Soil temperature regime

Intensity of physical, chemical, and biological processes in soil strongly depends on the temperature of the soil. Therefore, soil temperature regime is important since it affects characteristics and the dynamics of the soil. Heat directly affects many life functions of plants - sprouting, root size and activity, etc. It is therefore important to know the sources of heat, as well as thermal characteristics and temperature regime of the soil. The main source of heat for the soil is insolation. Other sources such as radiation from the core of the Earth, biological oxidation, and so on, are almost insignificant in practice. Heat directly affects plant growth, while indirectly - through humidity, aeration, structure, and microbiological activity – it affects availability of plant nutrients. (Resulović, H., Čustović, H., Pedology, University of Sarajevo, 2002)

Intensity of sunshine, as the primary source of soil warmth, mostly depends on the latitude. There are other local factors that can affect and modify solar energy by increasing or decreasing intensity of solar radiation. The most important are: relief, vegetation, and condition of the soil surface. Temperature is one of the most important factors of soil climate, particularly important for life activity of plants. Growth and development of plants are only possible within certain temperature ranges, different for each species and stage of development. Particularly important is the topsoil temperature regime, that is, mean dates of beginning and end of the period with soil temperatures above certain thresholds for the depths of 5 and 20 cm.

A soil temperature is defined by the relation between the heat received and the heat lost, and the thermal capacity of the soil. Soil absorbs 50–80% of sunshine, the rest is reflected. Absorbed radiation warms up the soil; however, some of the heat is lost when the soil emits it back into the atmosphere, and through evaporation of water. Numerous external and internal factors affect soil temperature regime. External factors include: solar radiation, altitude, exposure, inclination, vegetation, and snow cover. Internal factors include: soil colour, humidity, structure, and texture.

The relief affects the soil temperature through exposure and inclination, which is more obvious in sunny weather, and it decreases as cloudiness increases. South and SE exposures are the warmest, northern are the coldest, and the difference in temperature is greater as the slope is steeper. Vegetation is very important for the temperature regime, e.g., in summer months, it tampers the warming process, while in winter months the situation is reverse - soil under vegetation is warmer. Thermal capacity of soil depends on its colour as well. Dark soils absorb more sunshine and warm up faster and stronger than light soils of high albedo (reflection).

Temperature fluctuations on the surface, whether it is daily, 10-day, monthly, or annual range, decrease with deeper layers. Therefore, variability of mean soil temperatures recorded at the depth of 20 to 40 cm is lower than the variability of mean temperature of air. Daily fluctuations of soil temperature are measurable up to 50 cm of depth, concerning the measuring programme at meteorological stations. The highest amplitude of daily fluctuations occurs on the surface and up to several cm of depth, and it may range between 20 and 40°C; at the depth of 30 cm, it is approx. 3°C, with a tendency to decrease as the depth increases. This is the reason why, for example, soil temperature at the depth of 100 cm, regardless of the season, is measured only once a day, at 14:00 hours.

Temperature of soil in the Federation of Bosnia and Herzegovina, for the reference period 1961–1990, was measured at the main meteorological stations. Measuring was performed at the following depths: 2, 5, 10, 20, 30, 50, and 100 cm.

Different soils have different reactions to heat. Some absorb it well, and contain it well (Černozem), while others absorb it poorly, and lose it quickly through radiation (Pseudoglej – Stagnic luvisol). Thermal characteristics of a soil are defined by its ability to absorb the heat, its thermal capacity, its ability to conduct the heat from one layer to another, and its releasing of heat through radiation. Knowledge of temperature regimes of different types of soil helps us affect the change of its thermal characteristics, primarily, by choosing habitats depending on the exposure, and by selecting crops that are resistant to low temperatures.

The annual fluctuations of temperature in the surface layer of soil are characterised by a maximum typical of July or August, and a minimum occurring in January or February. There is a difference between summer and winter temperature distributions. After the vernal equinox and during summer, soil does not lose all of the absorbed heat - some of it is accumulated. The warmest is the surface layer, and the temperature decreases with depth (summer distribution). In the cold half of the year, the soil does not lose any more of its heat, so the surface is the coldest, and the temperature increases with depth (winter distribution). In autumn, soil temperature at all depths is higher than in spring.

In transition periods (autumn and spring), isotherm (almost identical temperatures at all observed depths) occurs frequently. Such distribution can be partially disturbed, so in autumn, some depths may occur as warmest layers from which the temperature decreases going upwards, and increases downwards, while in spring, some depths may occur as the coldest, and the temperature increases in all directions - both upwards and downwards, which is a result of major changes in the weather at given sites, over a short period.

Temperature thresholds

Calculated and mapped temperature thresholds for different depths are useful information, especially for sowing, but also for growth and development of plants, since different stages of these processes in plants require appropriate temperatures.

Analysis of temperature thresholds of 5, 10 and 15°C, according to the data from the main meteorological stations (see the attached table), shows that onset dates of certain temperature thresholds are earliest in the south, in the Neretva valley. In the mountain - basin part of the country, these dates are the latest, gradually becoming earlier as we move north.

Table 1 - Beginning and end of temperature thresholds for the depth of 5 cm (Ordinal number of days in a calendar year)

5°C

10°C

15°C

Station

beginning

end of the period

beginning

end of the period

beginning

end of the period

Capljina

34

350

83

314

114

292

Mostar

38

348

78

315

111

290

Tuzla

64

328

95

301

122

275

Zenica

65

328

96

300

123

274

Sanski Most

67

331

99

301

126

274

Bihac

69

332

101

300

128

272

Gorazde

70

327

102

298

129

271

Sarajevo

71

323

103

298

131

272

Butmir

73

326

101

296

127

269

Jajce

73

318

104

294

131

269

Bugojno

74

323

104

295

133

268

Drvar

75

322

105

298

131

271

Livno

76

325

106

296

133

268

Ivan-Sedlo

101

309

129

280

165

245

In the south of the country, soil temperatures at the depth of 5 cm exceed the 5°C threshold in early February, and the period ends in mid-December. Compared to the Neretva valley, the beginning of the period is about 30 days late up north, that is, 40 days late in the mountain - basin part of the country, while the end of the period comes early by 20 - 25 days.

Compared to the 5°C threshold, the period with soil temperatures above 10°C, in the south of the country, is shorter by 73 to 85 days. In the valleys and mountain-basin area, this period is shorter and ranging between 54 and 64 days.

Čapljina has the earliest mean date of crossing the 5°C threshold at the depth of 5 cm (3 February). The latest mean date of crossing this threshold in the Federation of Bosnia and Herzegovina is recorded in Livno (16 March). Ivan-Sedlo is a high-altitude station; therefore values recorded up there significantly differ from the others.

For temperature threshold of 10°C, at the same depth, the earliest date of temperature above 10 degrees is recorded in Mostar (17 March), and the latest date for this temperature threshold, if we exclude Ivan-Sedlo, is recorded in Livno (16 April).

The earliest date of crossing the 15°C threshold is recorded in Mostar, (21 April), and the latest is recorded in Livno and Bugojno, (13 May). On Ivan-Sedlo, the latest date for this temperature threshold is 14 June, 165th day in a year.

Table 2 - Beginning and end of temperature thresholds for the depth of 20 cm (Ordinal number of days in a calendar year)

5°C

10°C

15°C

Station

beginning

end of the period

beginning

end of the period

beginning

end of the period

Mostar

37

350

86

319

116

293

Zenica

66

334

99

303

129

275

Tuzla

66

337

100

306

129

277

Sanski Most

71

337

102

305

132

276

Gorazde

72

333

104

301

137

270

Bihac

73

339

99

305

135

274

Jajce

75

328

107

299

139

270

Sarajevo

76

330

107

300

137

271

Butmir

77

329

106

298

134

269

Bugojno

77

326

109

296

142

267

Drvar

80

329

112

298

143

268

Livno

81

330

113

298

147

267

Ivan-Sedlo

104

315

133

284

174

245

For the depth of 20 cm and temperature threshold of 5°C, the earliest multiannual mean beginning date is 6 February, in Mostar, and the latest is 20 March, recorded in Livno. On Ivan-Sedlo, mean date for this temperature threshold is 14 April.

Similarly, for thresholds of 10 and 15°C at the depth of 20 cm, earliest mean dates are recorded in Mostar - 25 March (for 10°C) and 26 April (for 15°C). If we exclude Ivan Sedlo, the latest mean dates for these temperature thresholds are recorded in Livno: 113th day, 23 April (for 10°C) and 147th day, 27 May (for 15°C).

The attached graph shows that end dates of all mentioned thresholds in autumn are always gravitating towards the end of the year, if we compare them to beginning dates and their position towards the beginning of the year, which confirms that soil temperatures in autumn are, on average, always higher than in spring.

In addition, it is obvious that only agrometeorological stations in the south (Mostar and Čapljina) constantly record the earliest beginning and the latest end of the period with certain temperature thresholds compared to other measuring stations.

At the Ivan-Sedlo station, periods with all temperature thresholds exceeded occur the latest and end the earliest compared to all other stations.

The remaining agrometeorological stations record uneven end dates of the periods. At some of them, certain thresholds occur earlier, but also end earlier, or vice versa, which is a consequence of climate variability.

Maps show not only beginning and end of temperature thresholds at the depth of 5 cm, but also mean monthly soil temperatures in March and October, at the same depth. The majority of measuring stations record soil temperatures in March exceeding 5°C, as shown in the table and the map below.

In the south, mean temperatures of soil in March are about 10°C. Mean monthly soil temperatures at 5 cm in October have practical use since they indicate optimal autumn sowing periods. These temperature values are, at the majority of agrometeorological stations in the continent, above 10°C, 15°C in the south, which is still high enough for sprouting and growth of winter crops.

Table 3 Mean monthly soil temperatures at the depth of 5 cm

Station

I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

Year

Bihac

1.6

2.6

5.6

10.7

16.3

20.3

22.5

21.6

17.4

12.1

6.6

2.7

11.7

Bugojno

0.3

1.8

5.1

10.1

15.3

19.2

21.5

21.1

16.8

11.3

5.6

1.5

10.8

Butmir

0.4

1.9

5.2

10.7

16.5

20.3

22.5

21.6

17.2

11.4

6.1

1.3

11.2

Capljina

3.8

5.7

8.7

13.5

18.7

22.5

25.1

24.2

19.6

14.2

9.2

5.2

14.2

Drvar

0.0

1.5

5.0

10.0

15.9

19.9

22.8

21.3

17.1

11.9

5.3

1.7

11.0

Gorazde

0.5

1.8

5.6

10.6

16.0

19.6

21.5

21.1

17.3

12.0

6.1

1.9

11.2

Ivan-Sedlo

-0.2

0.3

1.8

5.5

11.1

15.1

18.3

17.1

13.4

8.7

3.3

1.0

8.0

Jajce

0.1

1.5

5.3

10.1

15.7

19.5

21.9

21.2

17.1

11.2

4.9

1.5

10.8

Livno

0.4

1.6

4.9

9.8

15.3

19.3

21.9

21.1

16.8

11.5

5.8

1.6

10.8

Mostar

3.6

5.5

9.5

14.0

19.5

23.8

27.0

26.2

21.4

15.4

9.2

4.7

15.0

Sanski Most

1.0

2.4

5.9

11.1

16.7

20.7

22.9

22.0

17.9

12.5

6.7

2.4

11.9

Sarajevo

0.6

2.1

5.4

10.3

16.0

19.4

21.8

21.4

17.3

12.3

5.5

1.8

11.2

Tuzla

0.7

2.6

6.5

11.7

17.4

21.3

23.2

22.3

18.2

12.5

6.3

2.1

12.1

Zenica

0.6

2.4

6.2

11.9

17.2

21.0

23.3

22.7

18.2

12.3

6.3

1.9

12.0

Table 4 Mean monthly soil temperatures at the depth of 20 cm

Station

I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

Year

Bihac

2.4

2.9

5.2

11.3

15.0

19.0

21.2

20.6

17.5

12.8

7.6

3.7

11.6

Bugojno

0.9

1.9

4.7

9.4

14.2

18.0

20.3

20.2

16.5

11.5

5.9

2.1

10.4

Butmir

0.8

1.7

4.7

9.9

15.2

19.1

21.4

20.9

17.0

11.6

6.6

1.9

10.9

Capljina

4.6

6.0

8.3

12.5

17.2

21.1

23.6

23.3

19.6

14.7

10.0

6.0

13.9

Drvar

0.9

1.7

4.2

8.9

14.0

17.9

20.8

20.2

16.6

11.8

6.2

2.6

10.5

Gorazde

1.2

2.1

5.3

10.1

14.8

18.1

20.3

20.0

17.0

12.2

6.9

2.7

10.9

Ivan-Sedlo

0.6

0.7

1.6

5.1

10.3

14.3

17.1

16.8

13.7

9.4

4.3

1.6

8.0

Jajce

0.8

1.8

5.0

9.6

14.6

18.3

20.5

20.4

17.0

12.0

6.0

2.5

10.7

Livno

0.7

1.5

4.1

8.7

13.6

17.4

20.1

19.9

16.4

11.8

6.5

2.1

10.2

Mostar

4.0

5.4

8.2

13.1

18.3

22.4

25.3

25.1

21.2

15.9

10.0

5.5

14.5

Sanski Most

1.7

2.4

5.4

10.5

15.6

19.6

21.8

21.5

17.9

13.0

7.5

3.1

11.7

Sarajevo

1.1

2.2

4.9

9.6

14.8

18.1

20.5

20.4

17.0

12.5

6.3

2.6

10.8

Tuzla

1.7

2.9

5.9

10.8

16.0

19.8

21.9

21.6

18.1

13.3

7.5

3.3

11.9

Zenica

1.2

2.8

5.9

11.0

16.0

19.8

22.0

21.6

17.9

12.7

7.1

2.9

11.7