Forest Physiography (1911) pp.56-57
Isaiah Bowman

Influence of Water on Soil Temperature

Water has a predominating influence upon the temperature conditions of soils in humid regions. This is because the capacity of water for heat is about four or five times as great as the heat capacity of the average soil, weight for weight; so that while one unit of heat is required to raise one pound of water 1°, the same change of temperature is produced in a pound of dry sand by the expenditure of .19 unit, and a pound of pure clay requires about .224 unit. Indeed water has the greatest capacity for heat or the greatest specific heat among known substances. This means that when the sun shines upon moist sand or clay a large amount of heat is expended in evaporating the water in it, while a relatively small amount is expended in raising the temperature of the soil particles. A well drained field is therefore warmer on the whole than a poorly drained field and a dry soil warmer than a wet soil.¹ It also follows that a fine-grained soil like clay will have a lower temperature than a coarse-grained and easily drained soil like gravel or sand. Hence clay soils are "cold" and sand soils are "warm." Were the clay and the sand air dry, the clay soil would be the warmer because its volume weight is less than the volume weight of sand. Since, however, few soils in the humid region contain no water, it is clear that clay will always be relatively cold and sand relatively warm under comparable conditions of water content. The following table summarizes the temperature differences between clayey and sandy soils, the table representing observations on a well-drained clay loam and a well-drained sandy loam.

TEMPERATURE CONTRASTS BETWEEN SANDY AND CLAYEY SOILS²

1. The exception to this condition may be noted in the autumn when the warmer soils are those containing the more water on account of the slow radiation of heat by water. The condition may be compared to that of a lake in the temperate zones, which is colder in summer but warmer in autumn and winter than the adjacent land.
2. F.H. King, The Soil, 1905, p. 228.