THE BOOT AND THE SOIL 61 



90. The ideal soil for land plants must contain enough 

 plant-food and water fully to supply the plants, and yet 

 be so porous that air can circulate through it and come 

 in contact with the roots. ( Each particle of such a soil 

 is surrounded by a thin film of water, while between the 

 particles are spaces connected with each other, and filled 

 with moist air that is in communication with the air 

 above the soil. The root-hairs (100) apply themselves 

 intimately to the wet surfaces of the soil particles, or 

 reach out into cavities filled with saturated air, and are 

 thus able to draw in the well-aerated soil water, with its 

 dissolved food constituents, in sufficient quantity to re- 

 store the loss from transpiration (74) and to distend the 

 newly formed cells (62) .y 



91. Changes in the soil. — The soil is a scene of con- 

 stant changes. The part of the soil in which the roots of 

 plants grow is the field of most potent vital and chemical 

 activities. The dead remains of plants and animals it 

 chances to contain are undergoing decomposition during 

 the warm season, by serving as the feeding ground of 

 myriads of microscopic plants — bacteria (255). Through 

 their agency nitric acid, which supplies the higher plants 

 with their most valuable food element — nitrogen (254), 

 is formed in the soil. The carbonic acid which the 

 plants take from the air during growth is set free and helps 

 slowly to disintegrate the mineral soil constituents, ren- 

 dering these soluble and thus available as plant-food. 

 In winter, the frost separates the compacted particles of 

 clods, making the latter permeable to air and rootlets, or 

 flakes off new fragments of rock, thus unlocking new 

 supplies of mineral fertility. 



92. The importance of organic matter in the soil. — • 

 Crops secure a large part of their nitrogen, as well as 



