Moisture retention of a medium is determined by particle size, shape, and porosity. Water is retained on the surface of the particles and within the pore space. The smaller the particles, the closer they pack, the greater the surface area and pore space, and hence the greater the water retention. Irregular-shaped particles have a greater surface area and hence higher water retention than smooth, round particles. Porous materials can store water within the particles themselves; therefore, water retention is high. While the medium must be capable of good water retention, it must also be capable of good drainage. Therefore, excessively ne materials must be avoided so as to prevent excessive water retention and lack of oxygen movement within the medium.