Soil Health

Historical Context

In the 1930s, after the catastrophic dust storms in the United States (Dust Bowl), it became obvious: soil that had produced good harvests could, within a few years, become barren wasteland. The reason was not only the depletion of nutrients but also the destruction of structure, loss of organic matter, and the death of soil organisms. Fertility was measured by chemical tests, but it could not predict the catastrophe.

By the 1990s, ecological thought had formed a new view: soil is not just a substrate for plants, but a living ecosystem, part of a broader system that today is called the Earth's critical zone — the planet's surface layer where rocks, water, air, and living organisms interact (National Research Council, 2001; Richter & Tugel, 2012).

An understanding emerged: even fertile soil can be "sick" — for example, excessively compacted, salinized, with a disrupted microbial community, or with low moisture-holding capacity. Conversely, soil with low fertility can be "healthy" and capable of self-restoration.

Definition of Soil as a Living Body

One of the most comprehensive definitions of soil is given by the Soil Science Society of America. It defines soil as "the surface layer of the earth that has been subjected to the influence of climate (including water and temperature) and micro- and macro-organisms, with the participation of relief, acting on parent material over time" (Eash et al., 2016). Note: in this definition, organisms are not just inhabitants of the soil, but active agents of soil formation.

This fundamentally changes the perspective. Soil is not a passive medium, but a dynamic system in which living organisms continuously process organic matter, form structure, create pores, and maintain the cycling of elements.

Why Did the Old Concept of "Fertility" Become Insufficient?

There are several reasons:

1. Societal demands have expanded. People are interested not only in yield but also in water purity, air quality, biodiversity, and landscape resilience (White, 2006). Fertility does not answer the questions: "Will this soil contaminate groundwater?", "Can it recover after drought or fire?".

2. New challenges. Climate change, population growth, land degradation — all these require assessing soil not only by current productivity but also by its ability to maintain functions in the long term. As Richter & Tugel (2012) note, we live in the era of the Anthropocene, when humans have become the main force of global change, and soils can no longer be considered outside the context of human activity.

3. The need for forecasting. Traditional agrochemical methods are good for diagnosing the current state but do not provide information on how the soil will respond to stress: drought, over-compaction, the introduction of heavy metals, or organic pollutants.

4. Methodological limitations. Fertility assessment is based on chemical analyses, which are relatively simple and standardized. Soil health assessment requires a comprehensive approach, including physical, chemical, and biological indicators — the latter were unavailable for a long time due to a lack of methods (Weil, 2017).

Two Approaches to Defining Soil

The Soil Science Society of America has two definitions of soil, and they well illustrate the evolution of views:

1. Narrow definition: "The loose mineral or organic material on the Earth's surface that serves as a medium for the growth of terrestrial plants."

2. Broad definition: "The loose mineral or organic material on the Earth's surface that has been subjected to the influence of genetic and environmental factors: climate (including water and temperature effects) and macro- and microorganisms, conditioned by relief, acting on parent material over time" (Eash et al., 2016).

The second definition is the basis for understanding soil health. It emphasizes that soil is a historically formed system, the result of the interaction of many factors, and not simply a mixture of mineral particles with water and air.

Conceptual Breakthrough: From Resource to System

The key moment that led to the emergence of the Soil Health concept was the recognition that soil performs not one but many functions:

• supports the life of plants, animals, and microorganisms;
• regulates the flows of water, energy, and gases;
• processes and retains nutrients and pollutants;
• resists physical and chemical degradation;
• stores and transmits information in the form of genetic diversity;
• performs cultural and aesthetic functions (Richter & Tugel, 2012).

Fertility is just one of these functions. Soil health is an integral characteristic of how well the soil performs all its functions as a living system.

In their foundational paper, Doran & Parkin (1994) defined soil quality as "the capacity of soil to function within ecosystem boundaries, sustaining biological productivity, maintaining environmental quality, and promoting plant and animal health." Later, this definition was supplemented with the idea of self-regulation and resilience (Weil, 2017), leading to the formation of the concept of soil health.

Brief Chronology of Concept Development

Section Summary

The concept of Soil Health emerged as a natural stage in the development of soil science, when:

• data accumulated showing that fertility does not guarantee resilience;
• methods for studying microbial communities and their activity became available;
• society realized the need to assess ecosystem services;
• it became clear that soil is a living system requiring a systemic approach.

Key idea: soil health is not a replacement for fertility but a suprastructure above it. If fertility answers the question "how much?", then health answers the questions "for how long?", "how sustainably?", and "with what environmental consequences?".

In the next section, we will examine in detail the differences between key terms: fertility, quality, health, and land capability.

Definition

Soil fertility is its ability to supply plants with available forms of nutrients (nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, micronutrients) in quantities and ratios necessary for normal growth and development (Eash et al., 2016).

Key question: Can this soil produce a good yield without fertilization or with optimal fertilizer application?

When Used

Fertility is an agronomic category. It underlies:

• fertilizer systems;
• liming of acid soils;
• gypsuming of solonetz soils;
• assessment of the need for micronutrient application.

Fertility assessment traditionally relies on chemical methods: determination of available phosphorus, exchangeable potassium, pH, hydrolytic acidity, humus content, nitrate and ammonium nitrogen, etc.

Limitations

Fertility does not consider:

• physical properties (structure, density, water permeability);
• biological activity (microbial diversity, enzymatic activity);
• the soil's ability to self-recover after stress;
• ecological effects (nitrate leaching, greenhouse gas emissions).

Therefore, even a very fertile soil can be unhealthy if it is compacted, lacks biological activity, or is prone to erosion.

Definition

Soil quality is "the capacity of soil to function within ecosystem boundaries, sustaining biological productivity, maintaining environmental quality, and promoting plant and animal health" (Doran & Parkin, 1994; White, 2006).

A more detailed definition is given by Weil (2017): soil quality describes properties that make soil suitable for performing certain functions, such as:

• supporting plant growth;
• regulating water and air flows;
• processing and retaining nutrients and pollutants;
• providing habitat for living organisms.

Key question: How well does this soil perform its ecosystem functions?

When Used

Soil quality is a diagnostic tool for assessing soil condition for monitoring, certification, and sustainability assessment of land use. It is actively used:

• in monitoring programs (e.g., the US National Soil Assessment Program);
• in developing farming systems with minimal environmental impact;
• for assessing the impacts of climate change and anthropogenic pressure.

Quality assessment includes a set of indicators — physical, chemical, and biological (we will discuss them in Section 4).

Limitations

Soil quality is a functional characteristic. But it does not answer the question of how long the soil can maintain these functions under loads and stresses. Furthermore, the same soil may have high quality for one purpose (e.g., for growing grains) and low quality for another (e.g., for wastewater filtration).

Definition

Soil health is "the capacity of soil to sustainably function as a living system, maintaining its structure, biological diversity, and productivity under stress" (Weil, 2017).

This definition emphasizes three key aspects:

1. Integrity — the soil is viewed as a single system, not a set of individual properties.

2. Viability — emphasis on biological processes and self-regulation.

3. Resilience — the ability to resist degradation and recover after disturbances (Richter & Tugel, 2012).

Key question: Can this soil sustainably function as a living system, maintain its functions, and recover after stresses?

When Used

The concept of soil health goes beyond diagnostics and is oriented toward prediction and management. It is applied:

• for assessing the sustainability of agroecosystems;
• in developing organic and biological farming systems;
• for rehabilitating degraded lands;
• in environmental programs for biodiversity conservation.

Health assessment requires not only a set of indicators but also consideration of their dynamics over time, as well as mechanisms of resistance (ability to withstand change) and resilience (ability to recover) (Seybold et al., 1999; Weil, 2017).

Limitations

Soil health is an integrative but context-dependent characteristic. The same soil may be "healthy" for a natural forest and "sick" for arable land. Moreover, health assessment requires consideration of temporal scales: seasonal fluctuations, long-term trends, and threshold values beyond which irreversible degradation occurs.

Definition

Land capability (or land evaluation category) is the correspondence of soil and landscape properties to the requirements of specific types of land use (agriculture, forestry, construction, recreation, nature conservation, etc.) (White, 2006).

Key question: For what purposes is this territory suitable, and with what limitations?

When Used

Capability is a planning category. It is used:

• in land management and territorial planning;
• for cadastral land valuation;
• for determining environmental constraints and risk zones.

Capability assessment relies on comparative analysis of soil properties and the requirements of different uses. For example, under the USDA Land Capability Classes system, soils are divided into 8 classes — from lands without limitations to lands suitable only for pasture, forests, or wildlife.

Limitations

Capability is a normative characteristic that depends on adopted standards and technologies. The same soil may change its capability class with climate change, the emergence of new plant varieties, or new reclamation methods. Furthermore, capability does not consider biological activity or self-regulation capacity.

Support of Biological Productivity (the "Provider" Function)

This is the classical function that has been the focus of agronomists for centuries. Healthy soil provides:

• Physical support for plant roots (Eash et al., 2016);
• Supply of water and dissolved nutrients to the root zone;
• Availability of macro- and micronutrients (N, P, K, Ca, Mg, S, Fe, Zn, Mn, etc.) in required quantities and ratios;
• Favorable air regime (oxygen content in soil air for root respiration and aerobic microorganisms).

However, it is important to emphasize: in the context of soil health, this function is not limited to "yield" alone. It refers to supporting all life in the soil — from bacteria to earthworms — as well as supporting natural vegetation (forests, meadows, steppes). Thus, the productivity of healthy soil is manifested not only in the quantity of grain but also in the richness of biomass and species diversity (Weil, 2017).

Regulation of Water Flows (Hydrological Function)

Soil plays a key role in the global hydrological cycle (Eash et al., 2016). Healthy soil performs the following tasks:

• Water reception — infiltration of precipitation and meltwater;
• Water retention — capillary and film retention of moisture available to plants and microorganisms;
• Water redistribution — vertical and lateral filtration, groundwater recharge;
• Water purification — filtration of suspended particles and sorption of pollutants as water passes through the soil profile.

Especially important is the soil's ability to receive and retain water without surface runoff and erosion. Disruption of this function (e.g., due to compaction or structure destruction) leads to floods, droughts, and water body contamination. As Weil (2017) notes, good structure, high organic matter content, and a developed network of macropores are the foundation of a healthy hydrological function.

Participation in Biogeochemical Cycles (the "Recycler" Function)

Healthy soil is a reactor in which thousands of chemical transformations occur, ensuring the cycling of elements. Key processes:

• Transformation of organic matter — decomposition (mineralization) of plant residues and humification with the formation of stable forms of organic carbon (Weil, 2017);
• Nitrogen cycle — mineralization, nitrification, denitrification, nitrogen fixation by microorganisms (see Sections 27.3–27.4 in Huang et al., 2012);
• Phosphorus and sulfur cycles — mobilization of phosphorus from sparingly soluble forms, sulfur oxidation, immobilization and mineralization of organic compounds (Huang et al., 2012, Ch. 26);
• Redox reactions — transformations of iron, manganese, and sulfur under the influence of microorganisms, determining element availability and toxicity.

Importantly, healthy soil does not accumulate excessive amounts of mobile forms of nitrogen and phosphorus that could leach into groundwater or enter the atmosphere as greenhouse gases. It can retain elements in the biological cycle, binding them in organic matter or strongly sorbing them onto mineral particles (White, 2006).

Filtration, Buffering, and Detoxification of Pollutants

This function is especially important in the Anthropocene, when flows of pollutants — heavy metals, pesticides, petroleum products, pathogens — are becoming increasingly intense. Healthy soil can:

• Sorb toxic substances on the surface of clay minerals, organic matter, and iron and aluminum oxides (Huang et al., 2012, Ch. 26);
• Decompose organic pollutants (pesticides, petroleum products) using microbial enzymes (microbial degradation);
• Buffer pH changes, preventing sharp shifts in acidity or alkalinity;
• Immobilize heavy metals in the form of insoluble compounds or complexes with organic matter.

Disruption of this function manifests in groundwater contamination, accumulation of toxicants in plants, and reduced quality of food products. As Richter & Tugel (2012) emphasize, loss of detoxification capacity is one of the first signs of soil health degradation.

Provision of Habitat and Biodiversity

Soil is the most species-rich biotope on Earth. One gram of soil can contain up to 10–100 thousand species of bacteria and archaea, thousands of species of fungi, and hundreds of species of invertebrates (Weil, 2017). Healthy soil provides:

• Living space — pores of different sizes, aggregates, particle surfaces, root channels;
• Food resources — organic matter, root exudates, other organisms;
• Conditions for reproduction and dispersal — moisture, aeration, protection from predators;
• Sites for symbiotic interactions — mycorrhizae, nitrogen fixation, induction of systemic plant resistance.

Healthy soil is characterized by high functional diversity — the presence of several different organisms performing the same function (functional redundancy). This makes the system resilient to stress: if one species dies, its function can be taken over by another (Weil, 2017).

Cultural and Aesthetic Functions

Although this function is rarely discussed in textbooks, it is important for society. Healthy soil:

• Maintains landscape attractiveness (color, smell, structure);
• Serves as an object of historical and archaeological heritage (buried cultural layers, artifacts);
• Performs a recreational role (gardens, parks, nature reserves);
• Serves as a source of inspiration for artists, writers, and philosophers (Weil, 2017).

As Richter & Tugel (2012) note, these functions often remain "invisible" to economists, but they constitute an important part of human well-being.

Resistance (Stability Under Impact)

Resistance is the ability of soil to withstand degradation under the influence of external factors (Seybold et al., 1999; Weil, 2017). For example:

• Soil with high resistance to compaction maintains porosity even after heavy machinery passes.
• Soil with high resistance to salinization does not accumulate toxic salts under irrigation.
• Soil with high resistance to erosion retains aggregates and is not destroyed by raindrop impact.

Resistance is ensured by:

• good structure and aggregation;
• high organic matter content;
• a diverse and active microbial community;
• buffering properties (ability to neutralize acids and bases).

Resilience (Recovery Capacity)

Resilience is the ability of soil to restore its functions after disturbance (Seybold et al., 1999; Weil, 2017). For example:

• After drought, the soil quickly restores moisture and microbial activity;
• After compaction, it restores porosity through wetting-drying cycles, root activity, and soil fauna;
• After contamination, it decomposes toxic substances and returns to normal functioning.

Resilience depends on:

• the presence of a "bank" of viable microorganisms and plant seeds;
• reserves of organic matter as an energy source;
• spatial heterogeneity, allowing "refugia" of life to persist even under unfavorable conditions.

Functional Redundancy

The most important mechanism supporting resistance and resilience is functional redundancy (Weil, 2017). This means that in healthy soil, the same function (e.g., cellulose decomposition or nitrogen fixation) is performed by different groups of organisms. If one group disappears due to stress, others can compensate for its work.

This is precisely why biological diversity is one of the main indicators of health. The more different species and functional groups in the soil, the higher its resistance to adverse impacts (Weil, 2017; White, 2006).

Bulk Density

Definition: the mass of dry soil per unit volume, including pores (g/cm³ or Mg/m³).

Significance: bulk density reflects the degree of compaction, presence of macropores, and the overall physical condition of the soil. Optimal density for most loamy soils is around 1.2–1.4 Mg/m³. When exceeding 1.6–1.8 Mg/m³ for loams (or 1.7–1.9 Mg/m³ for sands), root growth is severely restricted (Weil, 2017).

How measured: by the core method or gamma densitometry. In health assessment, it is important to measure density several times per season, as it changes with wetting and drying.

Sensitivity to management: density increases rapidly with machinery traffic and decreases with tillage, organic matter addition, and earthworm activity.

Aggregate Stability

Definition: the ability of soil aggregates to retain their structure under the influence of water or mechanical loads.

Significance: aggregate stability is the "glue" of the soil. It determines how well the soil resists rain erosion, retains pores for air and water, and protects organic matter from rapid mineralization.

How measured: standard method is wet sieving, where a soil sample is exposed to water, and then the mass of aggregates remaining on sieves is measured. An alternative is the raindrop test, where the number of drops required to destroy an aggregate is counted (Weil, 2017).

Interpretation: the higher the percentage of stable aggregates (>60–70%), the better the physical health of the soil. Low aggregate stability (<30–40%) is a sign of degradation.

Infiltration and Hydraulic Conductivity

Definition: the rate of water entry into the soil (mm/hour or cm/hour).

Significance: infiltration reflects the soil's ability to receive precipitation, prevent runoff, and replenish moisture reserves. It is closely related to density, structure, and the presence of macropores (root channels, earthworm burrows).

How measured: by the double-ring infiltrometer method or using an infiltrometer. In field conditions, the "flooded square" method is also used.

Interpretation: high infiltration (>5–10 cm/hour for loams) is a sign of good physical health. Low (<1 cm/hour) indicates compaction, crust formation, or dispersion.

Available Water Capacity (AWC)

Definition: the amount of water that the soil can retain in a form available to plants between field capacity and wilting point.

Significance: this characteristic determines the water reserve available to plants during dry periods. It depends on texture, structure, and organic matter content.

How measured: by the difference between moisture content at -10 kPa (or -33 kPa) and at -1500 kPa. For rough estimates, pedotransfer functions are used (e.g., based on clay and organic carbon content data).

Interpretation: high available water capacity (>150–200 mm per 1 m of profile) is a sign of healthy soil, especially in regions with unstable moisture supply.

Soil pH

Definition: the negative logarithm of the hydrogen ion concentration in the soil solution.

Significance: pH determines the availability of almost all nutrients, microbial activity, solubility of toxic elements (aluminum, manganese), and enzyme function. Most cultivated plants prefer pH 6.0–7.5.

How measured: potentiometrically in a soil suspension with water (1:2.5) or with 0.01M CaCl₂. The latter method gives more stable and comparable results.

Interpretation: deviation from the optimal range is a sign of health disruption. Low pH (<5.0) indicates acidification, high (>8.0) indicates salinization or alkalization (Weil, 2017; White, 2006).

Total Organic Carbon (TOC)

Definition: total organic carbon content in the soil, expressed as a percentage or g/kg.

Significance: organic matter is the "core" of soil health. It is responsible for:

• aggregation and structure;
• moisture retention;
• cation exchange capacity;
• microbial nutrition;
• buffering and sorption of pollutants (Weil, 2017).

How measured: by dry combustion (catalytic oxidation) or the Tyurin method (wet combustion).

Interpretation: For mineral soils, TOC content >2% is considered good, <1% is low (depending on climate and texture). Not only the absolute value is important, but also the dynamics — a decline of 0.1–0.2% per year can be a signal of degradation.

Active Carbon (Permanganate-Oxidizable Carbon)

Definition: the fraction of organic matter that is readily oxidized by potassium permanganate and considered the most "labile" and biologically active (Weil et al., 2003; Weil, 2017).

Significance: active carbon is the "fast" pool of organic matter that responds sensitively to management changes, serves as food for microorganisms, and participates in aggregation. It changes much faster than total organic carbon and is therefore an early indicator of improvement or deterioration in soil health.

How measured: the soil is treated with a potassium permanganate solution (0.02M KMnO₄), and the amount of oxidized carbon is determined by the change in color (reduction in KMnO₄ concentration).

Interpretation: high values (>200–300 mg/kg) indicate good biological activity and rapid cycling. Low values (<100 mg/kg) indicate stress or insufficient energy for microorganisms.

Cation Exchange Capacity (CEC)

Definition: the total capacity of the soil to retain and exchange cations (Ca²⁺, Mg²⁺, K⁺, Na⁺, H⁺, Al³⁺), expressed in cmol(ec)/kg.

Significance: CEC determines the soil's ability to retain nutrients and prevent their leaching. It also affects buffering against acidification and pollutants.

How measured: by cation displacement methods (e.g., with ammonium acetate).

Interpretation: High CEC (>20 cmol/kg for loams) is a sign of good soil "capacity," low (<5–10 cmol/kg) indicates high leaching or a low proportion of clay minerals and organic matter.

Electrical Conductivity (EC)

Definition: the ability of the soil solution to conduct an electric current, which is proportional to the concentration of soluble salts.

Significance: EC is the main indicator of salinization. High salt concentration causes osmotic stress in plants, toxicity (Na, Cl), and dispersion of soil particles.

How measured: conductometrically in a 1:5 water suspension or in soil paste.

Interpretation: EC <2 dS/m — normal; EC >4 dS/m — salinization; EC >8 dS/m — severe salinization, threatening most crops (Weil, 2017).

Microbial Biomass Carbon (MBC)

Definition: the carbon contained in the cells of living soil microorganisms, expressed in mg/kg or kg/ha.

Significance: MBC is the size of the "working" microbial community. The higher the MBC, the more the soil can mineralize organic matter, cycle elements, and suppress pathogens.

How measured: by fumigation-extraction (chloroform fumigation followed by extraction) or fumigation-incubation methods.

Interpretation: MBC typically comprises 1–5% of TOC. In healthy soil, MBC is at the level of 0.3–1.0 g/kg (Weil, 2017). A decrease in MBC with the same TOC indicates stress — toxicity, acidity, compaction.

Soil Respiration (Basal and Substrate-Induced)

Definition: the rate of CO₂ release by microorganisms during the decomposition of organic matter. Basal respiration is measured without additions; substrate-induced respiration is measured after the addition of readily available carbon (glucose).

Significance: respiration is the "pulse" of the soil. It reflects overall microbial activity and the rate of organic matter mineralization. High basal respiration may indicate active decomposition, but also stress, where microorganisms spend energy on maintenance rather than growth.

How measured: incubation of soil in a sealed container followed by CO₂ analysis with a gas chromatograph or by titration.

Interpretation: Interpretation depends on soil type. The ratio specific respiration (qCO₂) = respiration / MBC is important. High qCO₂ is a sign of stress (Weil, 2017; Huang et al., 2012).

Enzymatic Activity

Definition: the activity of enzymes secreted by microorganisms and roots that catalyze the decomposition of organic substrates (Huang et al., 2012, Ch. 26).

Key enzymes:

Significance: enzymatic activity represents the "workhorses" of the soil. It reflects not just the presence of microorganisms but their functional activity — the ability to decompose specific substrates.

How measured: incubation of soil with a synthetic substrate (e.g., fluorescent or color-forming), followed by spectrophotometric or fluorimetric measurement of the reaction product.

Interpretation: A decrease in enzymatic activity with unchanged MBC is an early sign of stress (contamination, acidification, compaction). High activity is a sign of healthy, actively processing soil.

Biological Diversity (Microbial Community Composition)

Definition: the number and distribution of different taxa (species, genera, functional groups) of microorganisms and microfauna.

Significance: diversity is the basis of functional redundancy and resilience. The more diverse the community, the higher the resistance and resilience of the soil (Weil, 2017; Huang et al., 2012).

How measured: modern molecular methods:

• DGGE/TGGE (denaturing gradient gel electrophoresis) — DNA profiling;
• Amplicon sequencing of 16S rRNA (bacteria and archaea) and ITS (fungi);
• Metagenomics — the full set of genes of the entire community;
• Metatranscriptomics — active genes (RNA), indicating actually functioning processes (Huang et al., 2012, Ch. 24; Weil, 2017, Box 11.2).

Interpretation: A decrease in species number (especially the loss of functionally important groups, e.g., nitrogen-fixers or mycorrhizal fungi) is an alarming sign. However, interpretation requires caution: many organisms are unculturable, and their presence does not always indicate activity.

Soil Fauna Activity and Abundance

Definition: the number and activity of macro- and mesofauna — earthworms, springtails, mites, nematodes, and their impact on soil processes.

Significance: Soil fauna are the "engineers" of the soil. Earthworms create macropores, improve structure, and mix organic matter; nematodes and protozoa regulate microbial populations and release available forms of nitrogen (Weil, 2017).

How measured:

• Earthworms: counting in excavations (frames 50×50×30 cm) or assessing the number of casts and burrows.
• Nematodes: extraction from soil (Baermann method), counting under a microscope, classification by mouthpart types (bacterivores, fungivores, predators, plant parasites).
• Microarthropods: extraction in Berlese-Tullgren funnels.

Interpretation: High earthworm abundance (>100–200 individuals/m² in arable soils) and a diverse nematode community are signs of healthy, well-structured soil. The disappearance of earthworms or dominance of plant-parasitic nematodes is a signal of degradation.

Universal (Basic) MDS

This set is proposed for primary, general assessment of soil health, when there are no specific tasks (e.g., just need to understand whether the soil condition on a farm or in a region is improving or deteriorating).

Basic MDS includes indicators most frequently recommended in international guidelines (Doran & Parkin, 1994; Weil, 2017; White, 2006):

This set provides about 7–8 indicators, which is a compromise between completeness and cost. It allows identification of most common problems: compaction, loss of organic matter, acidification, reduced biological activity.

MDS for Assessing Production Function

If the main goal is ensuring yield, the emphasis shifts to indicators directly determining nutrient availability and physical conditions for roots.

Production MDS (Andrews et al., 2004; Weil, 2017):

Feature: biological indicators here are shifted toward those related to the N-cycle, since nitrogen most often limits yield (Weil, 2017; White, 2006).

MDS for Assessing Ecological Function (Filtration, Buffering, Detoxification)

If the soil is used for wastewater treatment, organic waste utilization, or is in a pollution risk zone, the set of indicators shifts toward buffering and sorption properties.

Ecological MDS (White, 2006; Richter & Tugel, 2012):

Feature: emphasis is placed on sorption capacity and biodegradation capacity — this is critical for assessing risks of groundwater contamination and ecological status (White, 2006).

MDS for Assessing Resistance to Degradation (Resistance and Resilience)

If the goal is to understand how protected the soil is from degradation (erosion, compaction, salinization) and whether it can recover, indicators reflecting structural integrity and functional redundancy are selected.

Resilience MDS (Seybold et al., 1999; Weil, 2017):

Feature: here, indicators that reflect not only the current state but also the potential of the soil to resist and recover are important. This is already a more advanced, prognostic level of assessment (Seybold et al., 1999; Weil, 2017).

SMAF (Soil Management Assessment Framework)

One of the most developed and applied systems for assessing soil quality/health is SMAF (Andrews et al., 2004). It includes:

• More than 20 possible indicators, but for a specific site, an MDS of 6–12 indicators is selected (depending on the purpose).
• For each indicator, SMAF provides interpretive curves that convert the measured value into a dimensionless score (from 0 to 1).
• The scores are integrated into a single index (SQI — Soil Quality Index) for comparing different sites or tracking dynamics.

SMAF flexibly adapts to different soil-climatic conditions and farming systems (Andrews et al., 2004; Weil, 2017).

Cornell Soil Health Assessment

The Cornell University (USA) system offers a standardized MDS for farmers and consultants, including:

• Physical: density, aggregate stability, available water capacity.
• Chemical: pH, organic carbon, active carbon, electrical conductivity.
• Biological: MBC, mineralizable nitrogen, respiration, enzymatic activity (β-glucosidase, phosphatase, arylsulfatase, N-acetyl-glucosaminidase).

This system is oriented toward temperate agricultural soils and includes about 12 indicators with clear interpretive scales and management recommendations (Weil, 2017).

Haney Soil Health Test

A commercial test proposed by R. Haney (USA) includes non-standard indicators:

• Respiration after addition of dry organic matter (SOLVITA).
• Active carbon (permanganate method).
• Carbon-to-nitrogen ratio in water extracts.
• "Health" assessment based on a composite index.

The hallmark of the Haney test is its focus on the biological component and fast, inexpensive methods suitable for mass application (Weil, 2017).

What Is SMAF?

SMAF (Soil Management Assessment Framework) is one of the most developed and validated soil quality assessment systems, created by a group of American scientists led by S. Andrews and D. Karlen (Andrews et al., 2004). It is actively used in the USA and other countries for monitoring, comparing farming systems, and making management decisions.

Key Idea

SMAF is based on the premise that soil quality is not an absolute characteristic but a function of three factors:

• Management objectives (e.g., maximizing yield or minimizing pollution);
• Soil type (texture, mineralogy, climate zone);
• Specific indicators selected for assessment.

Therefore, SMAF offers not a fixed set of indicators but a "framework" that allows flexible assembly of an assessment for a specific task.

SMAF Assessment Procedure

The SMAF assessment process includes four steps (Andrews et al., 2004):

1. Selection of indicators based on assessment objectives. SMAF offers a "library" of more than 20 indicators, from which the most relevant ones are selected (usually 6–12).

2. Measurement of indicators by standard methods.

3. Normalization (scoring) — for each indicator, an interpretive curve (scoring curve) is used that converts the measured value into a score from 0 to 1 (or 0 to 10). The curves differ depending on soil type and climate. For example, the optimal density for a loamy soil is 1.2–1.4 Mg/m³; for sandy soil, it may be higher. SMAF contains a database of such curves considering texture, climate zone, and depth.

4. Aggregation — scores for individual indicators can be:

• Simply averaged (if all indicators are equally important);
• Weighted (if it is known that some indicators are more important than others for the given purpose).

The result is a Soil Quality Index (SQI), a number from 0 to 1 (or 0–10) that allows comparison of sites over time and space.

Advantages of SMAF

• Flexibility — adapts to different purposes, soils, and management systems.
• Scientific rigor — interpretive curves are based on experimental data.
• Comparability — allows comparison of sites with different soil types.
• Transparency — methodology is published and available (Andrews et al., 2004; Weil, 2017).

Limitations of SMAF

• Complexity — requires experience for indicator selection and curve interpretation.
• Labor intensity — requires laboratory analyses, not "field" rapid methods.
• Does not consider dynamics — provides a "snapshot" at the time of measurement, not assessing trends or resilience (this goes beyond SMAF).

What Is CSHA?

Cornell Soil Health Assessment is a practical soil health assessment system developed at Cornell University (USA) specifically for farmers, consultants, and agronomists (Weil, 2017). Unlike SMAF, CSHA offers a standardized set of indicators and simplified interpretation.

CSHA Indicators

CSHA includes 12 indicators grouped into three categories (Weil, 2017):

Interpretation in CSHA

CSHA uses a "poor-medium-good" scale (analogous to "red-yellow-green"). Threshold values are defined for each indicator:

• "Good" (green) — value in the optimal range;
• "Medium" (yellow) — borderline, requires attention;
• "Poor" (red) — critical, requires corrective action.

The integral score is calculated as the arithmetic mean of the scores of individual indicators. Additionally, management recommendations are provided for each "red" indicator (e.g., add organic matter, reduce tillage, apply lime).

Advantages of CSHA

• Practicality — oriented toward farmers and consultants.
• Standardization — a fixed set of indicators simplifies comparison.
• Accessibility — methods do not require unique equipment (all indicators can be measured in a standard laboratory).
• User-friendly interpretation — color scale and recommendations are understandable to non-specialists.

Limitations of CSHA

• Regional specificity — developed for the temperate zone (northeastern USA); for other climate zones, interpretive thresholds may not be suitable.
• Static nature — does not consider dynamics (trends) and resilience.
• Biological bias — although this is a strength, chemical and physical indicators are represented in fewer numbers.

What Is the Haney Test?

The Haney Test is a commercial soil health assessment system developed by R. Haney (USA). It differs from SMAF and CSHA in its focus on biological processes and the use of non-standard, rapid methods (Weil, 2017).

Haney Test Indicators

The Haney Test includes:

1. Respiration after addition of dry organic matter (SOLVITA method) — CO₂ measurement over 24 hours after rehydration and addition of a readily available substrate.

2. Active carbon — permanganate method (similar to CSHA).

3. Ratio of organic carbon to organic nitrogen in water extract (C:N ratio of water-soluble organic matter).

4. "Health" assessment based on a composite index combining respiration, active carbon, and C:N.

Features and Limitations

• Speed and simplicity — methods designed for mass application.
• Focus on biology — provides early signals of change.
• Fewer physical and chemical indicators, so the assessment is incomplete.
• Commercial nature — methodology is not fully open to scientific scrutiny.
• Applicability — works best for soils with active biological cycling; less informative for highly leached or saline soils.

Spectroscopy and Remote Sensing

• Near-infrared and mid-infrared spectroscopy (NIR, MIR) allows rapid and inexpensive determination of organic carbon, nitrogen, clay, pH, and even some biological indicators (enzymatic activity — indirectly) from reflectance spectra.

• Gamma-ray spectrometry (measurement of natural radioactivity of K-40, Th-232, U-238) correlates with texture, clay content, and moisture.

• Electromagnetic induction (EMI) and ground-penetrating radar provide information on density, moisture, and presence of compacted layers.

These methods allow fast, spatially continuous assessment, but they do not yet replace direct laboratory measurements for biological indicators.

Machine Learning and Big Data

Modern approaches use machine learning for:

• Predicting health indicators from indirect data (spectra, climate, relief, management history);
• Identifying nonlinear relationships between indicators and soil functions;
• Clustering sites by health level and degradation risks.

Examples: using Random Forest to predict organic carbon content; neural networks for erosion risk assessment.

Field Rapid Methods (Microbiological Tests)

Rapid tests are being developed for field use:

• Test strips for enzyme activity (e.g., phosphatase, urease) — color reaction in 5–15 minutes;
• Portable respiration analyzers (CO₂ sensors);
• Tests for active carbon with potassium permanganate adapted for field conditions (Weil et al., 2003).

This makes soil health assessment accessible not only to laboratories but also to agronomists in the field.

Health for What?

One of the main limitations of the soil health concept is its context dependence. We cannot say that soil is "healthy" in general, without reference to for what purpose and under what conditions we are assessing it.

Consider an example:

• Soil with pH 5.0 — "sick" for a wheat field (where optimal is 6.0–7.0), but "healthy" for a blueberry plantation or pine forest, where these plants are adapted to acidic conditions and depend on mycorrhizal fungi that are active precisely at low pH (Weil, 2017).

• Soil with high density (1.6 Mg/m³) — "sick" for root crops (carrots, beets), but quite healthy for pastures, where grass roots can penetrate through cracks and earthworm channels (Weil, 2017).

• Soil with low organic carbon content (<1%) — "unhealthy" for intensive temperate arable land, but normal for many tropical soils, where organic matter mineralizes rapidly due to high temperatures (White, 2006).

Conclusion: Soil health is always defined in relation to a specific function or use purpose. The same soil can be healthy for one purpose and unhealthy for another.

Health as a "Norm" Relative to a Reference

In ecology, the concept of reference state is often used — for example, soil under natural vegetation (forest, steppe, savanna) without anthropogenic impact. By comparing with this reference, one can say how strongly the soil has been altered.

However, even here there are complexities:

• The "natural" state itself is not static. Climate changes, vegetation evolves, fires, floods, and other natural disturbances occur regularly. Which particular moment should be considered the reference?

• Humans are also part of the ecosystem. For more than 10,000 years, people have been affecting soils. Where to draw the line between "natural" and "anthropogenic" impact? As Richter & Tugel (2012) note, in the Anthropocene, soils are almost universally cultural-historical-natural systems, and a return to a "pristine" state is often impossible and undesirable.

• The reference is not always achievable. If all soils in a region are altered in one way or another, the reference state may be purely hypothetical, which reduces the practical value of comparison.

Conclusion: Health assessment through comparison with a "natural" reference has methodological and philosophical limitations.

Which Indicators to Choose?

As discussed in Section 5, the choice of MDS is always subjective. It depends on:

• Method availability;
• Budget;
• Researcher's objectives;
• Traditions and schools in which specialists work.

Different assessment systems can give different indices for the same soil if they use different sets of indicators or different interpretive scales (Andrews et al., 2004; Weil, 2017). This creates problems when comparing data from different sources.

Example: One system may consider phosphatase activity a key health indicator, while another may prioritize respiration. Both are right, but they measure different aspects. Therefore, a health index obtained from one system cannot be directly compared with an index from another system.

Biological Indicators: Measurement Problems

Biological indicators are the most informative but also the most difficult to measure (Weil, 2017; Huang et al., 2012). Main problems:

• High variability. Microbial biomass and respiration can change 2–3 times during a season. A single measurement may be random. Multiple measurements and averaging over several seasons are required.

• Sensitivity to storage and sample preparation. Within hours after sampling, microbial activity begins to change. Standardization of storage conditions (temperature, moisture, aeration) is a complex task.

• Not all organisms can be accounted for. Up to 99% of soil microorganisms are unculturable (Weil, 2017). Molecular methods (DNA sequencing) produce huge amounts of data, but their interpretation requires complex bioinformatics processing. Moreover, the presence of DNA does not mean the organism is active (dead cells also contain DNA).

• Functional redundancy complicates interpretation. If one group of microorganisms dies but its function is performed by others, the overall health index may not change — although the system has already lost part of its resilience (Weil, 2017). This is "hidden" degradation that is difficult to capture with standard indicators.

Spatial Heterogeneity

Soil health varies in space at scales from centimeters to kilometers. A single sample, even mixed from 10–20 points, may not reflect the true condition of the field.

• "Hot spots" — areas with high biological activity (e.g., around roots, in earthworm burrows, under manure piles) can give inflated values if included in the sample.
• "Dead zones" — compacted wheel tracks, salinization patches, areas with toxic residues — may go unnoticed.

Reliable assessment requires a systematic sampling grid and a sufficient number of replicates, which dramatically increases costs.