52 AERATION AND AIR-CONTENT. 



regarded as an important source of energy for the nitrogen-fixing 

 organisms, especially Azotobacter, and this serves to explain the 

 accumulation of unusually large amounts of nitrates in certain Colo- 

 rado soils. This has been confirmed by Hutchinson (1918), who has 

 shown that the activity of Azotobacter is appreciably increased by 

 the addition of plant residues to the soil. 



Moore and Karrer (1919 : 281) have studied the subterranean 

 algal flora of several soils from Missouri, Massachusetts, and Cali- 

 fornia, and have concluded that this flora is independent of the 

 nature of the soil and locality. The variety of algae was not large, 

 though comparable with that on the surface. Protoderma viride was 

 the most common species by far, while the blue-green algse supplied 

 the majority of the other forms. The former occurred to the great- 

 est depth, 1 meter, and in every soil, indicating its special ability to 

 live underground. It is concluded that the algal flora rises from 

 the surface forms, but that its persistence at considerable depths 

 indicates that the algse actually grow in the soil, since it is assumed 

 that surface waters are unable to carry them so deeply in such 

 compact soils as clay. 



Bristol (1920 : 35) has found 64 species of algse in soils from 

 widely separated locahties. Of these, 20 are diatoms, 24 blue-green 

 algse, and 20 yellow-green algse. The most common species are 

 Hantzchia mnphioxys, Trochiscia aspera, Chlorococcum humicola, 

 Bumillera exilis, and Ulothrix subtilis variabilis, with much moss 

 protonema. These were found to be able to withstand complete 

 desiccation for 4 to 26 weeks. The author suggests that the presence 

 of algae in the soil must affect the soil gases. 



Summary. — It has been shown by a number of workers that algae 

 may bring about the supersaturation of lakes and streams as a con- 

 sequence of photosynthesis. This frequently amounts to 200 to 300 

 per cent of saturation and has been found to reach 550 per cent. At 

 the same time, algae and other water-plants prevent the accumula- 

 tion of large amounts of CO2 as a result of the same process. Brizi, 

 and Harrison and Aiyer, have shown the importance of the algal film 

 of rice-fields for the aeration of the roots, and Bergmann has demon- 

 strated the value of Philotria and Spirogyra in furnishing a supply 

 of oxygen for submerged roots. Esmarch, Robbins, Moore and 

 Karrer, and Bristol have studied the algal flora of soils, and the 

 first two have concluded that the algae are an important source of 

 organic material for nitrogen-fixing bacteria. Bristol suggests that 

 the presence of algae may affect the soil-gases. It seems certain 

 that this must be the case in soils sufficiently moist to permit their 

 growth, and that they then increase the aeration and prevent the 

 harmful action of carbon dioxid in the same manner as in rice- fields. 



