Ag science - Physical characteristics of soil

What are the components of soil ?

Mineral matter, organic matter, water and air

What are mineral and organic matter responsible for ?

Providing nutrients for plants and providing the medium to anchor plants to the ground

What are the mineral matter particles from biggest to smallest ?

Gravel, coarse sand, fine sand, silt and clay

How is the drainage for gravel and sand ?

Free-draining due to large pore spaces

How is the aeration for gravel and sand ?

Well aerated due to large pore spaces

How is waterlogging/drought for gravel and sand ?

Can be prone to drought in prolonged periods of dry weather

How is fertility/ion exchange for gravel and sand ?

No contribution to fertility of ion exchange

How is drainage for silt and clay ?

Poor drainage due to small pore spaces

How is aeration for silt and clay ?

Poor aeration due to small pore spaces

How is waterlogging/drought for silt and clay ?

Do not suffer from drought, retain water in dry weather but can become waterlogged after prolonged rainfall

How is fertility/ion exchange in silt and clay ?

Clay is a source of K, P. Ca and Mg ions; its large surface area allows for ion exchange to take place

What is ion exchange ?

This process take place in soil where ions are attracted to soil particles and are held on the surface of these particles. AKA adsorption

What are the physical properties of soil ?

Texture, structure, porosity and density, colour and temperature (Timmy said pink can’t think)

What does an ideal soil for cultivation contain ?

40% sand, 40% silt and 20% clay

What does a loam soil contain ?

Equal amounts of sand, silt and clay

What is soil texture ?

A measure of the proportion of different-sized mineral particles that are found in a soil sample

What is sedimentation ?

The process that occurs when particles of a material settle at the bottom of a container of liquid

The largest particles settle at the bottom with the smaller particles forming layers above them

What is a soil triangle used for ?

To classify a soil sample by the proportions of clay, silt and sand it contains

Plasticity meaning

easily shaped or moulded

What does soil structure describe ?

The arrangement of soil particles within a soil

What is good soil structure necessary for ?

Drainage of excess water, retention of water for plant growth, emergence of seedlings, air movement within soil and root penetration

What is an aggregate ?

A substance formed by combining several separate elements

What are micropores ?

Pores that are formed between the particles within the aggregate

What are macropores ?

As aggregates cluster together, more pores are formed between the aggregate units

What does a poorly structured soil not form ?

Aggregates

When is an aggregate formed ?

When sand, silt and clay cluster together

What are aggregates held together by ?

Clay particles and other polymers (colloidal clay and colloidal organic matter particles)

What is flocculation ?

The clustering together of soil particles to create larger structures called floccules

What type of activities can impact soil structure ?

Climatic, biological, chemical or human activities

Can be classified as cementation of separation processes

What is cementation ?

The binding together of soil particles

What is separation ?

When soil aggregates are broken up within a soil

Large cracks may develop in the soil, which damages its overall structure

What are the factors that affect structural development ?

Freezing and thawing, wetting and drying, soil organic matter, plant root activity, animal activity and cultivation and tillage (Family warned Sarah Peters about cats)

Freezing and thawing

Water in soil expands and contracts, causing a change in the soil volume

This leads to aggregation of soil

Wetting and drying

This caused the soil volume to change as it expands and shrinks

As the soil dries out, the particles are cemented together

When the soil is wet again, the soil breaks again and cracks may form

Soil organic matter

This provides a substrate for building aggregates

The presence of organic matter in the soil will lead to aggregation of the soil, particularly in the upper horizons

Plant root activity

Small roots compact the soil and bind it together

The roots of larger plants and trees can break up the soil, forming cracks in the structure

Animal activity

Small burrowing animals, in particular earthworms, contribute to soil cementation, forming aggregates

Earthworms also contribute to the organic matter in the soil through ingestion and egestion of soil

Cultivation and tillage

Agricultural activities such as ploughing and harrowing break up soil and encourage aggregation

What does porosity mean ?

Describes materials that contain tiny holes or pores that allow gases and liquids to pass through the material

What is air needed for ?

Plant root respiration

What is the main difference between soil air and atmospheric air ?

Higher level of carbon dioxide in soil air and lower level of oxygen in soil air

Why is there low levels of oxygen in soil ?

Plant roots take in oxygen when they are respiring and release carbon dioxide

This leads to a depletion of oxygen in the soil and a build up of carbon dioxide

What is the result of a depletion of oxygen and build-up of carbon dioxide in the soil ?

Negatively affects crop growth and development

The plant would be starved due to a lack of oxygen but also be affected by carbon dioxide toxicity

Diffusion is necessary because of this

What is diffusion ?

The movement of a substance from an area of high concentration to an area of low concentratation, against a concentration gradient

How can compacted soil be broken ?

using a subsoiler

What is a plough pan ?

layer of compaction that can form on top of soil

When does soil compaction occur ?

when soil particles are pressed together and the aggregates are destroyed

Reduces the total pore space in soil

What are the impacts of soil compaction ?

Reduced rate of water infiltration and drainage

Reduced gaseous exchange, stunts plant growth

Plant roots can’t remove water from small capillary pores. This reduces water availability

What are the impacts of soil compaction ? (2)

Plant roots can’t penetrate dense compacted soils so root development is limited.

Repeated tillage operation can cause a plough pan- soil can form a crust after rainfall

Reduces optimum conditions for living organisms in soil habitat. Reduction in nematodes

What are large pores needed for ?

Adequate draining

What are small pores needed for ?

Water retention for plant uptake

How is water held in soil ?

By adsorption and capillary action

Why is water held on the surface of soil particles ?

Because it is polarised and attracted to the negative charges on the soil particles

The water forms a thin film around the surface of the soil particles

The water is not available for plants

Polarised meaning

Having a positive charge at one end and a negative charge on the opposite end of a particle

Hygroscopic water (adsorbed water)

Water that forms a thin film around a soil particle and is held on the surface of the particle by force of attraction

It cannot be removed by the soil and is unavailable to plants

When does capillary action occur ?

When water is drawn into the pores of the soil and is drawn upwards through the soil against the force of gravity

Water molecules are attracted to the soil particles and held to the soil by adsorption

Where is capillary water held ?

In pores with the soil aggregates and between the soil aggregates

What is capillary water available and unavailable for ?

It is unavailable to plants in small pores within the soil aggregates

Is available for plant uptake in large pores between the soil aggregates

Where is gravitational water found ?

in cracks in the soil and in soil pores

Saturation

Large pores are full of gravitational water after heavy rainfall

Field capacity

The amount of water in a soil after the gravitational water has drained away

Permanent wilting point

The point at which no more capillary water can be removed from a soil (by plant roots). Plants will die from drought if soil reaches this point

Available water capacity

The amount of water between the field capacity and permanent wilting point that is available for absorption by plant roots

Why are dark coloured soils warm ?

The can absorb more sunlight

Why is it advantageous to have a sandy/loam soil for crop growth ?

The soil will warm up quicker in springtime

What are the effects of soil temperature ?

Affects the germination and growth of crops

Affects water and mineral uptake by a plant as well as root growth

Can determine the length of a growing season

What are the three main methods of erosion ?

Wind, water and tillage

Water erosion

Precipitation on soil surfaces, particularly on soils with poor drainage, can break down soil aggregates leading to deterioration of the soil structure. Soil compaction and soil capping (hard crust that limits permeability) can increase erosion

Wind erosion

Fine soil particles can be carried away by wind. Hillsides are susceptible to wind erosion due to lack of shelter. Shelterbelts on farms can provide a windbreak and reduce soil erosion

Tillage erosion

the more passes over the soil the more soil is moved. Conservation measures such as min-till or no-till operations can reduce soil erosion on tillage farms

Minimum tillage (conservation tillage)

method of cultivation in which ploughing is not carried out at any stage during the seedbed preparation

Methods of reducing the effects of erosion and sedimentation

Use catch crops to provide vegetative cover on land

Return land to pasture

Install drainage to reduce heavy water run-off, which can erode

topsoil

Reduce effects of compaction as compacted soils have poor

infiltration rates so soil can be eroded by run off water.

Reduce tillage by employing a min-till or no-till system

Carbon sequestrion

The removal of carbon dioxide from the atmosphere

by plant photosynthesis and storing as plant biomass or organic soil matter