The Elements of economic geology

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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 193 The Ghirriya, the northern part of the Nile delta, was in Roman times the Garden of Egypt, and produced the rice that fed the slave populations in Rome and in the Serbian mines ; when the administration of Egypt was undertaken by Britain in 1883 the Ghirriya was a barren waste owing to the accumulation of sodium carbonate in the soil. This salt was removed by successive washings by the Nile floods, and the ground was restored to its former fertility. Some of the low land in Mesopotamia is faced with a greater difh- culty, as the slope to the Persian Gulf is too slight to remove the irrigation water and its removal by evaporation leaves a residue of sodium carbonate. Nitrogen, the most important organic constituent of soil, occurs mainly as a constituent of humus, which gives most soils their brown or black colour. Humus is due to bacteria, which prevents the organic matter being decomposed into carbon dioxide; as that change takes place most quickly with an excess of air humus is less abundant in light porous soils than in clay. The amount of organic matter may be as much as 20 per cent. in virgin soils, while it may be almost absent from sandy soils in the tropics where the humus is decomposed in the dry season. Knowledge of the chemical composition of the soil enables agriculture in some cases to be managed with the precision of an industrial operation, as the material removed in each crop can be replaced in fertilizers. In general farming, however, this knowledge proved less useful than was ex- pected. An average English wheat crop withdraws from each acre about 50 Ib. of nitrogen, 20 Ib. of phosphoric acid, and 30 lb. of potash. Ordinary soil has enough nitrogen to supply this amount for 50 crops, enough phosphorus for 120 crops, and potash for 70 crops. Only, however, a small fraction of these constituents can be withdrawn by the plants ; and then the soil requires rest until more has become available as plant food. Efforts to distinguish between the total and available amounts have been made by treating soil with a weak organic acid; but this test does not fully overcome the difficulty, as the clods are not penetrated by the soil waters, whereas during an analysis the solvent is brought into contact with all the particles. Soir TEXTURE AND WATER CaraciTry—Mechanical analysis 13

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