Origin of Surface Charge

A feature of all the soil mineral particles is that they bear surface charge sites. The charge sites arise because of:

1) Isomorphous Substitution: The substitution of one element for another in the structure of a mineral, without changing the basic structure of that mineral. If the replacing ion is of different valence (charge), the result is either the introduction of a deficit or a surplus charge at that site. For example, substitution of Al+3 for Si+4 in a tetrahedral site, or Fe+2 for Al+3 in an octahedral site, results in a local charge deficit of +ve charge – that is: the mineral attains a negative charge. These are permanent charge sites.

2) Unsatisfied Bonds and Dissociation of Ions: The tetrahedral and octahedral sheets come to an end at the edges of crystals, so the O in these positions both octahedra and tetrahedra have an unsatisfied valence electron. They must satisfy this by associating with cations in solution. These are pH-dependent charge sites.

pH dependence of edge charge sites on phyllosilicates:

Net charge = +1 Net charge = 0 Net charge = -2

at low pH at the isoelectric point at high pH.

(-1)

H+

NOTE: Organic matter also has pH dependent charge sites; they follow the same rules

Cation exchange capacity (CEC) Within a multiple-layer phyllosilicate, the net negative charge from

isomorphous substitution is satisfied by the presence of sufficient positive cations in the interlayers. The cations are the major force holding the layers together in 2:1 and 2:1:1 clays. At the outer surface, half of the net negative charge on the outermost layer is satisfied in the interlayer and half by cations attracted to the –ve surface. These outer cations are not firmly held, and, in water, they hydrate and can ‘stray’ from there positions – this allows them to be exchanged or replaced by other cations that are in the surrounding solution.

Likewise cations attracted to –ve edge charge is subject to cation exchange, as are anions attracted to +ve edge sites (anion exchange).

The amount of –ve surface and edge charge determines the CEC; the amount of +ve edge charge determines the AEC (anion exchange capacity) of the phyllosiliicate minerals

Function and dynamics of the CEC

The CEC serves to retain cations in the soil in a state in which they are readily available for plant uptake, while at the same time protecting

these soluble ions from being readily leached away by water passing

through the soil. This is very important in the soil fertility context.

Phyllosilicate clays: some physical and chemical properties

Property Illite Vermiculite Smectite Chlorite Kaolinite

Type 2:1 2:1 2:1 2:1:1 1:1

d-spacing (Ǻ) 10 14 Variable (>10) 14 7

Size (μ) 0.2 - 2 0.1 - 5 0.01 - 1 0.1 - 2 0.5 - 5

Surface area (m2g-1) 70 - 100 600 - 700 700 - 800 70 - 100 10 - 30

CEC (cmolckg-1) 15 - 40 150 80 - 100 15 - 40 2 - 5

Thickness:Width 1:100 1:1000 1:100 1:10

Oxide mineral and organic colloid properties

Property

Iron oxides

Gibbsite

Allophane

Organic colloids

Size (μ)

<0.1

<0.1

<0.1

0.1 - 1

Surface area

(m2g-1)

80 - 200

100 - 300

100 - 1000

high

CEC

(cmolckg-1)

<3

(pH dep)

<3

(pH dep)

25 – 50+ (pH dep)

50 – 300+ (pH dep)

AEC

(cmolckg-1)

<10

(pH dep)

<10

(pH dep)

5 – 30

(pH dep)

Function and dynamics of the CEC

The cations in the pore water compete for the ‘privilege’ of occupying

CEC sites according to their:

- valence: higher valence = greater competitiveness, hence +3 > +2 > +1

although H+ leads the pack because of its small size

- if valence is equal, the ion that is smaller when hydrated is more competitive the size order is reversed between hydrated and non- hydrated states, hence hydrated K+ is smaller than hydrated Na+.

Cation distribution out from surface

Increasing the concentration of the solution suppresses the distance at which the surface charge is neutralized.

Also, an increase in the

valence of the cation in solution suppresses the distance at which the surface charge is neutralized.

Suppression of the distance at which the surface charge is neutralized allows clay particles to more closely approach one another before they feel repulsion forces. The probability of flocculation increases.

Soil fertility implications

Organic matter and clays, both of which are colloids, are particularly important in the fertility and productivity of soil because:

1) Both have high cation exchange capacities and hold plant nutrients on their surfaces in available forms;

2) Both play a major role, individually and together, in the structural characteristics of the topsoil (A horizon) of soils;

3) Organic residues also release available nitrogen and phosphorus as they breakdown (a slow release process).

Soil Productivity Law of limiting factors

Plant production can be no greater than that level allowed by the growth factor present in the lowest amount relative to the optimum

amount for that factor. (Justus von Liebig) Light Heat Mechanical support Organic matter Nitrogen Phosphorus Potassium Other elements Water Air

Soil Fertility

Soil fertility refers to the chemical aspects of plant growth and the supply of macronutrients and micronutrients that are derived from the soil.

The individual essential elements must be in forms that are

available for plant uptake, where and when plants need them, or plant growth will suffer. If supply is sufficiently low, deficiency symptoms will appear.

The fertility of a soil is strongly dependent on its soil

texture, the amount and type of clay (CEC) and the organic matter content of the A horizon.

Fertility

Maintenance of a soil’s fertility is one of the important aspects of good soil management.

Nutrient uptake depends on the crop grown and yield. Yield/acre N (lb) P2O5 (lb) K2O (lb) Potatoes 800 bu 150 80 264 Wheat 80 bu 144 44 27 Brady and Weil (2008) reported that, for the previous 30 years, export

of agricultural and forest products from farms and forests has resulted in the loss of an average of 22 kg of N, 2.5 kg of P and 15 kg of K per hectare per year from about 200 million ha of Sub-Saharan African soils. The amount exported substantially exceeds the amount imported as fertilizers.

Fertility

The sequence in which deficiency symptoms normally appear when a soil is brought into agricultural production is normally: nitrogen; phosphorus; potassium; ?

Let us consider these BIG THREE elements

Element Reservoir Challenges N Atmosphere Adequate supply Losses – leaching - denitrification

P Rocks Availability

K Rocks Luxury consumption Leaching

The Nitrogen Cycle

From: Brady and Weil, 2008

Nitrogen cycle

Phosphorus cycle

From: Brady and Weil, 2008

Phosphorus additions and losses

From: Brady and Weil, 2008

Potassium cycle

From: Brady and Weil, 2008

Potassium additions and losses

From: Brady and Weil, 2008

Problems of nutrient excess

Nitrogen Excess N leads to weak and sappy growth: cereals are

subject to ‘lodging’ Leaching or erosion of N to surface waters is a major factor

in eutrophication of waterways (excessive algal growth)

Phosphorus While soils have a high ability to immobilize P, this ability

can be saturated by high applications of inorganic fertilizer and/or animal manures

Erosion and leaching of P into surface water greatly enhances eutrophication because P is commonly the

limiting nutrient in surface waters.