<P> If one cation is added in large amounts, it may replace the others by the sheer force of its numbers . This is called mass action . This is largely what occurs with the addition of fertiliser . </P> <P> As the soil solution becomes more acidic (low pH, and an abundance of H), the other cations more weakly bound to colloids are pushed into solution as hydrogen ions occupy those sites . A low pH may cause hydrogen of hydroxyl groups to be pulled into solution, leaving charged sites on the colloid available to be occupied by other cations . This ionisation of hydroxyl groups on the surface of soil colloids creates what is described as pH - dependent charges . Unlike permanent charges developed by isomorphous substitution, pH - dependent charges are variable and increase with increasing pH . Freed cations can be made available to plants but are also prone to be leached from the soil, possibly making the soil less fertile . Plants are able to excrete H into the soil and by that means, change the pH of the soil near the root and push cations off the colloids, thus making those available to the plant . </P> <P> Cation exchange capacity should be thought of as the soil's ability to remove cations from the soil water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution . CEC is the amount of exchangeable hydrogen cation (H) that will combine with 100 grams dry weight of soil and whose measure is one milliequivalents per 100 grams of soil (1 meq / 100 g). Hydrogen ions have a single charge and one - thousandth of a gram of hydrogen ions per 100 grams dry soil gives a measure of one milliequivalent of hydrogen ion . Calcium, with an atomic weight 40 times that of hydrogen and with a valence of two, converts to (40 / 2) x 1 milliequivalent = 20 milliequivalents of hydrogen ion per 100 grams of dry soil or 20 meq / 100 g . The modern measure of CEC is expressed as centimoles of positive charge per kilogram (cmol / kg) of oven - dry soil . </P> <P> Most of the soil's CEC occurs on clay and humus colloids, and the lack of those in hot, humid, wet climates, due to leaching and decomposition respectively, explains the relative sterility of tropical soils . Live plant roots also have some CEC . </P>

State four factors that play an important role in the formation of soil