2 SOILS AND FERTILIZERS 



it especially for the growth of roots is its permeable structure, 

 which furnishes innumerable channels through which roots 

 may ramify ; another property is its compressibility, which 

 makes it possible for the roots to grow in thickness by 

 forcing together the surrounding particles. The compacting 

 thus effected may be noted in a field of mangels or other 

 large roots at harvest. The firmness of this anchorage is 

 illustrated by the resistance that large trees offer to heavy 

 winds. 



2. Soil as a reservoir for water needed by plants. — The 

 leaves of land plants thrive without being in contact with 

 water, but their roots must have a nearly constant supply. 

 This the soil helps to maintain by catching and holding more 

 or less of the water that falls as rain. The water thus held 

 is in contact with the small roots and root-hairs of plants, 

 and may readily be absorbed by them. 



3. Uses of water by plants. — Plants require moisture for 

 several reasons : (1) Water acts as a solvent for substances 

 that are essential to plant growth, and these substances can 

 be absorbed by plants only when they are in solution. 

 (2) Water is itself a plant-food material and it either becomes 

 a part of the cell without change, or is decomposed and its 

 component parts are used in forming new substances. (3) The 

 cells, of which plants are composed, are kept filled and the 

 plant is more or less firm and erect when its cells are extended 

 with water. When not so filled, the plant wilts. (4) Nutri- 

 tive substances and substances formed from them in the 

 plant tissues are transferred from one part of the plant to 

 another, as occasion requires, by water in the plant. (5) The 

 evaporation of moisture from leaves (transpiration) causes a 

 reduction of temperature in plants, as does evaporation of 

 perspiration from animals. 



4. Soil as a source of plant-food materials. — Plants re- 

 quire for their growth certain nutrient substances, of which 



