THE SOIL 81 



down to 11.79 per cent. 1 In this case, the sandy soil 

 had actually been able to furnish more water for the 

 growth of corn than had the clay. 



79. Relation of Size of Particles to Plant Food. The 

 rock particles are very slowly soluble. Soil water can act 

 on the surface of the particles only. Since smaller par- 

 ticles have more surface for a given volume of soil, they 

 are able to furnish plant food more rapidly. The finer 

 soils are usually more fertile, but are less easily managed. 



80. Relation of the Size of Soil Particles to Air. About 

 half the volume of a dry soil is air; that is, a cubic foot 

 of such soil contains about half a cubic foot of air. The 

 small particles of which a clay is composed do not pack 

 so closely as do the larger sand particles, because they are 

 lighter. Therefore, there is more pore space in clay than 

 in sand. But the spaces in a sandy soil are larger, so that 

 the air moves more freely; hence, such a soil is better 

 aerated. 



81. Size of Particles in Relation to Temperature. The 

 temperature of the soil is influenced by its color, topog- 

 raphy, humus content, and by several other factors 

 But the chief factor is the water capacity. 



It requires about 20 heat units to raise the tempera- 

 ture of 100 pounds of dry soil 1 Fahr. To raise the tem- 

 perature of the same weight of water 1 Fahr. requires 

 100 heat units. But the effect of water is most striking 

 when it evaporates. To evaporate 100 pounds of water 

 requires 966.6 heat units. This explains why wet soils 

 are always cold soils. Clay soils are cold chiefly because 

 of the large amount of water that evaporates from them. 



King, The Soil, p. 161. 

 P 



