There isn't a single "softest" wood, as "softness" is relative and depends on how it's measured. However, Balsa wood (_Ochroma pyramidale_) is widely considered one of the softest woods in the world.

Here's why:

* Density: Balsa wood has an extremely low density, making it very lightweight and easy to compress.

* Cellular Structure: It has large, thin-walled cells, contributing to its softness and buoyancy.

Important Note: While Balsa is very soft, its lightness and strength-to-weight ratio make it useful for various purposes like model airplanes, surfboards, and insulation.

Keep in mind that other woods like Paulownia (Paulownia tomentosa) and Liriodendron tulipifera (Tulip Poplar) are also very light and soft. Ultimately, the "softest" wood depends on the specific application and desired properties.