The size of the soil particles will affect the level of porosity within the soil. Porosity is the pore space or small voids between the solid particles that contain either water or air. These pores or voids can be due to movement of worms, roots, expanding gases and soil additives.
There are two main categories of soil porosity: micropores and macropores. These categories are based of the pore size or spaces. Depending on the size we can predict the levels of oxygen, nutrients, and the ability to retain water. The optimal porosity for plant growth is a ratio of both micro and macro-pores.
Wool pellets expand as they absorb water creating a space. That not only increase oxygen but holds the nutrients in the soil for root development. As they begin to biodegrade that space is replaced by air, increasing soil oxygen levels.
Introducing worms to the soil will increase the porosity of the top inches of your soil. As they create deeper tunnels in the soil it will also help with drainage preventing root rot.
Consider adding lettuce that can be harvested at ground level and the root system left. Once harvested the roots will die off leaving spaces for water, oxygen, and other nutrients to fill their space.