Chemical and Physical Composition of Soils

General Composition of Soil

Fractional volumes of solids and pores varies with location and depth.

Thus, bulk density (ρB = mS / VT) varies depending on compaction / loosening.

Particle density (ρS = mS / VS) varies with relative proportions of different solids.

Together, ρB and ρS determine the porosity (VP / VT).

VP = VT – VS

VS = mS / ρB and mS = ρB VT

VP / VT = 1 – ρB / ρS

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ρS does not usually vary much among mineral soils since thedensities of common soil minerals does not vary much.

However, packing of discrete particles and their organization intolarger secondary bodies (aggregates) greatly affects ρB, porosity and distribution of pore sizes.

Pore-size distribution greatly affects fluid (air and water) flow.Consequently, it greatly affects biological and chemical processes.

Sand 2.000 – 0.050 mmSilt 0.050 – 0.002 mmClay smaller

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 = S3 and A = 6S2

A / V = 6 / S

This situation is exaggerated for sheet-like clay minerals.

V = λS2 A = 2S2

A / V = 2 / λ where 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-texturesoils, 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, ____________________

τS = η dv / dy ____________________

Upper plate movingArrow indicates velocity in fluid

Shear force is A τS

For fluid movement in a cylindrical pipe, tube or pore, at equilibriumpressure force = shear force.

Pressure force,

ΔP (π y2) = -2πyL (η dv / dy) for origin at center, i.e., dv / dy < 0

So, v = (ΔP / 4ηL) (R2 – y2) at any point y

Q = ΔP πR4 / (8ηL) Poiseulle’s Law

vAVG = Q / A = ΔP R2 / (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 orbelow 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.

OAE

B

C

Mineralogy

Primary or secondary

Primary less stable than secondaryand the latter predominate in soils that have been subject to weathering.

Primary Secondary layer aluminosilicates Al 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.

Al3+ exists where Si4+ would otherwise be or Mg2+ instead of Al3+.

In other cases, the co-precipitation phenomenon manifests as:

Inclusion (separate mineral)

Surface adsorbed and occluded species

Solid solution

Only ~ 2 % of soils are predominantlyorganic. Although the rest are calledmineral, this terminology diminishesthe importance of soil organic matterin affecting biological, chemical andphysical processes. The importance of organic matter is especially big incoarse soils.

Soil organic matter is a vague and broadly inclusive term. It includes everything organic in a soil from livingroots, etc., through their identifiable residues, to organic substances foundonly 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 acid C135H182O95N5S2

Humic acid C187H186O89N9S

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 neitheraffects 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) andlonger path length (tortuous connectivity through pores), but water also occludes pores.

O2 from the above ground atmosphere does dissolve in soil water

O2(aq) = KH PO2

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 itwill undergo much more rapid pedogenesis, including mineral weathering, andsomeday resemble the rightmost profile.

In passing through the Jackson-Sherman sequence, its constituent mineralswill 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 thatleave much of the parent structure intact

Assigned Problems (5)

K2[Si6Al2]Al4O20(OH)4(s) + 0.8Ca2+(aq) + 1.3Si(OH)40(aq) =

1.1Ca0.7[Si6.6Al1.4]Al4O20(OH)4(s) + 2K+(aq) + 0.4OH-(aq) + 1.6H2O(l)

This is Eq. 1.5. Coefficients are discussed on pages 20 – 21.

Starting with formulae given in Eq. s1.6, decide whether you like thesecoefficients. If you do not like them, say what set of coefficients you dolike.

Do problems 6, 8, 9 and 12.