150 SOIL 



are very important to the higher plants though just how important 

 cannot yet be said, at least in many cases. Every soil that contains 

 any humus material has some fungi. The role played by these 

 fungi is by no means well known. Some of them bring about decay, 

 some have mycorrhizal relations with seed plants, and there are 

 probably other interrelations that we do not yet understand. The 

 bacteria are probably of even greater importance than the fungi. 

 There are myriads of bacteria present in all soils: nitrogen-fixing 

 bacteria, nitrifying and denitrifying bacteria, bacteria of fermenta- 

 tion, of decay, etc. As many as a million bacteria have been found 

 in a single gram of soil. There are also many algse, especially blue- 

 green algae, that are found in soils and that probably are of impor- 

 tance to the higher plants. 



Earthworms play a very important role in soil fertility partly 

 through the fact that their tunnels aid in the aeration of the soil 

 and more largely through the fact that they mix the soil by bringing 

 the subsoil to the surface. It has been estimated that in some 

 localities earthworms bring up as much as 18 tons of subsoil per acre 

 annually and it is probable that every particle of soil to a consider- 

 able depth is worked over by earthworms within a few years. 

 Burrowing rodents serve in about the same way and where they are 

 very numerous they may be as important as earthworms. Moles aid 

 to a certain extent in mixing and aerating the soil and, on the other 

 hand, they destroy many plants by burrowing through their root 

 systems. 



Ants are also important soil mixers in some localities, it having 

 been estimated that they may bring as much as \ inch of new soil 

 to the surface each year. There are many other kinds of insects 

 found in the soil, but their activities are not well known. Some 

 counts of the numbers of animals in the surface layer of soil have 

 shown 2,250,000 per acre in a forest and 13,500,000 in a pasture. 

 In both cases a very large percentage of the animals were insects. 



Protozoa, which are microscopic animals, are present in soils in 

 numbers that are comparable to those of the bacteria. Many of them 

 feed upon bacteria. They undoubtedly have important roles to play 

 in the life within the soil but their interrelations with other animals 

 and with plants are still inadequately known. 



98. Soil Acidity and Alkalinity.— Water is composed of 2 parts 

 hydrogen and 1 part oxygen and the chemical formula for the water 

 molecule is HoO or HOH. In any sample of pure water some of the 

 molecules are always dissociated into hydrogen ions (H) and hy- 



