What is replacement fertility

Ion exchange

A property of soils that is important for plant nutrition is their ability to bind to their solid substances (mineral and organic) ions, molecules, organic substances (including pollutants) and water reversibly (exchangeably) (adsorption, desorption), since the surfaces of the solid substances are electrical are loaded. Nutrient ions that are important for plants, such as Ca2+ (Calcium), Mg2+ (Magnesium) or Na+ (Sodium) can thus be bound to the surfaces of the solid substances and released back into the soil solution as other ions replace it. This is why one speaks of ion exchange (see graphic). The electrically charged solid substances or soil particles thus act as ion exchangers. For the plants, this has the great advantage that nutrients are not immediately washed out of the root space by the seepage water, but can be more or less continuously absorbed in this from the soil solution.

Ion exchanger

In particular, mineral and organic soil components <2 µm (= soil colloids) with a high specific surface (internal and external) are able to do this and mainly contribute to the overall charge of the soil. These include above all clay minerals, in particular expandable three-layer clay minerals (e.g. illite, montmorillonite), and humic substances, and to a lesser extent also oxides and hydroxides. These solid substances can be referred to as reservoirs or stores for plant nutrients (and also for pollutants). This includes both cations (positively charged ions) and anions (negatively charged ions). Negatively charged exchangers are referred to as cation exchangers and positively charged as anion exchangers. The exchange of ions takes place as a process between the soil solution and the solid matter.

Due to their internal structure, clay minerals (= layered silicates) are usually negatively charged. Only the lateral fracture surfaces show a variable charge. The reason: The basic structure of the silicates is the SiO4-Tetrahedron, consisting of a central silicon ion and four oxygen ions. During the formation of clay minerals, however, it often happens that the central silicon ion (Si4+) z. B. against an aluminum ion (Al3+) is replaced (= isomorphic replacement), which creates a charge deficit (negative charge). Reversible cations (e.g. Na+, Approx2+) deposited. The negative charge is independent of the pH of the soil solution. Clays therefore have a predominantly permanent charge and are therefore cation exchangers. In humic substances, the negative charges are caused by COOH and phenolic OH groups, of which protons (H.+) are split off (dissociation). In an extremely acidic environment (3) into the groundwater in the event of excessive fertilization (e.g. manure) or in the event of excess mineralization after clear cutting in the forest.

Exchange process

During an ion exchange, lower-valued ions are displaced by higher-valued ions because of the stronger Coulomb force (electrostatic attractive forces): Al3+ displaces approx2+ and approx2+ displaces Well+. During the anion exchange z. B. PO43- (Phosphate) SO42- (Sulfate). With the same valence, the ion with the larger molar mass (M) displaces the ion with the lower. For example, K displaces+ (39 g / mol) Na+ (23 g / mol). The exchangeable cations are surrounded by a hydration shell. Among equivalent cations, the diameter of the hydrated ions is decisive (ion size). The thinner the hydration shell, the better the attachment to the exchanger: Mg2+ > Approx2+ and well+ > K+. This order is known as selectivity. The type of cation ion exchanger (clay minerals) also plays a role in the exchange. For example, montmorillonite prefers Na+, K+, But kaolinite is more likely to be Mg2+ and approx2+. If only the ion properties are taken into account, the following series results in terms of exchange strength and adhesive strength: AI3+ > Approx2+ > Mg2+ > K+ > Well+.

Exchange capacity

The sum of all exchangeable cations of a soil is the cation exchange capacity (KAK). The unit of measurement for the KAK is mmolc / kg (mmol = 0.001 mol, c = charge). A distinction is made between the potential cation exchange capacity (KAKpot) and the effective cation exchange capacity (KAKeff). The KAKpot indicates the maximum achievable amount of exchangeable cations of a soil under optimal conditions (pH 8.1 according to DIN ISO 13536) (maximum number of possible free cation binding sites). The KAKeff indicates the actual number of free cation binding sites at the current pH value of a soil. The higher the proportion of organic exchangers in the soil and the higher the pH value of the soil, the greater the KAKeff. In acidic soils (pH <7) the KAKeff is always lower than the KAKpot, since H+-Ions occupy exchanger sites to the detriment of cations. The ratio of basic cations to the total proportion of all cations in the exchangers is referred to as base saturation (BS) (in%). In soils containing carbonate (e.g. Rendzina, Terra fusca), base saturations of almost 100% are often reached, which means that at the KAK almost exclusively approx2+-, Mg2+-, K+- and well+-Ions are involved. Base saturation is an indicator of soil quality. If the proportion of basic cations is more than 70%, the base saturation is high, below 35% it is low.