Soil is made up of a diverse number of components. The smallest fraction of these are known as colloids, which are less than 1 micron in diameter. Being so small, they have a large surface area, are very reactive and are responsible for holding most of the nutrients in the soil.
The soil colloid has a negative charge, attracting positively charged nutrients (cations) and repelling negatively charged ions (anions). The strength of the bond depends upon the number of positive charges the ion has. Anions are free to move in the soil solution, being readily available to the plant and readily leached. In some cases (phosphorus), the anion will form compounds with cations, becoming insoluble and temporarily unavailable to the plant or for leaching. So, while phosphorus is an anion, it will usually combine with iron or aluminium and act like a cation. The finer colloids in the soil (such as clay), the greater the ability of the soil to bind cations.
Common cations and anions present in the soil
Cation Exchange Capacity (CEC) refers to the total number of exchangeable cations that a soil can hold at any one time. The strength of a cation’s positive charge varies, allowing a cation to be replaced by a stronger cation on the surface of a negatively charged soil particle. The likelihood of cations leaching through the soil profile decreases progressively with increasing CEC.
Factors that affect CEC
CEC is affected by the amount and types of clays and organic matter present. The more clay and organic matter available, the greater the ability of a soil to adsorb cations.
A soil is likely to have a low CEC if it has been highly weathered and has a low organic matter content. If the soil has been protected from erosion most of its life and has high levels of organic matter, then its CEC should be high.
Sandy soils (low CEC), adsorb smaller quantities of cations. This has important implications when deciding on a fertilisation program. For example, if nitrate-based fertilizers (such as urea) or products containing sulphate sulphur are applied at a time of heavy rain, then dramatic losses of nitrogen or sulphur may occur via leaching (especially if there is very little organic matter covering the soil). Split or delayed applications of fertilizer may be more appropriate, as this application is more effective in avoiding leaching or run-off loss.
Cation saturation is a concept that is being used to develop fertilizer regimes. The concept is based on the assumption that a specific nutrient ratio is required for correct nutrient uptake to ensure maximum yields. Evidence suggests that a healthy, high-yielding crop or pasture cannot be grown in a soil without this specific nutrient ratio. As long as each nutrient is present in adequate amounts, a high yield can still be obtained.
Cations and Lime
CEC has an effect on the liming requirement of a soil. The higher the soil CEC, the harder it becomes to change its pH level. The total amounts of clay and organic matter present in a soil, as well as the actual type of clay, control how strongly soils are buffered (produce a shift in pH). Rising clay and organic matter content leads to a higher buffering capacity. Soils such as these need more lime to raise the pH than soils with a low buffering capacity, such as sandy soils.
Lime, Gypsum and Potassium
Most soils contain 10kg/ha or less of potassium in solution. This can barely supply an actively growing crop for more than a few days. However, as the crop removes potassium that is in solution, some of the exchangeable potassium moves into solution. It is replaced on the soil colloid by another cation. This movement continues until a new equilibrium is established.
Therefore, via the cation exchange process, potassium is constantly available for plant growth, so long as the soil contains enough available potassium at the start of the growing season. However, when the plant is growing rapidly, the potassium removed from the soil solution may not be replaced quickly enough with potassium from the exchangeable pool. If this occurs, the crop or pasture may show signs of a potassium deficiency.
The absorption of potassium by plant roots is influenced by the activity of other cations, mainly calcium and magnesium. As the concentration of calcium and magnesium ions increases, the absorption of potassium decreases. Some potassium can be exchanged from the soil colloid to the plant root when the two come into direct contact with each other.
The addition of lime or gypsum will increase the supply of calcium, which may reduce the availability of potassium. Find out more about CEC here.