Sapropel
Sapropel (from Greek sapros — rotten and pelos — mud, silt) — bottom sediments of freshwater bodies, ranging from pink to dark brown in color; the natural color changes on exposure to air. It is an organomineral compound. Sapropel is formed from remains of plants and animals, contains mineral and organic impurities, dries slowly, after drying becomes solid and does not become wet again. It contains humic acids, fulvic acids, hemicellulose, cellulose, bitumens, ash (on average 20–60%).
Sapropel reserves in Russia are estimated at 92 billion t recalculated to 60% moisture; in the Republic of Belarus — 2.76 billion m3.
Sapropel is extracted by dredgers with pulp being pumped into settling tanks; in the first year it is dewatered, in the second, after freezing, which promotes loosening, it is dried and turns into a loose mass with moisture of about 50%. In the first year, it can be applied directly to fields, where after freezing and natural drying it turns into a loose mass with moisture of about 80%.
Chemical composition of sapropel
The composition of the organic mass of sapropel depending on deposits is: humic acids — 11.3–43.4%, fulvic acids — 2.1–23.5%, non-hydrolyzable residue — 5.1–22.6%, hemicellulose — 9.8–52.5%, cellulose — 0.4–6%, water-soluble substances — 2.4–13.5%, bitumens A — 3.4–10.9%, bitumens C — 2.1–6.6%, total nitrogen — 0.6–2.6%, phosphorus — 0.14–0.19%, calcium — 2.5–43.8%, magnesium — 0.3–2.3%. Content of organic matter — from 12 to 80%, ash — from 19 to 88% (in dry matter), including up to 20–30% calcium and magnesium carbonates. Technical specifications regulate requirements for sapropel fertilizers.
Table. Physical and chemical indicators of sapropel fertilizers.
| Indicator name | Norms by fertilizer types | ||
|---|---|---|---|
| Organic | Organosiliceous | Organo-calcareous | |
| Mass fraction of particles larger than 10 mm, %, max | 20 | 20 | 20 |
| Mass fraction of moisture, %, max | 60 | 60 | 50 |
| Ash content, %, max | 50 | 70 | 65 |
| Mass fraction of total nitrogen, % per dry product, min | 1.5 | 1.0 | not regulated |
| Exchange acidity, pH, min | 5.0 | 5.0 | not regulated |
| Mass fraction of calcium oxide, %, min | — | — | 17 |
| Specific activity of radionuclides (cesium-137), Bq/kg | not more than 300 | col | col |
Nitrogenous substances of sapropels are represented by high-molecular compounds; therefore, plant-available forms of nitrogen and phosphorus are 2–3 times less than in manure, and potassium is negligible.
Sapropels contain a large amount of trace elements: 1 kg of dry mass contains 200–1000 mg manganese, 10–400 mg zinc, 10–200 mg boron, 2–60 mg copper, and 1–20 mg molybdenum. Depending on the location of the water body, a large amount of heavy metals may also be present.
The composition of sapropel can vary greatly depending on location, even within the same water body.
Table. Average composition of various sapropels (from different sources).
| Type of sapropel | Content, % on dry matter | |||||
|---|---|---|---|---|---|---|
| Organic matter | Ash | N | P2O5 | CaO | MgO | |
| Low-ash | 80 | 19 | 3.4 | 0.14 | 2.5 | 0.5 |
| Medium-ash | 60 | 38 | 2.6 | 0.18 | 2.3 | 0.7 |
| Increased-ash: | ||||||
| - clayey, sandy | 37 | 63 | 1.9 | 0.19 | 2.7 | 1.5 |
| - calcareous | 40 | 60 | 1.6 | 0.14 | 16.0 | 1.2 |
| High-ash (lake lime) | 26 | 73 | 1.2 | 0.18 | 34.0 | 0.8 |
| Silt: | ||||||
| - clayey, sandy | 12 | 88 | 0.6 | 0.17 | 4.5 | 1.3 |
| - calcareous | 13 | 87 | 0.6 | 0.15 | 15.0 | 2.3 |
Classification
Depending on the content of silica (SiO2) and calcium oxide (CaO), sapropels are divided into:
- organic — with ash content less than 30%;
- siliceous — with silica content more than 50%;
- calcareous — with calcium oxide content more than 30%;
- mixed.
Calcareous sapropels are also used as lime fertilizer, not inferior to chalk and dolomite flour.
According to the classification of A. Ya. Rubinstein, by ash content sapropels are divided into:
- low-ash — with ash content up to 30%;
- medium-ash — with ash content 30–50%;
- increased ash content up to 70%;
- high-ash — with ash content 70–85%;
- silts — with ash content over 85%.
Application of sapropel
According to summarized VNIIUA data from field experiments, sapropels show relatively low efficiency: to obtain crop yield increases comparable to those from manure, the doses of sapropels on average must be 3 times higher.
Sapropel has some positive properties: high water-holding and low filtration capacity, which helps improve the water-physical properties of light soils. Due to its adhesive ability, sapropel, when interacting with soil, improves structure, gives a cloddy and loose texture, and increases air permeability.
Sapropel can be used as organic fertilizer in pure form and in composts with manure, liquid manure, and feces.
The timing of application and methods of incorporation of sapropel for crops do not differ from other organic fertilizers. Sapropel fertilizers do not necessarily have to be incorporated into the soil immediately after distribution over the field; incorporation can be done 3–7 days later. Sapropels are better suited to sandy and sandy loam soils, since their efficiency is higher than on soils of heavy granulometric composition.
In terms of fertilizer value, 1 t of sapropels is equal to 0.6–0.7 t of peat-manure composts. The use of sapropel as a local fertilizer is associated with costs for its extraction, transportation, and application. Transportation of sapropels over a distance of up to 20 km is economically justified.
Sapropel is applied to crops in doses determined for each field based on conditions, biological characteristics of the crop, and agrochemical characteristics of the fertilizer. It is optimal to determine the application rates of sapropel by the equivalent content of nutrients in it, primarily nitrogen.
Sapropel is applied in doses of 30–40 t/ha for cereal crops, 50–100 t/ha for vegetable crops. It is often used when there is a shortage of manure and in fields located near extraction sites.
List of sources
- Mineev V. G., Sychev V. G., Gamzikov G. P. et al. (2017). Agrochemistry. Textbook. Edited by Mineev V. G. Moscow: Publishing House of the D. N. Pryanishnikov VNIIA.
- Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Edited by B. A. Yagodin. Moscow: Kolos.
