52 SOIL ABSORPTION 



and harrowing break up the compact structure and tend to keep 

 the soil in a fine textural condition. It is surprising to note the 

 amount of surface exposed by the particles of various types of 

 soils. A coarse sand where the particles range from .i to .5 mm. 

 in diameter has no less than 40,000 sq. ft. of surface exposed in 

 each cu. ft. of soil and a clay soil where the particles are .005 mm. 

 in diameter or smaller has over 140 thousand sq. ft. of surface 

 per cu. ft. 



A second noteworthy feature of soil particles is their power to 

 retain certain elements that are brought in contact with them 

 in solutions. These elements are retained in some instances 

 much after the manner of film water but in other cases retention 

 is effected through chemical reactions. You see an illustration 

 of this property of vsoils when colored solutions, such as the 

 drainage w^ater from stables, after passing through a soil comes 

 out quite colorless. The old type of charcoal filter purified 

 drinking w^ater in this way. It is an important fact that many 

 of the important elements and compounds utilized by the plant, 

 such as potassium, ammonium, magnesium, calcium, phos- 

 phorus, etc., are retained in this way; and they are retained so 

 firmly that only small percentages of them are leached out and 

 lost through the action of running water. Certain aluminum 

 compounds (see above), also iron, calcium and the humus com- 

 pounds derived from dead plants and animals are very important 

 factors in retaining the above mentioned elements. 



21. Relation of the Root Hairs to the Soil Particles. — We are 

 now prepared to understand the significance of the structures 

 noted in the root and their relation to the physical properties of 

 the soil. In the first place we see that these microscopic root 

 hairs can readily penetrate any cavity and they are so delicate 

 and soft that they become moulded about any soil particle in 

 their line of growth (Fig. 30, B). So firmly are they attached 

 to these particles that it is impossible to detach them without 

 injuring the root hairs (Fig. 29, B). This arrangement admir- 

 ably adapts them to gathering up the water and other crude 

 material in the soil. The tubes are filled with organic acids and 

 other substances which have an affinity for w^ater and such earth 



