What process causes plant ion-uptake to acidify soil? What soil pH range generally promotes the greatest availability of cations? Solution The process that causes plant ion-uptake to acidify soil is known as soil acidification. It is a process by which the soil pH decreases over time without producing any visible signs. Slow soil acidification occurs under natural conditions over thousands of years, with high rainfall areas most affected, whereas, rapid acidification occurs over a few years under intensive agricultural practices. Soil acidification affects the surface and the subsoil simultaneously. However, areas that remain unaffected by industrial pollution, there soil acidification is mainly caused by the release of protons (H+) during the oxidation of carbon (C), sulphur (S) and nitrogen (N) compounds in soils. During nitrogen cycling, the major processes leading to acidification consists of the cation imbalance over anion during uptake in the rhizosphere of plants, during nitrification from fixation or from the fertilizers, and during the removal of plant and animal products by leaching. The uptake of excess cations over anions by plants results in the acidification of the rhizosphere which is a “localized” effect and can be balanced by the release of hydroxyl (OH-) ions during subsequent plant decomposition. Soils are considered acidic below a pH of 5, and alkaline above a pH of 7.5 and very alkaline above a pH of 8. It has been observed that the “ideal” soil pH is close to neutral, which fall within a range from a slightly acidic pH of 6.5 to slightly alkaline pH of 7.5. It has been determined that most plant nutrients are optimally available to plants within this range helpful for plant root growth. However, Nitrogen (N), Potassium (K), and Sulfur (S) are major plant nutrients that enhance plant growth and development. Most of the plant nutrients (cations) tend to be less available when soil pH is above 7.5, and are optimally available at a slightly acidic pH in the range of 6.5 to 6.8..