Rainforest soil is generally considered to be poor in terms of fertility. The reasons for this include:

1. Heavy rainfall: Rainforest areas receive heavy rainfall, which can lead to leaching of nutrients from the soil.

2. Rapid decomposition: The warm and humid climate of rainforests leads to rapid decomposition of organic matter, which means that nutrients are quickly released back into the atmosphere.

3. Dense vegetation: The dense vegetation in rainforests means that there is a lot of competition for nutrients among plants. This can further deplete the soil of nutrients.

However, it's important to note that not all rainforest soils are poor in fertility. Some rainforest soils, particularly those in areas with volcanic activity or in floodplains, can be quite fertile. Additionally, rainforest soils can improve over time as organic matter accumulates and is incorporated into the soil.