Skip to content

Algae (nacha/paaku) management in paddy

Algal mats, locally called nacha or paaku, are filamentous and colonial green algae that proliferate on the standing water of paddy paddies. They appear in fields with prolonged static flooding, excess nitrogen and warm, calm weather, where they smother seedlings, depress early tillering and form crusts that interfere with fertilizer absorption.

Principle

Copper sulphate (bluestone) is the standard algaecide because copper ions disrupt algal photosynthesis on contact. Knockdown is rapid but short-lived, so chemical treatment is paired with corrective agronomy: draining or lowering standing water, balancing nitrogen, and restoring potassium and zinc (Micronutrient Deficiency Paddy).

Implementation

Published extension recommendations describe surface application of copper sulphate to standing water at roughly 0.25-2.0 ppm, equivalent to about 0.5-2 kg per acre depending on water depth. Treatment is best done on warm, calm days so the dissolving copper stays in contact with the algal mat rather than drifting with wind-driven currents. Field drainage immediately afterwards reduces re-growth and limits aquatic toxicity.

Adoption context

Wet-establishment systems with extended ponding, including transplanted paddy (Transplanting Paddy) and wet direct-seeded systems (Direct Seeded Rice Broadcast), are most prone to algal blooms, particularly when basal urea is broadcast into stagnant water. Nursery beds (Paddy Nursery Management) and over-fertilised early-tillering fields are common problem spots.

Limitations

Copper sulphate is toxic to fish and other aquatic life above roughly 0.4 ppm, so fields adjacent to ponds, irrigation tanks or pisciculture units require caution and lower rates. Repeated copper use can also accumulate in soil. The longer-term solution is agronomic - draining the standing water, applying nitrogen in split doses, and avoiding the dense calm-water conditions that favour algal proliferation in the first place.

See also: Paddy Weed Management, Paddy Nursery Management, Micronutrient Deficiency Paddy.

References

  1. Copper Sulfate Applications for Filamentous Algae. Oklahoma State University Extension.
  2. Use of bluestone (copper sulfate) for algae control. University of Kentucky Aquaculture.