The soil temperatures that have usually been reported have been

those occurring on the soil surface or at depths less than 3 feet.McClatchie (MCCLATCHIE, A. J. 1904. RELATION OF WEATHER TO CROPS. Ariz. Agr. Expt. Sta. Bui. 48, p. 351-456, illus) conducted soil-temperature investigations in the Salt River Valley of Arizona by the use of three self-registering thermometers

situated underground at depths of 5, 10, and 15 feet. His results during the two years of investigation show that the annual range of temperature decreases with depth, for at 5 feet it varied from 20° to 25° F.; at 10 feet, from 15° to 20°; and at 15 feet, from 10° to 15°. He estimated that in the area under investigation, at a depth of about 50 feet, the soil temperature would remain constant throughout the year.

An Investigation of Temperature Variation at Soil Depths in Parts of Southern Nigeria (American Journal of Environmental Engineering,p-ISSN: 2166-4633    e-ISSN: 2166-465X2012;  2(5): 142-147,doi: 10.5923/j.ajee.20120205.05)

Abstract :Variations in soil temperature at depths have been investigated in different parts of Southern Nigeria. Measurements were carried out at different lithologies and at different times with the aid of a soil thermometer over a three-week period. Readings were taken in three major lithologies which include clay, sand and sandy loam. The daily soil temperature for clayey soil ranges from 27.7℃ – 28.9℃ with a simple mean of 28.3℃; 28.2℃– 29.1℃ for sandy soil and 28.3℃ – 29.0℃ for loamy soil. The mean temperature for all soil types is 28.7℃, indicating that these soils are favourable for farming. A feature of the results is that soil temperature varies more randomly in the sandy soil than the other soil types. The results can be valuable in planning and developing soil tillage practices and other agricultural activities in the areas studied. Some PDFs enclosed for further reading..

The soil temperatures that have usually been reported have been

those occurring on the soil surface or at depths less than 3 feet.McClatchie (MCCLATCHIE, A. J. 1904. RELATION OF WEATHER TO CROPS. Ariz. Agr. Expt. Sta. Bui. 48, p. 351-456, illus) conducted soil-temperature investigations in the Salt River Valley of Arizona by the use of three self-registering thermometers

situated underground at depths of 5, 10, and 15 feet. His results during the two years of investigation show that the annual range of temperature decreases with depth, for at 5 feet it varied from 20° to 25° F.; at 10 feet, from 15° to 20°; and at 15 feet, from 10° to 15°. He estimated that in the area under investigation, at a depth of about 50 feet, the soil temperature would remain constant throughout the year.

An Investigation of Temperature Variation at Soil Depths in Parts of Southern Nigeria (American Journal of Environmental Engineering,p-ISSN: 2166-4633    e-ISSN: 2166-465X2012;  2(5): 142-147,doi: 10.5923/j.ajee.20120205.05)

Abstract :Variations in soil temperature at depths have been investigated in different parts of Southern Nigeria. Measurements were carried out at different lithologies and at different times with the aid of a soil thermometer over a three-week period. Readings were taken in three major lithologies which include clay, sand and sandy loam. The daily soil temperature for clayey soil ranges from 27.7℃ – 28.9℃ with a simple mean of 28.3℃; 28.2℃– 29.1℃ for sandy soil and 28.3℃ – 29.0℃ for loamy soil. The mean temperature for all soil types is 28.7℃, indicating that these soils are favourable for farming. A feature of the results is that soil temperature varies more randomly in the sandy soil than the other soil types. The results can be valuable in planning and developing soil tillage practices and other agricultural activities in the areas studied. Some PDFs enclosed for further reading..

Thermometer might just solve the problem. Although that has to be done several times in different locations of the study area.