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Handbook of Soils for Landscape Architects$
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Robert F. Keefer

Print publication date: 1999

Print ISBN-13: 9780195121025

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195121025.001.0001

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PRINTED FROM OXFORD SCHOLARSHIP ONLINE (oxford.universitypressscholarship.com). (c) Copyright Oxford University Press, 2021. All Rights Reserved. An individual user may print out a PDF of a single chapter of a monograph in OSO for personal use. date: 26 September 2021

Chemical Properties of Soils for Growing Plants

Chemical Properties of Soils for Growing Plants

(p.91) 9 Chemical Properties of Soils for Growing Plants
Handbook of Soils for Landscape Architects

Robert F. Keefer

Oxford University Press

Soil reaction is the amount of acids (acidity) or bases (alkalinity) present in a soil and is indicated by a term called “pH”. By definition, pH is the logarithm of the reciprocal of the hydrogen ion (H+) concentration, or When a number has a smaller superscript number with it, the number is raised to that power which is called the “logarithm.” Raising a number to a power means multiplying that number by itself the number of times indicated by the superscript. . . . Examples: 102 means 10 x 10 = 100; 103 means 10 x 10 x 10 = 1,000. The logarithm (log) is 2 for the first example and 3 for the second. . . . Logarithms are used as these are more convenient in expressing the amount of hydrogen ions present. Under neutral solutions the pH is 7.0. Any pH that is less than 7 is acid and any pH above is alkaline. When changing from a pH of 7 to a pH of 6, the H ion concentration increases ten times, and when going from a pH of 7 to a pH of 5, the H ion concentration increases 100 times because pH uses a geometric scale and not an arithmetic scale. Thus, pH changes by steps of ten times the next adjacent number. The logarithmic scale used for pH is the same type, but opposite in direction, as that used to measure earthquakes. For each larger number of earthquake, the severity increases ten times; for each smaller number of pH, the acidity increases ten times. Some plants can tolerate very low pH (4.5) and others can withstand a pH of 8.3, but the optimum range for growth of most plants and microbes is between 6 and 7. Availability of most nutrients is affected by pH changes. Charts have been constructed to show this relationship. On these charts the pH at which most nutrients are readily available is from 6 to 7. At extremes of pH, availability of nutrients to plants often is reduced considerably.

Keywords:   acidity of soil, alkalinity, base-forming factors, clays (secondary minerals), leaching, saline soils, sodic soils

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