414 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



crop up. Whenever the grass is weeded out, the algae attain a 

 vigorous growth which gradually ceases with the renewed appear- 

 ance of the grass. 



Various scientists have described the presence of algal crust on 

 large areas of soil during drought and how, by the addition of 

 organic matter to the soil due to their decay, the land is made ready 

 for the growth of grasses. Very recently. Booth (1941) has written 

 of a series of scientific experiments that attempted to discover the 

 use of algae in the control of erosion. Several species of algae 

 belonging to the Myxophyceae grow in algal crusts on the residual 

 Red Plains soils of central Oklahoma. This crust of algae forms 

 during the wet seasons on sandy, wind-blown soil and is able to 

 hold the soil in place during heavy winds. The crust is often 

 broken by trampling and then is easily undermined which, together 

 with covering by sand, leads to its final destruction. Hundreds of 

 acres of badly eroded land in the south-central United States were 

 chosen for study. The badly eroded land was due to overgrazing, 

 to cultivating the land for a brief period and then abandoning it, 

 or to frequent burning. Excessive erosion on the cultivated fields 

 had changed the top soil so that the native climax plants were un- 

 able to become established until the slow process of plant succession 

 and the building up of humus in the soil would recreate a suitable 

 environment. In such places soil algae are of great importance as 

 after a short time they are accompanied by mosses and annual seed 

 plants. After study and experimentation on this area. Booth came 

 to the conclusion that several species of soil algae constitute an 

 initial stage in plant succession by the formation of a complete algal 

 layer over these badly eroded acres. This plant cover may last for 

 many years until higher perennial plants are able to form an abun- 

 dant ground cover. The algal stratum does not slow down the rate 

 of infiltration of water into the soil except in one case studied where 

 there is a slight retardation for about the first 7 mm. of water. The 

 soil losses from plots protected by the algal stratum were greatly 

 reduced as compared with the losses from bare areas. The resistance 

 of the algal crust to erosion is evidently the result of the union of 

 the surface particles of soil into a nonerosible layer which is found 

 to be very effective in breaking the force of falling water. Experi- 

 mental tests indicated a higher moisture content in the top inch of 

 soil which has had the protection of an algal layer, than in bare 

 soil. 



Fritsch sums up the chief benefits which the very important soil 

 algae contribute as follows: Their faculty of withstanding drought 

 without appreciable change and without the assumption of special 

 resting stages; their ability to absorb atmospheric vapor as well as 



