SOILS 



37 



of plants live on the surface of rocks. They feed 

 on the rocks and when they die and decay they keep 

 the surface of the rocks moist and also produce 

 carbonic acid which dissolves the rocks slowly just 

 as the vinegar dissolved the limestone in our ex- 

 periment. 



Dead decaying roots, stems, and leaves of plants 

 form largely the organic matter of the soil. When 

 organic matter has undergone a certain amount of 

 decay it is called humus, and these soils are called 

 organic soils or humus soils. The black soils of 

 the woods, swamps and prairies, contain large 

 amounts of humus. 



Work of Animals. Earth worms and the larvae 

 of insects which burrow in the soil eat soil particles 

 which pass through their bodies and are partially 

 dissolved. These particles are generally cast out 

 on the surface of the soil. Thus these little animals 

 help to move soil, to dissolve soil, and to open up 

 passages for the entrance of air and rain. 



SOIL TEXTURE 



We have seen that the soil particles vary in size 

 and that for the best development of the plant the 

 particles of the soil must be so arranged that the 

 delicate rootlets can readily push their way about 

 in search of food, or, in other words, that the soil 

 must have a certain texture. By the texture of 

 the soil we mean the size of its particles and their 

 relation to each other. The following terms are 



