30 



DESEKT BOTANICAL LABORATORY 



A combination of the above mechanical and physical conditions of 

 the soil and of the presence of harmful substances is offered by the 

 White Sands district in the Tularosa desert in New Mexico, of which 

 a more detailed description is given on pages 5 to 10. The sand dunes in 

 this distinct consist chiefly of gypsum, the principal remaining con- 

 stituents being silicates and calcium chloride in the proportions of 

 3 per cent and i per cent respectively. The gypsum is slowly soluble 

 in cold water and retains the greater amount of .the water which 

 falls upon it. Consequently the dunes are really moist hillocks of a 

 granular structure, the surface layers of which are dried out by the heat 

 of the sun to a depth of a few inches. The dried layer is constantly 

 drifted by the wind, and the exposed layers are dried in turn so that 

 the progressive action of sand dunes is manifested. The underlying 

 layers at some depth often become solidified and stratified, but are 

 easily broken up when exposed to the action of the sun and wind. 

 The moisture included is sufficient for a number of species of plants, 

 but the mineral substances in solution makes it possible for only those 

 forms which are adapted to an alkaline substratum to gain a foothold. 

 The White Sands absorb the entire precipitation and give rise to no 

 distinct streams, but occasional small pools or tanks of water highly 

 charged with calcium salts are to be found in areas among the dunes. 

 In western Austi'alia extensive areas of gypsum desert are to be found 

 which, it is reported, form a distinct harder surface crust instead of a 

 granular layer as in the instance described above but no exact anal- 

 yses of the substratum are at hand. 



A number of districts in America show enclosed pockets or basins 

 forming the extreme lower depressions of ancient lake and river beds, 

 in which the soil is highly charged with salts, the most of which is 

 sodium chloride. Examples of this character are offered by the region 

 around Great Salt lake, Utah, and by the Salton district in the Colo- 

 rado desert of southern California. The characteristic vegetation in 

 both instances is composed of species showing halophytic adaptations, 

 resembling those found near the seashore. 



Limited areas in various regions show soils impregnated with sodium 

 sulphate, sodium carbonate, potassium sulphate, sodium phosphate, 

 sodium nitrate, calcium sulphate, calcium chloride, magnesium chloride, 

 and magnesium sulphate. In agricultural operations two types of 

 such soils are recognized, namely : white alkali and black alkali. 



The nature of the mixture in the soil constituting white alkali is 

 illustrated by the following analyses : 



