Agronomic characteristics and use of podzolic and sod-podzolic soil

In practical soil science, soil type is determined by the nature of organic matter input, mineral weathering processes, the type of substance migration, and profile structure. Each soil zone requires its own strategy for tillage and nutrition. For efficient land use, it is important for an agronomist to distinguish the characteristics of podzolic and peat-bog soils.

Podzolic and sod-podzolic soils: managing nutrition and acidity

Podzolization destroys minerals in the upper part of the profile and leaches decay products into deep layers and groundwater. As a result, an acidic, whitish podzolic horizon forms beneath the forest litter. It is characterized by a low content of nutrients and poor physical properties. The underlying illuvial horizon has maximum density and minimum porosity, which hinders the development of the plant root system.

Sod-podzolic soils are more agronomically valuable. In the upper part of the profile, they have a humic-eluvial (sod) horizon, and a podzolic one below. Their base saturation degree is higher than that of classic podzolic soils, and exchangeable bases are represented mainly by calcium and, to a lesser extent, magnesium.

• Humus in podzolic soils — from 1.0–1.5 to 2–4 %
• Exchange capacity — from 20–40 to 120–170 mmol-eq/kg
• Base saturation — less than 50 %
• Available phosphorus (by Kirsanov) — 0–50 (rarely 50–100) mg/kg
• Exchangeable potassium (by Peive) — 70–150 mg/kg

Although sod-podzolic soils are suitable for most field crops, their natural soil fertility is limited. Almost all nitrogen is locked in organic matter and is released only as it mineralizes. Total phosphorus and potassium reserves also require constant adjustment.

Element	Total content in the arable layer	Availability to plants
Nitrogen	from hundredths of a percent to 0.2 %	Available only after mineralization in nitrate and ammonium forms
Phosphorus	from 0.05–0.07 to 0.1–0.16 %	Bound to sesquioxides and clay, availability is limited
Potassium (as K₂O)	from 1.0 to 2.5 %	Found mainly in poorly available mineral forms

Peat-bog soils: specifics of raised and low-lying types

This soil type is formed under conditions of constant or prolonged excessive humidity, causing semi-decomposed peat to accumulate in the profile. The effective profile depth is limited by the summer water table level, which drops to a depth of 30 to 50–80 cm. Based on nutritional conditions and properties, peat-bog soils are divided into raised and low-lying types.

Characteristic	Raised bog soils	Low-lying bog soils
Water supply source	Non-mineralized atmospheric precipitation	Mineralized groundwater and surface water
Acidity (pH aqueous)	3.5–4 (strongly acidic reaction)	5.5–6 (slightly acidic) or neutral
Ash content	less than 6 %	over 6 % ash elements
Physical properties	High water-holding capacity, not base-saturated	Less water-retentive, more balanced

For proper planning of land reclamation and crop selection, it is important to accurately determine the soil subtype in a specific area:

• Raised peat: the profile consists entirely of peat.
• Raised peat-gley: have a pronounced gleyed mineralized horizon in the lower part of the profile.
• Low-lying depleted peat-gley: a loamy horizon lies beneath a 20–50 cm peat layer, underlain by a gley horizon.
• Low-lying depleted peat: the profile is composed entirely of peat.
• Low-lying typical peat-gley: beneath the 30–50 cm upper peat-humus layer lie humus-gleyed and gley horizons.
• Low-lying typical peat: the entire soil profile consists of peat.

Agronomic potential of brown forest and gray forest soils

Brown forest soils form in a moderately warm, humid climate under forest vegetation. Their main value for agricultural production is high humification, reaching 5–10% in the upper horizon, and a stable granular or nut-like structure. These soils have a slightly acidic reaction and a moderately deep profile of up to 100 cm, which is poorly differentiated into horizons. In agriculture, they are actively used for sowing soybean, grain, and vegetable crops, as well as for establishing vineyards.

Gray forest soils develop in a continental climate on loess-like loams and carbonate moraines. They are distinguished by a deep 150–200 cm profile, high biological activity, and good accumulation of nutrients in the upper horizons. Depending on the subtype, the humus content varies significantly, which directly affects the choice of fertilizer application rates. A wide range of crops is successfully grown on such lands: from grains and forage crops to industrial, vegetable, and fruit crops.

Grey forest soil subtype	Humus content, %
Light grey	1.5–2.0
Grey	3.5–6.0
Dark grey	6.0–8.0

Chernozems: profile structure and classification by soil fertility

Chernozems are the most valuable soils of the steppe and forest-steppe zones, forming on carbonate parent materials under non-leaching or periodically leaching water regimes. The main driver of their soil fertility is a powerful humus-accumulative process. It is maintained by an active biological cycle with the annual return of nitrogen and ash elements through plant litter. Physical properties of chernozems are optimal for most crops: they possess high water-holding capacity, excellent porosity, and a near-neutral soil reaction.

The chernozem profile is clearly divided into genetic horizons. A dark-colored humus horizon (A) lies on top, transitioning into the B1 horizon with a characteristic brown hue. Below lies the horizon of humus tongues (B2) and the carbonate horizon (Bk), which smoothly transitions into the parent material (C).

• Humus in the top horizon — up to 15%
• Solid phase density — 2.4–2.5 g/cm³
• Super-thick humus layer — more than 120 cm

To accurately assess the potential of a specific field, chernozems are classified by the thickness of the humus layer and the percentage of organic matter. By texture, clay and loam varieties predominate among them. Genetic diversity includes podzolized, leached, typical, ordinary, and southern subtypes.

Humus layer thickness class	Horizon thickness, cm
Super-thick	more than 120
Thick	120–80
Medium-thick	80–40
Thin	40–25
Very thin	less than 25

Humus content class	Humus share, %
High-humus (rich)	more than 9
Medium-humus	9–6
Low-humus	6–4
Very low-humus	less than 4

Chernozem soils represent the main value of the land fund of the Russian Federation. They are particularly widely and diversely represented in the North Caucasus region. Here, agronomists work with almost all known subtypes of chernozems, including podzolized, leached, typical, ordinary, southern, and mountain varieties.

Chestnut soils and solonchaks: specifics of working in arid conditions

Chestnut soils form in dry steppe conditions with moisture deficiency and sparse grass cover. Compared to chernozems, biomass accumulates more slowly here, and the depth of precipitation wetting is shallow. The profile consists of a greyish-chestnut humus layer with a fine-crumb structure and a compacted underlying horizon with carbonate inclusions in the form of white eyes, pseudomycelium, or powdery accumulations. These lands are divided into three subtypes: dark chestnut, chestnut, and light chestnut. They are mainly used for pastures and hayfields, but under tillage they are used to grow durum wheat, corn, millet, sunflower, and gourd crops.

Solonchaks contain an excess of easily soluble sulfates and chlorides, which are toxic to most crops. Automorphic solonchaks form on outcrops of ancient saline rocks, while hydromorphic ones develop due to the capillary rise and evaporation of mineralized groundwater. Secondary salinization often occurs due to irrigation errors in the fields. The profile of solonchaks is weakly differentiated, has an alkaline reaction, and characteristic salt efflorescence is visible on the surface.

The development of solonchaks requires mandatory reclamation. The technology of their desalination is based on the following sequential operations:

1. Installation of desalination drainage for stable lowering of the groundwater level.
2. Performing leaching to wash out easily soluble salts from the root zone.

After successful reclamation and provided there is regular irrigation, former solonchaks can be used to cultivate cereals, vegetables, cotton, and forage crops, or used as pastures for sheep and camels.

Solonetzes, solods, and sierozems: reclamation and agrochemical properties

Solonetzes form when an excessive amount of sodium or magnesium accumulates in the soil adsorption complex. Based on the water regime, solonetzes are divided into three groups:

• Automorphic solonetzes: chernozem, chestnut, and semi-desert.
• Semi-hydromorphic solonetzes: meadow-chernozem, meadow-chestnut, meadow-semi-desert, and permafrost.
• Hydromorphic solonetzes: chernozem-meadow, chestnut-meadow, meadow-bog, and meadow permafrost.

To bring solonetzes into active agricultural rotation and grow cereals, sugar beet, and forage crops on them, an agronomist must carry out a range of reclamation works:

1. Gypsuming to displace adsorbed sodium with calcium and reduce alkalinity.
2. Deep ploughing (autumn) to mechanically break down the dense illuvial horizon.
3. Application of organic and mineral fertilizers to compensate for nutrient deficiency.
4. Introduction of grass sowing and leaching of the soil profile.

Solods are the result of the desalination of solonetzes under leaching or periodically leaching water regimes, most often in micro-depressions of the relief. During this process, exchangeable sodium in the upper layers is replaced by hydrogen, which leads to the formation of a weakly acidic eluvial horizon. Meadow-steppe, meadow, and meadow-bog solods are distinguished. In their natural state, they are used for hayfields, but for growing crops, they require mandatory chemical amelioration.

Serozems (grey desert soils) are formed under subtropical semi-desert vegetation in foothills and sloping plains. Soil formation occurs on loess, loess-like loams, and ancient alluvial deposits under non-leaching and capillary-rise water regimes. These soils are characterized by an alternation of active biological processes of humus accumulation during the wet period and complete mineralization of organic matter with the accumulation of readily soluble salts during the summer dormant period.

Below are the main agrochemical indicators and critical parameters of the considered soil types:

• Sodium during solonetzic process — from 10–15 to 70% of exchange capacity
• Magnesium during solonetzic process — more than 30% of exchange capacity
• Salts in automorphic solonchaks — 0.5–1% in the upper horizon
• Salts in hydromorphic solonchaks — 6–8% or more in the upper horizon
• Depth of the solodized horizon of solods — 5–20 cm

Soil type	Humus content, %	Soil reaction (pH)	Specific profile properties
Chestnut	1.0–4.5	Weakly alkaline; close to neutral in the upper horizon	Illuvial-carbonate horizon with inclusions of "white eyes" (carbonate nodules) or pseudomycelium; readily soluble salts and gypsum in the lower part.
Solonchaks	0.5–10	Alkaline	Weakly differentiated profile with streaks or patches of salts; salt efflorescence on the surface.
Solonetzes	0.5–8 (in the top of the horizon)	Alkaline	Illuvial horizon with columnar, prismatic, or blocky structure; high mobility of colloids.
Solods	2–8	Weakly acidic in the humus horizon	Eluvial solodized horizon, often gleying; clearly defined profile with a depth of 120 cm or more.

Yellow soils (zheltozems) and red soils (krasnozems): specifics of acidic soils in humid subtropics

Yellow soils and red soils are formed under forest vegetation in a humid subtropical climate under conditions of an intensive leaching regime. The main feature of red soils is the ongoing allitization of their mineral part, where the ratio of silica to alumina (SiO2 to Al2O3) in the clay fraction drops below 2. Yellow soils, by contrast, contain more silica and have pronounced signs of podzolization. Both soils are characterized by an acidic reaction, low availability of nitrogen and potassium, and high phosphorus fixation.

• Acidity of red soils — pH 4.2–4.5
• Acidity of yellow soils — pH 5–6
• Humus in yellow soils — 2–7 %
• Humus in red soils — 5–12 %

Indicator	Yellow soils	Red soils
Parent materials	Shales (acidic and intermediate), loose clayey rocks on terraces	Weathering products of igneous rocks (basalts, tuffs), shales, clay-sandy and pebble-boulder deposits
Cation exchange capacity	From low (40–50 mmol-eq/kg) to medium (250–300 mmol-eq/kg)	Low (CaO and MgO content is low)
Base saturation	From 4–7 % to 60–70 % (calcium predominates in the cation composition — 60–80 % of capacity; magnesium and hydrogen are present)	Unsaturated with bases
Physical properties	Predominantly clay or loam texture	Well-defined water-stable structure, high water permeability, high water capacity and porosity
Mineral composition	Silica — 55–65 %, sesquioxides — 15–30 %	Rich in iron; SiO2 to Al2O3 ratio in the clay fraction is less than 2

The profile of yellow soils is clearly divided into genetic horizons. The forest litter (A0) has a thickness of 1 cm, the humus horizon (A1) — 10–15 cm, and the transitional humus-metamorphic (AB) — 15–20 cm. The illuvial-metamorphic horizon (B) reaches 30–40 cm, and the transition to the parent rock (BC) — 20–40 cm. With depth, the humus content in this soil decreases sharply.

Four sub-types are distinguished in the yellow soil category: yellow soils, podzolized yellow soils, unsaturated podzolized yellow-gley soils, and podzolized yellow-gley soils. These soils are most suitable for tea cultivation.

The profile of red soils also has a clear structure. The thickness of the turf or forest litter (A0) is 3–4 cm. The reddish-grey cloddy-granular humus horizon (A1) has a thickness of 20–25 cm, and the brownish-red transitional horizon (B) — 80 cm. Below lies the heavily weathered parent rock (C) of red color with iron-manganese concretions and silica patches.

Red soils are divided into two sub-types: typical and podzolized. They are favorable for growing high-quality grapes and subtropical crops.