Verification

First, make sure that you are indeed dealing with fungus gnats. They are small, black, and resemble tiny mosquitoes.
Their larvae are tiny with a shiny black head and an elongated, white to clear body.

Storage

Always store your soil amendments in a cool, dry place. Make sure the storage container is sealed well to prevent moisture and pests from entering.

Monitoring

Place yellow sticky traps near the affected area. This will help you monitor the extent of the infestation and also catch adult gnats.

Non-Toxic Treatments

Diatomaceous Earth (DE)

1. Sprinkle a thin layer of food-grade diatomaceous earth over the surface of the amendment.
2. The DE works by dehydrating and physically injuring the insects, thus killing them over time. It should not harm beneficial microbes.

Beneficial Nematodes

1. Purchase beneficial nematodes (Steinernema feltiae) from a reputable supplier.
2. Follow the instructions to mix them with water.
3. Drench the amendment with the nematode mixture.
4. The nematodes will seek out and kill gnat larvae but won't harm beneficial microbes.

Neem Oil Drench

1. Create a mixture of neem oil and water according to the instructions on the neem oil package.
2. Drench the amendment with the neem oil mixture.
3. Neem oil is organic and should not harm beneficial microbes, but it will disrupt the life cycle of the gnats.

Biological Control

Bacillus thuringiensis var. israelensis (Bti)

1. Purchase a Bti-based product from a gardening supply store. Or buy Mosquito Dunks on Amazon
2. Mix it with water according to the package instructions.
3. Drench the amendment with the Bti mixture.
4. Bti is a natural bacterium that is toxic to the larvae of fungus gnats but safe for beneficial microbes.

Follow-Up

Monitoring

Continue to use sticky traps to monitor the situation. You should see a reduction in the number of adult gnats caught over time.

Repeat Treatments

If necessary, repeat the treatments until you are certain that the problem has been eradicated.

Maintain Dry Conditions

Allow the top layer of the amendment to dry out between waterings. Fungus gnats thrive in moist conditions, so letting the surface dry can help prevent future infestations.