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 Soil temperatures 
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 Post subject: Soil temperatures
PostPosted: Tue Feb 16, 2016 3:14 pm 
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Joined: Mon Nov 02, 2009 10:23 pm
Posts: 1864
Location: Maulds Meaburn, Cumbria
Altitude: 183
Weather Station: Davis VPro2
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Found this below - and it is a case of a higher water content increasing the temp'.

Temperature[edit]

Further information: Soil thermal properties, Heat capacity and thermal conduction

Soil temperature depends on the ratio of the energy absorbed to that lost. Soil has a temperature range between -20 to 60 °C. Soil temperature regulates seed germination, plant and root growth and the availability of nutrients. Below 50 cm (20 in), soil temperature seldom changes and can be approximated by adding 1.8 °C (2 °F) to the mean annual air temperature. Soil temperature has important seasonal, monthly and daily variations. Fluctuations in soil temperature are much lower with increasing soil depth. Heavy mulching (a type of soil cover) can slow the warming of soil, and, at the same time, reduce fluctuations in surface temperature.

Most often, agricultural activities must adapt to soil temperatures by:
1.maximizing germination and growth by timing of planting
2.optimizing use of anhydrous ammonia by applying to soil below 10 °C (50 °F)
3.preventing heaving and thawing due to frosts from damaging shallow-rooted crops
4.preventing damage to desirable soil structure by freezing of saturated soils
5.improving uptake of phosphorus by plants

Otherwise soil temperatures can be raised by drying soils or the use of clear plastic mulches. Organic mulches slow the warming of the soil.

There are various factors that affect soil temperature, such as water content, soil color, and relief (slope, orientation, and elevation), and soil cover (shading and insulation). The color of the ground cover and its insulating properties have a strong influence on soil temperature. Whiter soil tends to have a higher albedo than blacker soil cover, which encourages whiter soils to have cooler soil temperatures. The specific heat of soil is the energy required to raise the temperature of soil by 1 °C. The specific heat of soil increases as water content increases, since the heat capacity of water is greater than that of dry soil. The specific heat of pure water is ~ 1 calorie per gram, the specific heat of dry soil is ~ 0.2 calories per gram and the specific heat of wet soil is ~ 0.2 to 1 calories per gram. Also, tremendous energy (~540 cal/g) is required and dissipated to evaporate water (known as the heat of vaporization). As such, wet soil usually warms more slowly than dry soil - wet surface soil is typically 3 to 6 °C colder than dry surface soil.

Soil heat flux refers to the conduction (movement) of energy (heat) in response to a temperature gradient. The heat flux density is the amount of energy that flows through soil per unit area per unit time has both magnitude and direction. For the simple case of conduction into or out of the soil in the vertical direction, which is most often applicable:
q_x = - k \frac{\delta T}{\delta x}
In SI units
q is the heat flux, W·m−2k is the soils' conductivity, W·m−1·K−1. The thermal conductivity is sometimes a constant, otherwise an average value of conductivity for the soil condition between the surface and the point at depth is used.\delta T is the temperature difference (temperature gradient) between the surface and some point within the soil, K.\delta x is the depth to some point beneath the surface, at which the temperature is measured m, and where x is measured positive downward.
Heat flux is in the direction opposite the temperature gradient, hence the minus sign. That is to say, if the temperature of the surface is higher than at depth x the negative sign will result in a positive value for the heat flux q, and which is interpreted as the heat being conducted into the soil.

[45]


Component

thermal Conductivity (W.m‐1.K‐1)

Quartz 8.8
Clay 2.9
Organic matter 0.25
Water 0.57
Ice 2.4
Air 0.025
Dry soil 0.2‐0.4
Wet soil 1-3

Soil temperature is important for the survival and early growth of seedlings. Soil temperatures affect the anatomical and morphological character of root systems.[59] All physical, chemical, and biological processes in soil and roots are affected in particular because of the increased viscosities of water and protoplasm at low temperatures. In general, climates that do not preclude survival and growth of white spruce above ground are sufficiently benign to provide soil temperatures able to maintain white spruce root systems. In some northwestern parts of the range, white spruce occurs on permafrost sites[60] and although young unlignified roots of conifers may have little resistance to freezing,[61] less than half of the "secondary mature" root system of white spruce was killed by exposure to a temperature of 23.3 °C in multiple year experiment with containerized trees from local nurseries in Massachusetts.[62]

Optimum temperatures for tree root growth range between 10 °C and 25 °C in general[63] and for spruce in particular.[64][65][66][67] In 2-week-old white spruce seedlings that were then grown for 6 weeks in soil at temperatures of 15 °C, 19 °C, 23 °C, 27 °C, and 31 °C; shoot height, shoot dry weight, stem diameter, root penetration, root volume, and root dry weight all reached maxima at 19 °C.

However, whereas strong positive relationships between soil temperature (5 °C to 25 °C) and growth have been found in trembling aspen and balsam poplar,[68][69][70] white and other spruce species have shown little or no changes in growth with increasing soil temperature.[68][69][70][71][72] Such insensitivity to soil low temperature may be common among a number of western and boreal conifers.[73]

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Darren Rogers - he who submits the most interesting posts, AKA Mr Data
Maulds Meaburn


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