Tropical deserts are mostly found in the western margins of the country because the rainfall rate of that region is well to support the growth of tropical forests. The fertility of the soil is also good.

Tropical deserts are ecosystems characterized by arid and dry conditions, with very little rainfall and high temperatures throughout the year. These deserts are typically located in specific regions around the world, and in the case of the given question, they are found in the western margins of the country.

1. Tropical Desert Characteristics:
- Low precipitation: Tropical deserts receive very little rainfall, usually less than 10 inches (250 mm) per year. This lack of rainfall leads to arid conditions and limited water availability.
- High temperatures: These deserts are known for their hot climates, with temperatures often exceeding 100°F (38°C) during the day.
- Sparse vegetation: Due to the lack of water, vegetation in tropical deserts is limited and consists mostly of drought-resistant plants, such as cacti and succulents.
- Sand dunes and rocky terrain: The landscape of tropical deserts is often characterized by sand dunes, rocky outcrops, and sparse vegetation cover.

2. Western Margins of the Country:
- The question specifies that tropical deserts are found in the western margins of the country. This means that the country in question has a westward-facing coastline or border.
- In general, western margins of countries are more likely to experience arid conditions due to the presence of prevailing winds and ocean currents. These factors can contribute to the formation of deserts along the western coastlines.
- For example, the western margins of countries like the United States (e.g., the Mojave Desert) and Australia (e.g., the Great Sandy Desert) are known for their tropical desert ecosystems.

3. Factors Affecting Desert Formation:
- There are several factors that contribute to the formation of deserts, including global wind patterns, topography, and proximity to large bodies of water.
- In the case of western margins, the prevailing winds (such as the trade winds) tend to blow from east to west. As these winds move across the land, they lose their moisture content, resulting in arid conditions.
- Additionally, the presence of mountain ranges along the western margins can create rain shadows, where the mountains block the moisture-laden winds from reaching the inland areas, further exacerbating aridity.
- Ocean currents, such as the cold California Current along the western coast of the United States, can also contribute to aridity by cooling the air and reducing its moisture-holding capacity.

In conclusion, tropical deserts are found in the western margins of the country due to the combined influence of prevailing winds, topography, and ocean currents. These factors create arid conditions with low rainfall and high temperatures, which are characteristic of tropical desert ecosystems.