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

THE SOIL 195 control, so that none is wasted and the crop is supplied at the right time. The loss of water and chilling by evaporation are prevented by a “ mulch ” or loose cover of farm refuse, or by so hoeing or harrowing the soil that it serves as a mulch. The dry farmer keeps the surface broken after every fall of rain until sufficient for a crop has soaked underground. He then rolls the ground so as to connect the surface with the water-table, so that the plants may be nourished. This method was practised in early times and its adoption has extended wheat cultivation in Australia, beyond the former supposed limit of * Goyder's Line,” which marked the edge of the area with a rainfall of 14 inches. The colour of soils affects their earliness. A black soil, such as the chernozen of Russia, and the black cotton soil of India, absorbs more heat and is warmer than a pale soil. The Indian black soils are coloured by a colloidal silicate of iron and aluminium containing some organic matter (Harrison and Swan (Mem. Agric. Research Inst., Pusa, No. 5, 1913). Some black soils, such as those of the highlands of Benguella, though apparently promising, are infertile, because they consist of coarse quartz grains coated by a thin film of colloidal silicate. Other black soils, such as those of the lava plains of East Africa, are darkened by the high proportion of iron and humus; they are chemically rich, but are diffi- cult to work as the clay in the rainy season becomes semi- fluid. It is then impassable to wheeled traffic and horses. In the dry season the soil shrinks and a network of deep cracks tears across the plant roots and allows the water to sink deeper and thus increase the depth with these incon- venient properties. Soir CoMPOSITION AND SurvVEYs—Agricultural chemistry in its early days formed exaggerated expectations of the help it could give the farmer; recognition of the limitations of soil analysis led to the view that the biological factors are most important. The most accurate test available of them is nitrification ; but, as pointed out by Burgess (Soil Science, vi, 1918, pp. 449-62), it does not explain the differences in soils or indicate how poor soils can be improved. The address by Dr. Crowther, an agricultural biologist, to the British Association in 1923, shows the return to greater faith in soil composition; he expressed the * conviction

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