Lacustrine deposits sapropel and gyttja as an agrochemical resource

Lake deposits are sedimentary formations on the bottom of lakes, resulting from biochemical, microbiological, and physical-mechanical processes involving the remains of animals and plants inhabiting them, as well as organic and mineral impurities brought into the water bodies by water and wind.

According to the nature of their formation, lake deposits are subdivided into:

• gyttja, which is a subhydrogenated humus form found in oxygen-rich waters;
• sapropel (putrefying organic silt), which arises under oxygen-deficient conditions.

There are gradual transitions between both forms of lake silt. Gyttja is a loose material transitioning into a plastic state, enriched with fine-grained soil particles; it contains lime in most cases and has about 15–25% organic matter calculated on a dry mass basis.

Sapropel externally resembles a gelatinous mass with a consistency close to sour cream, which gradually compacts as the depth of the deposits increases. The color of sapropel is very diverse: brown, dark olive, almost black, gray-yellow, greenish, bluish, pink, and even reddish. Color plays a major role in evaluating sapropel, as it indicates the presence of certain organic and inorganic substances:

• greenish – chlorophyll;
• pink – carotene;
• bluish – vivianite;
• black, darkening rapidly in air – reduced iron;
• grayish – lime.

Once removed from the deposit, sapropel oxidizes rapidly in the air and loses its natural color. Its distinctive features are its colloidal structure and high moisture saturation in its natural state.

The natural humidity of the bulk of the deposits is 84–96%. The lower limit of sapropel humidity is 60%, the upper is 97%, and the statistical average humidity is 88.4%.

The organic matter content in sapropel reaches up to 80% of the dry matter. The composition of the organic mass of sapropel varies widely depending on the deposits:

One of the main classification indicators of sapropel is ash content, which averages from 20 to 60%. The upper limit of the ash residue is 85%, and the lower is 4–7%. Sapropel containing less than 10% ash is particularly valuable. The magnitude and nature of the ash-forming components of sapropel are determined by the specifics of its formation and depend on the chemical composition of the waters feeding the water bodies, the influx of mineral substances with river sediments as a result of erosive and biochemical processes, as well as anthropogenic influence.

Based on ash content or organic matter content, A.Ya. Rubinstein (1964) divides sapropels into:

• low-ash – up to 30% ash;
• medium-ash – 30–50%;
• high-ash – 50–70%;
• very high-ash – 70–85% ash.

If the ash content exceeds 85%, the deposits are called silt.

The chemical composition of sapropels from different deposits, and even within the same deposit, is not uniform. Sapropel fertilizers are very diverse, containing 1.3–2.9% nitrogen (N), 0.18–0.43% phosphorus (P2O5), 2.1–36.6% calcium (CaO) per dry matter, with a pH of 2.4–8.5. Their content of macronutrients varies within wide limits (Table 116; Vasiliev V.A., 1984).

Table 116 – Average chemical composition of lake silt, % of dry matter Organic Group of sapropel matter Ash N P2O5 CaO MgO deposits

 Low-ash 80 19 3.4 0.14 2.5 0.50 Medium-ash 63 38 2.6 0.18 2.3 0.70 High-ash: clayey, sandy 37 63 1.9 0.19 2.7 1.50 calcareous 40 60 1.6 0.14 16.0 1.20 Silt: calcareous 13 87 0.6 0.15 15.0 2.30 clayey, sandy 12 88 0.6 0.17 4.5 1.30

According to their composition and properties, sapropels are subdivided into siliceous, containing more than 50% silica; carbonate, containing more than 30% calcium oxide; and organic, with an ash content of less than 30%. Carbonate sapropels are not inferior in effectiveness to chalk and dolomite flour, especially on sandy and sandy loam soil.

The use of sapropel in agriculture has been known since ancient times. In Russia, lake silt was used in agriculture as early as the 18th century. The widespread development of market gardening on lands around such large lakes as Nero in the Yaroslavl region and Galichskoye in the Kostroma region is connected to the use of sapropel from these water bodies. The best lakeside garden lands were created on poor sandy loam podzolic soil through centuries of cultivation and the systematic application of lake silt.

The first research paper on the use of sapropel as a fertilizer in Russia dates back to 1912. Beginning in the 1930s, its use became more widespread. Experimental data proved the fertilizing value of sapropel when growing barley, winter rye, potatoes, onions, and cabbage.

The effectiveness of sapropels as a fertilizer most often depends on their nitrogen, phosphorus, and potassium content. Their nitrogenous substances are represented mainly by high-molecular compounds that are difficult for plant nutrition to access, and are strongly bound to humic substances. The content of available phosphorus is very low, and potassium is negligible. Available nitrogen and phosphorus in freshly extracted sapropel are usually 2–3 times lower than in manure.

Physicochemical properties of sapropel and its effect on soil

Sapropel is a valuable source of organic matter formed at the bottom of lakes. Its primary reserves are concentrated in regions of former glaciation, mainly in the Leningrad, Arkhangelsk, Pskov, and Sverdlovsk regions, as well as in Karelia. The thickness of lake deposits typically ranges from 3 to 10 meters, but in some water bodies, it can reach 20–40 meters. All reserves are divided into balance reserves (feasible for extraction with current technologies) and off-balance reserves, which may be converted into balance reserves in the future.

• Total reserves (at 60% humidity) — 92 billion tons
• Typical thickness of deposits — 3–10 m
• Maximum thickness of deposits — 20–40 m
• Target humidity of the finished fertilizer — 50–60%

Sapropel possesses unique physical properties that determine the specifics of its application. In its natural state, it releases water slowly. After conventional drying, the material becomes extremely hard, and after being ground into powder, it completely loses its ability to absorb moisture (the only exceptions are some types of calcareous sapropel, which remain loose). Frozen sapropel behaves quite differently: it dries quickly to a humidity of 18–20% and acquires a stable, loose structure that is not lost even after thawing.

For an agronomist, sapropel is valuable because it radically improves the hydro-physical properties of light soils. Due to its high water-holding capacity and low filtration capacity, it binds sandy soils. Its natural adhesive properties help form a cloddy soil structure and increase its looseness and air permeability. This fertilizer can be used either in its pure form or for preparing composts with manure, liquid manure, or feces.

Technology for extraction and processing of lake deposits

Sapropel is extracted using excavators or hydromechanization equipment. Depending on production conditions, the raw material is applied directly to fields or coastal meadows, transported to settling tanks, or used for the colmatage of waterlogged floodplains. The specific method is chosen based on the viscosity and thickness of the silt, its degree of mineralization, and the capabilities of the equipment. The hydromechanized method using suction dredgers is considered the most technologically advanced.

In the hydromechanized extraction method, the technological process is divided into several mandatory stages.

1. Extraction of sapropel from the lake bottom using dredgers.
2. Hydrotransport of pulp through pipes to special thickening and drying fields.
3. Layer-by-layer hydraulic filling of the mass into settling tanks in 20 cm layers.
4. Thickening of the mass, draining of settled water at the end of the season, and winter freezing.
5. Drying under field conditions to a humidity of 50%, followed by milling and loosening of the top layer.
6. Harvesting the finished loose material for storage.

The filling technology requires strict compliance with regulations. The filled 20 cm layer settles for 4–5 days, during which its humidity decreases to 82%, and its thickness reduces to 4.5 cm. Only after this is the next layer applied — to fill one meter of sapropel, the operation is repeated up to 22 times. Water cannot be drained during the filling process; it is released from the thickening fields only before the winter frosts. During subsequent drying, the humidity drops to 75%, and the height of the one-meter layer decreases to 0.75 meters.

Layer-by-layer filling, combined with milling and turning, ensures excellent aeration of the mass. Due to constant contact with oxygen, ferrous and manganese compounds are completely oxidized and become safe for plants. The activation of microorganisms during the drying process increases the amount of available nitrogen and other nutrients. The result is a loose crumble with a particle size mainly of 3 mm (fractions of 1–3 mm are also encountered) and a humidity of 50–60%. Such fertilizer does not contain weed seeds, is easily applied by standard organic fertilizer spreaders, and is quickly incorporated into the tillage horizon.

The main disadvantage of this technology is the high labor intensity of constructing settling tanks and the two-year production cycle. The process also requires significant costs for loading and unloading operations and transportation of the finished product. The entire complex of operations for applying the obtained fertilizers is carried out by standard machinery for organic matter.

The timing of application and methods of incorporating sapropel for all crops do not fundamentally differ from those for other organic fertilizers. At the same time, sapropel fertilizers do not necessarily need to be incorporated into the soil immediately after spreading in the field; this can be done after 3–7 days. It is more efficient to use sapropel on sandy and sandy-loam soils, as its effectiveness is significantly higher there than on soils with a heavier particle-size distribution.

Sapropel is applied to crops under fertilization at rates determined for each field, based on specific conditions, biological characteristics of the crop being grown, and the agrochemical characteristics of the fertilizer. It is advisable to determine the application rates of sapropel based on the equivalent of its plant nutrients, primarily nitrogen. The estimated application rate of sapropel for fields is from 210 to 150 t/ha.

Hydraulic dredging directly from a water body onto fields allows for applying from 200 to 1000 t/ha of sapropel into the soil, calculated at 60% humidity. In such quantities, sapropel is not only a fertilizer but also a soil conditioner for poorly cultivated soils. The optimal rate ranges from 600 to 1000 t/ha. Such a quantity is very effective, and the costs of application are recouped within 2–4 years. Sapropel, applied in large quantities, gradually mineralizes and creates a high agro-background in the improved soils for a long period.