TEMPERATURE




Soil temperature plays an important role in many processes, which take place in the soil such as chemical reactions and biological interactions. Soil temperature varies in response to exchange processes that take place primarily through the soil surface. These effects are propagated into the soil profile by transport processes and are influenced by such things as the specific heat capacity, thermal conductivity and thermal diffusivity.

At site X, see map, 2 one meter square plots were roped off. Thermocouple thermometers were placed at depths 1cm, 10cm and 25cm in one plot. In the other plot thermocouples were placed at 1cm and 10 cm. The thermocouple thermometers were recorded every 10 minutes throughout the experiment. Three mercury thermometers were also used to test the accuracy of the thermocouples. The thermometers were placed at 1cm, 10 cm and in the air in the un-mulched plot. The experiment was run for 24 hrs recording temperature to check that every thing was working correctly, then mulch was added to the plot containing only two electrodes. And the experiment was run for another two days to test the effect of the mulch.

The mercury thermometers were read every hour during the time that we worked in the field.

A thermocouple is a combination of two metals, that produce a differing resistance that is temperature dependant. A common thermocouple used today is a combination of Copper and Canstantan. With proper calibration this electrode is accurate in the temperature range -262 C – 350 C (Dike, 1958.).

Copper of high electrical conductivity and oxygen content is very homogenous, and gives a highly reproducible thermoelectric power against platinum.

The constantan used with iron for thermocouples cannot be employed with copper, because of different voltage requirements. It is therefore necessary to use a constantan especially formulated for copper. This alloy is known as Adams Constantan and is not specified as a definite composition, but is any copper-nickel alloy that, combined with copper conforms to ASTM B3-45, matches the Adams copper-constantan table (Dike, 1958.).

It is important to calibrate the electrode to temperature. The EMF produced from the electrode increases linearly with increases with temperature. In the field temperature range, the thermocouple would increase approximately 40 V per degree Celsius that the temperature raises.

‘The simplest mathematical representation of the fluctuating thermal regime in a soil profile, is to assume that at all depths in the soil the temperature oscillates as a pure harmonic (sinusoidal) function of time around an average value’ (Hillel, 1980). At each succeeding depth, the peak temperature is dampened and shifted progressively in time. The degree of damping increases with depth and is related to the thermal properties of the soil and the frequency of the temperature fluctuation.

Plots of temperature verses time were fitted with a sinusoidal function for depths of 1cm, 10cm and 25cm.

The highest peak at 42 degrees is the temperature at 1cm, the second highest peak is the temperature at 10cm and the lowest amplitude is the temperature at 25cm below the soil surface. This data clearly shows how damping increases with depth.

Modifying the soils thermal regime has great advantages for agricultural practices. These include controlling germination and plant growth through increasing or decreasing the soils temperature. This is most effectively done by manipulating the soil surface. To do this such things as mulches or other coverings as well as tillage practices may be used to warm or cool the soil. The overall effect is to modify the temperature amplitude at the soil surface and consequently throughout the profile. Mulches, depending on what they are and their albedo, will either warmer or cool the soil. A highly reflective material will lower the temperature by reducing the radiant flux reaching the soil. A more dense and less reflective material will increase the soil temperature by inhibiting evaporation.

At Narabri, a mulch consisting of straw was applied to the soil surface and the effects were monitored. Figure below shows that the temperature at 1 cm depth was decreased due the mulch absorbing the heat and not conducting it through the profile.
The effects of the mulch include:


Introduction
Moisture content
Moisture Potential
Infiltration & Hydraulic Conductivity
Soil Strength
Soil EC, using EM devices
Overall discussion
References

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