The Elements of economic geology

Bibliographic data

Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part III. Earthy minerals

Collection:: Economics Books

[04 " ECONOMIC GEOLOGY . is attended by a corresponding difficulty as it deals with powdered soil which does not indicate the actual texture. Heber Green, and Ampt (¥. Agric. Sci., iv, 1911, pp. 1-24) have shown that the essential factors can be determined and expressed by formule which indicate the amount of pore space, the permeability to air and water, and the capillarity. This method represents the facts for a soil as it is, and not when it is artificially broken up. The texture of soils controls their fertility in various ways. Soils may be barren owing to the absence of water, which drains away quickly from coarse sands; or to its excess in water-logged clay; or to deficiency of air in stiff impene- trable clay; or to acidity due to absence of carbonate of lime. The water capacity of soil depends on its interspaces or pores. The pores in stiff clay amount to 50 per cent. of the bulk, and the total surface of the particles is about 3 acres per cubic foot. A coarse sand, on the other hand, has a pore space of only from 25 to 30 per cent., and the surface area per cubic foot is about half of an acre. As water spreads through soils in a film covering the particles, the larger their surface the more water the soil will hold. Hence clay absorbs more water than sand, and holds it more firmly. A clay soil may hold an excess of water, and being water-logged, air is excluded, and the soil is barren. A sand on the contrary may be drained quickly and may not hold sufficient water to feed a crop during dry weather. Clay may be relieved of the excess of water by drainage, which allows air to enter and aerate the roots, lightens the soil by washing away clay particles, and renders it warmer by avoiding the chilling process of evaporation. The soil is nourished from the water-table {cf. p. 224) during drought as the film of water spreads from particle to particle. The water rises higher through fine grained rock with numerous pores and uniform closely packed grains than through loose coarse material. Hence crops separated from the water- table by a few feet of coarse gravel may perish from drought ; while the soil above an even-grained rock may be well- nourished. The principle of dry farming is to till the land so that the svaporation of water from the surface is kept under full

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