THE SOIL 315 



than in moist soils, and it also contains a larger per- 

 centage of nitrogen, so the plants receive the proper 

 supply of food in either case. The light color of soils 

 in arid regions is due to the lack of humus and not to the 

 lack of plant food. Dark soils sometimes lack one or 

 more important elements of plant food. This must be 

 supplied before large crops can be grown. 



218. Bacteria and "Other Living Organisms. Soil is 

 not an entirely dead substance. It is much more than a 

 collection of rock particles containing some water. It 

 is full of life, and without this life the valuable grains 

 and vegetables could not be grown. In order to keep 

 the soil productive it is necessary to keep the helpful 

 living organisms in the soil healthy and well supplied with 

 food. Some of these living organisms are animals, but 

 most of them are the lowest forms of plants, such as 

 molds, and the one-celled plants such as bacteria and 

 yeast. Some bacteria live on the waste products of 

 molds and other bacteria, and thus the plant food is worked 

 over and over until it finally becomes available for use 

 in proper form. If there is not enough humus, or if the 

 humus is not properly decomposed by the living organ- 

 isms, the growing plants will surfer and small crops will 

 result. 



Molds are very effective in breaking down woody 

 organic matter. The root-like portions of the mold 

 soften the woody tissue of plants and then bacteria can 

 work on it with good results. The effects of mold can 

 be seen by keeping some old bread moist for a few days 

 at a temperature of 70 to 90 F. 



Earthworms and a few other small animals help to 

 work up the soil, decompose organic matter, and keep 

 the soil porous by making holes as they travel through it. 



