic matter is a vague and broadly inclusive term. It includes everything organic in a soil from living roots, etc., through their identifiable residues, to organic substances found only in soils –humic substances.

Humic substances are understood to derive from organic residues through partial (maybe substantial) structural degradation and concurrent synthesis of other structures. People say there are three types of humic substances – Fulvic acidC 135 H 182 O 95 N 5 S 2 Humic acidC 187 H 186 O 89 N 9 S which are extractable from soils by base and Humin which is not extractable. Some speak of fulvic acids and humic acids to be more precise because these are like snowflakes, i.e., each is unique.

Mineral matter and organic matter are the two general types of solids in soils. Air and soil water are the two types of fluids that fill pores. The soil is alive and respiring. Questions: 1.Gaseous diffusion in soil is rapid, comparable to diffusion in this room (True / False). 2.Therefore, the composition of soil air is much the same as in this room (True / False). 3.Furthermore, the extent to which soil pores are saturated with water neither affects rate of gaseous diffusion nor the composition of soil air (True / False).

Obviously, false to all. Not only is diffusion restricted by limited cross sectional area (pores) and longer path length (tortuous connectivity through pores), but water also occludes pores. O 2 from the above ground atmosphere does dissolve in soil water O 2 (aq) = K H P O2 but diffusion is orders of magnitude slower than in air. As for soil water, the other fluid, it is a solution, like soil air is a solution, and some people call it the soil solution.

It is conceivable that the climate will change where the leftmost profile occurs and it will undergo much more rapid pedogenesis, including mineral weathering, and someday resemble the rightmost profile.

In passing through the Jackson-Sherman sequence, its constituent minerals will be altered by general weather reactions— Dissolution, including acidic dissolution Hydrolysis, in which water is a reactant and is cleaved Complexation, in which structural elements are abstracted into solution Oxidation-reduction, which is structurally destabilizing Hydration-dehydration Structural modifications more extensive than hydration-dehydration but that leave much of the parent structure intact

Assigned Problems (5) K 2 [Si 6 Al 2 ]Al 4 O 20 (OH) 4 (s) + 0.8Ca 2+ (aq) + 1.3Si(OH) 4 0 (aq) = 1.1Ca 0.7 [Si 6.6 Al 1.4 ]Al 4 O 20 (OH) 4 (s) + 2K + (aq) + 0.4OH - (aq) + 1.6H 2 O(l) This is Eq Coefficients are discussed on pages 20 – 21. Starting with formulae given in Eq. s1.6, decide whether you like these coefficients. If you do not like them, say what set of coefficients you do like. Do problems 6, 8, 9 and 12.