Chalk and chalk debris as a medium for plant growth, with particular reference to Channel Tunnel spoil

. Four lithological types of chalk are recognized. Normal white chalk has 34–50% porosity, nearly all of which holds water conventionally‘available’to plants. Hard chalk and grey chalk have less porosity and available water; chalk marl has very little available water. Significant capillary rise from below the root zone can be expected in normal white chalk only, and is not halted by fragmentation. Chalk has little N and usually little P. Only grey chalk and chalk marl have much ettractable K and Mg. These also slake, impeding roots. Thus, fertilized plants tolerant of high pH can grow well on normal white chalk debris, but on raw chalk marl, e.g. Channel Tunnel spoil, they need irrigation. Interesting plants volunteer on chalk debris, even on slaked chalk marl by the sea.