It will be noticed that the dry soils are warmer than the wet ones. Why is this? Scientists tell us that it takes a great deal more heat to warm water than it does to warm other substances. Therefore when soil is wet it takes much more heat to warm it than if it were dry. It will be seen that of the dry soils the humus is the warmest. Why? =Experiment.=--Take two thermometers, wrap the bulb of one with a piece of black or dark colored cloth and the bulb of the other with a piece of white cloth, then place them where the sun will shine on the cloth covered bulbs. The mercury in both thermometers will be seen to rise, but in the thermometer with the dark cloth about the bulb it will rise faster and higher than in the other. This shows that the dark cloth absorbs heat faster than the white cloth. In the same manner a dark soil will absorb heat faster than a light colored soil; therefore it will be warmer if dry. Why was the dry clay warmer than the dry sand? Because its darker color helped it to absorb heat more rapidly than the sand, and, as the particles were smaller and more compact, heat was carried into it more rapidly by conduction. Why were the wet humus and clay cooler than the wet sand? As they were darker in color and the clay was more compact than the sand, they must have absorbed more heat, but they also held more water, and, therefore, lost more heat by evaporation. [Illustration: FIG. 32. Charts showing average temperature of a set of dry and wet soils during a period of five days. _H_, humus; _C_, clay; _S_, sand.] [Illustration: FIG. 33. To show the value of organic matter. 1 contains clay subsoil; 2, clay subsoil and fertilizer; 3, clay subsoil and organic matter. All planted at the same time.] Of the dry soils, then, the humus averaged warmest, because, on account of its dark color, it absorbed heat more readily than the others. The dry clay was warmer than the sand on account of its color and compact texture. Of the wet soils the sand was the warmest, because, on account of its holding less moisture, less heat was required to raise its temperature and there was less cooling by evaporation, while the other soils, although they absorbed more heat than the sand, lost more on account of greater evaporation, due to their holding more moisture. Why are sandy soils called warm soils and clay soils said to be cold? How may we check losses of heat from the soil? If we make a mulch on the surface of the soil evaporation will be checked and therefore loss of heat by evaporation will be checked also. The mulch will also check the conduction of heat from the lower soil to the surface and therefore check loss of heat by radiation from the surface. VALUE OF ORGANIC MATTER