Physical and Chemical Soil Degradation

Last updated: July 26, 2026 Русский Español

1. What is Soil Degradation? A Modern Definition.

Soil as a Living System and the Basis of Fertility

Before we talk about degradation, let us recall: what is soil and what is its main property?

Soil is a complex bio‑abiotic system consisting of solid (mineral particles and organic matter), liquid (soil solution), and gaseous (soil air) phases. It is the unique combination of these three phases that creates the conditions for plant life and the entire soil community. But the most important property of soil, which distinguishes it from any parent rock, is fertility.

Fertility is the ability of soil to meet the needs of plants for nutrients, water, air, and heat, ensuring their growth and productivity (Valkov et al., 2004).

Fertility is not a fixed property. It develops as a result of a long soil‑forming process that can take centuries and millennia. Under natural conditions, there is a balance between soil formation and its destruction: the rate of soil formation roughly corresponds to the rate of geological erosion. However, human intervention disrupts this balance.

Modern Definition of Degradation

In the broadest sense, soil degradation is the process of gradual deterioration of soil properties, leading to a reduction or loss of its fertility and ability to perform ecological functions (Richter & Tugel, 2012).

A more formal definition is given by international organisations: land degradation is the reduction or loss of the biological or economic productivity and complexity of land, caused by a combination of processes resulting from human activities (FAO, 2011).

In the Russian scientific tradition, soil degradation is understood as “a set of processes that lead to the deterioration of soil composition, properties, and regimes, reducing their fertility and resilience as a result of natural or anthropogenic impacts” (Valkov et al., 2004).

Note the key points of this definition:

1. Decline in fertility — the main criterion of degradation.

2. Process nature — degradation is not a one‑time event but a gradual deterioration.

3. Anthropogenic origin — although degradation can also occur naturally (e.g., during a volcanic eruption), the main cause in the modern world is human activity.

Classification of Degradation Processes

All soil degradation processes are conventionally divided into three large groups (Valkov et al., 2004; Weil & Brady, 2017):

1. Physical degradation — deterioration of physical properties: destruction of structure, compaction, crust formation, loss of porosity, erosion.

2. Chemical degradation — deterioration of chemical properties: acidification, salinisation, alkalisation (sodification), nutrient depletion, contamination with toxicants.

3. Biological degradation — decline in biological activity: reduction in the number and diversity of soil organisms, decrease in enzymatic activity.

In this lecture, we will focus on physical and chemical degradation — processes that often start first and create the preconditions for biological degradation.

Why Do Soils Lose Properties Faster Than They Form?

This is the central question of the entire topic. To answer it, we need to compare the rates of two opposing processes:

The rate of soil formation under natural conditions is approximately 0.5–2 cm per 100 years (or 5–20 t/ha per year) (Weil & Brady, 2017). This means that it takes 1000 to 6000 years to form a 20–30 cm fertile layer.

The rate of degradation under anthropogenic impact can reach 20–40 t/ha per year during erosion, and with intensive mechanical tillage, compaction develops over several years or even one season.

Thus, soil is destroyed tens to hundreds of times faster than it is formed. As one of the founders of soil science figuratively put it, “soil is a slowly renewable resource, practically non‑renewable on the scale of a human lifetime.”

It is important to understand: degradation does not always mean complete destruction of the soil. More often it is an imperceptible deterioration of properties that accumulates year after year. This is why many agricultural lands, which at first glance appear normal, are actually losing fertility every year (Richter & Tugel, 2012).

Degradation as a Loss of Functions

The modern view of degradation links it not so much to changes in individual properties but to the loss of the soil’s key functions (Weil & Brady, 2017). Soil performs five main functions:

1. Productive — supplying plants with nutrients, water, and physical support.

2. Water‑regulating — filtering and purifying water, regulating the water regime.

3. Buffering — neutralising and binding pollutants.

4. Climatic — participating in global cycles of carbon, nitrogen, and other elements.

5. Habitat — supporting a huge diversity of organisms.

Degradation reduces the soil’s ability to perform these functions. For example, compaction impairs the productive and water‑regulating functions; contamination impairs the productive and buffering functions; loss of organic matter impairs all functions simultaneously.

In the following sections, we will examine in detail the main mechanisms of physical and chemical degradation and understand how these processes are interrelated.

2. Physical Degradation of Soils

2.1. Definition of Physical Degradation

Physical degradation of soils is a set of processes that lead to the deterioration of the soil’s physical properties: its structure, density, porosity, water permeability, and resistance to external influences (Valkov et al., 2004; Scheffer et al., 2018). Unlike chemical degradation, which changes the composition and reaction of the soil environment, physical degradation primarily affects the soil framework — the part that determines the living conditions for root systems, water movement, and gas exchange.

Physical degradation is dangerous because it is often invisible in its early stages: the soil may look normal, but its internal structure is already impaired. Moreover, the restoration of physical properties, especially in deeper horizons, takes many years and is often impossible without special reclamation measures.

The main processes of physical degradation include:

compaction;
destruction of aggregate structure;
slaking and surface crust formation;
loss of porosity;
erosion (water and wind).

We will consider each of these processes in turn.

2.2. Soil Compaction

Compaction (or densification) is an increase in soil bulk density due to a reduction in total pore volume, especially large pores (macropores). As a result, the density of the solid phase per unit volume increases, i.e., soil bulk density rises (Eash et al., 2016).

Causes of Compaction

The main factors causing soil compaction (Foth, 1990; Weil & Brady, 2017):

1. Mechanical impact of agricultural machinery. Modern tractors and harvesters weigh from 5 to 20 tonnes or more. The pressure on the soil can reach 100–200 kPa, which significantly exceeds its bearing capacity, especially when wet. Repeated passes of machinery along the same track are particularly dangerous.

2. Tillage when the soil is too wet. Water between particles acts as a lubricant, reducing internal friction and allowing particles to move closer together under pressure. Upon drying, such soil becomes very dense (Eash et al., 2016).

3. Livestock grazing, especially at high animal densities and on wet soil. Hooves not only compact the surface layer but also damage the sod, increasing erosion.

4. Natural compaction under the force of gravity, especially in sub‑tillage horizons; however, under natural conditions it is usually compensated by biological activity (worms, roots) and freeze‑thaw cycles.

Mechanism of Compaction

When external pressure is applied, soil aggregates move closer together, large inter‑aggregate pores are destroyed, and the aggregates themselves may deform. In this case, total pore volume decreases, but more importantly, the pore‑size distribution changes: the number of macropores (diameter > 50 µm) drops sharply, while the proportion of micropores increases (Eash et al., 2016).

Particularly serious consequences arise from the formation of a compacted sub‑tillage horizon — the so‑called “plough pan”. This is a layer at a depth of 20–40 cm where repeated passes of machinery and constant tillage to the same depth create a dense layer that is almost impermeable to roots and water (Scheffer et al., 2018).

Consequences of Compaction

Compaction triggers a chain of negative changes:

Reduced water permeability — water cannot penetrate deeply, surface runoff increases, leading to erosion and waterlogging of the upper layers.
Impaired aeration — roots and aerobic microorganisms suffer from oxygen deficiency; anaerobic processes, including denitrification, intensify.
Restricted root growth — dense soil creates mechanical resistance; roots become distorted, thickened, and cannot access nutrients and moisture from deeper layers.
Yield reduction — according to various estimates, compaction can reduce cereal yields by 15–30% or more.

2.3. Structure Destruction

Soil structure is the ability of soil particles to combine into aggregates (peds) of various shapes and sizes: granular, crumb, prismatic, platy, etc. (Eash et al., 2016). Good aggregate structure is the foundation of fertility: it provides an optimal combination of macro‑ and micropores necessary for water, air, and roots.

Causes of Structure Destruction

Soil structure is not a permanent property. It is very sensitive to external influences:

1. Intensive mechanical tillage. Ploughs, disc harrows, and cultivators not only mix the soil but also mechanically destroy aggregates, especially with repeated passes. “Long‑term tillage leads to pulverisation and degradation of structure” (Scheffer et al., 2018).

2. Loss of organic matter. Humus is the main cementing agent that binds mineral particles into aggregates. When humus content decreases, aggregates become less stable to mechanical impacts and water action.

3. Impact of raindrops and freeze‑thaw cycles. The energy of falling drops destroys surface aggregates, and freeze‑thaw cycles also contribute to their breakdown (Weil & Brady, 2017).

4. Chemical factors. High sodium content (especially in solonetzic soils) promotes the dispersion of clay particles and structure breakdown (Eash et al., 2016).

Structure Destruction and Loss of Aggregate Stability

Aggregates have different resistance to external forces. Water‑stable structure — the ability of aggregates not to disintegrate when wetted — is particularly important for erosion control. When aggregates lose stability, under rainfall they break down into individual particles. Fine particles (clay and silt) clog pores, forming a crust on the surface (see next section).

2.4. Slaking and Surface Crust Formation

Slaking is the process by which the surface layer of soil loses its structure and becomes structureless and massive. Upon drying, this layer turns into a soil crust — a hard, dense film from a few millimetres to 2–3 cm thick (Troeh & Thompson, 1993; Weil & Brady, 2017).

Mechanism of Crust Formation

When raindrops hit the soil surface, their kinetic energy destroys aggregates. The released fine particles (clay, silt) move downwards with infiltrating water and clog the pores in the uppermost layer. After drying, this layer cements into a crust. “The destructive effect of raindrops on soil was underestimated for a long time; today it is recognised that it is raindrops that are the main agent of particle detachment for water and wind erosion” (Troeh & Thompson, 1993).

Consequences of Crusting

Sharp reduction in infiltration — water cannot enter the soil; almost all rainfall runs off. The runoff coefficient can increase from 5–10% to 50–60% or more.
Increased erosion — runoff water acquires greater destructive power.
Impeded seedling emergence — seedlings cannot break through the hard crust, especially small‑seeded crops.
Impaired gas exchange — the crust prevents oxygen from entering the soil, slowing root and microbial respiration.

2.5. Loss of Porosity

Porosity is the total volume of pores (voids between solid particles) in the soil, expressed as a percentage of the total soil volume. In a well‑structured soil, porosity is 40–60% (Eash et al., 2016). Not only the total amount of pores matters, but also their size distribution:

Macropores (> 50 µm) — provide drainage, aeration, and root penetration;
Mesopores (0.2–50 µm) — hold capillary water available to plants;
Micropores (< 0.2 µm) — contain water that is hardly available to plants.

In physical degradation, loss of porosity occurs primarily through the loss of macropores. This means that the soil loses its ability to quickly drain excess water and supply oxygen to roots, even if total porosity does not decrease very much. “Changes in the volumetric ratio of solid, liquid, and gaseous phases are the most important indicator of physical degradation” (Scheffer et al., 2018).

A decrease in porosity directly correlates with:

reduced water‑holding capacity;
lower hydraulic conductivity;
increased soil density;
worsening conditions for soil biota.

2.6. Erosion as a Physical Process of Soil Change

Erosion is the process of detachment, transport, and redeposition of soil particles by water, wind, or gravity (Foth, 1990; Weil & Brady, 2017). In the context of physical degradation, erosion is important not so much as the removal of soil beyond the field, but as a process of change in the soil profile itself at the erosion site.

Water Erosion

Water erosion includes three stages:

1. Detachment of particles under the kinetic energy of raindrops and the turbulence of runoff.

2. Transport of particles by water (in suspension, by traction, or rolling).

3. Deposition when flow velocity decreases.

Three main types of water erosion are distinguished (Weil & Brady, 2017; Troeh & Thompson, 1993):

Sheet (surface) erosion — relatively uniform washing of the top horizon. It is the most insidious because it is hardly noticeable, yet it removes the most fertile layer — the humus horizon.
Rill erosion — formation of small rills that can be ploughed over, but the soil is already lost.
Gully erosion — formation of deep gullies that dissect fields and make them unfit for cultivation.

During erosion, selective removal of the finest and most fertile particles occurs: clay, silt, humus, nutrients. What remains is coarser, sandier, and less fertile material. “The organic matter and nitrogen content in eroded sediment can be 5 times higher than in the original soil, and phosphorus and potassium 2–3 times higher” (Weil & Brady, 2017).

Wind Erosion (Deflation)

Wind erosion occurs in dry and poorly sheltered conditions. Mechanisms:

Saltation — bouncing and rolling of sand‑sized particles (0.1–0.5 mm), which break aggregates and knock out fine particles.
Surface creep of larger particles.
Suspension — fine particles (clay, dust) are lifted into the air and can be transported hundreds and thousands of kilometres.

Like water erosion, wind erosion removes the most valuable components — organic matter and nutrients — and also worsens the textural composition: the surface layer becomes more sandy and less fertile.

Effect of Erosion on Physical Properties

Erosion leads to:

reduction in the thickness of the humus horizon;
exposure of less fertile sub‑horizons;
coarsening of the particle‑size distribution;
reduction in water‑holding capacity and deterioration of structure;
formation of new micro‑relief forms (rills, gullies) that hinder machinery use.

2.7. Interconnection of Physical Degradation Processes

All physical degradation processes are closely linked and often act together. For example:

Loss of organic matter → structure destruction → compaction → reduced porosity → impaired infiltration → increased runoff → intensification of erosion → further structure destruction and humus loss.
Compaction of the sub‑tillage horizon → water stagnation above it → waterlogging of upper layers → impaired aeration → changes in chemical and biological processes.
Surface slaking → reduced infiltration → increased erosion → removal of the fertile layer.

Thus, physical degradation is a self‑accelerating process. The more physical properties are disturbed, the faster they deteriorate further. It is this positive feedback that makes physical degradation particularly dangerous and difficult to reverse.

3. Chemical Degradation of Soils

3.1. Definition of Chemical Degradation

Chemical degradation of soils is a set of processes that lead to adverse changes in the chemical composition and chemical properties of the soil, reducing its fertility and impairing its ecological functions (Valkov et al., 2004; Scheffer et al., 2018). If physical degradation destroys the “skeleton” and “pores” of the soil, chemical degradation alters the “chemical environment” for plants and microorganisms — the conditions that determine the availability of nutrients, acid‑base status, and presence of toxic substances.

The main processes of chemical degradation include:

acidification;
salinisation;
alkalisation (sodification, solonetzation);
loss of organic matter (dehumification);
contamination with heavy metals, organic toxicants, and other pollutants.

All these processes are generally the result of anthropogenic impact, although some (e.g., natural acidification during long‑term leaching) occur in nature, but much more slowly.

3.2. Soil Acidification

Acidification is the process of decreasing the pH of the soil solution, i.e., accumulation of hydrogen ions (H⁺) and, in acid soils, aluminium (Al³⁺) on the exchange complex (Eash et al., 2016; Weil & Brady, 2017).

Causes of Acidification

Acidification can be natural and anthropogenic.

Natural acidification occurs under conditions of excessive moisture and good drainage, when bases (Ca, Mg, K, Na) are leached from the soil profile and replaced by hydrogen and aluminium ions. This process takes millennia and is characteristic of soils in humid regions — podzolic, sod‑podzolic, and grey forest soils (Foth, 1990).

Anthropogenic acidification proceeds much faster and is associated with several factors:

1. Acid rain. Burning fossil fuels releases sulphur oxides (SO₂) and nitrogen oxides (NOₓ) into the atmosphere, which form sulphuric and nitric acids. In the middle of the 20th century, the pH of rain in Central Europe reached 4.0–4.5, and sometimes lower (Scheffer et al., 2018). This led to rapid acidification of forest soils, especially on base‑poor rocks.

2. Use of physiologically acid fertilisers, especially ammonium and urea fertilisers. During nitrification of ammonium nitrogen, hydrogen ions are released (Eash et al., 2016; Weil & Brady, 2017):

2NH_4^+ + 4O_2 \rightarrow 2NO_3^- + 4H^+ + 2H_2O

Each kilogram of applied ammonium nitrogen produces about 3–4 kg of CaCO₃‑equivalent acidity upon oxidation. Therefore, systematic application of such fertilisers without liming leads to significant acidification of the topsoil.

3. Removal of bases with harvest. Plants remove considerable amounts of Ca and Mg from the field. In natural ecosystems, these elements return to the soil with litter. Under agricultural use, the balance is disturbed, and the soil gradually loses bases, which also contributes to acidification.

4. Intensification of nitrification and mineralisation of organic matter due to tillage.

Mechanism of Acidification and the Role of Aluminium

In acid soils, hydrogen ions do not simply accumulate in solution — they actively interact with the soil exchange complex. When pH drops below 5.5, dissolution of aluminosilicates and release of Al³⁺ begin, which in turn hydrolyses to form additional H⁺ (Foth, 1990; Weil & Brady, 2017):

Al^{3+} + H_2O \rightarrow Al(OH)^{2+} + H^+

Al(OH)^{2+} + 2H_2O \rightarrow Al(OH)_3 + 2H^+

Thus, aluminium becomes a powerful generator of acidity. In very acid soils, 80–100% of exchangeable cations may be Al³⁺ (Scheffer et al., 2018). Free aluminium ions are toxic to plants; they damage root tips, inhibit calcium and phosphorus uptake. For most agricultural crops, a concentration of 0.1–0.5 mg Al/L in the soil solution already causes growth depression (Eash et al., 2016).

Consequences of Acidification

Reduced availability of nutrients: phosphorus is bound into insoluble aluminium and iron phosphates; molybdenum becomes less available; basic cations (Ca, Mg, K) are displaced from the exchange complex and leached.
Increased availability of toxic elements: Mn, Fe, Al become easily soluble and can reach toxic concentrations for plants and microorganisms.
Suppression of soil biota: bacterial activity, especially nitrifiers and nitrogen fixers, drops sharply at pH below 5.5; more acid‑tolerant fungi dominate. In forest soils at pH < 4, earthworm numbers are greatly reduced (Eash et al., 2016).
Mineral destruction: under strongly acid conditions, secondary clay minerals and iron oxides begin to break down, leading to irreversible soil degradation.

3.3. Salinisation

Salinisation is the accumulation of readily soluble salts (chlorides, sulphates, carbonates, and bicarbonates of sodium, calcium, and magnesium) in the soil in concentrations that inhibit plant growth (Weil & Brady, 2017; Scheffer et al., 2018). The threshold for salinisation is considered to be a salt content giving an electrical conductivity of the saturation extract (ECₑ) greater than 4 dS/m (some sources say > 2 dS/m for sensitive crops).

Causes of Salinisation

Salinisation most often occurs in arid and semi‑arid regions where rainfall is insufficient to leach salts from the soil profile. The main anthropogenic factors are:

1. Irrigation without adequate drainage. Irrigation water always contains some salts. When water evaporates, pure water goes into the atmosphere, while salts remain and accumulate in the soil. Especially dangerous is irrigation with mineralised water (1–2 g/L or more) and the absence of a drainage water disposal system (Weil & Brady, 2017; Valkov et al., 2004).

2. Rise of mineralised groundwater due to disruption of the water balance (e.g., replacing deep‑rooted natural vegetation with shallow‑rooted crops).

3. Use of saline fertilisers or contaminated wastewater for irrigation.

4. Secondary salinisation — a process that occurs on already irrigated soils due to improper operation of irrigation systems. This is one of the most serious problems in modern irrigated agriculture.

Mechanisms of Salt Effects on Plants

Salts affect plants in two ways (Eash et al., 2016; Weil & Brady, 2017):

1. Osmotic effect. A high salt concentration in the soil solution lowers its water potential. Roots have more difficulty taking up water; the plant spends extra energy to maintain osmotic pressure inside cells, leading to reduced growth and productivity. Externally, this manifests as wilting even when soil moisture is adequate.

2. Specific ion toxicity. Na⁺, Cl⁻, and boron (in boron‑containing waters) can have direct toxic effects on cells, disrupt metabolism, cause leaf necrosis, and even plant death.

Legumes, many vegetables, and fruit trees are most sensitive to salinisation. Relatively tolerant are barley, sugar beet, cotton, and some wheat varieties (Weil & Brady, 2017). Table 10.3 in Weil & Brady (2017) shows that at ECₑ 4–8 dS/m, sensitive and moderately sensitive crops suffer severely, while at ECₑ > 16 dS/m only very salt‑tolerant species survive.

3.4. Alkalisation (Solonetzation, Sodification)

Alkalisation (solonetzation) is the process of accumulation of exchangeable sodium on the soil exchange complex, leading to a sharp deterioration in physical properties and an increase in pH to 8.5–10 or more (Weil & Brady, 2017; Scheffer et al., 2018). Essentially, alkalisation is a special type of chemical degradation that is closely intertwined with physical degradation: high sodium content causes dispersion of colloids, destruction of structure, and a sharp decrease in water permeability.

Difference Between Salinisation and Alkalisation

These terms are often confused, but the difference is fundamental:

Saline soils contain many readily soluble salts, but sodium may not dominate on exchange sites. In such soils, pH is usually < 8.5, and aggregates are stable.
Alkaline (sodic) soils have low total salt content (or salts have been leached), but a high proportion of exchangeable sodium (ESP > 15%). pH rises to 8.5–10, colloids are dispersed, and structure is destroyed.

Sodium causes dispersion of clay particles because its monovalent ions are weakly held on colloid surfaces and give a wide diffuse layer. This causes particles to repel each other and not form aggregates (Weil & Brady, 2017). The soil becomes structureless, sticky, and “soapy” to the touch. When dry, it cracks into large polygonal blocks.

Causes of Secondary Alkalisation

Irrigation with waters high in sodium bicarbonate (sodic waters) or with a high sodium adsorption ratio (SAR). Upon evaporation of such waters, carbonate ions react with calcium, precipitating it as CaCO₃, while sodium remains in solution and starts to enter the exchange complex (Valkov et al., 2004).
Rise of alkaline groundwater containing soda (Na₂CO₃ and NaHCO₃) to the surface.

Consequences of Alkalisation

Almost zero water permeability — water hardly enters the soil, strong surface runoff, which prevents leaching and deepens degradation.
Very poor aeration — oxygen does not reach roots, suppression of respiration and microbial activity.
Toxic effect of high concentrations of Na⁺ and OH⁻ on roots, especially young plants.
Formation of a black crust on the surface upon drying (dissolved and dispersed humus rises with capillary water and creates a dark coating — “black alkali”).
Restoration of such soils requires application of gypsum (CaSO₄), which replaces sodium with calcium, followed by leaching to remove the resulting neutral salts (Weil & Brady, 2017).

3.5. Loss of Organic Matter (Dehumification)

Soil organic matter is the totality of all organic compounds in the soil, including plant residues, microbial metabolites, and humus. Humus is the basis of soil fertility: it provides structure, water‑holding capacity, cation exchange capacity, and serves as a reservoir of nutrients (Eash et al., 2016; Foth, 1990).

Causes of Humus Loss

1. Ploughing and long‑term arable use. The transition from natural ecosystems (forest, steppe) to agroecosystems leads to a sharp decline in organic matter content. In the first 20–30 years after ploughing, humus stocks can decrease by 30–50% (Foth, 1990). The reasons are increased aeration and mineralisation, reduced input of fresh organic material, and structure disturbance.

2. Erosion. Water and wind erosion remove the uppermost, most humus‑rich layer. The concentration of humus in eroded material is often 2–5 times higher than in the soil (Weil & Brady, 2017).

3. Removal of crop residues (straw removal, stubble burning) — reduces the input of fresh organic matter.

4. Intensive tillage accelerates humus mineralisation, especially in warm, well‑aerated conditions.

5. Drainage of waterlogged and peat soils — leads to rapid mineralisation of organic matter, sometimes with a loss of 2–3 cm of peat layer per year (Weil & Brady, 2017).

Consequences of Dehumification

Structure deterioration — without cementing agents (humic substances), aggregates do not form.
Reduced cation exchange capacity (CEC) — humus carries a large negative charge; its loss sharply reduces the soil’s ability to retain basic cations (Ca²⁺, Mg²⁺, K⁺, NH₄⁺).
Reduced water‑holding capacity — organic matter can hold up to 20 times its weight in water. A loss of 1% humus reduces available water by about 1.5–2 mm per 10 cm layer.
Reduced buffering — soil is less able to resist acidification and chemical pollution.
Suppression of microflora — reduction of energy substrate for soil biota, decline in biological activity.

It is important to emphasise that humus loss triggers a positive feedback: the less humus, the worse the structure, the easier erosion, and the faster the remaining humus disappears.

3.6. Soil Contamination (Pollution)

Soil contamination is the introduction of chemical substances (or changes in their concentration) that exceed the soil’s capacity for immobilisation and transformation, leading to negative changes in properties, reduced fertility, and deterioration of product quality (Valkov et al., 2004; Scheffer et al., 2018).

The most hazardous types of contamination include:

Heavy Metals

Heavy metals (Cu, Zn, Ni, Pb, Cd, Hg, Cr, etc.) accumulate in soils from industrial emissions, sewage sludge, mineral fertilisers (especially phosphate), and pesticides. Their danger lies in the fact that they do not decompose, but only redistribute among forms. In soil, they are sorbed onto clay particles, organic matter, oxides, form their own minerals, or exist in exchangeable form. When pH decreases, many of them (especially Cd, Zn, Pb) become mobile and enter plants (Eash et al., 2016; Weil & Brady, 2017). Many heavy metals are toxic to plants and microorganisms and can enter food chains, affecting animals and humans.

Organic Pollutants

Organic pollutants include:

pesticides and herbicides (especially persistent organochlorines, e.g., DDT, although many are banned);
polycyclic aromatic hydrocarbons (PAHs) — products of incomplete fuel combustion;
polychlorinated biphenyls (PCBs) and dioxins — extremely toxic and persistent compounds;
petroleum products and solvents (benzene, toluene, etc.);
pharmaceuticals (antibiotics, hormones) from livestock waste.

The fate of organic pollutants in soil depends on their adsorption (on humus and clays), degradation (chemical, photochemical, or microbial), volatility, and water solubility. Some are very persistent and can remain for decades (Weil & Brady, 2017; Scheffer et al., 2018). Many organic pollutants are toxic, carcinogenic, mutagenic, and disrupt the endocrine system.

Radionuclide Contamination

Although radionuclides occur naturally in soils (potassium‑40, radium‑226, uranium), anthropogenic sources — nuclear accidents (Chernobyl, Fukushima), nuclear tests — create local areas of elevated radioactivity (⁹⁰Sr, ¹³⁷Cs, ¹³¹I, etc.). These isotopes can enter the biological cycle and pose a serious risk to human health (Weil & Brady, 2017; Valkov et al., 2004).

3.7. Interrelation Between Chemical and Physical Degradation

We have already mentioned several times that chemical and physical degradation are inseparable. This is especially evident in two examples:

1. Alkalisation — a purely chemical process (accumulation of exchangeable sodium) produces profound physical consequences: dispersion, structure destruction, drastic deterioration of water permeability, aeration, and filtration. In turn, the destroyed structure prevents salt leaching, reinforcing alkalisation.

2. Loss of organic matter — a chemical process (degradation and removal of humus) causes physical degradation: aggregate destruction, compaction, reduced resistance to erosion. And the deterioration of physical properties, in turn, slows down the recovery of humus stocks.

Thus, chemical and physical degradation form a single degradation complex in which all processes are interconnected. Therefore, when developing measures to combat degradation, it is necessary to address both physical and chemical factors simultaneously, taking their mutual influences into account.

4. Interrelation of Physical and Chemical Degradation Processes

4.1. Degradation as a Systemic Process

So far we have considered physical and chemical degradation separately. But in real soil, these processes never act in isolation. They are intertwined in a single degradation complex, where each process reinforces the others, creating positive feedback loops. This means that once started, degradation tends to self‑accelerate: the more the soil is damaged, the faster it deteriorates further.

This is why degradation should be understood not as a sum of separate processes, but as a systemic phenomenon, in which physical and chemical changes are mutually conditioned and sustain each other (Valkov et al., 2004; Weil & Brady, 2017).

At the centre of this degradation complex is organic matter — the key factor that ties together the physical and chemical properties of the soil. Let us examine this central role in more detail.

4.2. The Central Role of Organic Matter

Organic matter (humus) is not just a nutrient reserve. It is a structure‑forming agent, a regulator of chemical processes, and the energy basis of biological activity. Its loss triggers a chain of interrelated degradation changes.

Loss of organic matter → structure destruction → compaction

Humus cements mineral particles into aggregates. When it decreases (dehumification), aggregates become fragile and lose water stability. Under raindrop impact and mechanical tillage, they break down into individual particles. The soil becomes structureless, leading to:

reduction in macropores and total porosity,
increased bulk density,
reduced water permeability (Eash et al., 2016; Weil & Brady, 2017).

Compaction → impaired aeration → changes in chemical processes

Compacted, structureless soil has poor gas exchange. In it:

oxygen supply to roots and microorganisms is reduced — root respiration and aerobic mineralisation are suppressed,
anaerobic conditions develop in micro‑zones, especially within wet aggregates,
anaerobic processes are activated — denitrification (loss of nitrogen as N₂ and N₂O), reduction of iron and manganese, formation of phytotoxic compounds.

Anaerobic conditions shift the redox potential (Eh) to negative values, changing the forms and mobility of many chemical elements (Fe, Mn, S). This, in turn, affects nutrient availability and the toxicity of some metals (Weil & Brady, 2017).

4.3. Specific Chains of Interrelations

Let us consider several typical chains of degradation changes.

Chain 1: Humus loss → structure destruction → erosion → further humus loss

This is a classic positive feedback. With ploughing and long‑term agriculture, humus content decreases. The soil loses aggregation. Surface aggregates are more easily destroyed by rain, slake, and form a crust. Infiltration drops sharply, runoff increases. Water erosion removes the upper, most fertile layer, taking away the remaining humus. The soil becomes even poorer and even more erosion‑prone. Each subsequent cycle accelerates.

Chain 2: Compaction → acidification → aluminium mobilisation → deterioration of physical properties

Compaction impairs aeration and water permeability, but it can also promote acidification (especially under ammonium nutrition). When acidification reaches pH < 5.5, aluminium ions (Al³⁺) begin to dominate on the cation exchange complex. High exchangeable aluminium is toxic to roots and microorganisms, and can also interact with clay minerals, altering their charge and enhancing dispersion. This affects physical properties and further worsens conditions for plants (Weil & Brady, 2017).

Chain 3: Irrigation → salinisation → alkalisation → structure destruction → sharp reduction in filtration

When irrigating with even moderately saline water (1–2 g/L), salts gradually accumulate in the upper layers. If sodium dominates in the salts and bicarbonate is present, alkalisation begins: sodium displaces calcium and magnesium from the exchange complex. Sodium causes clay particles to disperse, aggregates collapse, structure disappears. The soil becomes almost impermeable. Salt leaching becomes impossible, salinisation deepens, and the soil completely loses productivity (Valkov et al., 2004; Weil & Brady, 2017).

Chain 4: Heavy metal contamination → biota suppression → structure deterioration → acidification intensification

Heavy metals are toxic to microorganisms and plants. Reduced biological activity slows humus formation and weakens structure‑forming processes. Moreover, some heavy metals (e.g., aluminium and iron) upon oxidation can form acids, contributing to acidification. Acidification, in turn, increases the mobility of many metals, making them even more toxic (Scheffer et al., 2018; Weil & Brady, 2017).

4.4. The Role of Chemical Factors in the Physical State of Soil

Two chemical factors deserve special attention because they strongly influence physical properties:

Type of Exchangeable Cations

The degree of soil aggregation depends on which cations dominate on the exchange complex:

Ca²⁺ and Mg²⁺ promote flocculation of clay particles and the formation of stable aggregates. Soils with high contents of these cations (chernozems, chestnut soils) have good structure.
Na⁺, by contrast, causes dispersion of clay particles, structure destruction, and slaking (Eash et al., 2016; Weil & Brady, 2017).

This difference is explained by the different thickness of the diffuse layer (see Section 3.4). The higher the sodium proportion, the worse the physical properties, and the faster physical degradation develops.

Soil pH

pH affects many physical properties:

The higher the pH (especially > 8.5), the more dispersed the clay particles, especially in the presence of sodium.
At pH > 9, organic matter may dissolve (“black alkali”), reducing its structure‑forming role.
Under strong acidification (pH < 4), aluminosilicate minerals begin to break down, changing the particle‑size distribution and impairing aggregation.

4.5. Spatial Heterogeneity of Degradation

It is important to understand that degradation does not always affect the entire soil uniformly. In the soil profile, “hot spots” of degradation may form, where certain processes are particularly intense:

Sub‑tillage compacted horizon (plough pan) — here physical degradation is localised, but it blocks water and air exchange between upper and lower horizons, intensifying degradation throughout the profile.
Surface crust — confined to the upper millimetres, but it radically changes the hydrological regime of the entire top layer.
Sodic patches (solonetz, solonchak) — within one field there may be areas with a high degree of alkalisation and almost normal areas. This creates a mosaic pattern of degradation.

4.6. Cumulative Effect: Combined Impact Greater Than the Sum of Individual Processes

When several degradation processes act simultaneously, their combined effect often exceeds the simple sum of their individual effects (synergy). For example:

Acidification + compaction together suppress the root system, reducing the input of organic residues, thus lowering humus accumulation and worsening structure.
Salinisation + heavy metal contamination can be particularly toxic to microflora, as salts increase the mobility of some metals.
Erosion + dehumification accelerate each other: the removal of the top layer carries away humus, while humus loss weakens resistance to erosion.

Thus, degradation is not a linear but a non‑linear, self‑accelerating process. The more factors act simultaneously, the faster fertility declines, and the more difficult it is to restore.

4.7. Section Conclusion: Degradation as a Systemic Syndrome

We see that physical and chemical degradation do not exist separately. They form a single pathological syndrome of the soil, in which:

1. Disruption of one link triggers the whole complex. For example, loss of organic matter is a trigger for physical and chemical degradation.

2. Processes reinforce each other through positive feedbacks.

3. The combined effect is greater than the sum of individual effects (synergy).

4. Irreversibility increases with each stage.

Therefore, when developing soil protection measures, we cannot limit ourselves to individual measures (e.g., only liming or only structure improvement). A systemic approach is needed, taking into account all links of the degradation complex. Only a comprehensive intervention aimed simultaneously at improving physical, chemical, and biological properties can stop and reverse degradation.

5. Rate of Degradation. Modern Estimates

5.1. Why Is the Rate of Degradation Critically Important?

We have already formulated the key question of the entire lecture: why do soils lose their properties faster than they form? To answer it quantitatively, we need to compare the rates of two opposing processes — soil formation and degradation. It is this comparison that gives us an understanding of the scale of the problem and its time horizons.

In this section, we will consider:

the rate of natural soil formation under different conditions;
the rate of the main degradation processes (erosion, humus loss, salinisation, acidification, compaction);
factors that accelerate or slow down degradation;
modern global estimates of degradation rates.

5.2. Rate of Soil Formation

Natural soil formation is the process of forming a soil profile from parent material under the influence of climate, organisms, topography, and time. The rate of this process is highly variable and depends on many factors (Weil & Brady, 2017; Troeh & Thompson, 1993).

Estimates of Soil Formation Rates

The most common estimate: 0.5–2 cm of soil per 100 years. This means that forming a 20–30 cm fertile layer (the thickness of the plough layer) requires 1000 to 6000 years (Troeh & Thompson, 1993; Weil & Brady, 2017).
In terms of mass: 5–20 tonnes per hectare per year (at an average soil density of about 1.3 g/cm³). This figure serves as a guide for determining allowable (tolerable) soil losses.
On carbonate rocks (limestones) and in warm, humid climates, soil formation can be faster — up to 1 cm per 20–30 years (e.g., in the tropics).
On acid crystalline rocks (granites, gneisses) and in cold climates — slower, up to 1 cm per 200–500 years.

It is important to emphasise: the rate of soil formation is not constant. It is highest at the initial stages, when fresh rock is actively weathered, and slows down as the soil profile accumulates and begins to act as a protective screen (Troeh & Thompson, 1993).

Rate of Humus Accumulation

Humus accumulation is particularly slow. Under natural conditions, increasing humus content by 1% in the upper 20‑cm layer may take from 200 to 800 years (depending on climate and vegetation type). In agroecosystems, the loss of the same 1% humus can occur in 5–20 years.

5.3. Rate of Degradation Processes

Water Erosion

Water erosion is one of the fastest degradation processes. The rate of soil loss varies greatly depending on climate, topography, soil, and land use:

Natural (geological) erosion under undisturbed vegetation: 0.02–0.2 t/ha per year (Weil & Brady, 2017; Troeh & Thompson, 1993). This corresponds to a loss of about 1 cm of soil over 2000–5000 years.
Accelerated erosion on arable land: on average 5–15 t/ha per year in the USA, 20–40 t/ha per year in Europe, and up to 50–100 t/ha per year in tropical regions under intensive agriculture (Weil & Brady, 2017).
Extreme values: on highly erodible slopes during heavy rainfall, losses can reach 100–200 t/ha in a single season, corresponding to a loss of 5–10 mm of soil layer in one year (Weil & Brady, 2017; Troeh & Thompson, 1993).

The average rate of water erosion on arable land worldwide is estimated at 20–30 t/ha per year. This means that 1 cm of soil is lost in 5–7 years (Troeh & Thompson, 1993).

Wind Erosion

Wind erosion is particularly intense in arid and semi‑arid regions, as well as on light‑textured soils:

On arable land in dry regions of the USA, losses from wind erosion average 5–15 t/ha per year, and in some areas (New Mexico) up to 29 t/ha per year (Weil & Brady, 2017).
In years of severe droughts and dust storms, losses can reach 50–100 t/ha in a single storm.
On drained peat soils (under wind erosion), the layer can lose 1–2 cm per year.

Loss of Organic Matter (Dehumification)

When virgin lands are ploughed:

In the first 20–40 years after ploughing, humus content may decrease by 30–50% of the initial level (Foth, 1990). In temperate chernozems, this means a loss of 0.5–1.5 t/ha of humus per year (in terms of carbon — 0.3–1.0 t C/ha per year).
Under erosion, humus loss rates are even higher: the eroded material contains 2–5 times more organic matter than the original soil.

Salinisation and Alkalisation

These processes increase over several years after the start of improper irrigation:

Secondary salinisation can reach critical levels (ECₑ > 4 dS/m) in 5–15 years under irrigation without drainage.
When using waters with high mineralisation (2–3 g/L), the process may take 2–5 years (Valkov et al., 2004; Weil & Brady, 2017).
Alkalisation develops more slowly, but also within 10–30 years, especially on soils with high sodium in the irrigation water.

Acidification

Natural acidification lasts millennia. Anthropogenic acidification (under the influence of acid rain and fertilisers) proceeds much faster:

With application of physiologically acid fertilisers (ammonium nitrate, ammonium sulphate, urea), pH can decrease by 0.3–0.5 units in 5–10 years.
In forest soils under acid rain, over 30–40 years the pH of upper horizons could drop from 4.5–5.0 to 3.5–4.0 (Scheffer et al., 2018).

Compaction

Compaction develops very quickly — sometimes in one season of intensive heavy machinery use:

Soil bulk density can increase by 10–20% after a few passes on wet soil.
Restoration of a compacted sub‑tillage horizon (even under favourable conditions) takes 5–10 years in the absence of new loads, and sometimes longer.

5.4. Modern Global Estimates of Degradation

Global Extent of Degradation

According to the Food and Agriculture Organization (FAO, 2011, 2015):

About 25% (approximately 2 billion hectares) of all land worldwide is affected by some form of degradation. This is an area twice the size of Russia.
Every year, 10–12 million hectares of land are lost from agricultural production due to degradation — roughly 1% of the world’s cultivated land.
Land degradation affects more than 1.5 billion people (almost 20% of the global population), especially in developing countries.
Annual economic losses from land degradation are estimated at $40–100 billion (Weil & Brady, 2017; FAO, 2011).

Main Types of Degradation Worldwide

According to various international assessments:

Erosion (water and wind) — 85% of all soil degradation (Weil & Brady, 2017; Valkov et al., 2004).
Chemical degradation (salinisation, acidification, contamination) — about 12%.
Physical degradation (compaction, structure destruction) — about 3%.

However, these figures vary greatly by region:

In Africa and Asia, erosion and dehumification dominate.
In Australia and parts of North America — salinisation.
In Europe — compaction and contamination (up to 50% of agricultural land in Europe is affected by compaction) (Scheffer et al., 2018).

Degradation in Russia

According to Russian studies (Valkov et al., 2004; various departmental estimates):

About 40–50% of agricultural land in Russia is degraded to some degree.
Main processes: water erosion (in the south and in the Central Chernozem region), wind erosion (in steppe and forest‑steppe areas), dehumification (almost everywhere), secondary salinisation (in irrigated agriculture areas).
In the chernozem zone, humus content has decreased by 15–25% over the last 30–40 years.
Annual humus losses on arable land in Russia are estimated at 30–50 million tonnes.

5.5. Factors Determining the Rate of Degradation

The rate of degradation is determined by a combination of natural and anthropogenic factors (Weil & Brady, 2017; Troeh & Thompson, 1993):

Natural factors accelerating degradation:

Climate: intense rainfall, strong winds, arid or, conversely, excessively humid climate.
Topography: steep slopes, long slopes (increase erosion).
Texture: sandy loams and loams are most vulnerable to erosion; clays to compaction; sands to wind erosion and dehumification.
Humus content and humus type — the less humus, the weaker the structure and the faster degradation proceeds.
Type of clay minerals: montmorillonitic clays are more prone to compaction and swelling, kaolinitic clays to erosion.

Anthropogenic factors accelerating degradation:

Tillage intensity — the more tillage operations, the faster structure is destroyed and humus is lost.
Use of heavy machinery — direct compaction.
Irrigation without drainage — salinisation.
Application of high mineral fertiliser rates without considering balance — acidification, contamination.
Monoculture and lack of crop rotations — soil depletion, accelerated dehumification.
Insufficient organic fertilisers and crop residues — slowed humus accumulation.

5.6. Time Scales

Comparing the rates of natural soil formation and anthropogenic degradation gives striking figures (Troeh & Thompson, 1993; Weil & Brady, 2017):

Process	Rate (approximate)
Soil formation	0.5–2 cm per 100 years (5–20 t/ha·yr)
Water erosion on arable land	5–40 t/ha·yr (average)
Wind erosion on arable land	5–30 t/ha·yr
Humus loss upon ploughing	0.3–1.0 t C/ha·yr
Acidification (with fertilisation)	pH decreases by 0.3–0.5 in 5–10 years
Salinisation (under irrigation)	critical level in 5–15 years
Compaction	10–20% density increase in 1 season

Thus, degradation proceeds 10–1000 times faster than soil formation.

This is the key figure that determines our attitude to soil as a resource. At an erosion rate of 20–30 t/ha·yr, 1 cm of soil is lost in 3–5 years. To restore it under natural conditions would take 50–200 years. This means that in one human generation, we can lose what took many centuries to form.

5.7. Non‑linearity of Degradation

It is important to note that degradation often proceeds non‑linearly. In its early stages, it may be slow and almost imperceptible. However, as changes accumulate, it can accelerate sharply, crossing threshold values (Richter & Tugel, 2012).

Examples of such thresholds:

Erosion: as long as the top humus layer is not completely removed, erosion may be moderate. But once the less structured subsoil is exposed, its erosion resistance may be lower, and the rate of loss increases sharply.
Alkalisation: as long as the exchangeable sodium percentage does not exceed 15%, structure may be preserved. Once this threshold is exceeded, colloids disperse, structure is destroyed almost instantly, and the soil becomes virtually impermeable.
Acidification: at pH above 5.5, aluminium is immobile. When pH drops below 5.0–5.5, a sharp mobilisation of aluminium, toxic to plants, begins, leading to a rapid decline in productivity.

This non‑linearity makes degradation especially dangerous: critical changes can occur suddenly, when lagging indicators (e.g., visual plant condition or yield) do not yet signal that a threshold is approaching.

6. Reversibility of Degradation Processes

6.1. Introduction: Degradation as Loss of Time

We have come to the most important and perhaps most alarming question of the entire lecture: can we stop degradation and restore the fertility of lost soils?

The answer to this question is not straightforward. It depends on which specific process of degradation has occurred, how far it has progressed, and what resources (time, money, technology) we are willing to invest in restoration. One of the main concepts in modern soil science states: degradation is not only a loss of soil properties, but above all a loss of the time required for their formation (Richter & Tugel, 2012; Weil & Brady, 2017).

Throughout this lecture, we have repeatedly emphasised that soil formation is an extremely slow process, measured in centuries and millennia. Degradation, however, can develop in years or even months. Therefore, when discussing reversibility, we must always keep in mind the time scales: what can be restored in 10–20 years, and what takes 200–500 years or is not restored at all.

6.2. The Concept of Thresholds and Irreversibility

A key idea for understanding reversibility is the concept of thresholds in degradation (Richter & Tugel, 2012; Weil & Brady, 2017). By analogy with a disease: at an early stage, many processes are reversible; after crossing a certain threshold, they become practically irreversible or require enormous expenditures.

Two types of thresholds are distinguished:

1. Physical threshold: destruction of aggregate structure to such an extent that it cannot recover even after the load ceases; complete removal of the humus horizon by erosion.

2. Chemical threshold: a drop in pH to a level where active destruction of clay minerals begins; accumulation of exchangeable sodium above 15% of the cation exchange capacity (CEC), leading to colloid dispersion.

When a threshold is crossed, the mechanisms of processes change: the soil shifts to a new state from which return to the original state without substantial external intervention is very difficult or impossible (Richter & Tugel, 2012).

6.3. Reversibility of Major Degradation Processes

Let us examine each of the main degradation processes from the standpoint of restoration potential.

Erosion

Reversibility: extremely low. Practically irreversible on a human lifetime scale.

Erosion is the most destructive and least reversible process. When a soil layer is washed or blown away, it cannot be “put back” by natural means. One can only:

Slow down or stop further erosion.
Restore fertility on the remaining soil by adding fertilisers and organic matter (but this does not return the lost profile thickness).
In exceptional cases, carry out reclamation (spreading imported topsoil), but this is extremely expensive and applicable only on small areas.

Estimates show that 1 cm of soil lost by erosion takes 100–400 years to form naturally (Weil & Brady, 2017; Troeh & Thompson, 1993). Therefore, in practice, erosion is considered a practically irreversible process within a human generation.

Loss of Organic Matter (Dehumification)

Reversibility: moderate, but requires long time and systematic efforts.

Humus restoration is possible, but requires:

Regular input of organic residues (green manures, manure, composts, crop residues).
Minimisation of tillage (preserving structure, slowing mineralisation).
Optimisation of crop rotations (including perennial grasses).

Under optimal conditions (temperate climate, adequate moisture, regular organic amendments), humus content can increase by 0.05–0.1% per year. This means that restoring 1% humus (lost over 10–20 years) may take 10–20 years of intensive measures. However, in arid conditions or on light soils, this process may take 50–100 years (Foth, 1990; Weil & Brady, 2017).

Compaction

Reversibility: moderate on the surface, low in the sub‑tillage horizon.

Surface compaction can be alleviated in 2–5 years by:

Reducing heavy machinery use or using low‑pressure tyres.
Applying organic fertilisers (stimulating worms and structure formation).
Deep loosening (chiselling) (Eash et al., 2016; Scheffer et al., 2018).

However, sub‑tillage compaction (plough pan) recovers extremely slowly because it lies below the zone of active biogenic structure formation. Even after deep ripping (40–50 cm), restoration of natural porosity may take 5–15 years, and under repeated loads, the process may not complete at all. Recovery of bulk density to the original level in heavy clay soils may take more than 20 years (Scheffer et al., 2018).

Acidification

Reversibility: high, but requires regular liming.

Acidification is one of the most manageable processes. Application of liming materials (CaCO₃, CaMg(CO₃)₂) quickly (within months) neutralises acidity and raises pH. However:

The rate of recovery depends on the soil’s buffer capacity. On light sandy soils, 1–2 t/ha of lime every 3–5 years may suffice; on heavy clay and humus‑rich soils, 3–5 t/ha or more are required (Eash et al., 2016; Weil & Brady, 2017).
Natural recovery (without liming) when anthropogenic impact ceases is very slow (hundreds of years) due to mineral weathering and input of bases with atmospheric precipitation.

It is important to remember: if acidification has led to the destruction of clay minerals and the leaching of bases from the entire profile, even liming will not restore the lost cation exchange capacity (CEC) and structural condition — these changes are irreversible on a decade scale (Weil & Brady, 2017).

Salinisation

Reversibility: high on drained soils; low in the absence of drainage or under high mineralised groundwater tables.

Salinisation is reversible if good drainage and sufficient leaching water (fresh or low‑mineralised) are available. Salt leaching can take 1–3 seasons with adequate irrigation rates.

However, restoration of strongly saline soils (ECₑ > 16 dS/m) requires more time (5–10 years) and large water volumes. In the absence of natural drainage or under high saline groundwater, salinisation becomes irreversible (Weil & Brady, 2017; Valkov et al., 2004).

Alkalisation (Solonetzation)

Reversibility: low; requires chemical reclamation (gypsum application) and many years of efforts.

Alkalisation (accumulation of exchangeable sodium, ESP > 15%) is an extremely difficult process to reverse. To eliminate it, one needs:

Application of gypsum (CaSO₄) to displace sodium.
Subsequent leaching to remove the formed Na₂SO₄.
Restoration of the structure destroyed by dispersion.

This process takes several years (3–10 years) and can be carried out only on soils with good drainage. In the absence of drainage or insufficient leaching water, alkalisation is practically irreversible (Weil & Brady, 2017; Valkov et al., 2004).

Contamination with Heavy Metals and Organic Toxicants

Reversibility: extremely low; in most cases irreversible.

Heavy metals do not decompose in soil. The only removal method is phytoremediation (extraction by hyperaccumulator plants), but this is a very slow process (decades) and only applicable at moderate contamination levels (Weil & Brady, 2017). An alternative is isolation of the contaminated layer (removal to landfill, covering with clean soil), which is not restoration but merely containment.

Organic pollutants (PAHs, PCBs, pesticides) can be degraded by microorganisms, but the degradation rate is often very low (years to decades). For some persistent compounds (dioxins, certain PCBs), natural degradation is practically absent.

Thus, chemical contamination is generally considered a practically irreversible form of degradation.

6.4. Restoration vs. Reclamation

It is important to distinguish two concepts (Valkov et al., 2004; Richter & Tugel, 2012):

Restoration — returning the soil to its original (or close to original) state with a full set of functions. This is possible only for reversible processes (acidification, early salinisation, shallow compaction) and requires time measured in years to decades.
Reclamation — a set of measures to create a new soil or replace lost properties on disturbed lands (quarries, dumps, eroded lands). This is not a return to the original state, but the creation of a new soil body, often less fertile and requiring constant maintenance.

Reclamation may include:

Selective excavation and storage of the humus horizon prior to development (e.g., in mining).
Spreading a fertile layer over the restored surface.
Application of large doses of organic fertilisers, lime, and mineral fertilisers.
Sowing perennial grasses to restore structure.

But even with the most intensive reclamation works, full restoration of original fertility on severely disturbed lands is impossible in the foreseeable future (Weil & Brady, 2017).

6.5. Managed Reversibility: What Can Humans Do?

Despite the pessimistic tone of the previous sections, it is important to emphasise: many forms of degradation can be halted, and some can be reversed with proper management.

Halting Degradation

The first task is to stop further deterioration. This requires:

Erosion control (contour ploughing, buffer strips, minimum tillage, maintaining plant cover).
Stopping over‑compaction (low‑pressure tyres, limiting machinery passes, tillage only at optimal moisture).
Stopping acidification (liming according to need).
Stopping salinisation (installing drainage, regulating irrigation rates).
Stopping contamination (compliance with environmental standards, replacing toxic pesticides with less hazardous ones).

Fertility Restoration (Managed Increase)

After halting degradation, active measures to improve fertility are possible:

Process	Restoration measures	Time scale
Dehumification	Organic amendments, green manures, mulching, minimum tillage	5–20 years
Surface compaction	Deep loosening (chisel, cultivator), organic matter, reduced heavy machinery	3–10 years
Acidification	Liming (CaCO₃, dolomite)	1–3 years (effect), repeat every 3–5 years
Salinisation (initial)	Leaching with water, drainage installation	1–3 years
Alkalisation (initial)	Gypsum application + leaching	3–10 years

6.6. Examples of Successful Restoration

Humus Restoration in Agroecosystems

Numerous long‑term experiments (e.g., at Rothamsted, England, and in the USA) show that with systematic application of organic fertilisers and use of perennial grasses in crop rotations, humus content can be increased by 0.5–1.0% over 20–30 years (Foth, 1990; Weil & Brady, 2017). However, reaching the initial level of virgin chernozems (5–8% humus) under modern agriculture is practically impossible — the difference in organic matter input is too great.

Structure Improvement Under Minimum Tillage

In the USA and Western Europe, numerous examples of improved soil structure have been documented when switching to minimum or no‑till systems. After 5–10 years of abandoning ploughing:

Earthworm numbers increase (2–5 times).
Aggregation of the top layer improves.
Water permeability increases.
Erosion decreases (Weil & Brady, 2017; Eash et al., 2016).

However, restoration of the sub‑tillage horizon structure does not occur; it remains compacted for years (Scheffer et al., 2018).

Restoration of Saline Soils

In several countries (Israel, Egypt, California), successful restoration of saline irrigated lands has been achieved through the construction of drainage systems and leaching. The process takes 2–5 years and requires significant investments. However, full restoration of original fertility (especially under secondary alkalisation) may take 10–20 years or more (Weil & Brady, 2017).

6.7. Time Aspect of Restoration

We can distinguish three time levels of degradation reversibility:

1. Rapid restoration (1–5 years)

Elimination of surface compaction (loosening).
Neutralisation of acidification (liming).
Removal of initial salinisation (leaching).

2. Medium‑term restoration (5–20 years)

Significant increase in humus content.
Recovery of aggregate structure in the top horizon.
Reclamation of sodic soils (gypsum application).

3. Long‑term or irreversible (more than 50–100 years)

Restoration of profile thickness lost by erosion.
Restoration of the sub‑tillage horizon (plough pan).
Clean‑up of persistent pollutants (heavy metals, dioxins).

6.8. Conclusion: Managing Time

Thus, the key conclusion regarding the reversibility of degradation is as follows:

1. The earlier measures to combat degradation are started, the higher the chances of successful restoration. At early stages, most processes are reversible within 5–20 years.

2. The further degradation has progressed, the more difficult and expensive restoration becomes. After crossing certain thresholds (loss of the entire humus horizon, destruction of sub‑tillage structure, critical salinisation), restoration becomes either impossible or economically unfeasible.

3. The best strategy is to prevent degradation (prevention) rather than to cure it. As the old soil science saying goes: “It is easier to preserve soil than to restore it” (Troeh & Thompson, 1993).

4. Restoration is an investment in the future, which pays off through increased yields, improved water quality, reduced fertiliser and reclamation costs, and through the preservation of biodiversity and agroecosystem resilience.

6.9. Lecture Conclusion

We have travelled the full path — from the definition of degradation to the analysis of its reversibility. Let me summarise the main ideas of this lecture:

1. Soil degradation is a systemic deterioration of physical, chemical, and biological properties, leading to a decline in fertility and ecological functions of the soil.

2. Physical degradation (compaction, structure destruction, loss of porosity, erosion) alters the “skeleton” of the soil — its ability to transmit water, air, and support root growth.

3. Chemical degradation (acidification, salinisation, alkalisation, humus loss, contamination) changes the chemical environment for plants and microorganisms, reducing nutrient availability and creating toxic conditions.

4. Physical and chemical degradation are inextricably linked in a single degradation complex. Loss of humus destroys structure; compaction changes chemical processes; salinisation and alkalisation destroy aggregates. Degradation is a self‑accelerating process.

5. The rate of degradation (years to decades) is 10–1000 times higher than the rate of soil formation (centuries to millennia). This is the main reason why soil is a practically non‑renewable resource on a human lifetime scale.

6. Reversibility varies greatly. Erosion and deep contamination are irreversible; compaction, acidification, and initial salinisation can be reversed with proper management; dehumification requires long‑term and systematic efforts.

7. The main practical takeaway: the best strategy is to prevent degradation rather than to fight its consequences. Any investment in soil conservation is an investment in the future of food security and environmental sustainability.

6.10. Self‑Check Questions

To reinforce the material, I suggest you answer the following questions:

1. Why is erosion considered a practically irreversible process? What time scale is needed to restore 1 cm of soil lost by erosion?

2. What is a “plough pan” and why is its restoration more difficult than that of the surface layer?

3. What is the difference between salinisation and alkalisation? Which of these processes is easier to reverse and why?

4. Which degradation processes (among those considered) can be regarded as reversible within 5–10 years under proper management?

5. Why is humus loss considered the central link of the degradation complex? Explain using one of the chains of interrelations as an example.

6. What is a threshold in degradation? Give examples of physical and chemical thresholds.

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