Explanation:

Loamy soil is a type of soil that is composed of a balanced mixture of sand, silt, and clay particles. It is considered to be the ideal type of soil for gardening and agriculture due to its ability to retain moisture while still allowing for proper drainage.

Sand:
- Sand particles are the largest among the three types of soil particles.
- It has large spaces between the particles, allowing water to drain quickly.
- Sand particles do not retain moisture very well.
- However, when mixed with silt and clay particles, it contributes to the overall texture and structure of the soil, making it more loamy.

Silt:
- Silt particles are smaller than sand particles but larger than clay particles.
- Silt is known for its ability to retain moisture and nutrients.
- It has moderate water drainage capabilities.
- When mixed with sand and clay particles, it helps improve the texture and fertility of the soil.

Gravel:
- Gravel is composed of larger-sized rock fragments.
- It is not a soil particle and does not contribute to the soil's texture or fertility.
- Gravel has poor water-holding capacity and drains quickly.
- It is not suitable for loamy soil composition and is considered the odd one out in the given options.

Clay:
- Clay particles are the smallest among the three types of soil particles.
- Clay has excellent water-holding capacity but poor drainage.
- It can become compacted easily, resulting in poor aeration and root growth.
- When mixed with sand and silt particles, it helps improve the soil's structure and nutrient-holding capacity.

Conclusion:
Considering the characteristics of loamy soil, the odd one out in the given options is Gravel. Gravel is not a soil particle and does not contribute to the soil's texture, fertility, or water-holding capacity.