The Cation Exchange Capacity (CEC) is a fundamental parameter in soil chemistry and in the management of soil fertility, particularly regarding its ability to adequately nourish crops, preventing both nutrient deficiencies and excessive availability or inefficiencies (losses) in nutrient use.
But, as always, let’s proceed step by step.
The soil solution refers to the liquid phase composed of water and the substances dissolved in it. Plants absorb almost all nutrients from this solution, in quantities and proportions that depend on the complex equilibrium connecting it to the solid phase.
The composition and concentration of the soil solution are two important characteristics because:
Several factors can modify the composition of the soil solution:
Fluctuations in soil solution composition—potentially very intense due to the high number of influencing factors—are moderated by the equilibria established between the liquid and solid phases of the soil, thanks to an important characteristic of the solid phase: the Ion Exchange Capacity.
Both cations (such as Mg²⁺, Ca²⁺, K⁺, etc.) and anions (PO₄³⁻, SO₄²⁻, etc.) tend to restore any alteration in equilibrium between the two phases through adsorption (movement from the liquid to the solid phase) and desorption (movement from the solid to the liquid phase).
The soil’s ability to fix certain ions is called adsorption power, a crucial property for ensuring proper nutrient availability to crops and for preventing losses of unused elements (evaporation, leaching, erosion), with negative environmental consequences.
In this context, modern agronomy highlights that, due to the widespread lack of organic matter in agricultural soils, the limiting factor described by the law of the minimum is often not a mineral element, but organic matter itself.
The Cation Exchange Capacity (CEC) of soil is the total amount of positively charged ions (cations such as Ca²⁺, Mg²⁺, K⁺, Na⁺…) that the colloidal complex can retain per unit of dry soil, expressed in cmol(+)/kg or meq/100g (equivalent units).
This capacity derives from the negative charges—both permanent and variable—present on the surfaces of mineral colloids (clays) and organic colloids (humus). These charges attract and bind positive ions through a reversible ion exchange process.
In agricultural soils, CEC values between 15–50 cmol(+)/kg are typical of clayey soils, rich in active colloids, while sandy soils usually show values below 10. However, the amount of organic matter present can significantly modify values otherwise predictable solely from soil texture.
CEC classification for cultivated soils
| CEC (cmol(+)/kg or meq/100 g dry soil) | Assessment |
|---|---|
| <6 | Low |
| 6–12 | Medium-low |
| 12–18 | Medium |
| 18–32 | High |
| >32 | Very high |
(Source: Luigi Giardini, “Agronomia generale, ambientale e aziendale”)
Phenomena of anion adsorption in soil mainly involve phosphorus-containing ions (PO₄³⁻, HPO₄²⁻, H₂PO₄⁻) and involve the soil organic matrix and iron and aluminum hydroxides, giving rise to an Anion Exchange Capacity.
It is important to note that the nitrate anion (NO₃⁻) escapes the soil’s adsorption power, acquiring peculiar characteristics that must always be considered when choosing nitrogen fertilizers and that also underpin the so‑called Nitrates Directive.
The Cation Exchange Capacity of soil is primarily influenced by the content of colloids and their types: clays and humic substances.
With clays, soil CEC varies depending on mineral type (much higher in montmorillonite and vermiculite than in kaolinite), type of exchangeable cation (e.g., Ca²⁺ is more strongly adsorbed than Na⁺ and K⁺), its concentration in the soil solution, and the pH of the soil solution (adsorption decreases at low pH).
Organic matter increases CEC far more than clays. This is mainly due to stable, highly humified organic matter. The humic and fulvic acids contained in this fraction exert a chelating action on the ions in the soil solution, transporting them toward plant roots, enhancing nutrient uptake and reducing losses.
Phenomena of soil degradation, increasingly widespread, are systematically linked to the loss of stable organic matter (humus), primarily due to intensive agriculture combined with the effects of climate change.
As a result, degraded soils show—among other issues—a reduced Cation Exchange Capacity, limiting their ability to support crop nutritional needs. In such conditions, mineral fertilization is inefficient and environmentally impactful due to significant nutrient losses.
Conversely, adopting regenerative practices, including the use of organic and organo‑mineral fertilizers, helps counteract soil degradation.
Cultivated soils, especially degraded ones, benefit greatly from organic and organo‑mineral fertilizers capable of supplying humic substances to the soil, provided they are produced from high-quality organic matrices.
For this reason, all organic matrices used in Agribios fertilizers are highly humified, thanks to a long, slow natural fermentation process that increases their content of humic and fulvic acids.
Fumier Sulfer is derived from the intimate and homogeneous mixture of humified manure, partially hydrolyzed animal proteins, micronized sulfur, and ferrous sulfate heptahydrate. The humic acids in the humified manure bind, through carboxylic and phenolic groups, to bivalent iron (ferrous ion, Fe²⁺), forming iron humates. Chelation makes iron available through the complex cycle of organic matter. Fumier Sulfer is ideal for iron- and sulfur-demanding crops such as vegetables (spinach, chard, lettuce) and ornamentals. It is also ideal for tree crops such as kiwifruit, cherry, and vine. The sulfur content promotes soil acidification.
Fertilsoil is an inoculum of mycorrhizal fungi, specifically designed to enhance soil microbial fertility. Produced with high-quality humified organic substances, Fertilsoil contains a specific native microbial consortium, selected from fertile cultivated soils. Its main function is to increase the fertility of tired soils, thanks to the activity of added microorganisms, which perform nitrogen fixation, phosphorus solubilization, siderophore production, and indoleacetic acid production. The microorganisms, the supplied organic matter, and the naturally present humic acids promote an optimal microbiota and microbial fertility.
Humotech is a 100% natural organic amendment, an alternative to manure, produced from a blend of selected bovine and equine manures. The organic matrices undergo a long fermentation and humification process to increase their humic and fulvic acid content. Humic acids improve the physical-mechanical properties of soil (water retention, friability, permeability), the chemical properties (cation exchange capacity and element chelation), and the buffering capacity of soil. They also exert important biostimulant effects on crops. Humotech also contains natural zeolites, which interact with the soil’s CEC, improving nutrient availability both in the soil and from fertilization.
Organic fertilizers
Fertilizers with microorganisms
Organic fertilizers
