16 SOILS AND FERTILIZERS 



This table represents merely two cases, and is not meant 

 to imply that these losses always occur in just these propor- 

 tions whenever rocks of this type are converted into soil. 

 It will be noticed that some of the most valuable plant-food 

 materials are lost in large quantities. For instance, practi- 

 cally all the lime has been lost, as has also a large propor- 

 tion of the magnesia and potash. Phosphoric acid shows 

 great variation in respect to loss. 



Other changes that occur in weathering include the forma- 

 tion of extremely fine particles that give plasticity to soils, 

 and that have the property of absorbing certain substances, 

 like fertilizers, from solution and holding them in a condition 

 in which they do not leach readily from the soil, and yet in 

 a form in which roots may make use of them. As these 

 particles are very small, we find a relatively large propor- 

 tion of them in a clay soil, but a very small proportion in 

 a sand. 



Another operation that accompanies soil formation is 

 the incorporation of vegetable matter or animal remains — 

 together called organic matter — with the soil particles. 

 This adds greatly to the crop-producing power of a soil, 

 for as the organic matter decays it makes more soluble 

 the inorganic constituents. 



QUESTIONS 



1. Name the agencies concerned in soil formation and trans- 

 portation. 



2. In what way do heat and cold act to decompose rock ? 



3. What is the action of frost on rock ? 



4. How does water aid in the transportation of soil ? 



5. What part did the great glaciers play in soil formation ? 



6. Has wind been more potent as a soil former or as a trans- 

 porter ? 



7. Describe the ways in which roots aid in the decomposition 

 of rocks. 



8. Explain the difference between powdered rock and soil. 



