Abstract1. Root room is defined and its downward limitation due to the occurrence of different kinds of impediments in the soil profile is discussed. 2. The main avenues for root penetration are enumerated and explained. 3. The quantitative relationship between particle size and effective pore space is stated and its application to the selection of sandy media for rooting cuttings and hydroponics is considered. 4. The geological origin of sandy soils having adequate porosity for supporting crop plants is discussed. 5. The determination of pore space by the measurement of apparent and true specific gravity is described. 6. The usefulness of sticky point constant as an approximate measure of capillary pore space in investigations of soil aeration and root room is discussed. An example of its application to a cacao soil profile is explained.