Soil Fertility

Definition of the Concept

Fertility is the most important and specific property of soil, distinguishing it from parent rock. Valkov et al. (2004) give the following definition: fertility is the ability of soil to meet the needs of specific plants for nutrients, water, and to provide their root systems with air and heat.

Note the word “specific.” Fertility always manifests itself in relation to a particular plant species. As the Roman philosopher Pliny the Elder noted, “soil adorned with tall and stately trees is far from the best, if we do not consider its suitability for the trees themselves” (Valkov et al., 2004). This observation contains profound meaning, which we will explore throughout this lecture.

Thus, fertility is not an abstract “wealth of the soil” but its functional ability to support plant life under specific conditions.

Historical Understanding of Fertility

The understanding of fertility as a special property of soil did not emerge in science all at once. In the second half of the 19th century, the great Russian scientist V.V. Dokuchaev substantiated the concept of soil as a special natural-historical body possessing its own properties, distinct from those of rocks. It was Dokuchaev and his followers who showed that soil is not merely fragmented rock but a product of the interaction of soil-forming factors.

The Swiss soil scientist H. Jenny (Jenny, 1941) expressed this in his famous formula, which remains the theoretical foundation of genetic soil science:

(where S – soil, cl – climate, o – organisms, r – relief, p – parent material, t – time; though the original formula is often given in different notation). In the text, the formula is given as:

Translated to English, it becomes:

Where:

• S — soil (as a natural body)
• P — parent material (rock)
• C — climate
• O — organisms (vegetation, animals, microorganisms)
• R — relief
• H — human (anthropogenic factor)
• t — time

It follows from this formula that soil fertility is the result of long-term interaction of all these factors, not merely an inherited property of the parent material (Scheffer et al., 2018).

Why Fertility Is Not Just a Set of Elements?

Let us return to the key question of our lecture: why can soils with the same nutrient content differ in fertility?

Imagine two soils. In both, chemical analysis shows the same amount of phosphorus and potassium. However, on one soil plants develop normally, while on the other they suffer from nutrient deficiency. What is the reason?

The answer lies in the fact that fertility is determined not only by the quantity of elements but also by their availability, and availability, in turn, depends on many factors:

1. In what chemical compounds are the elements present? In readily soluble salts or in sparingly soluble minerals?

2. How are these elements bound to soil particles? Are they free in the soil solution or strongly sorbed onto mineral surfaces?

3. In what form are they present? For example, phosphorus can be bound to calcium (in alkaline soils) or to iron and aluminium (in acidic soils), and in each case its availability differs (Weil & Brady, 2017).

4. What are the conditions for root growth? If the soil is compacted or waterlogged, roots cannot effectively use even the available elements.

As Foth and Ellis (1997) note, “a good soil test for a nutrient element should effectively measure the amount that will be closely correlated with the amount that plants can take up.” This is the crux of the problem: the total content of an element in the soil and its availability to plants are not the same thing.

Fertility as an Integral Property

From the above follows an important conclusion: fertility is an integral property that arises from a set of soil characteristics. Valkov et al. (2004) emphasise that fertility is determined not only by the content of nitrogen, phosphorus, potassium, and other nutrients, but also by the reaction of the medium (pH), physical properties, salt content, particle-size distribution, stoniness, and moisture conditions.

Thus, fertility is a systemic property emerging from the interaction of all soil components and environmental factors. That is why fertility cannot be reduced to a simple sum of nutrients or to any single indicator.

Fertility Is Tied to Plant Species

The ecological requirements of plants are extremely diverse. Some crops need acidic soils, others alkaline. Some require heavy, humus‑rich soils; others prefer light sandy soils.

Here are a few examples from the textbook by Valkov et al. (2004):

• Tea and lupine grow only on acidic soils.
• Alfalfa prefers neutral and slightly alkaline soils.
• Cereal crops perform best on heavy structural soils.
• Potatoes, melons, and sweet cherry grow better on light soils.
• Vineyards and tobacco produce lower‑quality products on nutrient‑rich soils.

This means that the same soil may be fertile for some plants and marginally fertile for others. This feature underlies the rational placement of agricultural crops according to soil conditions.

Fertility and Productivity of Biogeocenoses

It is important to distinguish between soil fertility in the agronomic sense and the productivity of natural plant communities. Valkov et al. (2004) draw attention to obvious contradictions: extremely poor, leached soils can serve as substrates for extra‑productive biogeocenoses.

A classic example is humid tropical forests (hylaea) on red and yellow ferrallitic soils. These soils are so poor in nutrients that when brought into agricultural production, they yield only 2–4 satisfactory harvests. Yet under natural forest, they support enormous biomass.

What is the explanation? In forests, the biological cycle of matter approaches a direct exchange between organisms and their dying residues. Nutrients do not have time to leach out because the forest “holds” them in biomass. In agroecosystems, however, this mechanism is disrupted, and fertility begins to be determined by the properties of the soil itself.

Two Forms of Fertility Expression

As Valkov et al. (2004) note, soil fertility manifests in two forms:

1. In the productivity (yield) of the plants growing on it. Example: the Chernozems of the Kuban region, which possess excellent natural properties and are justifiably considered among the most fertile soils in the world.

2. In the richness of nutrients, humus, and plant‑ecological properties and their quantitative‑qualitative characteristics. Here, fertility is determined by the content of nitrogen, phosphorus, potassium, and other nutrients essential for plant nutrition.

It is important to understand that high yields can also be achieved through increased artificial fertility—for example, the soils of Western Europe are world record holders in yield, even though their natural fertility is inferior to that of Chernozems.

Content of the Law of the Minimum

The Law of the Minimum (or Liebig’s law) states: plant productivity is determined by the factor that is in minimum relative to the plant’s need (Weil & Brady, 2017).

This law is figuratively explained by the Liebig barrel: yield is determined by the shortest stave (factor). If one stave of a barrel is shorter than the others, the water level cannot rise above that short stave, no matter how much water we pour in.

Practical Significance of the Law

The law of the minimum leads to important practical consequences:

1. It is useless to increase the content of a factor that is not limiting. If plants suffer from phosphorus deficiency, adding nitrogen will not help—it may even worsen phosphorus starvation.

2. The effect of jointly applying deficient elements may be greater than the sum of the individual effects. This phenomenon is called synergism or interaction.

3. When diagnosing low yields, one must identify the specific limiting factor, rather than simply “improving the soil” in general.

Limitations and Refinements

The law of the minimum, however, has its limitations. As Weil and Brady (2017) note, if a plant is limited by phosphorus deficiency (the first limiting factor), then adding available phosphorus will allow the plant to respond positively to subsequent nitrogen application. In other words, factors can interact, and removing one limitation can “open” access to other resources.

Moreover, in real conditions, plant growth is influenced not by one but by a complex of factors, and the limiting factor may be one we simply overlooked—for example, a micronutrient deficiency or adverse physical soil properties.

Soil Structure and Fabric

Soil structure is the ability of soil particles to combine into aggregates of various shapes and sizes. As Scheffer et al. (2018) point out, soil structure determines the pore space, which in turn regulates water and air regimes.

A well‑structured soil has an optimal ratio of pores of different sizes:

• Macropores (> 50 µm) provide aeration and drainage
• Mesopores (0.2–50 µm) hold plant‑available water
• Micropores (< 0.2 µm) contain water that is unavailable to plants (Scheffer et al., 2018)

Foth and Ellis (1997) emphasise that under natural vegetation, soil structure develops under the influence of root systems, soil fauna activity, and wetting‑drying cycles. Particularly important is biogenic structure—aggregates formed through the activity of microorganisms and earthworms.

Soil fabric refers to the degree of compaction, characterised by bulk density and porosity. High density (> 1.6 g/cm³ for loamy soils) impedes root growth and worsens gas exchange. As Weil and Brady (2017) note, soil compaction is one of the most widespread physical factors limiting yield in modern agriculture.

Water Regime

Water is one of the most important components of fertility. Scheffer et al. (2018) emphasise that field capacity (the amount of water the soil can hold against gravity) and available water (the difference between field capacity and wilting point) determine plant water supply.

Different soils have different water‑holding capacities:

• Sandy soils — low water‑holding capacity; water percolates easily
• Loamy soils — optimal water‑holding capacity due to mesopores
• Clay soils — high water‑holding capacity, but a significant portion of water may be unavailable to plants

Foth and Ellis (1997) point out that water availability for plants is determined not only by its quantity but also by the force of retention, which depends on pore size and the mineralogical composition of the soil.

Air Regime

Plant roots respire, consuming oxygen from the soil air. As the textbook by Scheffer et al. (2018) notes, oxygen diffusion in soil is 10,000 times slower than in the atmosphere, so soil aeration is critical.

Signs of poor aeration:

• Slowed root growth
• Reduced uptake of water and nutrients
• Accumulation of toxic products of anaerobic respiration (ethylene, organic acids)

The optimal oxygen content in soil air for most crops is at least 10–12% (Scheffer et al., 2018).

Thermal Regime

As Scheffer et al. (2018) indicate, soil temperature affects all life processes of plants and microorganisms. Each crop has its own temperature optimum for seed germination and root growth:

• Rye — 1–2 °C
• Wheat and barley — 2–4 °C
• Maize — 8–10 °C
• Potato — 8–10 °C (Scheffer et al., 2018)

Warm sandy soils warm up faster in spring but cool down faster in autumn. Cold clay soils warm up more slowly but retain heat better (Scheffer et al., 2018). This difference significantly affects sowing dates and yields.

Soil Reaction (pH)

Valkov et al. (2004) emphasise that soil reaction (pH) is one of the key factors determining the availability of almost all nutrients.

The dependence of nutrient availability on pH can be summarised as follows (Weil & Brady, 2017):

• Nitrogen, phosphorus, potassium, sulphur, calcium, magnesium — most available in slightly acidic to neutral conditions (pH 6.0–7.0)
• Phosphorus — has two “peaks” of availability: at pH 6.0–6.5 (associated with aluminium and iron) and at pH 7.5–8.0 (associated with calcium); however, in the range pH 6.0–7.0 maximum availability is achieved (Weil & Brady, 2017)
• Micronutrients (iron, manganese, zinc, copper) — more available in acidic conditions
• Molybdenum — more available in alkaline conditions (Weil & Brady, 2017)

Valkov et al. (2004) give concrete examples: tea and lupine grow only on acidic soils, while alfalfa prefers neutral and slightly alkaline soils.

Special attention should be paid to aluminium toxicity in acidic soils (pH < 5.0). As the textbook by Scheffer et al. (2018) notes, under such conditions aluminium ions dissolve and damage root systems, leading to yield reduction even when other nutrients are sufficient.

Sorption Capacity and Cation Exchange Capacity (CEC)

Foth and Ellis (1997) emphasise that the ability of soil to retain cations (positively charged ions) on the surface of colloidal particles is a fundamental property determining reserves of available elements.

Cation Exchange Capacity (CEC) is the total amount of cations that the soil can retain in exchangeable form. This indicator depends on:

• Clay mineral content (especially montmorillonite and vermiculite)
• Organic matter content (humus)
• Soil pH (CEC increases with rising pH)

Weil and Brady (2017) note that soils with high CEC have greater buffering capacity—they deplete more slowly and retain nutrients against leaching better. Sandy soils with low CEC require more frequent fertilisation.

Forms of Element Compounds

One of the key reasons for differences in fertility among soils with the same total element content is the different chemical forms in which these elements occur.

Phosphorus fixation is a classic example. As Weil and Brady (2017) note, phosphorus applied to soil as soluble salts reacts with:

• Calcium (in alkaline soils) to form sparingly soluble calcium phosphates
• Iron and aluminium (in acidic soils) to form sparingly soluble aluminium and iron phosphates

As a result, within a few weeks after application, most of the phosphorus becomes unavailable to plants (Foth & Ellis, 1997).

Potassium fixation is related to its entry into the interlayer spaces of certain clay minerals (vermiculite, illite), where it is held so firmly that it cannot be extracted by ordinary cation exchange (Foth & Ellis, 1997; Weil & Brady, 2017).

Ammonium fixation is a similar process, where NH₄⁺ ions become fixed in the interlayers of 2:1 clay minerals (Scheffer et al., 2018). This can be both beneficial (protecting nitrogen from leaching) and undesirable (reducing availability).

Availability of Nutrients

The most important concept for understanding the chemical component of fertility is nutrient availability. Even if an element is present in large quantities, it may be poorly available to plants due to:

1. Strong sorption onto soil particle surfaces (Foth & Ellis, 1997)

2. Formation of sparingly soluble compounds (Weil & Brady, 2017)

3. Unfavourable pH (Scheffer et al., 2018)

4. Ion antagonism (high concentration of one element hindering uptake of another)

Transformation of Organic Matter

Microorganisms are the main agents of mineralisation—the conversion of organic compounds into mineral forms available to plants. Scheffer et al. (2018) distinguish two key processes:

• Ammonification — conversion of organic nitrogen into ammonium (NH₄⁺)
• Nitrification — oxidation of ammonium to nitrate (NO₃⁻)

Here, the carbon‑to‑nitrogen ratio (C/N) of decomposing plant residues plays a crucial role:

• Narrow C/N (< 25) — nitrogen mineralisation occurs; the element becomes available to plants
• Wide C/N (> 25) — nitrogen immobilisation occurs; microorganisms “bind” available nitrogen, competing with plants (Scheffer et al., 2018)

Foth and Ellis (1997) emphasise that this process is key to understanding nitrogen dynamics in soil. That is why adding straw with a wide C/N ratio can cause temporary nitrogen starvation in plants.

Atmospheric Nitrogen Fixation

A most important biological function of soil is symbiotic nitrogen fixation by bacteria of the genus Rhizobium in the root nodules of legumes. As noted in the textbook by Huang et al. (2012), this ability allows plants to assimilate atmospheric nitrogen (N₂) and is a key mechanism for natural fertility reproduction in agroecosystems.

In addition, free‑living nitrogen‑fixing bacteria (Azotobacter, Beijerinckia, Clostridium) also contribute to the nitrogen balance, fixing from 1 to 30 kg N/ha per year (Huang et al., 2012).

Mycorrhiza

As Weil and Brady (2017) note, about 90% of higher plant species enter into symbiosis with mycorrhizal fungi. This symbiosis:

• Increases the absorbing surface of roots tenfold
• Facilitates access to phosphorus that is strongly sorbed by the soil
• Promotes the uptake of micronutrients (especially zinc and copper)

Mycorrhizal fungi are especially important on soils with low available phosphorus, where roots alone cannot ensure adequate uptake (Weil & Brady, 2017).

Microbial Loop

An important discovery of recent decades is the so‑called microbial loop (Clarholm, 1981, cited in Scheffer et al., 2018). Its essence is as follows:

1. Plants release organic substances into the rhizosphere (root exudates)

2. These substances stimulate microbial growth

3. Microorganisms, in turn, serve as food for protozoa and nematodes

4. When microorganisms are consumed, mineral nitrogen is released and can again be used by plants

Thus, the biological component of fertility is a self‑renewing system in which nutrients are cyclically redistributed.

Activity of Soil Fauna

Earthworms, insects, nematodes, and other soil animals play a vital role in fertility. Scheffer et al. (2018) highlight the following functions:

• Bioturbation — mixing and loosening of soil
• Aggregate formation — creating stable structural units
• Accelerating mineralisation — passing organic residues through the digestive tract

Thickness of the Root‑Bearing Layer

The depth to which roots can penetrate determines the volume of soil accessible for water and nutrients. As Scheffer et al. (2018) indicate, this varies greatly:

• Many grasses have most roots concentrated in the upper 5–10 cm
• Cereal crops penetrate to 80–100 cm
• Alfalfa and some woody species reach 2 m or more

Particle‑Size Distribution

Soil texture (the ratio of sand, silt, and clay) determines many properties. Valkov et al. (2004) give examples of ecological preferences of crops for particular textures:

• Cereal crops grow better on medium‑loamy structural soils
• Potatoes and melons — on light soils
• Vineyards — on well‑drained, often stony soils

Salt Regime

The salt content of soil is an important ecological factor. As Valkov et al. (2004) note, at high salt concentrations (especially sodium), plants suffer from osmotic stress and cannot take up water even when it is present. This phenomenon—physiological drought—is characteristic of saline soils in arid regions.

Ecological Plasticity of Plants

Different plant species have different ecological plasticity—the ability to tolerate variations in environmental factors. Scheffer et al. (2018) note that varieties of the same crop can differ significantly in tolerance to:

• Drought
• Salinity
• Acidic conditions
• Soil compaction

Definition

Natural fertility is the fertility that a soil possesses in its natural (virgin) state, under the influence of natural soil‑forming factors, without human intervention (Valkov et al., 2004). It is the result of long‑term interaction of climate, parent material, relief, organisms, and time—the very factors incorporated in Jenny’s formula.

Natural fertility develops through natural soil formation and reflects:

• The degree of humus and nutrient accumulation in the upper horizons
• The fabric and structure of the soil profile
• The balance of water, air, and thermal regimes characteristic of the given natural biocenosis

As Valkov et al. (2004) note, under natural conditions, soil fertility is inseparably linked to the biocenosis corresponding to that soil and is a result of the natural soil‑forming process. Therefore, one must be very cautious when assessing natural fertility by the productivity of virgin vegetation.

Examples of Differences in Natural Fertility

The world of soils shows a huge range of natural fertility:

• Steppe Chernozems have high natural fertility due to their thick humus horizon, neutral reaction, and favourable physical properties.
• Podzolic soils of the taiga have low natural fertility because of their leaching regime, acidic reaction, and thin humus horizon.
• Ferrallitic soils of humid tropical forests are poor in nutrients, although under natural forest they support enormous biomass through rapid biological cycling (Valkov et al., 2004).

Modern Understanding of Natural Fertility

In modern soil science, natural fertility is considered not as a static characteristic but as a potential embedded in the soil by evolution. This potential is manifested only under conditions close to natural, that is, when the biological cycle characteristic of that ecosystem type is preserved.

Foth and Ellis (1997) emphasise that assessment of natural fertility based on total nutrient content is often uninformative: much more important are the rate of organic matter mineralisation and the intensity of biological cycling.

In modern conditions, soils that have fully preserved natural fertility are practically non‑existent, because even in natural landscapes, humans affect soils through atmospheric deposition, climate change, and species invasion.

Definition

Artificial fertility is the fertility created by humans in the course of agricultural use of soil through tillage, fertilisation, amelioration, and other agronomic practices (Valkov et al., 2004).

Valkov et al. (2004) give vivid examples: irrigated lands in arid regions, drained bogs, cultivated sod‑podzolic soils—all demonstrate artificial fertility that can significantly exceed natural fertility.

Relationship between Natural and Artificial Fertility

An important principle: artificial fertility does not replace natural fertility but builds upon it. In any cultivated soil, the natural foundation remains and continues to influence the response to fertilisers and ameliorations.

Valkov et al. (2004) note that within the same farming system, different land users treat the soil differently: careless farmers cause double damage—they forgo production and degrade soil fertility, which recovers slowly over years and decades.

The degree of change in natural soil properties upon cultivation depends on the ecological similarity or distance between the natural biocenosis and the agroecosystem. Chernozem‑type soils undergo less change than podzolic‑type soils because agroecosystems in the steppe are closer to natural grassland communities (Valkov et al., 2004).

Forms of Artificial Fertility

Artificial fertility can be created in two ways:

1. Direct influence on soil properties—liming, gypsum application, addition of organic and mineral fertilisers, irrigation, drainage, tillage.

2. Indirect influence through crop selection, crop rotations, green manuring—that is, managing the biological cycle.

However, it is important to remember that artificial fertility requires continuous maintenance. As soon as agronomic measures cease, the soil begins to revert to a state close to natural, but this process may involve degradation (erosion, compaction, loss of humus) if previous use was excessively intensive.

Definition

Potential fertility is the maximum possible level of soil productivity that can be achieved under full optimisation of all manageable factors (water supply, nutrient regime, physical properties) within the given climatic conditions (Foth & Ellis, 1997; Weil & Brady, 2017).

Potential fertility is a theoretical limit determined by:

• Climatic resources (heat, light, growing season length)
• Properties of the soil itself that cannot be radically changed (e.g., particle‑size distribution, thickness of soil profile, depth to parent rock)
• Biological potential of varieties and hybrids.

Why Potential Fertility Is an Important Concept

It allows us to:

1. Compare soils by their genetically determined capacities, regardless of current management level. Potential fertility underlies soil bonitation—a comparative assessment of soil quality (Valkov et al., 2004).

2. Evaluate the efficiency of agronomic practices: the closer the actual yield is to potential, the better natural resources are used.

3. Plan necessary capital investments: if potential fertility is low, even large expenditures will not lead to high, stable yields.

Factors Limiting the Realisation of Potential Fertility

Besides climate and genetic soil properties, the realisation of potential fertility is limited by:

• Water deficit (in arid zones)
• Insufficient thickness of the soil profile (shallow soils on bedrock)
• Presence of toxic elements (aluminium in acid soils, salts in saline soils) (Scheffer et al., 2018)

It is important that potential fertility cannot be raised indefinitely—each soil has its own natural limit. This means that when choosing a site for intensive agriculture, we must consider the soil’s potential, not try to “squeeze” more out of it than it can provide.

Definition

Effective fertility (or actual fertility) is the fertility that is actually expressed under given conditions, at a given level of agronomic practice and meteorological conditions (Foth & Ellis, 1997). It is that part of potential fertility that is realised in a particular year on a particular field.

Valkov et al. (2004) indicate that fertility manifests itself in the productivity (yield) of the plants growing on it. It is effective fertility that we measure when we harvest.

Difference between Effective and Potential Fertility

The difference between potential and effective fertility is the reserve we are not yet using. This reserve may be related to:

• Insufficient farming practices (poor timing, lack of fertilisers)
• Unfavourable weather conditions
• Temporary constraints (e.g., young plantations)
• Low level of amelioration (lack of irrigation, drainage)

Significance of Effective Fertility

For practice, effective fertility is paramount: we deal with what we actually harvest from the field. Therefore, all recommendations on fertilisers, tillage, and variety selection are aimed at increasing effective fertility.

Weil and Brady (2017) emphasise that in industrialised countries, yield increases over recent decades have been achieved mainly by bringing effective fertility closer to potential fertility, rather than by raising potential fertility itself.

Example: Dependence on Weather

A classic example is cereal yields in different weather years. In a dry year, effective fertility may be low even on a soil with high potential fertility because moisture becomes the limiting factor. In a wet year, the same field may give a high yield. That is why it is said that effective fertility is a function of weather (Foth & Ellis, 1997).

Definition

Economic fertility is fertility assessed from the standpoint of economic efficiency of production: the ratio of the value of output to the costs of obtaining it (Valkov et al., 2004; Foth & Ellis, 1997).

Unlike the previous three categories, which are natural‑scientific, economic fertility is an economic category. It reflects how profitable it is to use a given soil for producing a given product.

Criteria of Economic Fertility

The main criterion is profitability. A soil is considered economically fertile if the costs of fertilisers, tillage, harvesting, and other operations are covered by the value of the produce obtained.

Weil and Brady (2017) show that the optimal fertiliser dose from the viewpoint of economic fertility is always lower than the dose giving the maximum yield. This is due to the law of diminishing returns: each additional kilogram of fertiliser gives a smaller increment, and at some point the cost exceeds the value of the increment.

Practical Significance

Economic fertility is a key guide for the farmer. It forces one to:

• Choose crops that are most profitable for the given soil and climate
• Apply fertilisers and ameliorations only in economically justified doses
• Adopt technologies that reduce costs (e.g., minimum tillage, precision farming)

Valkov et al. (2004) introduce the concept of economic land evaluation as the comparative value of land as a means of production, taking into account not only yield but also labour and material costs. This is the practical embodiment of the economic fertility category.

Practical Conclusion

For sustainable agriculture, it is necessary to maintain a balance:

1. Do not deplete natural fertility—preserve humus, structure, biological activity.

2. Develop artificial fertility, but without excessive costs.

3. Strive to bring effective fertility closer to potential fertility, but with economic feasibility in mind.

4. Understand that on different soils and in different climatic zones this balance will differ.

Valkov et al. (2004) emphasise that at high yields, the volume of organic matter remaining in the soil becomes close to the volume created under natural meadow steppes—that is, those communities that form the most fertile soils in nature, Chernozems. This is an example of how wise fertility management can maintain it at a high level.

Fertility Categories and Sustainable Development

The concept of sustainable development requires us to use fertility in such a way as not to reduce it for future generations. This means:

• Moving from maximum exploitation of potential fertility to its balanced use
• Including not only yield but also ecological consequences in the assessment
• Recognising that economic fertility cannot be the sole criterion for decisions in the agri‑food sphere

Classic Formulation

In the mid‑19th century, the German chemist Justus von Liebig formulated the law of the minimum, which became the foundation of the scientific approach to fertility. In its simplest form, it states: yield is determined by the nutrient element that is in minimum supply relative to the plant’s needs (Weil & Brady, 2017).

A vivid illustration is the “Liebig barrel”: the water level in a barrel cannot rise above the shortest stave, regardless of the length of the others. Likewise, yield cannot exceed the level allowed by the most deficient factor.

Foth and Ellis (1997) emphasise that this principle remains the basis for understanding fertiliser efficiency. If phosphorus is at a minimum, applying nitrogen will have no effect—it will merely waste resources.

Limitations of the Classic Law

However, the classic law of the minimum has serious limitations that must be taken into account under modern conditions:

1. Not only nutrients but also other factors. As Scheffer et al. (2018) note, the limiting factor may be not only a nutrient deficit but also:

• Water deficit (drought)
• Oxygen deficit (waterlogging)
• Unfavourable temperature (cold soil in spring)
• Soil compaction (root obstruction)
• Toxicity (aluminium, salts, heavy metals)

2. Interaction of factors. Weil and Brady (2017) show that factors can enhance or weaken each other’s effects. For example, under phosphorus deficiency, plants cannot efficiently use nitrogen even if it is present in sufficient amounts.

3. Change of limiting factor over time. During the growing season, the limiting factor may change: cold in spring, water in midsummer, phosphorus or potassium deficiency for grain filling at the end (Foth & Ellis, 1997).

4. Quality, not just quantity. The content of an element in the soil is not yet availability. Bound elements may be poorly available to plants. That is why analyses of total nutrient content often do not correlate with yield (Weil & Brady, 2017).

Modern Understanding: The Law of the Minimum in a System of Factors

Modern soil science considers the law of the minimum as a special case of a more general principle: plant growth and development are limited by the factor that is farthest from its optimal value. Moreover:

• The optimum for each factor is a range of values, not a single point
• Factors interact, and the “bottleneck” may be the result of a combination of several unfavourable conditions
• Removing one limitation may reveal another

It is this diversity of limiting factors that explains why even on soils with good chemical composition yields can be low.

Chemical Limiting Factors

Macronutrient Deficiencies

The most obvious type of limitation, usually the first to come to mind. However, even here nuances matter.

• Nitrogen — the most common limiting factor in most soils. Nitrogen deficiency shows as pale colouring and retarded growth. As Scheffer et al. (2018) note, nitrate content in soil is subject to strong seasonal fluctuations, and even if sufficient nitrogen reserves are present at the start of the season, by the period of rapid growth it may become insufficient.

• Phosphorus — particularly often limits on acidic and strongly weathered soils. The paradox of phosphorus is that its total content may be high, but availability low (Weil & Brady, 2017). As Foth and Ellis (1997) vividly express it, phosphorus is “fixed” in the soil, converting into sparingly soluble compounds with iron, aluminium, or calcium. Therefore, even when phosphorus fertilisers are applied, only 10–15% of phosphorus is taken up by plants in the year of application.

• Potassium — often limiting on light sandy soils or under intensive removal with harvest. The complexity is that potassium exists in several forms: available in solution and exchangeable, slowly available in interlayer spaces of clay minerals, and unavailable in the crystal lattice of primary minerals (Foth & Ellis, 1997; Weil & Brady, 2017).

• Sulphur, calcium, magnesium — less frequently, but can also be limiting. Sulphur is important for protein synthesis; calcium for cell wall structure; magnesium for chlorophyll.

Micronutrient Deficiencies

Micronutrients are needed in small amounts, but their deficiency can sharply limit yield:

• Iron — often deficient on calcareous soils (lime‑induced chlorosis)
• Zinc — widespread deficiency on many soils, especially in arid regions
• Manganese, copper, boron, molybdenum — deficiencies occur on specific soils and for specific crops (Weil & Brady, 2017)

Unfavourable Reaction of the Medium

As the textbook by Scheffer et al. (2018) indicates, soil pH affects the availability of almost all elements:

• Strongly acidic soils (pH < 5.0) — aluminium and manganese toxicity, phosphorus, calcium, magnesium deficiency
• Alkaline soils (pH > 8.0) — iron, zinc, manganese, copper deficiency
• Optimal range for most crops — pH 6.0–7.0

Alkalinity and Salinity

High salt content, especially sodium, creates osmotic stress in plants: water cannot enter roots even when present in the soil. As Valkov et al. (2004) note, this phenomenon is called physiological drought and is characteristic of saline soils in arid regions.

Physical Limiting Factors

Water Deficit or Excess

As Foth and Ellis (1997) emphasise, water is the most important factor determining the availability of all nutrients. Without water, there is no transport of elements to roots, no dissolution, no movement.

• Water deficit — limits organic matter decomposition, reduces element supply to roots (diffusion and mass flow slow down)
• Water excess — reduces aeration, suppresses aerobic microorganisms, causes denitrification and nitrogen losses, promotes reduction of iron and manganese to toxic forms (Scheffer et al., 2018)

Soil Compaction

As Weil and Brady (2017) note, soil compaction is one of the most common and underestimated limiting factors in modern agriculture. Consequences of compaction:

• Mechanical obstruction to root growth
• Reduced aeration and consequent oxygen starvation of roots
• Slower water infiltration — increased surface runoff and erosion
• Reduced activity of soil fauna, especially earthworms (Scheffer et al., 2018)

Poor Structure

Soils with poor structure (e.g., slaking, structureless) have:

• Low water permeability
• Tendency to crust formation
• Poor water‑air regime
• Limited root system development

Thin Root‑Bearing Layer

Roots can develop only in the volume of soil physically accessible. Shallow soils on bedrock or with a dense subsoil horizon limit access to water and nutrients from lower horizons (Scheffer et al., 2018). For most agricultural crops, the maximum root penetration depth is 1–2 m, and the effective depth is 30–80 cm.

Biological Limiting Factors

Low Biological Activity

Soil fertility is largely provided by microorganisms that:

• Mineralise organic matter, converting it into plant‑available forms
• Fix atmospheric nitrogen (symbiotic and free‑living fixers)
• Participate in phosphorus and sulphur cycling
• Decompose toxic compounds

Lack of Beneficial Symbionts

As Weil and Brady (2017) note, many plants depend on:

• Nodule bacteria for nitrogen fixation (especially legumes)
• Mycorrhizal fungi for phosphorus and micronutrient uptake
• Rhizosphere microorganisms for protection against pathogens

In the absence of these symbionts, plants cannot efficiently use even the elements that are present in the soil.

Pathogenic Organisms

Soil pathogens (phytopathogenic fungi, bacteria, nematodes) can limit plant growth by causing root diseases, reducing uptake of water and nutrients. As Valkov et al. (2004) note, soil is a reservoir for many pathogens that can accumulate under monoculture.

Unbalanced C/N Ratio in Plant Residues

As Scheffer et al. (2018) emphasise, when plant residues with a wide C/N ratio (e.g., straw) decompose, microorganisms actively immobilise nitrogen from the soil, temporarily creating a deficiency for crop plants—so‑called nitrogen immobilisation (N‑immobilisation).

Ecological and Climatic Limiting Factors

Temperature Constraints

Each crop has its own temperature minimum for germination and growth (Scheffer et al., 2018):

• Winter rye: 1–2 °C
• Wheat, barley, rapeseed: 2–4 °C
• Sugar beet: 6–8 °C
• Maize, potato: 8–10 °C

Late spring frosts and early autumn frosts can seriously limit yields even under good soil conditions.

Light and Day‑Length Constraints

This factor is not directly related to soil but determines the potential productivity of any agroecosystem. In northern regions, light may be insufficient for high yields of some crops.

Wind and Water Erosion

As Valkov et al. (2004) note, erosion leads to:

• Loss of the top, most fertile horizon
• Deterioration of physical properties
• Reduction in humus and nutrient reserves
• Impaired water regime

Unfavourable Relief Position

Landscape position determines water and heat supply, groundwater depth, risk of erosion and flooding.

Synergy Effect

In real practice, limiting factors rarely act in isolation. More often, there is interaction among them.

Weil and Brady (2017) give an example with maize: on compacted soil, even with adequate nitrogen and phosphorus application, yield will be low because roots cannot develop and absorb elements efficiently. Conversely, even on a well‑structured, aerated soil without fertiliser, yield will be limited by nutrient deficiency.

Examples of Factor Interaction

1. Phosphorus and mycorrhiza: On soils with low phosphorus availability, plants with well‑developed mycorrhiza can obtain sufficient phosphorus, while plants without mycorrhiza will suffer from deficiency (Weil & Brady, 2017).

2. Nitrogen and phosphorus: Nitrogen deficiency often exacerbates phosphorus deficiency, and vice versa. When nitrogen is applied, plants grow more vigorously and require more phosphorus, which may become deficient.

3. Moisture and nutrition: Under dry conditions, plants cannot use even available nutrients because their supply via mass flow of water is severely reduced (Foth & Ellis, 1997).

4. Compaction and aeration: Soil compaction reduces oxygen supply to roots, impairing water and nutrient uptake even when sufficient amounts are present.

Removing One Limitation Reveals Another

This is one of the most important practical conclusions: when we remove one limiting factor, the next one takes its place. Weil and Brady (2017) illustrate this with the Liebig barrel: lengthening one stave does not raise the water level above the next short stave.

Practical consequence: when diagnosing yield problems, one must identify the whole complex of limitations, not just the most obvious one.

Visual Symptoms and Field Observations

The first step is careful observation of plants:

• Characteristic deficiency symptoms on leaves (chlorosis, necrosis, purple colouring)
• Spatial patterns: problems on certain parts of the field (hilltop, depression, wheel tracks)
• Root system development: poor development, deformations, darkening, rotting
• Presence or absence of nodules on legumes, signs of mycorrhiza

Soil Diagnostics

As Foth and Ellis (1997) emphasise, soil analysis is the most important tool for identifying limiting factors. However, it is crucial to interpret results correctly:

• Element content does not equal its availability
• Temporal dynamics may change during the season
• pH, organic matter content, cation exchange capacity must be taken into account

For nitrogen, special attention must be paid to the soil’s mineralisation capacity, which varies greatly with temperature and moisture (Scheffer et al., 2018).

Plant Diagnostics

Plant analysis (tissue testing) allows:

• Assessing actual element uptake
• Detecting latent (hidden) deficiency before visual symptoms appear
• Evaluating element ratios (e.g., N/S, K/Mg, Fe/Mn)

Weil and Brady (2017) note that for many crops, critical concentrations of elements in specific organs have been developed (e.g., the leaf opposite the ear in maize).

Consideration of Weather and Agronomic Practices

Any diagnosis must take into account:

• Current year weather (dry, wet, cold, hot)
• Preceding crop and tillage systems (what organic residues were added, what tillage was done)
• Applied fertilisers and pesticides (whether applied, in what doses)

Integrated Approach

The main principle of fertility management in modern conditions is an integrated approach, based on knowledge of all potential limiting factors for a given soil and crop. As Weil and Brady (2017) note, modern agriculture must consider the whole system of factors.

Sequential Removal of Limitations

Practical strategy:

1. Remove gross physical limitations (compaction, drainage, irrigation)

2. Adjust pH (liming or acidification)

3. Provide major macronutrients (especially nitrogen, phosphorus, potassium)

4. If necessary, micronutrients

5. Maintain biological activity (organic fertilisers, green manures, crop rotations)

Foth and Ellis (1997) note that this approach—systematic removal of limitations—gives the maximum return from fertilisers and other agronomic measures.

Adaptive Management

Since limiting factors change from year to year and even within the season, it is necessary to:

• Regularly monitor soil and plant status
• Use precision agriculture methods for differentiated fertiliser and amendment application
• Quickly adjust agronomic practices according to changing conditions

Why Are Soils Heterogeneous?

Spatial variability of soil properties is not accidental but a regular consequence of the soil‑forming process. Scheffer et al. (2018) emphasise that the soil cover is always mosaic, and this mosaic reflects the interaction of all soil‑forming factors.

Main causes of heterogeneity:

1. Heterogeneity of parent material. Even within a single field, the parent material can vary: sandy lenses among loams, gravel interlayers, differences in mineralogical composition. Foth and Ellis (1997) note that this is especially characteristic of glacial and alluvial deposits.

2. Relief. Slope position determines:

• Water supply (upper part of slope — drier, lower — wetter)
• Erosion intensity (on tops and slopes — erosion, in depressions — accumulation)
• Soil profile thickness (on tops — thinner, in depressions — thicker)
• Heat distribution (south‑facing slopes warmer than north‑facing) (Scheffer et al., 2018)

3. Vegetation. Under natural conditions, even slight changes in species composition lead to changes in soil properties. As Scheffer et al. (2018) note, different plant species affect humus accumulation, acidity, and element distribution down the profile differently.

4. Soil fauna activity. Earthworms, insects, rodents create biopores, mix soil, locally enrich it with organic matter. This leads to the formation of microsites with enhanced fertility (Scheffer et al., 2018).

5. Anthropogenic impact. Humans create additional variability: differences in tillage, fertilisation, crop rotations, compaction from machinery wheel traffic. Valkov et al. (2004) emphasise that within the same farming system, different land users treat the soil differently, and this generates substantial differences in fertility between fields and even within a field.

Scales of Variability

Spatial variability of fertility occurs at different scale levels (Foth & Ellis, 1997; Weil & Brady, 2017):

Microscale (mm — cm):

• Differences in the immediate vicinity of the root (rhizosphere)
• Microaggregates with different porosity and organic matter content
• Biopores created by earthworms and roots
• Heterogeneous distribution of microorganisms and enzymes

Mesoscale (m — tens of m):

• Natural variation within a field (patchiness)
• Influence of microrelief
• Differences in tillage, fertiliser application (tractor wheel track, fertiliser spread pattern)

Macroscale (hundreds of m — km):

• Differences between fields
• Influence of macrorelief (slopes, watersheds, floodplains)
• Changes in soil types and subtypes
• Landscape differentiation

Global scale (thousands of km):

• Climatic zones
• Biogeochemical provinces (deficiency or excess of elements in soils)

Variability of Chemical Properties

Nutrient Elements

As Foth and Ellis (1997) note, the content of mobile forms of nutrients can vary within a single field by several times. This is especially true for:

• Nitrate nitrogen — mobile, easily transported with water, its content changes strongly in space and time
• Phosphorus — its availability depends on pH and content of iron and aluminium oxides, which are also heterogeneous
• Potassium — its exchangeable content depends on the type of clay minerals, which may change within the field

Reaction (pH)

pH can vary significantly even over short distances. Reasons:

• Heterogeneity of parent material (presence of carbonate interlayers)
• Differences in tillage and fertiliser application (physiologically acidic fertilisers acidify the soil)
• Differences in erosion processes (removal of the top, often more acidic horizon) (Scheffer et al., 2018)

Organic Matter Content

As Scheffer et al. (2018) emphasise, humus content within a field can range from 1 to 5% or more. This is related to:

• Distribution of plant residues
• Erosion intensity (humus is less on hilltops)
• Soil fauna activity

Variability of Physical Properties

Particle‑Size Distribution

Even within a field, areas with different sand, silt, and clay content can be found. This is especially characteristic of:

• Glacial deposits (moraine with sand lenses)
• Alluvial deposits (alternating layers of different composition)
• Aeolian deposits (loess with interlayers)

As the textbook by Scheffer et al. (2018) notes, particle‑size distribution determines water‑holding capacity and water availability for plants, so its variability directly affects fertility.

Density and Porosity

Soil compaction is extremely heterogeneous in space:

• Tractor wheel track — density can increase by 20–30%
• Subsoil horizon — often denser than the topsoil
• Zones of active bioturbation (earthworms) — looser

Weil and Brady (2017) emphasise that compaction is one of the most widespread and underestimated limiting factors, and its spatial variability must be taken into account in differentiated fertility management.

Water Regime

Soil moisture varies greatly in space due to:

• Microrelief (depressions — wetter, elevations — drier)
• Particle‑size distribution (sands dry quickly, clays retain water long)
• Groundwater depth (shallower in depressions)

Thermal Regime

As Scheffer et al. (2018) note, soil temperature depends on:

• Slope aspect (south‑facing slopes warmer than north‑facing)
• Surface colour (dark soils warm faster)
• Moisture (wet soils are cooler)
• Presence of vegetation and mulch

Variability of Biological Properties

Microbial Abundance and Activity

Microorganisms are distributed extremely unevenly in soil (Scheffer et al., 2018):

• Hot spots — zones of elevated activity near roots (rhizosphere), in biopores, around organic residues
• Cold zones — areas with low biological activity (compacted, dry, acidic)
• Differences between microaggregates — anaerobic conditions may occur inside aggregates

Distribution of Symbionts

• Nodule bacteria — their abundance depends on the presence of legumes in the rotation, soil acidity, molybdenum and cobalt content
• Mycorrhizal fungi — their root colonisation depends on phosphorus content, tillage, preceding crop (Weil & Brady, 2017)

Variability of Ecological Conditions

Influence of Relief

Relief position determines a whole set of conditions:

• Water supply (tops — drier, footslopes — wetter)
• Soil profile thickness (thinner on tops)
• Groundwater depth (shallower in depressions)
• Erosion risk (higher on slopes)

Influence of Vegetation (under natural conditions)

Different plant species affect soil differently:

• Root system depth (deep‑rooted vs. shallow‑rooted)
• Chemical composition of litter (C/N ratio)
• Intensity of biological cycling
• Acidity of root exudates

Natural Variability

Natural variability of fertility is the result of long‑term interaction of soil‑forming factors (Valkov et al., 2004; Scheffer et al., 2018):

1. Primary variability of parent material. Heterogeneity of the original material is preserved in the soil profile, determining differences in particle‑size and mineralogical composition, nutrient content.

2. Secondary differentiation during soil formation. Soil processes (leaching, gleying, podzolisation, solonetzation) create new differences between horizons and between sites.

3. Differences in vegetation cover. Even small changes in species composition over time lead to the formation of different soil properties.

4. Influence of relief. As Scheffer et al. (2018) note, soils within a single catena (a sequence of soils from summit to base) form a regular series reflecting the redistribution of water and matter.

Anthropogenic Variability

Human activity creates additional, often substantial, variability:

1. Tillage. Differences in intensity and depth of tillage, especially when shifting from one farming system to another, create heterogeneity in density, structure, and distribution of organic matter.

2. Fertiliser application. Even with uniform spreading, fertilisers are distributed unevenly due to:

• Machine inaccuracies
• Speed variations
• Heterogeneity of the fertiliser itself

As Weil and Brady (2017) note, modern precision agriculture systems aim to minimise this variability, but it remains significant.

3. Ameliorative measures. Liming, drainage, and irrigation are often not applied uniformly over the whole area, creating patches with different properties.

4. Compaction. As Foth and Ellis (1997) emphasise, compaction from agricultural machinery varies greatly in space (wheel track — denser, between tracks — looser), creating a “striped” heterogeneity of fertility.

5. Differences in crops and rotations. Different crops affect soil differently, and even within a field there may be patches with different use histories.

Patterns of Spatial Variability

Despite apparent randomness, spatial variability of fertility exhibits certain patterns (Foth & Ellis, 1997; Weil & Brady, 2017):

1. Autocorrelation. Soil properties at nearby points are more similar than at distant ones. This allows the use of geostatistical methods for mapping.

2. Directional variability (trend). Along a slope, from top to bottom, soil properties change regularly (catena).

3. Periodic variability. Related to regular agronomic operations (e.g., wheel track patterns).

4. Patchiness (mosaicity). A random combination of different factors creates a mosaic of patches with different fertility.

Traditional Farming: Averaging

In traditional farming, the field is treated as homogeneous:

• Uniform fertiliser rates applied across the whole field
• Uniform tillage
• Single variety sown

As Weil and Brady (2017) note, averaging leads to double losses: on poor patches, fertilisers are insufficient; on rich patches, they are excessive and may harm the environment. This is especially critical for nitrogen, excess of which can leach into groundwater.

Precision Farming: Accounting for Heterogeneity

The modern approach — precision agriculture — involves differentiated fertility management based on:

1. Mapping of soil properties. Using:

• Grid sampling
• Proximal sensing (electrical conductivity, gamma‑spectrometry)
• Remote sensing (satellite images, vegetation indices)
• Yield data (yield mapping)

2. Creation of management zone maps. Areas of the field with similar properties are grouped into zones for differentiated fertiliser application, tillage, and variety selection.

3. Variable‑rate fertiliser application. Different rates for different zones, allowing:

• Reduced fertiliser costs (on rich patches)
• Increased yield (on poor patches)
• Reduced environmental impact (Weil & Brady, 2017)

Example: Nitrogen Variability and Precision Nitrogen Management

As Foth and Ellis (1997) and Weil and Brady (2017) note, nitrogen is one of the most spatially variable elements. Its content in the topsoil can vary several‑fold.

Precision technologies allow:

• Differentiated nitrogen top‑dressing based on vegetation indices (NDVI)
• Using sensors to determine crop nitrogen needs in real time (SPAD meters, chlorophyll meters)
• Site‑specific nitrogen application, only where needed

This is particularly important for environmental protection: nitrogen excess on some parts of the field does not compensate for deficiency on others.

Traditional Methods

1. Soil survey (soil mapping). As the textbook by Scheffer et al. (2018) notes, this is the basis of soil cartography. However, traditional maps, even large‑scale ones, generalise information. They give a discrete picture—delineating polygons with a dominant soil type, but do not show continuous changes within the polygon.

2. Agrochemical survey. Sampling on a regular grid (e.g., 5×5 ha or 1×1 ha) followed by analysis and mapping. Classic approach, but labour‑intensive and expensive, especially for large areas.

3. Yield assessment. Combines with GPS systems allow creation of yield maps, which reflect the integrated influence of all factors on productivity. This is one of the most accurate methods for identifying patches with different effective fertility.

Modern Methods (Remote and Proximal)

1. Remote sensing (satellite, UAV):

• Vegetation indices (NDVI, NDRE) reflect plant condition and, indirectly, nitrogen status
• Thermal imagery shows heat stress and water deficit
• Multispectral and hyperspectral data allow estimation of chlorophyll and element content

2. Proximal sensing (sensors on machinery):

• Soil electrical conductivity measurement (geophysical methods)
• Visible and near‑infrared spectroscopy (Vis‑NIR) for estimation of organic matter, moisture, particle‑size distribution
• Gamma‑spectrometry for potassium, thorium, uranium content (indirect indicators of soil properties)

3. Precise geopositioning (GPS/GNSS). All measurements are georeferenced, allowing creation of digital maps of properties with high accuracy.

Geostatistics and Digital Soil Mapping

As Weil and Brady (2017) note, the modern approach to accounting for spatial variability is digital soil mapping. Based on a limited set of measurements (reference points), using geostatistical methods (kriging), continuous maps of properties with accuracy assessment are created.

Advantages of digital soil mapping:

• Use of a limited number of samples (cost reduction)
• Possibility of updating maps as new data arrive
• Assessment of prediction uncertainty
• Creation of user‑defined maps for specific purposes (phosphorus, nitrogen, pH, etc.)

From Sum to System

In the first lectures, we identified four components of fertility: physical, chemical, biological, and ecological. This was necessary for analysis, so that we could understand the structure of fertility. However, in real soil these components do not exist in isolation.

As Scheffer et al. (2018) vividly put it, soil is not just a mixture of mineral particles, water, air, and organic residues. It is a highly active reactor in which all components continuously interact. Fertility is the result of this interaction, not an arithmetic sum of properties.

Integrality of fertility means that:

1. Properties are not additive. Improving one property does not compensate for deterioration of another. For example, high nitrogen content will not help if roots cannot develop due to compaction (Weil & Brady, 2017).

2. They manifest only in the system. Fertility cannot be measured in an isolated soil sample—it manifests only in interaction with plants, microorganisms, and climate (Foth & Ellis, 1997).

3. They have emergent character. This means that the system (soil as a whole) acquires properties that its individual parts do not possess. For example, the ability of soil for self‑regulation (buffering) arises only when all components are present.

System Approach in Soil Science

Valkov et al. (2004) emphasise that soil is a special bio‑inert shell in which organic and mineral components are inseparably linked. It is in this unity that the key to understanding fertility lies.

Modern soil science considers fertility as a property of the soil system, manifested through:

• Functioning (ability to support plants)
• Resilience (ability to maintain functions under external impacts)
• Reproduction (ability to recover after disturbances)

Physical and Chemical Properties

Structure → Chemistry → Structure

As the textbook by Scheffer et al. (2018) notes, good soil structure (aggregation) creates an optimal pore ratio, providing:

• Aeration, necessary for oxidative processes and microbial life
• Water regime, determining dissolution and movement of elements

In turn, chemical properties affect structure:

• Calcium promotes aggregation (bridges between particles)
• Sodium (in excess) destroys structure, causing dispersion of clay particles
• High organic matter content improves structure

Foth and Ellis (1997) emphasise that liming of acidic soils improves not only pH but also physical properties by increasing earthworm activity and aggregate formation.

Chemical and Biological Properties

Chemistry → Biology → Chemistry

Soil pH determines nutrient availability and microbial species composition:

• Acidic soils — fungi dominate, bacterial activity suppressed, mineralisation slowed
• Neutral and slightly alkaline — high bacterial activity, intense element cycling (Scheffer et al., 2018)

Microorganisms, in turn, change chemical properties:

• Nitrogen fixation enriches soil with nitrogen
• Nitrification acidifies the soil
• Mineralisation converts organic nitrogen into available forms
• Production of organic acids can mobilise phosphorus and micronutrients

Weil and Brady (2017) show that mycorrhizal fungi, by increasing the absorbing surface, change the availability of phosphorus, zinc, and other elements for plants even at low soil contents.

Biological and Physical Properties

Biology → Physics → Biology

Biological processes create and maintain physical structure:

• Earthworms create biopores, improving aeration and drainage (Scheffer et al., 2018)
• Plant roots compact soil around them but leave loose zones after death
• Microorganisms secrete polysaccharides that bind particles into aggregates

Physical properties determine conditions for biological activity:

• Aeration necessary for aerobic microorganisms
• Moisture determines substrate availability
• Temperature affects the rate of all biological processes

Ecological Conditions as an Overall Framework

All these interrelations are realised under specific ecological conditions:

• Climatic regime (temperature, precipitation)
• Relief position (water supply, erosion)
• Groundwater depth (water regime)
• Parent material properties

Valkov et al. (2004) emphasise that different soils of the same type but located in different ecological conditions may have different fertility. For example, a Chernozem on a plateau and a Chernozem in a river floodplain will differ in moisture regime, which will determine different nutrient availability.

Example 1. Fertility and pH

Soil pH (chemical component) affects:

• Availability of almost all elements (chemistry)
• Activity of microorganisms and enzymes (biology)
• Structure and aggregation (physics)
• Root system development (ecology)

If pH is not optimal (e.g., acidic soil), then even with high nutrient content they will be poorly available. At the same time:

• Phosphorus binds to aluminium and iron (chemical fixation)
• Microorganisms are suppressed (biological activity reduced)
• Structure deteriorates (physical properties suffer)

As a result, effective fertility is low, although potential (by total content) may be high. This is a classic example of a systemic effect (Weil & Brady, 2017).

Example 2. Tropical Forests on Poor Soils

This example is discussed in detail by Valkov et al. (2004). Humid tropical forests (hylaea) grow on extremely poor ferrallitic soils. Almost all nutrients are in the biomass, not in the soil. The biological cycle is so intense that elements do not have time to leach out.

Here:

• Chemical properties (element poverty) do not reflect the soil’s ability to support the forest
• Biological component (rapid cycling) becomes dominant
• Physical properties (deep weathering) allow deep root penetration and access to elements

When this forest is cleared for agriculture, the system breaks down: the biological cycle is disrupted, elements are not returned to the soil, and fertility drops sharply within 2–4 years. These soils prove to be ecologically specific—highly productive for forest and low‑productive for agroecosystems.

This example shows that fertility is not an absolute property of the soil. It manifests only in a specific system (here, in the tropical forest) and is lost when the system changes (Valkov et al., 2004).

Example 3. Fertiliser Efficiency and the System Approach

Foth and Ellis (1997) analyse the reasons for low fertiliser efficiency. One of the main reasons is ignoring the systemic nature of fertility.

Suppose we apply phosphorus fertiliser to an acidic soil with poor structure. Phosphorus immediately binds to aluminium and iron (chemical fixation) and becomes unavailable. We add more—the same happens. Yield does not increase, but costs rise.

The system approach suggests a different sequence:

1. First — liming (pH optimisation)

2. Then — structure improvement (organic fertilisers, tillage)

3. Only then — phosphorus application

The effect of each measure increases because they mutually reinforce each other. This is how systemic fertility management works.

Fertility Buffering

Buffering is the ability of soil to maintain its functions under changing external conditions (Foth & Ellis, 1997). It manifests at different levels:

Chemical buffering — ability to resist changes in pH and salt concentration. Provided by:

• Presence of carbonates (in alkaline soils) or exchangeable cations (in acidic)
• High cation exchange capacity
• High humus content

Physical buffering — ability to maintain structure and water regime under compaction, waterlogging, drought. Provided by:

• Good aggregation
• High porosity
• Organic matter presence

Biological buffering — ability to maintain activity and diversity of microorganisms under stress. Provided by:

• High biodiversity
• Presence of resistant forms (spores, cysts)
• Reserve nutrients

Buffering is an integral property that arises only when all components are present. That is why well‑structured, humus‑rich soils with neutral reaction and high biological activity can “withstand” both drought and waterlogging much better than degraded soils.

Fertility Resilience

Fertility resilience is the ability of the soil to restore its functions after disturbances (erosion, compaction, pollution, drought). It is also an integral property and depends on:

• Self‑regulation ability — buffering
• Self‑restoration ability — speed of biological processes (microbial activity, root growth, structure recovery)
• Safety margin — reserves of humus, nutrients, biota

Foth and Ellis (1997) emphasise that fertility resilience is a key characteristic for sustainable agriculture. Soils with low resilience degrade quickly under intensive use; soils with high resilience can withstand pressures without loss of productivity.

Seasonal Dynamics

Over a single growing season, fertility changes:

• Spring — reserves of available moisture and elements after winter accumulation
• During active growth — rapid mineralisation, high element consumption
• Autumn — depletion of available reserves, accumulation of organic residues

This dynamics determines the need for differentiated management of fertility during the season (top‑dressing, irrigation) (Foth & Ellis, 1997).

Multi‑Year Dynamics (in Agroecosystems)

Under the influence of crop rotations and farming systems, fertility can:

• Increase (under proper management, organic farming, humus accumulation)
• Stabilise (under balanced farming)
• Decrease (under depletion, erosion, improper agronomy)

Valkov et al. (2004) note that soils, not having time to reach equilibrium with a new farming system, again acquire the ability to move and change. This means that fertility is a dynamic characteristic that always tends toward equilibrium with the conditions of use.

Evolutionary Dynamics (in Natural Ecosystems)

Over thousands and millions of years, fertility changes under climate, tectonics, and biota evolution:

• Humus accumulation — in steppes and forest‑steppes
• Leaching — in humid conditions (Podzolic soils)
• Ferrallitisation — in the tropics (depletion, accumulation of oxides)

These processes determine potential fertility and its changes on a geological scale (Scheffer et al., 2018).

Soil Bonitation

As Valkov et al. (2004) note, bonitation is a comparative assessment of soil quality and productive capacity. This is an integral indicator based on:

• Thickness of the humus horizon and humus reserves
• Particle‑size distribution
• Moisture and aeration conditions
• Reaction and nutrient content

Bonitation scores are a synthesis of many properties and allow comparison of soils of different genesis by their potential fertility.

Soil Quality

As Weil and Brady (2017) show, the modern approach is to assess soil quality. This is an integral characteristic that includes:

Productive function — ability to support plant and animal growth

Ecological function — ability to filter, buffer, and transform pollutants

Biological function — ability to support diversity and activity of organisms

Soil quality assessment is based on a minimum data set of indicators, which includes:

• Physical: bulk density, aggregate stability, infiltration, water‑holding capacity
• Chemical: pH, organic matter content, cation exchange capacity, available nutrients
• Biological: microbial biomass, enzyme activity, soil respiration

Soil Ecosystem Services

The modern concept of ecosystem services views fertility as part of a broader notion. Soil provides society with:

• Provisioning (crops, timber)
• Regulating (water filtration, carbon storage)
• Supporting (biodiversity, nutrient cycling)
• Cultural (aesthetics, recreation)

Integral fertility assessment should consider the full range of these services.