Soil Temperature

Soil temperature is an important environmental factor in plant growth and distribution. In comparison to air temperature, the amplitude of variation in soil surface temperature is much more pronounced because of the varying characteristics and composition of soil.

Factors Affecting Soil Temperature

• Aspect and slope: These factors are of great importance in determining soil temperature outside the tropics. In the Northern Hemisphere, a south-facing slope is always warmer than a north-facing slope or a level plain. The reverse is the case in the Southern Hemisphere. The difference in soil surface temperature exceeds the difference in air temperature.

• Tillage: By loosening topsoil and creating mulch, tillage reduces the heat flow between the surface and subsoil. Because a mulched surface has a greater exposed area and the capillary connection with moist layers below is broken, cultivated soil has greater temperature amplitude than uncultivated soil. At noon, the air temperature 2.5 cm above the soil surface can be 5 to 10°C higher in cultivated soil as compared to uncultivated soil.

• Soil texture: Because of lower heat capacity, sandy soils warm up and cool down more rapidly than clay soils; hence, they are at a higher temperature during the day and a lower temperature at night.

• Organic matter: Organic matter reduces the heat capacity and thermal conductivity of soil, increases its water-holding capacity, and has a dark color which increases its solar radiation absorptivity. In humid climates, because of a large water content, peat and marsh are much cooler than mineral soils in spring and warmer in winter. However, when organic soils are dry, they become warmer than mineral soils in summer and cooler in winter.

Soil Temperature and Crop Germination

Soil temperatures influence the germination of seeds, the functional activity of the root system, the incidence of plant diseases, and the rate of plant growth (Singh, Singh, and Rao, 1998). Living tissues of many temperate plants are killed when they are exposed to a surface temperature of about 50oC (Chaurasia, Mahi, and Mavi, 1985). Excessively high soil temperatures are also harmful to roots and cause lesions on the stem. Extremely low temperatures are equally detrimental. Low temperatures impede the intake of nutrients. Soil moisture intake by plants stops when they are at a temperature of PC. Root growth is generally more sensitive to temperature than that of aboveground plant parts, meaning that the range between maximum and minimum temperature for roots is less than for shoots and leaves.

In numerous cases soil temperature is more important than air temperature to plant growth. In Canada, sowing of agronomically important crops takes place during the early months of spring when temperatures are well below the optimum. This often results in reducing the rate and success of germination, slow, asynchronous seedling emergence, and poor stand establishment (Nykiforuk and Flanagan, 1998).

Several rice varieties do not emerge as long as the soil temperature is below IPC (Kwon, Kim, and Park, 1996). Germination of warm-season grasses is very poor during the winter season. Slower germination rates during cooler seasons require long periods of soil water availability at the surface to enable germination (Roundy and Biedenbender, 1996). Figure 3.1 shows that cassava plants of variety MAus 10 did not emerge below 14.8°C or above 36.6°C, whereas those of variety MAus 7 did not emerge below 12.5°C or above 39.8°C (Yinet al., 1995). Germination of sunflower, maize, and soybean is very poor when day/night soil temperature is above 21/12°C and soil water content is too low during the first week after sowing (Helms, Deckard, and Gregoire, 1997; Hernandez and Paoloni, 1998).

FIGURE 3.1. Soil temperature and rate of development from sowing to emergence in two cassava cultivars (Source: Reprinted from Agricultural and Forest Meteorology, 77, X.Yin et al., A nonlinear model for crop development as a function of temperature, pp. 1-16, 1995, with permission from Elsevier Science.)

FIGURE 3.1. Soil temperature and rate of development from sowing to emergence in two cassava cultivars (Source: Reprinted from Agricultural and Forest Meteorology, 77, X.Yin et al., A nonlinear model for crop development as a function of temperature, pp. 1-16, 1995, with permission from Elsevier Science.)

In the tropics, high soil temperature causes degeneration of the tuber in potato. Optimum soil temperature for this crop is 17°C. Tuber formation is practically absent above a soil temperature of 29°C. Preconditioning of potato seed under specific temperatures has an important impact on germination. Seed stored at 27°C showed the best germination, while that stored at 45°C failed to germinate even after eight days of lowering the temperature in the germination environment to 17°C (Pallais, 1995).

Impact of Soil Temperature on Plant Growth

After germination, soil temperature is important for the vegetative growth of crops. For each species, a favorable soil temperature is needed for ion and water uptake. The daytime soil temperature is more important than the nighttime temperature, because it is necessary to maintain a favorable internal crop water status to match the high evaporation rate.

Maize yield is closely related to soil temperature at planting. Some cultivars sown at soil temperatures above 30°C show reduced final seedling emergence (Arachchi, Naylor, and Bingham, 1999). Soil temperature controls the rate of maize development while the meristem is underground. Increased soil temperature accelerates the rates of leaf tip appearance and full leaf expansion, enabling the crop to more rapidly attain maximum green leaf-area index. This enables a better synchrony between the time of peak radiation interception and peak radiation incidence. The extent to which soil temperature affects yield will therefore vary with sowing time and the latitude of the crop's location (Stone, Sorensen, and Jamieson, 1999).

Tomato seed germination, plant growth, and fruit yields are governed by the prevailing soil temperature conditions. Germination is completely inhibited at low temperatures (up to 5°C) as well as high temperatures (40°C). Germination is highest at 25 to 30°C. At 10°C, plant growth is slow, almost no fruit formation occurs, and plants start to die off prematurely. At 18°C, the highest growth rates and earliest fruit formation are recorded (Sakthivel and Thamburaj, 1998; Nieuwhof, Keizer, and Van Oeveren, 1997).

Studies on the effect of temperature on root yield and quality of sugar beet show that soil temperature correlated positively with root yield and negatively with sugar content (Hayasaka and Imura, 1996).

The root zone temperature significantly affects the quality and yield of sweet pepper. Growth is more inhibited by low temperature than high temperature. Sugar content is influenced by root zone temperature. Phosphate content is lower at 13 and 33°C root zone temperatures than at other temperatures. Higher numbers of fruits are obtained at 18 to 28°C, and higher yields are obtained at 23 to 28°C than at other root zone temperatures. A 23°C root zone temperature is considered optimal for economic production of sweet pepper (Kim et al., 2001).

Optimal soil temperature for growth of wheat plant roots during the vegetative stage is below 20°C and is lower than that for the shoots. Temperatures higher than 35°C have been shown to reduce terminal root growth and accelerate its senescence. Root growth may cease altogether if soil temperatures drop below 2°C. Studies have shown (Porter and Gawith, 2000) that an air temperature of -20°C is lethal for root survival, although this must be translated into a soil surface temperature, which would, in most cases, be higher.

Cardinal Temperatures

Three temperatures of vital plant activity have been recognized, which are often termed cardinal points.

1. A minimum temperature below which no growth occurs: For typical cool-season crops, it ranges between 0 and 5°C, and for hot-season crops between 15 and 18°C.

2. An optimum temperature at which maximum plant growth occurs: For cool-season crops, it ranges between 25 and 31°C, and for hot-season crops between 31 and 37°C.

3. A maximum temperature above which the plant growth stops: For cool-season crops, it ranges between 31 and 37°C, and for hot-season crops between 44 and 500C.

The cardinal temperatures for germination of some plants are given in Table 3.1. The cardinal points can be measured only approximately because their position is related to external conditions, the duration of exposure, the age of the plant, and its previous treatment.

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