To understand how colloidal clay particles and humus contribute to the chemical reactions of the soil, how the soil’s pH can be modified, and how the application of chemical fertilizer can increase the nutrients in soil requires an understanding of cation exchange. The term refers to the capacity of colloidal particles to attract positively charged ions (cations) and to exchange one ion for another. Without cation exchange, nutrients would be readily leached from the soil. With cation exchange, the hydrogen cations held by the colloidal particles can be replaced by cations furnished through the decomposition of organic material, the weathering of rocks, or the application of fertilizers. It follows that soils having a higher percentage of colloidal particles, such as clay soils and organic soils, have a higher capacity for cation exchange than sandy soils that are lower in colloidal particles. Therefore, they have a higher capacity to hold available nutrients and resist loss due to leaching.

Role in pH

It is the replacement of hydroxyl cations on the colloidal particles of the soil by hydrogen ions that makes a soil acidic. To make a soil more acidic (lower the pH), elemental sulfur is usually added. In the soil, bacteria convert the sulfur to sulfuric acid. To make the soil more alka­line (increase the pH), calcium or calcium-magnesium compounds are commonly used.

The influence of pH on cation exchange is centered around the availability of nutrients as described earlier and illustrated by iron chlorosis at high soil pH. There are other elements that become bound tightly within the soil and are unavailable to plants when the pH is high, just as there are elements made unavailable when the pH is too low. At extremely low or high pH ranges the mere excess of H+ or OH – ions can be toxic to the plants.

Figure 3-6 illustrates how various nutrients become more or less available to the plants as the pH of the soil increases or decreases.