PLANTS AND ANIMALS IN SOILS 155 



soil. To increase the humus of soils, manure is added and green 

 crops, such as Clover, Rye, and others, are plowed under. 



Humus gives soils a dark color and adds much to their fertility. 

 The rich loams of the prairie states have much humus mixed 

 through the sand and clay. Soils properly supplied with humus 

 are loose, thus being well aerated, and they hold moisture well. 

 Such soils afford the conditions for roots and soil organism to thrive. 

 Humus adds a number of substances to soils, some of which are 

 harmful such as organic acids if allowed to accumulate, but usu- 

 ally, due to the activity of soil organisms or other influences in 

 soils, they are either destroyed or changed into useful forms. 



Plants and animals in soils. — Plants and animals, mostly 

 minute and innumerable, make their home in soils and soil fertil- 

 ity depends much upon them. In soils many kinds of Fungi, 

 Algae, Bacteria, Protozoa, worms and insects live. 



The soil Fungi, chiefly Molds, are of many kinds and often 

 several thousand per gram of soil are present. The molds con- 

 sist of thread-like filaments, as Bread Mold illustrates, and these 

 thread-like filamemts spread through the soil, attacking and 

 decomposing the organic matter which is thereby converted into 

 simpler and more soluble substances. Some kinds so act upon 

 the organic matter as to cause an accumulation of ammonia in 

 soils. Ammonia contains nitrogen, which when combined in 

 salts, is the source of nitrogen for higher plants. The larger 

 Fungi, such as Toadstools and Mushrooms, have underground 

 thread-like filaments which also work on the organic matter of 

 the soil. A number of disease-producing Fungi live in the soil. 



A special group of Fungi, called the Mycorrhizal Fungi, live 

 in soils. They grow their thread-like filaments around the young 

 portions of the roots of certain plants, forming mat-like coverings, 

 and in some cases branches of the filaments grow into the cells of 

 the roots. There is evidence that the Fungus performs in part 

 the function of root hairs, absorbing water and mineral substances 

 from the soil and transmitting them to the root. Such a struc- 

 ture, consisting of Fungus and root so united, is called a mycor- 

 rhiza. (Fig. 137.) Pine trees, Beeches, Oaks, Blueberries, and 

 Orchids are some of the familiar plants having mycorrhizas. 

 Some plants, as the Blueberries, are so dependent upon mycor- 

 rhizas that they cannot grow unless the proper mycorrhizal Fungus 

 is present in the soil. 



