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The soil is a major storage media of heat. It is a reservoir of energy during the day and source of heat to the surface at night. The soil also stores energy during the warm season and releases it to air during the cold season.
Soil temperature is one of the physical properties of the soil caused by variations in air temperature and solar radiation. The amount of radiation received by the soil affects soil temperature .
Soil temperature refers to the measurement of the ground’s inherent warmth or coldness. It can also be defined as the function of heat flux in the soil as well as heat exchanges between the soil and atmosphere. It is also defined as the function of the internal energy of the soil.
Soil temperature is an important factors that influence soil properties and processes involved in plant growth. It governs the soil physical, chemical and biological processes. It is a physical property ( that is, soil temperature) that controls the chemistry and biology of the ground and the atmospheric-ground gas exchange. 
Just as temperature is important to plant growth so is temperature important to air and soil. Soil temperature has a direct effect on plant growth and also influence the  physical, chemical, and microbiological processes that take place in soil. Many biological processes, including seed germination, plant emergence, microbial activity, nutrient cycling and soil respiration are a function of soil temperature. Cropping practices including use of cover crops, crop rotations, grazing of crop residues or cover crops, removal of crop residues and tillage have an effect on soil temperature. It is an essential factor that determines the ability of plants to thrive in an environment. It determines the time to sow seeds, whether a seed will germinate and develop, seedling emergence, plant root growth and the availability of nutrients.
The temperature of the surface soil usually fluctuate greatly both during a 24hr period and with the season ( that is ,varies daily and seasonally). This variation in soil temperature is due to changes in radiant energy and energy changes taking place through the soil surface. 

SOURCES OF SOIL TEMPERATURE

1. solar radiation
2. microbial decomposition of organic matter
3. respiration by soil organisms
4. internal source of heat

Solar radiation is the primary source of soil temperature. The rate of solar radiation reaching the earths atmosphere is called solar constant and it has a value of 2cal/sq.cm/min. major parts of this energy is absorbed in the atmosphere, absorbed by plants and also scattered. Only a small part reaches the earth.

HEAT TRANSPORT PROCESSES
Soil temperature is determined by transport process of heat to and within the soil. Heat can be transported in the soil through the following processes:

1. latent heat transfer. 2. Radiation. 3. Conduction. 4. Convection
RADIATION: The main source of soil heat is the energy of sun’s radiation that reaches the earth surface. The exposure of the earth to the heat of the sun warms the surface of the soil on which the sun’s rays falls. Both the earth and the sun emit radiation but because the sun is hotter ( at a higher temperature), the radiant energy is greater for the sun’s surface than for the Earths surface.

CONDUCTION: The interior of the earth is very hot ( core). The conduction of heat within the soil is very slow. The core produces heat that gradually travels within the soil to the surface layer. Generally, at night, the surface of the soil becomes cooler than the subsurface soils. the heat flows from the subsoil layers (warm layers) to the surface soil(cooler layer).

LATENT HEAT: This is the heat that can change matter without raising its temperature . example include humidity. Humid day snows will melt at high soil temperature.

FACTORS AFFECTING RATE AT WHICH SOILS WARMS OR COOL
Soil can become warm up or cool down through the following factors:
A. energy input
B. energy loss
A. ENERGY INPUTS

1. solar radiation absorbed depend on the amount of the radiation received and surface albedo. 2. convection: movement of a heat carrying mass.
a. air mass becomes warmer than soil surface( parking lot effect)
b. Rain that is warmer than the soil
c. condensation of water vapour. 3. conduction: Movement of heat in a body.
a. Heat conductance from warmer soil layer below to above or vice versa.

B. ENERGY LOSS. 1. radiation to colder atmosphere (at night) 2. convection losses
a. to colder air( gaseous exchange)
b. Rain or snow that is colder than the soil
c. evaporation. 3. conduction losses
a. Heat flow to colder soils above or below
Heat conduction
movement of sensible heat
Q= quantity of heat
A= cross sectional area K=thermal conductivity
T= temperature
X= distance
Q=AK(dT/dX)

A soil receives most of its energy inputs during the day and loses energy by radiation and convection at all times, but generally more at night.
The effect of energy gains and/or losses on soil temperature are greatest at the surface and decrease with depth.
The diurnal fluctuation in soil temperature in most soils does not penetrate below 50cm. While the annual fluctuations in soil temperature does not extend below 3m.

FACTORS THAT AFFECT HEAT DOWN THE SOIL PROFILE
Some of the factors that affect the amount of heat dissipated from the soil down the profile include; moisture content and bulk density.
a. SOIL MOISTURE CONTENT: Moisture influences soil heat dissipation down the profile. The flow of heat is higher in a wet soil than in a dry soil where the pores are filled with air. The rate of heat dissipation increases with moisture content.

b. BULK DENSITY: High bulk density increases the soil surface by increases the amount of heat dissipated through the soil surface by increasing the rate at which heat energy passes through a unit cross-sectional area of the soil.

FACTORS AFFECTING SOIL TEMPERATURE
Soil temperature is not a universal value and depends on several constituents.
The average annual soil temperature is about 1°C higher than mean annual air temperature.
Soil temperature is influenced by climatic conditions.

1. environmental factors. 2. soil factors:
a. thermal heat capacity of soil
b. heat of vaporization
c. thermal conductivity and diffusivity
d. biological activities
e. radiation from sun
f. soil colour
g. soil structure, texture and moisture
h. soluble salt
I. slope,
j. vegetation cover,
k. compaction,.

1. ENVIRONMENTAL FACTOR
SEASON AND ATMOSPHERIC CONDITIONS: The distribution of solar energy depends on the season and the absence/presence of sunlight, clouds, and air warmth/coldness. Naturally, the warmer the day is, the warmer the earth is.
In the tropics, during rainy season, the soil are usually cooler due to less sunshine. while during dry season, the land are hotter. In the temperate region, during cold weather, the soil are usually cooler especially during december winter but during summer or hot weather, the ground is usually hotter due to the high intensity of sunshine.

2. SOIL FACTORS
a. THERMAL HEAT CAPACITY OF SOIL:
The amount of energy required to raise the temperature by 1°C is called heat capacity. When it is expressed per unit mass (Calories per gram), then it is called specific heat. The specific heat of water is 1.00cal/g where the specific heat of dry soil is 0.2cal/g.
b. HEAT OF VAPOURIZATION: The evaporation of water from soil requires a large amount of energy, 540kilocal/kg soil
c. THERMAL CONDUCTIVITY AND DIFFUSIVITY
This refer to the movement of heat in the soil.
××The ration between the incoming( incident energy) and the outgoing ( reflective energy) radiation is called albedo.
Albedo=incident energy/reflective energy.

d. SOIL TEXTURE: Clay usually have a higher heat capacity than sand, with equal water content and density . However, sand heats more quickly than clay due to the lower volume of water (lower porosity). Thermal conductivity increases in finer grounds. Nonetheless, factors that influence soil temperature are complex and depend on how they interact. For example, water produces a reverse effect on thermal conductivity.
e. SOIL STRUCTURE: soils with good structure warm up more rapidly because it does not waterlog.
freezing and thawing of the soil water also affect soil structure. Slow and occational freezing and thawing (like under mulch) is beneficial for soil structure.
f. SOIL COLOUR: Physics proves that darker objects absorb more sunlight, and the earth is no exception. Dark coloured soils usually warm up more rapidly than light coloured soils. Dark coloured soils posses a greater capacity to absorb the sun heat. Because dark coloured soils absorb more radiant heat than light coloured soils, dark coloured soils have a higher soil temperature than light coloured soils.
g. VEGETATIVE OR GROUND COVER: Ground cover can be inform of mulch cover, cover crops, crop residue, or vegetative cover or plant canopy cover. Bare lands heat up faster, while any additional layer like mulch or fallen leaf covers on the earth prevents evaporation and cools the ground down .
Mulch materials for example, inhibit evaporation and increase soil moisture. Consequently, these materials also reduce the temperature on the soil surface. Therefore, mulching the surface of the soil serves to insulate heat thereby reducing soil temperature. Generally less heat will flow into a mulched soil compared to bare soils.
Also, A bare soil quickly absorbs heat, becomes hot during the hot season and becomes cold during the cold season. Vegetation acts as a thermal insulator, they significantly block the solar rays from reaching the ground and thus, significantly affects the soil temperature. It does not allow the soil to become either too hot during the dry season or too cold during the rainy season .

h. CLIMATE SEASON: soils in cooler regions are cooler than those in tropical regions. During the cold months of december and January when the air temperature are low, the soil temperature are also very low
I. ORGANIC MATTER : Organic matter increases the water holding capacity, increases water retention and darkens the earth. For these reasons, organic matter content increases the absorption of heat , thereby increasing the soil temperature .
j. ANGLE OF SLOPE: Soil temperature decreases as the slope of land increases. Solar radiation penetrates the ground more intensively when the angle is around 90 degree, that is, right angle and disseminates or scatter more if the field is on a hill. Therefore the amount of radiation per unit area of the land surface decreases as the slope of the land increases. This gives reason why Terrace farming is a great agricultural practice to control the intensity of solar radiation on hilly farmlands.
k. COMPOST AND MANURE: Decomposition is a chemical process with a certain volume release of heat. As soil microbs act on the plants and animal wastes to cure or decompose them, a lot of heat is released, thus, creating heat during decomposition process. In this regard, the temperature of soil on which the decomposition takes place also increases.

L. SOIL MOISTURE: Wet grounds conduct heat vertically better than dry ones . It means dry earth heats up faster during the day and cools down faster at night. However, the water content may affect double ways depending on the earth’s compaction and density – either evaporating from the surface or dissipating in the profile underneath. Cold precipitation cools the earth.
In addition, moist soil has lower temperature than dry soils. most soils get heated very slowly. Also, sandy soils warm quicker than clayey soils. As fine grained soils carry a large amount of water, they are for this reason slow to warm. Specific heat of dry soil is less than moist soil. Hence, moist soils are cooler than dry soils
m. SOIL COMPOSITION: soils partly contain mineral matter and partly organic matter. Hence mineral soils get heated up more rapidly than organic soils.
n. EVAPORATION: The evaporation of water from the soil requires a large amount of energy. Soil water utilises the energy from solar radiation to evaporate thereby rendering it unavailable for heating up of the soil. Thus the greater the rate of evaporation, the more a soil is cooled and its temperature decreases.

EFFECT OF SOIL TEMPERATURE ON PLANT GROWTH
SOIL TEMPERATURE REQUIREMENT OF PLANTS:.
Soil temperature has a great effect on plant growth by influencing water and nutrient uptake, root and shoots growth.
Growing plant requires a certain amount of heat right from the seed germination stage up to the stage where the plants reaches maturity. The soil temperature requirement of plants varies with the species. Each species of plant has a minimum soil temperature below which no germination or elongation (growth) occurs. Above that minimum, elongation rate increases to a maximum with increasing temperature, then falls rapidly. The temperature at which a plant thrives and produce best growth is called optimum range (temperature).
The entire range of temperature under which a plant can grow including the optimum range is called growth range. The maximum and minimum temperature beyond which the plant would die are called survival limits.

EFFECT OF SOIL TEMPERATURE ON
AVAILABILITY OF SOIL WATER AND PLANT NUTRIENTS
Low temperature reduces the nutrient availability, microbial activities and root growth and branching. The ability to absorb nutrients and water by plants reduces at low temperature.

WATER UPTAKE: Temperature plays a significant role in water uptake. Higher temperatures can increase the rate of evaporation from the soil and the plant’s surface, reducing the amount of water available for uptake. Conversely, lower temperatures can slow down the rate of evaporation, allowing the plant to absorb more water. However, extremely low temperatures can freeze the water in the soil, making it unavailable for the plant.
The viscosity of water is also affected by soil temperature. At low temperatures the water viscosity increases and the rate of water uptake by roots decreases.

NUTRIENT UPTAKE: The metabolic activities of micro-organisms play an important role in the cycling of nutrients in the soil and ensuring the nutrients are in a form available to plants. Therefore increased metabolic activities of micro-organisms as a result of increase in soil temperature will stimulate the availability of nutrients for plants. Soil temperature also affects nutrient uptake by changing soil water viscosity and root nutrient transport. Studies had shown that at low soil temperature, nutrient uptake by plants reduce as a result of high soil water viscosity and low activity of root nutrient transport. While at high soil temperatures, soil dryness conditions increases, thus, reduces the rate of nutrient diffusion from the rhizosphere to the absorbing surface of the roots . Generally, higher temperatures promote the decomposition of organic matter and nutrient release, thus providing plants with more nutrients for absorption. Additionally, nutrients in the soil are typically present in a dissolved state, and the increase in soil temperature aids in enhancing the solubility of nutrients. Consequently, the roots of plants can more readily absorb nutrients from the soil. 

ROOT GROWTH: Growth of roots is affected by root-zone temperature. Root-zone temperature can influence plant growth and affect plant productivity. Temperature affects important functions of the root, such as metabolism, growth and morphology, and nutrient and water absorption. Studies had shown that by providing optimum temperature to the roots of crops will result in increase productivity by promoting plant growth and flowering.
Increase in soil temperature will improve root growth due to increase in metabolic activity of root cells and the development of lateral roots. Low soil temperature results in reduced tissue nutrient concentrations and as such decreases root growth..

EFFECTS OF SOIL TEMPERATURE ON WATER MOVEMENT
Water moves from soil with high temperature ( the soil is more dryer) to soil at lower temperature. If freezing occurs, water movement can be substantial. Freezing is similar to drying and water will flow towards a freezing zone, resulting in almost solid packing of the pores with ice, if the water movement to the freezing zone is fast enough

IMPACT OF SOIL TEMPERATURE ON SOIL PROPERTIES

The ground’s thermal conditions can either decrease or increase the biological, chemical, and physical characteristics of various types of soil.

BIOLOGICAL PROPERTIES: The average soil temperatures for bioactivity range from 50 to 75°F (10-24°C). These values are favorable for the normal life functioning of microorganisms that aid the proper decomposition of organic matter , increased nitrogen mineralization, uptake of soluble substances, and metabolism. These temperature range influence soil respiration by increasing the activity of extracellular enzymes that degrade polymeric organic matter in soils and increase microbial respiration rates. On the contrary, conditions near or below freezing point slow down the activities of soil microorganisms. Below freezing points, the microbs are kill, microbial activity are also inhibited thus, decreases mineralization process and decrease in diffusion of soluble substrates in the soil. Decreased microbial activities are the reason for reduced organic matter decomposition and excessive accumulation of undecomposed litter materials and animal wastes on the soil.
a. SOIL MICRO-ORGANISMS: Most soil micro-organisms require temperatures between 10℃-35.6℃ to survive and carry out their activities. At low temperature and freezing points, soil microbial activities decrease and cease respectively.
At soil temperature between the range of 10℃-24℃, soil macro-organisms rate of metabolism increases, therefore, they feed more and rapidly and burn their fat store. At an extreme high temperatures of 58℃ and above, soil macro-organisms die because of the unfavourable temperature of the soil. Soil macro-organisms do not survive in temperatures below freezing point.

b. ORGANIC MATTER DECOMPOSITION: At a temperature below 0℃ , the rate at which microbs carry out their activities decreases. Accumulation of plant matter increases on top of the soil due to the slow rate of decomposition. Also, Organic matter decomposes slowly due to decrease in microbial activities and biochemical processes. At soil temperature between the range of 2℃-38℃, activities of soil microbs are rapid, thus, increasing the rate of organic matter decomposition. Therefore, there will be increase in the movement of soluble substrates in the soil.

CHEMICAL PROPERTIES: Due to decomposed organic matter, high soil temperature regimes show higher cation exchange capacity. The warmer the ground, the more water-soluble phosphorus it contains for plants. Vice versa, low-heated earth is poor in phosphorus available for vegetation. As to pH levels, the acidity rises to a greater degree due to organic acid denaturation.

a. CATION EXCHANGE CAPACITY (CEC)
Increasing temperature generally decreases soil organic matter content. Warmer temperatures accelerate the rate of decomposition of organic matter by soil microbes. This leads to faster breakdown and mineralization of organic compounds, releasing carbon dioxide and nutrients back into the soil. In contrast, colder temperatures slow down microbial activity and organic matter decomposition, allowing organic matter to accumulate in the soil.
When soil temperature increases, organic matter decreases due to its combustion. This decrease in organic matter and reduction in clay size fraction leads to a decrease in the cation exchange capacity of the soil  This gives reasons why increase in soil temperature result in decrease in cation exchange capacity of the soil.

b. AVAILABLE PHOSPHORUS
Increasing soil temperature result in decreased in total phosphorus. In contrast , soil available P increased with temperature.
Studies has shown that between 50°C-250°C, water-soluble phosphorus increases with soil temperature due to the increase in the movement of phosphorus in the soil. Soils with low temperature have low availability of phosphorus because the release of phosphorus from organic material is hindered by low temperature.

c. SOIL pH.
pH is inversely proportional to temperature. An increase in temperature results in lower pH levels while a drop in temperature results in higher pH levels. When soils are saturated and the temperature of the soil increases, the pH of the soil decreases ( becomes more acidic).
 The increase in temperature increases the kinetic ionization of water and molecules within the soil. Due to an increase in kinetic energy, the molecules collide with each other and their molecular vibration increases which causes more ionization in the soil solution. The water will dissociate into hydroxide ions and hydrogen as represented below.

H2O (l) ⇌ H+ (aq) + OH− (aq)

Ionization produces more hydrogen ions which results in a decrease in pH value.
At a soil temperature ranges of 25℃- 39℃ the soil pH increases as a result of organic acid denaturation which increases at high temperature.

EFFECTS OF SOIL TEMPERATURE ON SOIL PHYSICAL PROPERTIES

a. SOIL STRUCTURE
Some of the cementing agents of soil particles include organic matter, clay particles, aluminium oxides etc. These cementing agents can easily be affected by high soil temperature. When soil temperature increases, the binding and cohesive forces between the particles and aggregates become weakened. When organic matter is the main stabilizing agent in soils, this organic matter are very prone to degradation produced by high heating temperatures, thus, producing the destabilization of aggregates.
On the other hand, soils in which inorganic compounds like iron and aluminium oxides and also carbonates are the main soil stabilizing agents, heat can cause increase in soil aggregation . For example, at soil temperature above 30°C, the aggregate stability of the soil increases. This is as a result of thermal transformation of iron and aluminium oxides, causing them to act as cementing agents for clay particles that then form strong silt-sized particles in the soil.

b. SOIL TEXTURE. Soils pores are usually filled with water and air. When soil temperature increases, high rate of evaporation will occur in the soil, crating more air spaces. In clay soil for example, an increase in temperature enhances clay formation. The temperature factor mainly affects clay formation in the Great Soil Groups in the following order: Podzols > Gray Brown Podzols > Brown Earths > Chernozems > Prairies > Red and Yellow Earths (Latosols) > Laterites.
Increase in soil temperature causes temperature induced dehydration of 2:1 clay minerals in the soil leading to strong interactions among the clay particles which in turn yield less clay and more silt-sized particles in the soil. High soil temperature also leads to heat-induced cracks in the sand-sized particles that eventually lead to breakdown and consequently a reduced amount of sand-sized particles in the soil. Therefore, increase in soil temperature lowers the clay, sand contents and increases the silt content.

c. SOIL MOISTURE CONTENT. Reductions in soil moisture occur when increased soil temperatures decrease water viscosity, thus allowing more water to percolate through the soil profile. In addition, reduced shade combined with increased soil temperatures also results in higher evaporation rates which in turn restrict the movement of water into the soil profile.

d. SOIL AERATION. Temperature influences the carbon dioxide content in the soil air. High temperature causes high rate of evaporation of water in the water filled pore spaces creating a more dryer and cool soil. These pore spaces are then filled with more air. Along side with this, high temperature encourages micro-organism activity which results in higher production of carbon dioxide in the soil.

SOIL TEMPERATURE MANAGEMENT. The following are ways of managing high soil temperature.

1. USE OF ORGANIC AND SYNTHETIC MULCHES
: Mulches keep soil cooler in hot weather or summer and warmer in cool weather or winter.

2. SOIL WATER MANAGEMENT: High moisture content in humid temperate regions lower soil temperature

3. TILLAGE MANAGEMENT: Tilling soil to break the natural structure reduces the heat conductance and heat loss. A highly compacted soil looses heat faster than loose friable soil.
METHODS USED FOR MEASURING SOIL TEMPERATURE.

1. Mercury soil thermometer: thermocouple and thermistor based devices are used. 2. infra red thermometer. measures the soil surface temperature. 3. automatic continuous soil thermography. records the soil temperature on a time scale.
The international meteorological organization recommends standard depths to measure soil temperatures. The standard depths are : 10, 20, 40 and 100cm.