THE FACTORS OF DISTRIBUTION 47 



extent of surface is important, as it determines the rate 

 at which the particles could be dissolved, the amount of 

 water that can adhere as a film round the soil-particles, 

 and the absorbent capacity of the soil. It has been 

 roughly calculated that a cubic foot of light loam has 

 thus an acre of surface; clay more, and sand less. 



Speaking generally, it may be said that the finer the 

 particles of a soil and consequently the larger the total 

 surface of the particles the more fertile it is. Ordinary 

 clay contains 400,000 million particles per ounce; land 

 suitable for potato cultivation from 250,000 to 350,000 

 million ; for corn about 280,000, and for onions 350,000 

 to 450,000 million. 



As the pore-spaces are filled either with air or water, 

 the air-content and water-content of a soil necessarily 

 vary inversely. Many physiological processes take 

 place in the soil that depend on the presence of air. 

 Oxygen is required for the germination of seeds, for the 

 respiration of roots; and for the bacteria which manu- 

 facture nitrates, whether from free atmospheric nitrogen, 

 or from humus, manure, or other decaying organic 

 matter. Nitrogen is of course also necessary for the 

 former of these last-named processes. About half the 

 pore-space in the soil should, for the healthy nutrition 

 of ordinary plants, be occupied by air; and plants, 

 especially those grown in pots, often suffer from an excess 

 of water. 



The water-content, or total amount of water in soil, 

 depends mainly upon rainfall; but the texture of the 

 soil determines its power of retaining water the slope 

 of the surface influences the rate at which it drains off 

 and the moisture of the air controls evaporation. The 

 water occurs in the soil under three conditions. It 

 may be (i) free, percolating downwards in porous, 

 coarse-textured soils, so filling the pore-space as to check 

 the respiration of the roots, and thus proving injurious 

 to most plants other than bog-plants. The removal of 

 this free water is the object of drainage. 



(2) Capillary water is that which adheres to the soil- 

 particles, especially in fine-grained soils, in films thick 

 enough to move slowly, by capillarity or surface-tension, 

 upwards, or in any direction where the soil is relatively 

 drier. This is the chief part of the water available to 

 the root-hairs of plants. 



