Explanation:

Organic farming is a type of agricultural practice that focuses on the use of natural materials and methods to improve soil fertility, control pests, and promote plant growth. It avoids the use of synthetic chemicals such as pesticides, fertilizers, and genetically modified organisms (GMOs).

Organisms used in organic farming:

1. Glomus: Glomus is a type of mycorrhizal fungus that forms a symbiotic relationship with plant roots. It helps in nutrient absorption, particularly phosphorus, and enhances the plant's resistance to diseases. The fungus also improves soil structure and increases water-holding capacity.

2. Oscillatoria: Oscillatoria is a filamentous cyanobacteria commonly found in water bodies. It is used in organic farming as a biofertilizer. Cyanobacteria fix atmospheric nitrogen and convert it into a usable form for plants, thereby increasing soil fertility. Oscillatoria also helps in improving soil structure and water retention.

3. Earthworm: Earthworms play a crucial role in organic farming as they enhance soil fertility through their burrowing activities and the breakdown of organic matter. They consume organic materials such as plant debris, animal manure, and microorganisms, and excrete nutrient-rich castings that improve soil structure, aeration, and nutrient availability.

Organism not used in organic farming:

1. Snail: Snails are not typically used in organic farming practices. While snails can be beneficial in some ecosystems, they are often considered pests in agricultural settings. They can cause damage to crops by feeding on plant leaves and stems, leading to reduced yields. Therefore, snails are not intentionally introduced or encouraged in organic farming.

In conclusion, snails are not used in organic farming due to their potential for crop damage, while Glomus, Oscillatoria, and earthworms are beneficial organisms that improve soil fertility and plant health.