Here's why:

* Porosity is the amount of empty space within a soil.

* Sand particles are much larger than silt or clay particles. This means there are larger gaps between sand grains, leading to a higher percentage of empty space.

* Clay soils have the lowest porosity because clay particles are very small and tightly packed, leaving little room for air or water.

* Loam soils fall somewhere in between, with a moderate level of porosity.

Important Note: While sandy soil has the highest porosity, it doesn't necessarily mean it holds the most water. Water retention is also influenced by other factors like particle size distribution and soil structure.