LEACHED AND SOUR SOILS 45 



In cool, hximid climates, organic matter decays slowly, hence 

 soils in such regions are generally well supplied with organic matter. 



In warm, hmnid climates, vegetable matter in soils decays 

 rapidly, hence the soils are usually low in organic matter. This 

 accounts in part for the many red soils found in the South and 

 the Tropics. ^ 



Leached and Sour Soils. — Soils in hiunid regions are generally 

 well leached of their soluble salts. Moreover, many of them, espe- 

 cially the upland soils, have had the carbonate of lime so completely 

 leached out of them that they have developed a condition generally 

 referred to as ^^ acidity.'^ Soils which lack carbonate of hme are 

 thus commonly termed "sour" or "acid." 



lEustration Material for Lessons. — ^A few hand specimens of igneous, 

 sedimentary and metamorphic rocks. (Include a sample of lava rock.) Speci- 

 mens of the common rock-forming minerals. A sample of kaolin (true clay). 

 Limestone and other natural carbonates. 



If possible show samples representmg different stages in the formation of 

 soil from an igneous rock; from a sandstone; shale; and from a limestone. 



A sample of an alkah soil and of a red soil commonly found in the South. 



Demonstrations. — Material Needed. — ^A tablespoonful each of common salt 

 and sodium carbonate; 4 tumblers; and about two-thirds of a cupful of a 

 black soil. 



To Make Clear the Meaning of " White '* and " Black " Alkali.— Pro- 

 cedure. — Place about a handful of black soil in each of two tumblers. Fill 

 one about half full of a strong solution of common salt (a white alkali) and the 

 other about half full of a strong solution of sodium carbonate (black alkah). 

 Stir the contents of each tumbler and let stand at least twelve hours. Strain 

 the Mquid into two other tumblers and note difference in color. (Show the 

 class samples of the two salts.) 



Questions, — (a) Which salt dissolves organic matter and which does not? 



i})\ What would be the color of the drainage water in a section of ''black" 

 alkah? In an area of ''white" alkah? 



(c) Why is the one salt called ''white" and the other "black" alkah since 

 both salts are white in color? 



{d) Where are alkah soils foimd? Why? 



Laboratory Exercises. — Materials Needed, — Specimens of common rock- 

 forming minerals — ^feldspar, hornblende, quartz, white and black mica, calcite, 

 gypsum, apatite, pyrite, etc.; specimens of common rocks, such as granite, 

 trap rock, schist, shale, slate, limestone, marble, sandstone and quartzite. 



To Learn to Recognize the CoTnmon Rock-forming Minerals. — Procedure. 

 — ^Examine carefuUy the samples of rock-forming minerals provided for study. 



Eecord observations in tabular fprm with headings as follows: Name of 

 mineral, color, essential plant-food element it contains, plains in which mineral 

 can be spht (plains of cleavate), common end product in weathering, and rela- 

 tive hardness. 



The relative degree of hardness can be determined by scratching each 

 with the other. (See Rocks and Rock Minerals — ^Pirsson.) 



Questions. — (a) Which are the two most common rock-forming minerals? 



(h) Suppose a sand (soil) contains many sand particles of apatite. What 

 of its phosphorus supply? 



