152 SOIL 



acidity in a soil is not well understood but it is known to check the 

 activities of many soil bacteria and other soil organisms and to have 

 a marked effect upon the solubility, and thus upon the availability, 

 of many salts. On the other hand such plants as blueberries {Vac- 

 cinnim), mountain laurel (Kalniia) (Fig. 65) and azaleas {Rhodo- 

 dendron) , grow only in acid soils. It has been found that these plants 

 can be grown in ordinary soils if the soil is kept acid by some means, 

 as by the addition of aluminum sulphate or of tannic acid. These 

 acid-requiring plants, however, are all endotrophic mycorrhizal 

 plants and it seems likely that it is the mycorrhizal fungi rather 

 than the higher plants that require acid conditions. This is 

 made to appear all the more probable by the fact that large num- 

 bers of non-mycorrhizal plants, including many ferns, spring flow^ers, 

 and others, have been found growing in soils ranging from definite 

 alkalinity to high acidity. There are also certain species, such as 

 field sorrel, Rwnex acetosella, which are usually found growing in acid 

 soil but w^hich will grow just as well or better in a neutral soil pro- 

 vided competition with better adapted species is artificially pre- 

 vented. 



In many places in the arid southwestern portion of the United 

 States the concentration of salts in the soil is so great that most 

 plants are prohibited from growing in it. A plant cannot absorb 

 water from a soil in which the soil solution has a greater concentra- 

 tion of salts than that of the cell sap within the root cells of the plant. 

 Very few seed plants can endure a concentration of salts of more 

 than 1.5 per cent of the dry weight of the soil, although the saltwort, 

 Salicornia rubra, has been found growing in a concentration as high 

 as 6.5 per cent near Great Salt Lake in Utah. In White Sands 

 National Monument in New Mexico there are places where gypsum, 

 or calcium sulphate, is so concentrated that only two species of 

 plants, inkweed, Allenrolfea occidentalis, and sand verbena, Abronia 

 angustifolia, are able to grow in it. 



All soils that contain an excess of soluble salts are called alkali 

 soils although the salts concerned may or may not have an alkaline 

 reaction. Chlorides, nitrates, and sulphates have a neutral reac- 

 tion and when such salts are present in excess they usually produce a 

 w^hite incrustation on the surface of the soil. Such a soil is called 

 white alkali. Carbonates of sodium or potassium, on the other hand, 

 are highly alkaline and are likely to produce a dark colored incrusta- 

 tion because of their solvent action on the organic matter in the 

 soil. A soil containing an excess of such salts is called black alkali. 



