Introduction to Soil Systems ESS (5.1) GR12 S1T1

Soil Profile

A vertical section through a soil, and is divided into horizons.

The different types of horizons

1. O organic Horizon

2. A Mixed Mineral organic horizon

3. E eluvial or leached horizon

4. B illuvial or deposited horizon

5. C bedrock or parent material

Soil-forming processes

1. Gains and losses of material to and from the profile

2. Movement of water between the horizons

3. Chemical transformations within each horizon

Gains and losses of material

Inputs: Litter, and inorganic matter
Outputs: uptake by plants and soil erosion

Translocation

Movement of nutrients and minerals within a plant

Leaching

Downward movement of soluble material

Evapotranspiration (EVT)

When water is transferred from land to atmosphere through evaporation, and transpiration

Arid, and Semi-Arid environments with EVT

EVT is greater than preciptation, so the movement of soil solution is upwards through the soil. Water is drawn to the drying surface by capillary action and leaching is generally ineffective apart from during occasional storms.

Functions of Soil

1. Medium for plant growth

2. Soils are able to store accessible fresh water

3. Soils are able to filter the materials that are added onto the soil thus ensuring water quality

4. Soils provide raw materials (eg. peat, clays, sands)

Fertile soils are non-renewable because,

the current rate of usage is faster, compared to the rate it is restored.

Chemical Transformations

1. Decomposition

2. Weathering

3. Nutrient Cycling

Decomposition

1. It changes organic litter onto dark mass (humus).
2. Decomposers & detritivores (fungi, algae, small insects) help w/ degradation. In wet conditions, humification forms peat.
3.In long-term humus then decomposes due to mineralization, which releases nitrogenous compounds.

Weathering

1. It is the process of decomposition and disintegration of rocks in situ.

2. Disintegration (Mechanical weathering) produces smaller, angular fragments of the same rock (eg. scree).

3. Biological weathering, and chemical weathering are interrelated.

4. Weathering helps break down rock and forms regolith—regolith helps form soil.

Biological weathering

Is a process where plants and animals chemically alter rocks and physiclly break rocks through growth and movement.

Nutrient Cycling

1. Nutrient cycles can be sedimentary (nutrients from rock), can also be atmospheric (nitrogen cycle).

2. Gaseous cycles are more complete compared to sedimentary cycles because they are more susceptible to disturbance, especially by human acitivity.

Benefits of soil

1. Supply of roots

2. Supply of water

3. Supply of oxygen

4. Supply of mineral nutrients

Physical Soil Conditions that limit root growth

1. Absence of cracks

2. Dryness

3. Temp are too high/ low

4. Shortage of oxygen due to waterlogging

Chemical Soil Conditions that limit root growth

1. Low nutrient supply

2. Phytotoxic chemicals in anaerobic soil

3. High aluminimum concentration—due to low pH

Soil Texture

1. Shape and arrangement of individual soil particles (called peds)

2. The ideal soil for cultivation is a loam—a balance between water-holding ability and freely draining, aerated conditions.

Loam

A well-balanced soil with significant proportions of sand, silt, and clay.

Advantages of Triangular Data

1. A large number of data can be shown on one graph

2. Groupings are easily recognizable

3. Dominant characteristics can be shown

4. Classifications can be drawn up

Limitation of Triangular graphs

Difficult to interpret and easily confusing

Factors that effect the agricultural potential of a soil

1. The porosity and permeability of the soil

2. The surface area of the soil peds

Pore space

Determines the rate at which water drains through a soil

Surface area of the peds

Determines the amount of water and nutrients in solution that can be retained against the force of gravity.

Terms used to refer to the workability of a soil

1. light

2. medium

3. heavy

Heavy clay soil

holds twice as much water as a light soil/

Light soil (80% sand)

Coarse textured and are easily drained of water and nutrients. Warm up faster compared to heavy clay soils

Heavy soil (>25% clay and are fine textured)

pores are very small, and they have large chemically active surface area which means that these soils are water and nutrient retentive. Clay absorbs water, so that the soil swells when wet and shrinks when dry.

Silt

1. have larger particles than clay, and smaller than sand.

2. It drains faster than clay but slower than sand.

3. Doesnt retain water/ chemicals well, so its potential for farming is reduced.

Ideal soil structure is a crumb structure

peds are small.

How to measure soil structural condition?

porosity—it determines its air capacity and water availability

Types of primary productivity

1. Sandy soil

2. Clay soil

3. Loam soil

Sandy soil

Low primary productivity due to poor-water holding capacity and low nutrient status

Clay soil

Quite low primary productivity due to poor aeration and poor water infiltration

Loam soil

High primary productivity due to medium infiltration rate, water holding capacity, nutrient status, aeration, and ease of working.

Factors that effect primary productivity of soil

1. Mineral content

2. Drainage

3. Water-holding capacity

4. Air spaces

5. Biota

6. Potential to hold organic mattereeeeee

Suitability of Soils for food production (Limiting factors)

1. Light soils—Drought during the growing season because these soils have a poor nutrient, and water holding capacity.

2. Heavy soils—Difficult for arable cultivation. Highly water retentive, have low permeability and field drainage is slow. Drying out is slow. Heavy soils can become waterlogged when wet/ hard when too dry. Number of days they can be ploughed is smaller than other soils.