The Earth Contents

5. Soils

Ion exchange affects soil structure

Adding positively charged cations, in particular protons (+ve charged hydrogen ion), neutralizes negatively charged particles. Adding negatively charged anions, in particular negatively charged hydroxide ion, neutralizes positively charged particles. Once this happens the neutral particles aggregate together, forming a gel. The extent to which coagulation is brought about by ions depends on their valency and the degree of hydration of the ions, but the relative coagulating capacity of (+ve charged sodium ion):(+ve charged calcium ion):(+ve charged aluminium ion) is 1:20:350. Therefore, it can be seen that removal of calcium (e.g. by leaching) leads to a shift from a gel to a sol state, and less stability in the soil. Where there is abundant calcium, soil structure is good, and the small particles form stable aggregates with pore spaces between them. When the volume of a soil is 40-50% composed of pore space it is an ideal structure for plant growth as roots can penetrate into the soil, water can move through it and there is air present for root and microbial respiration.

Soil structure
<Click on the diagram for more detail>

What is the effect of adding hydroxide ions to a soil?