20 



moist atmosphere. Man naturally observes first the latter feature, 

 which is so important to him, and then associates it with the budding 

 of the plant, but he recognizes his mistake when he considers that the 

 plant is firmly established in the earth and that its nourishment and 

 growth must depend primarily on the condition of the soil and roots. 



TEMPERATURE AND MOISTURE OF THE SOIL. 



The temperature of the soil a short distance below the immediate 

 surface does not depend, by way of cause and effect, priniarily on the 

 temperature of the air. It is not warmed or cooled appreciably by 

 conduction of atmospheric heat, but by direct absorption or loss of 

 the radiation that falls upon it. To a slight extent (perhaps 5 per 

 cent) this sunshine is reflected from the surface particles of the 

 ground according to the laws of simple reflection; the remainder is 

 absorbed by the surface and warms it. This warmed surface layer 

 immediately radiates back a small quantit}^ (10 per cent) as long- 

 waves into the atmosphere and through that into space, since the 

 atmosphere does not absorb these long waves, but it gives up a larger 

 part, perhaps ^0 per cent, by conduction to the adjacent lowest 

 layer of air, which being thus warmed quickly rises and by convection 

 distributes this 50 per cent of heat throughout the atmosphere, whence 

 it is eventually radiated back into space. The remaining 40 per cent 

 of the solar heat is by conduction carried downward through the solid 

 earth; a large portion is consumed in the evaporation of soil water 

 and returns to the atmosphere with the aqueous vapor ; the rest goes 

 on downward, warming up the soil until it arrives at a layer 30 to 50 

 feet below the earth's surface, where the gi'adient of temperature 

 just in front of it is the same as that just behind it. Here the heat 

 would accumulate and push its way still deeper were it not that by 

 this time, in most cases, the diurnal and annual changes of tempera- 

 ture at the earth's surface, where this heat wave started, have brought 

 about a deficiency just below the earth's surface; consequently the 

 heat that had reached the depth of 30 or 50 feet now finds the tem- 

 perature gradient just above it beginning to reverse, wherefore this 

 heat begins to flow back, upward, and outward. In this manner the 

 temperature of the ground increases downward to a depth of a few 

 yards during certain months and then upward during other months, 

 in diurnal and annual fluctuations interspersed with irregular 

 changes, depending on cloud and wind and rain, all of which are easily 

 recognized by examining any system of curves representing the earth 

 temperatures at different depths throughout the year. 



The ground is warmed by the air only in case the temperature of 

 the surface soil is lower than that of the air, and, although this 

 happens frequently, yet the quantity of heat thereby communicated 



