188 The Plant World. 



thoroughly discussed by Patten. * Essentially the heat equa- 

 tion of the soil is the balance of, (1), the heat added by radiation 

 plus, (2), the heat added in rain water; against, (1), the heat 

 lost by radiation plus, (2), the heat loss in the drainage 

 water plus (in some cases), (3), the heat "absorbed" by evap- 

 oration from the soil surface. 



This, however, is not the whole of the matter. We are 

 interested not only in the temperature of the soil at any given 

 moment but also in the rapidity with which this temperature 

 will change under any given rates of heat supply and removal. 

 For instance one of the most important questions in soil ther- 

 mics is that of the rapidity with which various soils become 

 warmed in the spring. These questions of rate of change are 

 largely questions of specific heat.t The more heat required 

 to warm a body the more slowly will its temperature rise under 

 given heat supply. Now the solid constituents of the soil have 

 all about the same specific heat, but the specific heat of water 

 is about five times as great. Hence it takes much more heat to 

 warm water than to warm dry soil, and much more heat to 

 warm a wet soil than a dry one. Other things equal the wetter 

 soil will warm more slowly and conversely, cool more slowly. 

 This is a matter of no small practical importance and upon it 

 depends the necessity of arranging that soils be well drained 

 in the spring when tbeir rapid warming is most desirable. 



Turning now to the influence of the nature of the soil on 

 these various thermal questions, we see, first, that the heat 

 addded or subtracted as a result of warm or cold rains need 

 receive no consideration. Such heat movements have nothing 

 to do with the soil as such and are not sensibly affected by its 

 nature, nor under the control of those who handle it. This 

 leaves only the radiations, and of these,the positive radiation only 

 is affected by the soil character. The amount of heat radiated 

 f r o m a soil depends only upon its temperature and not at 

 all upon its individual properties. The amount of radiant 

 energy absorbed is, however, considerably affected by one soil 

 property — the color. In general darker colored bodies absorb 

 and retain a larger proportion of the radiant energy which falls 



*Bull. 59. Bureau of Soils, U. S. Department of Agriculture. 



tThe specific heat of a substance is defined as the quantity of heat necps-ary to 

 raise a given quantity of the substance through a given temp raturc interval. This .same 

 quantity of heat vn\l be given up by the substance in cooling though this same interval. 



