Soil Temperature and Plant Growth
In order to achieve maximum seed germination and growth, soil temperatures must be at optimum. Crops have different optimum temperatures for germination and growth.
For example, wheat and peas have 4-1000C, corn 10-290C, potatoes 16-210C sorghum and melons 270C, and cabbage, and spinach 8-110C.
By and large, the influence of soil temperature on seedling emergence and growth emanates largely from its influence on the physical, biological and chemical processes going on in the soil.
The decomposition of organic matter and therefore the release of plant nutrients such as nitrogen, phosphorus, and sulphur has the most favorable temperature limits of 27-320C. Likewise, absorption and transport of water and nutrient ions by higher plants are adversely influenced by low temperatures.
Crops adapted to hot regions such as maize, sorghum, or cotton give active root growth during the warm seasons.
The quantity of heat absorbed by the soil is dependent on the amount of solar radiation reaching the soil. A portion of solar radiation is reflected back into the atmosphere by clouds, and atmospheric gases, or is scattered into the atmosphere before it reaches the earth. On the global average, about 50% of solar radiation reaches the ground.
Furthermore, over 30% of the radiation energy reaching the earth is reflected back into the atmosphere or is lost by thermal radiation. The remainder is termed net radiation. About 3% of this net radiation is utilized for photosynthesis, while only about 5-15% of the net radiation is commonly stored as heat in the soil and plant cover.
In general, the factors which affected the temperature of a given soil are the amount of radiation it receives, its albedo, a fraction of incident radiation that is reflected by the land surface, its moisture and air contents, and any factor that affects the rate of evaporation of water from the moist soil. These factors are elaborated upon below:
Effect of Mulch: Mulch of dead vegetation such as cereal straw, stover, legume haulms, etc. immobilize the air within the mulch and still air has a very low thermal conductivity; therefore, heat is only slowly transmitted from the surface of the mulch to the soil surface. Thus the soils under the mulch remain cool, and the organic coverings on the soil act as insulators.
Read Also: Movement of Nutrients in Soil Moisture and Measuring Soil Water
In temperate regions, the use of plastic mulches makes the soil hotter when the sun is shining, but transparent plastic allows greater warming than opaque plastic (Miller and Donhue, 1990).
Soils under organic residue mulches also remain relatively moist. In areas where conservation tillage is practiced much of the crop residues left on the soil surface in this non–tillage practice usually give low soil temperatures.
Effect of Moisture: The heat capacity of water is known to be 3 to 5 times more than for soil minerals. Therefore heat that is absorbed by soil can be lost in evaporating soil water.
Moist soil conducts heat upwards and downwards much better than dry soil, but during a sunny day the surface of dry soil warms up much quicker, and during a clear night cools much quicker than a wet surface.
A temperature difference of 3 to 60C has been recorded between poorly and well-drained soils, and this difference is significant in terms of crop performance, especially in temperate regions.
The soil has to be drained in this instance in order to raise the temperature. Irrigation has been known to increase the thermal conductivity of the soil.
Soil Type: Sandy soils warm up faster than clay soils thus sands are usually referred to as warm soils and clay as cold soils.
Sandy soils have much greater diurnal variations in surface temperature but at depths of 5 to 10cm, the temperature difference between the soils can be small.
Effect of Vegetation: Owing to its insulating property, vegetation reduces both the diurnal and seasonal fluctuation because it intercepts a part or all of the incoming solar radiation and of the back-radiation from the soil.
The effect of vegetation depends on the degree of shading and canopy cover. For example, under complete canopy cover, the leaves will absorb all the incoming solar radiation.
Furthermore, vegetation influences seasonal changes in the soil surface temperatures. Soils under vegetation warm up slowly and cool down slower than uncovered soils.
In temperate and cold regions where crops are usually damaged by low temperatures or frost, the blanketing effect of vegetation cover is used to reduce the penetration of frost into the soil.
In the tropics, vegetation reduces fluctuations in the soil surface temperature, especially maximum temperature at about 14 hours universal time. Tree crops such as coffee and cocoa which do not give complete canopy cover are inter-planted with food crops such as plantain, banana, and cocoyam which are shade-loving.
In conclusion, the composition of soil air affects the nature of the transformation that soil nutrient elements undergo. This, in turn, dictates the usefulness or toxicity of the transformation products to plants.
Read Also: Soil Aeration on Biological Activities and Plant Growth
