Factors Affecting Growth and Reproduction 221 



Bog soils, which are naturally acid, must be neutralized when 

 reclaimed for the growing of celery, onions, cabbage, and mint. 

 These soils are also deficient in potassium, and this element must 

 be supplied in some form to obtain the best yields. 



Alkalinity of soils. In arid regions, the evaporation of water 

 may cause salts to accumulate in the surface layers of the soil to 

 such an extent that most or all plants are excluded. About many 

 of the lakes in the Great Basin region are alkali lands of this 

 kind. Various salts have been leached from the rocks and 

 minerals of the mountains, and washed down into these lake 

 basins, which have no outlets. This has been going on for 

 thousands of years and the water that carried them has evapo- 

 rated, leaving the salts behind. Some of these salts, like sodium 

 chloride (common table salt), sodium carbonate (washing soda), 

 sodium sulfate (black alkali), and borax, are poisonous to plants. 

 Others are not poisonous, but when present in considerable 

 amounts interfere with the absorption of water by roots. When 

 the concentration of salt is slight and relatively pure water is 

 available, these lands may be irrigated and drained and a part 

 of the alkah removed. These lands then become valuable for 

 agriculture. The cultivated sugar-beet and alfalfa lands near 

 Salt Lake City are of this character. 



Humus. Another soil factor of great importance is humus. 

 This material, which gives the brown and black colors to rich 

 agricultural land, is composed of the partially decayed remains 

 of plants. Leaves and other plant organs that fall to the ground 

 are slowly changed and broken up by bacteria, fungi, and other 

 agencies until only the brown, powdery humus remains. Moist 

 or wet grasslands accumulate more humus than forested lands, 

 because so large a part of the plant is underground where the 

 decay is slower, and because these lands are covered with water 

 during a part of the year so that there is less oxygen available 

 for completely oxidizing the plant remains. 



Humus favors plant growth by increasing the water-holding 



