SOIL: The farmer's most important asset PART 8:
Soil Acidity by Martiens du Plessis, Soil Scientist, NWK Limited and Prof Cornie van Huyssteen, Lecturer: Soil Science, University of the Free State
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oil is the most fundamental resource for the farmer, without which food and natural fibre cannot be produced. This article forms part of a series to highlight this resource. In this article, soil acidity and the management thereof is discussed. Introduction The soil’s acidity level (pH) is arguably the most important chemical parameter in the measurement of the soil’s general “chemical health”. It significantly influences plant growth and the uptake of a variety of plant nutrients. The maintenance of a favourable soil pH is therefore of vital importance for the soil utiliser
and has an intrinsic influence on the sustainable utilisation of the soil. What is pH? An important property of the soil solution is the soil reaction – in other words whether the soil is acid, neutral or alkaline. Acid refers to an excess of H+ ions compared to OH- ions and in the same context alkaline refers to an excess of OH- ions compared to H+ in the soil solution. With a neutral solution, there are equal quantities of H+ and OH- ions. The soil reaction is measured as pH. It is the negative logarithm of the hydrogen ion concentration [H+] expressed as “mole per litre” (pH = - log [H+] and it ranges
ProAgri Botswana / Namibia / Zimbabwe 11
from 0 to 14. A pH of 1 is strongly acidic and a pH of 14 is strongly alkaline, with a pH of 7 being neutral. For example: A pH of 6 implies an H+ ion concentration 10-6 mole litre-1 (0,000001 mole litre-1) and an OH- ion concentration of 10-8 mole litre-1 (0,00000001 mole litre-1) (the difference between the pH value and 14 is the OH- concentration). There is thus 10x10=100 times more H+ than OH- ions in the soil solution and the soil is therefore acidic. At a pH of 7 there are 10-7 mole litre-1 H+ and 10-7 OH- ions and the soil is neutral. At a pH of 9 there are fewer H+ ions (10-9 mole litre-1) than 17
