SOIL AS A MEDIUM FOR PLANT GROWTH 5 



rock, and hence soil may be expected to have a somewhat 

 similar composition to that of the earth's crust. 



It will be seen that two of the important nutrients, as far 

 as plants are concerned, namely phosphoric acid and sulfur, 

 are present in relatively small quantities. Potash, magnesia, 

 lime and iron are present in much larger proportions. This 

 is somewhat the relation in which we are likely to find them 

 in soils, and emphasizes the probable need of phosphoric 

 acid and sometimes sulfur for the maximum production of 

 crops. Potash, in spite of its greater quantity, is often not 

 available in sufficient amount and must be applied as a 

 soluble fertilizer. 



Lime, being easily soluble in soil water, has frequently 

 been leached out of soils in such quantities that it must 

 be replaced. Magnesia is less soluble and hence is rarely 

 lacking. 



6. Soil-forming rocks. — As the earth, which was once 

 a molten mass, cooled, the crust became solid and this solidi- 

 fied material formed igneous rocks, so called to distinguish 

 them from rocks that were formed in other ways. Some 

 examples of igneous rocks are granite, syenite and basalt. 

 Other kinds of rocks, called sedimentary, have been formed 

 from material derived from igneous rock by solution and 

 sedimentation, and later solidified into rock, often under 

 pressure. Limestone, dolomite, shale and sandstone repre- 

 sent some rocks of sedimentary origin. The first two are 

 quite readily soluble in soil water, having been deposited 

 from solution in the process of their formation. Shale 

 is a more or less hardened clay. Sandstone, as its name 

 implies, consists of sand grains cemented together. 



Metamorphic rocks have been formed by heat, pressure, 

 solution and other processes acting on either igneous or sedi- 

 mentary rocks. These forces have frequently produced 

 rocks quite unlike those originally involved in the process. 



