Week 8 - Soil Temperature Flashcards
What does soil temperature influence?
- plant processes
- breakdown of organic matter
- chemical reactions in soils
- evaporation of water from the soil profile
- soil formation
List the factors that influence the temperature of field soils either directly or indirectly
- the net amount of heat energy arriving at the soil surface;
- the heat energy required to bring about a given change in the temperature of a soil;
- the transmission of heat to or from depth in the soil
- dissipation of heat from soil surface.
How does soil temperature influence plant processes?
Plants vary widely in their optimum temperature requirements and this is also dependent on their stage of development, for example:
- Maize requires a minimum soil temperature of 7-10ºC for germination but the optimum temperature preferred for root growth is 25ºC.
How does soil temperature influence the breakdown of organic matter?
- Microbial activity is regulated by soil temperature and therefore the decomposition of soil organic matter is directly influenced by soil temperatures.
- For example, the microbial mineralisation of soil organic matter has an optimum temperature between 20 - 30ºC.
- Below a temperature of 10ºC little mineralisation occurs.
How does soil temperature influence chemical reactions in the soil?
- Temperature influences the rate of chemical reactions occurring in soils.
- Generally reaction rates double for each 10ºC rise. Above 30 to 35ºC, enzyme reactions are reduced.
How does soil temperature influence evaporation of water from the soil profile?
- As temperature rises and humidity of the atmosphere decreases, evaporation of soil moisture increases.
How does soil temperature influence soil formation?
- Both physical and chemical weathering of parent materials and soils is enhanced by increasing temperature.
- As parent materials heat, the different mineral fractions expand at different rates leading to cracking of rocks. Also with increasing temperature, chemical reaction rates increase.
- As a result, the rate of soil formation increases with increase in temperature.
The temperature of soils in the field is dependent directly or indirectly on what factors?
- the net amount of heat energy arriving at the soil surface;
- the heat energy required to bring about a given change in the temperature of a soil;
- the transmission of heat to or from depth in the soil
- dissipation of heat from soil surface.
How does the net amount of heat energy arriving at the soil surface impact soil temperature?
Solar radiation (sunlight) is the primary source of energy to heat soils.
Soils normally gain heat by absorbing sunshine (short wave radiation)
What are the non-soil factors affecting heat accession by soils?
- Gaseous composition of the atmosphere
- Angle of the sun to the soil surface - the closer the angle of the sun is to 90° at the soil surface, the greater is the amount of short wave radiation received.
- Amount of vegetative mulch
How does the heat energy required to bring about a given change in the temperature of a soil influence the temperature of soils?
- The specific heat capacity of soils of individual soil constituents is the amount of energy necessary to raise heat of a given mass of soil a given temperature, usually 1°C.
- The higher the specific heat capacity of a soil component, the greater the amount of energy required by the material to raise the temperature by the specified amount.
- Air has a very low heat capacity, water has a relatively high heat capacity, considerably higher than that of the soil solid component. Therefore if two soils have different moisture contents, the soil with the greater amount of moisture will require a greater input of energy to heat to the same temperature as the drier soil.
How does the transmission of heat to or from depth in the soil influence soil temperature?
The flow of heat through soils is influenced by three components:
- Cross sectional area of the heated surface;
- Temperature gradient between the heated surface and the lowest surface; and
- The thermal conductivity of the soil minerals.
What does the rate of heat flow through soil depend on?
- Soil moisture content
- Bulk density
- Dissipation of heat from soil surface
What are the main management strategies for managing soil temperature?
- Aspect: In the southern hemisphere, especially in the higher latitudes, north and west facing slopes have higher temperatures than adjacent flat lands. South facing slopes are cooler.
- Cultivation: Cultivation opens up the soil increasing the proportion of air filled pores, that is the bulk density is decreased.
- Irrigation: Management of the soil moisture content can change the rate of and depth to which soils heat up or cool down.
- Organic mulches: Organic mulches decrease the extent of temperature variations. Mulches contain a large proportion of air filled pores reducing the thermal conductivity.
- Plastic covers: These increase temperature by allowing heat energy in preventing its exit, decreasing evaporation and creating an air pocket under the plastic
Which factors are important in the acquisition of heat by soil.
- Gaseous composition of the atmosphere
- Angle of the sun to the soil surface
- Amount of vegetative mulch
