Soil in the tropics is often nutrient-poor because a. nutrients are tied up in plants. b. it is mostly sand. c. it has a high pH. d. rainfall leaches out minerals.

Let's go through the options one by one: a. nutrients are tied up in plants It's true that plants absorb nutrients from the soil, but this does not necessarily make the soil nutrient-poor. b. it is mostly sand Sandy soil can be nutrient-poor, but mentioning that tropical soil is mostly sand is an overgeneralization because it can vary depending on the specific location. c. it has a high pH A high pH can affect nutrient availability in the soil, making some of them less available for plants. However, the pH of tropical soils can vary greatly, which makes this option not a comprehensive explanation. d. rainfall leaches out minerals Heavy rainfall can cause leaching of nutrients and minerals in the soil, making it nutrient-poor. Tropical regions often experience heavy and frequent rainfall, which can lead to this issue.

Since tropical regions are known for heavy rainfall and the leaching process, the correct answer is: d. rainfall leaches out minerals