Summary SOQ-22803 soil 2 19/20

Soil 2 summary
what you need to know of soil 1

- Soil formation factors
- Weathering (physical and chemical)
- Soil horizons
- Classification

Chapter 1

Weathering is hastened by the appearance of primitive plants on rock surfaces. These plants can store
sun as chemical energy of photosynthesis. It also:

 Decreases free energy
 Increases entropy
 Enhanced by plants (photosynthetic products)
 Results in simpler and small molecules and soluble ions

Soil formation under extensive deciduous forest on calcareous rock in the temperature climate zone:

1. Thin layer of weathered material stabilized by primitive plants
2. Organo-mineral material accumulates
3. Sedge and grass appear that are adapted to harsh environment
4. The litter is mixed in the soil by burrowing animals and decomposition is hastened
5. Larger plants etc. appear and with deeper roots, pushes the zone of rock weathering below the
soil
surface.




Time as a soil forming factor
Parent material  entisoil  mollisol  Alfisol

,1.2 Concepts of soil

The pedosphere is at the interface between the atmosphere, hydrosphere, biosphere and lithosphere.
Soil means a different things to different people: example: hydrologist may see storage reservoir,
farmers the stuff in which to grow plants.

Plant growth

Soil has been used for food production a long time ago. There was a rise in demand for agricultural
products.

Von Liebig determined that plants absorbed nutrients as inorganic compounds from the soil. The
nitrogen is obtained from the soil, cation exchange takes place.

Contemporary view

A compromise between maximizing crop production and conserving a valuable natural resource has
been made. The emphasis is placed on maintaining the soil natural
condition by minimizing the disturbance. The fertilizer has to be as
close to the crop demand.

A soil can be distinguished from inert rock by (alfisol).

1. The presence of plant and animal life
2. Structural organization that reflects the action of pedogenic
processes
3. Capacity to respond to environmental change. E.g. gains and
losses in the profile.

1.3 Components of the soil
Mineral matter

 Clay, silt, sand
 The particle density P p = 2.65 g/cm 3 (average)

Organic matter
3
 P0=1−1.3 g / cm
Water

 Pw = 1.0
 Soil solution = soil water that contains dissolved organic and inorganic solutes

, Soil air = N2, O2, CO2 and other gasses



Chapter 2 the mineral component of the soil
2.1 The size range

- Fine earth passes through 2 mm sieve (zeef)
- The stones or gravel > 2 mm
- Boulders > 600 mm
- Colloidal < 0.001 mm

Distribution

The distribution of particle sizes determines the soil texture.
Frequency distribution = continuous range of particle sizes, which obtained by plotting number/mass
against the size of particles.

There are different ways to determine the texture.

Field texture

Texture class can be estimated in the field by taking a moist sample, squeezing and feeling the soil
constituents.

 Clay; sticky, formable, soft, shiny surface, high plasticity
 Silt: non-sticky, rough and floury surface after squeezing
 Sand: cannot be deformed, feels grainy.

Large stone content (2 mm >)

 Decreases water holding capacity
 Increases drainage
 Promotes uptake of heat energy
 Make cultivation (tillage) hard.

2.3 Mineralogy of the sand and silt fractions
Sand and silt consists almost entirely of the resistant residues of primary rock minerals:

 Predominantly silicates
 Quartz (siO2) or olivine

Isormorphous substitution

Isomorphous = elements of the same valency and coordination number ate substituted/replaced in a
silicate structure.

When elements of same coordination number but different valanecy exchanged  imbalance of charge.
3+ ¿∈ tetrahedron¿
 Si 4+¿ replaced by Al ¿
3+ ¿∈ octahedron ¿
2+ ¿, Fe ¿


 3 +¿replaced by Mg2+¿,Fe ¿
¿
Al