Fertilizer application for increasing the productivity of water bodies and rice systems

Breeding herbivorous fish in ponds and rice paddies allows for obtaining valuable products with minimal costs. The productivity of a water body directly depends on its food base. It is formed through a chain: from phytoplankton (primary production) via zooplankton and benthos (intermediate feed) to fish.

How the chemical composition of water affects fish development

The development of ichthyofauna is determined by the depth of the water body, the type of soil, temperature, and water quality. The main indicator of the environmental condition is nitrogen in all its forms: protein, ammonium, and in the form of nitrites and nitrates. Aquatic organisms excrete ammonia, and dead plankton decomposes, saturating the water with nitrogenous compounds. Plants absorb these salts to build cells, initiating a new feeding cycle.

Indicator	Optimal nitrogen content in fish pond water
Total nitrogen	1–2 mg/l

• Optimal content of soluble phosphorus (P2O5) — up to 1 mg/l
• Threshold for severe water contamination with phosphorus — more than 5 mg/l
• Dose of lime for preliminary reclamation — 1–2 centners/ha
• Application rate of manure along the water edge — 150–200 centners/ha

It is important to control the content of other elements in the water. An excess of chlorides and sulfates indicates pollution and reduces the oxygen level, which suppresses the fish. Ferrous iron at a high concentration oxidizes and settles on the fish gills as a reddish-brown coating, leading to their death from suffocation. To prevent die-offs, the water and the soil of the water body must have a neutral or slightly alkaline reaction.

Technology of liming and fertilizer application

Preparation of a water body begins with reclamation work. In order for fertilizers to work as efficiently as possible, it is important to follow the correct sequence for preparing the pond bed.

1. Perform liming of the water body bed if the analysis showed an acidic reaction of the environment.
2. Completely remove "hard" vegetation: lake bulrush, sea club-rush, and reedmace.
3. Evenly distribute fertilizers over the surface of the water body during the summer period.

The best results are provided by organic fertilizers, containing the entire complex of nutrients. Use well decomposed manure (pig, cow, sheep, poultry), composts, liquid manure, or excrement. Solid manure is ploughed in autumn or spring before filling the water body to a depth of no more than 15 cm. It can also be laid out in piles along the water edge, and liquid manure can be applied in portions every 8–10 days.

As an alternative, green manure is used. For this, the bed of the drained water body is sown with a vetch-oat mixture, lupine, sweet clover, or clover. Young seedlings are flooded with water: they decompose rapidly, stimulating the vigorous development of forage microorganisms and algae.

Mineral fertilizers in fish ponds and rice paddies stimulate the growth of phyto- and zooplankton — the natural food base for fish. First and foremost, water bodies need phosphorus, which is used for skeleton formation, consumed during muscular and nervous activity, and is also part of blood plasma. Fish assimilate phosphorus ions directly from the water through gills, the oral cavity, and skin. Practically all types of bottom soils need phosphorus, especially sandy loams, clays, loams, peatlands, and podzols.

For phosphorus top dressing, superphosphate, phosphorite meal, bone meal, and precipitate are used. Nitrogen fertilizers are applied for the development of plants and microorganisms serving as food for aquatic invertebrates. Nitrogen gives the maximum effect in combination with phosphorus and potassium, and ammonium sulfate, ammonium nitrate, and urea are used as sources. Before application, nitrogen fertilizers are diluted with water and sprayed over the water mirror: the dosage is increased in deep areas and decreased in shallow waters.

Potassium is required for the balanced development of all flora and fauna of the water body. Most ponds contain a sufficient amount of potassium, but sandy and acidic soils are poor in it. An abundance of Elodea ("water thyme"), arrowhead, frogbit, and water plantain indicates a sufficient level of potassium in the water. With its deficiency, plants turn yellow and brown, and their leaves become brown.

As potassium fertilizers, sylvinite, kainite, potassium chloride, and potassium sulfate are used. It is recommended to apply them in small doses and together with phosphorus fertilizers. This is done only in those water bodies where a potassium deficiency is actually confirmed.

• Phosphorus fertilizer application rate — P20–30
• Nitrogen fertilizer application rate — N4–5
• Potassium fertilizer application rate — K30–100

How to increase treatment efficiency and what equipment to use

The efficiency of top dressing directly depends on the density of hard aquatic vegetation and the flow rate of the water body. Under the influence of fertilizers, reeds, cattails, and sedge begin to grow much faster. If their spread is not controlled, an excess of hard flora will drastically reduce the benefits of the applied substances.

Specialized equipment is used for the mechanization of work. Mineral fertilizers, lime, and portioned dough-like feed are distributed over the water using catamarans. On small water bodies, mineral fertilizer solutions are sprayed with an OSSH-15 mounted sprayer. If the reservoir bed is drained, a self-loading broadcast seeder of the STS-15 type is used for fertilizer application.

It is important to organize the entire technological chain of fertilizer application and chemical ameliorants without pauses or losses. Work is planned in advance, taking into account logistics and storage. The entire process consists of five sequential stages:

1. Production or harvesting of organic and local fertilizers.
2. Transportation to the water body.
3. Storage under proper conditions.
4. Preparation of fertilizers for application.
5. Application into the water or distribution over the drained bed.