* Not too compact: Air and water can move easily through the soil, allowing for good drainage and aeration.

* Not too loose: It holds together well enough to support plant roots and prevent erosion.

* A balance of different particle sizes: This allows for good water retention and nutrient availability.

Here's a more detailed explanation:

* Crumb structure: Fertile soil is made up of small clumps called aggregates or crumbs. These crumbs are formed by the interaction of soil particles with organic matter, microorganisms, and other soil components. They create small air pockets within the soil, allowing for good aeration and drainage.

* Porosity: The spaces between soil particles and aggregates make up the pore space. This space allows for air and water to circulate, which is essential for plant growth.

* Texture: The texture of soil refers to the relative proportions of sand, silt, and clay particles. Fertile soils typically have a balanced mixture of these particles, which contributes to the crumb structure and good drainage.

A good way to test the texture of fertile soil is to squeeze a handful in your hand. It should hold its shape when compressed, but crumble easily when released.