IMPORTANCE OF SOIL 



213 



etc.) to enrich the soil with nitrogen. This method of 

 farming is called rotation of crops. 



Nitrogen, phosphorus, potassium, and calcium, the 

 chemical elements most needed by plants, may be 

 added to the soil by the use of commercial fertilizers. 

 About a half billion dollars is spent in the United 

 States each year for commercial fertilizers. 



Fertilizers containing nitrogen are produced in sev- 

 eral ways. Animal waste products of slaughter houses 

 are ground up and sold. Large quantities of sodium 

 nitrate (saltpeter) are imported into this country from 

 Chile, where it occurs in natural deposits. A method 

 has been discovered in recent years whereby nitrogen 

 is extracted from the atmosphere and then used in 

 the manufacture of nitrates valuable for fertilizing the 

 soil. 



Phosphorus is applied to the soil in the form of 

 phosphates. Some fertilizers containing phosphates 

 are made from the bones of animals and fish scraps. 

 Others are made from phosphate rocks mined in the 

 South. 



Potassium comes from wood ashes or mined potas- 

 sium salts. The Stassfurt mines of Germany are the 

 greatest single supply of potassium fertilizer in the 

 world. During the World War, when this supply of 

 potassium was not available to the United States, 

 American scientists developed methods of manufac- 

 turing potassium compounds from various sources of 

 potassium in this country. 



Lime, made from limestone, is a calcium compound 

 sometimes added to the soil. In some cases the lime- 

 stone itself is ground fine and used. If a soil is too 

 acid, lime is added to neutralize the acid. When the 

 acidity of the soil is reduced, certain beneficial bac- 

 teria are able to thrive better. These bacteria release 

 more of the potassium, phosphorus, and other ele- 

 ments from the soil for use of the plants the farmer 

 is raising. 



Exercise. From your study of this topic, cite evidence 

 in support of each of the following statements. 1 



1. Soil is formed in different ways. 



2. There are different types of soil. 



3. Soil fertility depends upon several factors. 



4. Soil water is important in crop raising. 



5. Soil moisture may be conserved. 



6. Soil fertility may be conserved in several ways. 



7. Nitrogen goes through a cycle in nature. 



REFERENCES FOR FURTHER STUDY 



Texts 



Caldwell and Curtis, Science for Today, Chaps. 20, 37 (part) 

 Clement, Collister, and Thurston, Our Surroundings, Chap. 

 21 



1 See workbook, p. 77. 



Hunter and Whitman, My Oivn Science Problems, Unit 9; 



Science in Our Social Life, Unit 10 

 Lake, Harley, and Welton, Exploring the World of Science, 



Chap. 14 

 Pieper and Bcauchamp, Everyday Problems in Science, 



Unit 2 

 Powers, Neuner, and Bruner, The World around Us, Chaps. 



14, 15 



Skilling, Tours through the World of Science, Tour 5 

 Van Buskirk and Smith, The Science of Everyday Life, 



Chap. 7 



Watkins and Bedell, General Science for Today, Unit 3 

 Webb and Beauchamp, Science by Observation and Experi- 

 ment, Unit 4 (part) 

 Wood and Carpenter, Our Environment: Its Relation to 



Us, Topics 5, 6; Our Environment: How We Use and 



Control It, Topic 16 



Special references 



Washburne, The Story of the Earth 

 Fabre, This Earth of Ours 

 Thomson, Outline of Science 

 Fall, Science For Beginners 



WHAT YOU SHOULD AIM TO ACQUIRE FROM 

 THIS STUDY 



1. The importance of fertile soil to mankind. 



2. A thorough understanding of how to maintain 

 and improve the fertility of soil. 



3. How waste lands such as deserts and swamps 

 have been made fertile for crop raising. 



TEST OF MASTERY OF THE TOPIC 



In your notebook complete the statements and comply with 

 the instructions. 



1. The United States government has undertaken some 

 irrigation projects in 



2. The water used for irrigation is usually supplied from 



3. Supplying water to the soil artificially is called . 



4. Soil that has been built up from deposits by rivers is 

 called 



5. Two leguminous plants are 



6. Humus is composed chiefly of 



Select the word or group of words which most satisfac- 

 torily completes each of the following statements. 



7. Soils are classified according to their 



density color location size of particle 

 capillarity 



8. Soil that will hold water best is 



black light fine coarse full of gravel 



9. The soil that is most likely to be sticky after a rain is 

 sandy stony loam clay 



10. When different crops are planted in the same field in 

 successive years the method of farming is called 



dry farming irrigation rotation of crops 

 reclamation 



11. Our best all-around fertilizer is probably 

 manure lime phosphate potash nitrate 



12. Plants on which bacteria grow and which are able to 

 take nitrogen from the air are called 



vegetables legumes nitrodes cereals 



13. Soil to be fertile must contain 



oxygen carbon copper phosphorus hydrogen 



