508 



STATE BOARD OF AGRICULTURE. 



ences in magnitude among the dififerent mineral soils in the air dry state 

 is not very great, as is the case between these and the peat; while in 

 the moist condition, the difToronce among the various mineral soils 

 is also not highly large but there is a very great ditference between 

 them and the peat. This latter anomaly finds its explanation in the 

 large difference in the water content of the two classes of soils. The 

 variations are indeed very great, as shown in the table below which 

 contains the moisture content of the different soils determined at vari- 

 ous periods, also at the time when the soil cores were taken. 



TABLE 11.— MOISTURE CONTENT OF THE DIFFERENT SOILS UNDER FIELD CONDITION 



AT DEPTH OF 7 INCHES. 



These figures show that the peat usually contains about 25 times as much 

 water as sand or gravel do and about 5 times and 4 times as much water 

 as clay and loam respectively. Now it has been demonstrated that water 

 exerts a tremendous influence upon the thermal conductivity of soil 

 materials both on account of its high specific heat and because of its 

 influence on the structure of the soil. It has been shown also that 

 Avater is a poorer conductor of heat than the soil material in the ratio 

 of 1 :7 when the latter is compacted into a rock ; yet water is by far 

 a better thermal transmitter than air. In a well moistened state a 

 soil may transmit heat many times as fast as in the air dry state. 

 In the dry state the soil grains are surrounded by air which acts r.s 

 an insulation to the passage of heat, consequently the heat transmission 

 is retarded. In the well-moistened condition, however, the air is replac- 

 ed by the water and the soil grains are enclosed by the water films. 

 These water films perform two functions (1) they replace or expel the 

 air as already stated, and (2) they reduce the pore space and thereby 

 bring the soil particles in nioi-e intimate contact or in a more con- 

 tinuous massive condition. It is by producing these effects that water 

 increases so greatly the thermal conductivity of soils so that its own 

 opposing effect, on account of its poorer power of heat transmission, is 

 offset or overshadowed. If water, however, is present in a soil above 

 a certain amount which amount varies with the different soils due to 

 their different water holding cai)acities, the temperature of this soil 

 will rise more slowly because of the very large heat capacity of wat'^r, 

 which is almost five times that of the soil, yet heat will travel faster 

 thru the soil in the more saturated condition than in the moist state. 

 It is on account of the greater -water content and hence of the larger 

 specific heat that the peat shows poorer heat transmitting power than 

 the mineral soils in their natural state as compared in their air dry 

 condition. It is an interesting coincidence that the mineral soils and 

 especially the sandy .soils have both a better thermal conducting po\\'er 

 and a smaller water holding capacity than the peat. These factors 



