lime in the forms of slaked lime (calcium hydroxide) or ground limestone (calcium carbonate), may be added to soils to neutralize soil acidity. Ground limestone is immobile in the soil, except in light sands, and must therefore be thoroughly incorporated to be fully effective. Liming is therefore most appropriately used, if needed, before vine planting. It is easy to achieve in the topsoil but very difficult in the subsoil (below 50cm/20 in). The calcium in lime or limestone also helps to give the soil greater crumb structure and friability (see soil structure). Gypsum (calcium sulfate) can be used for the latter purpose on soils where acidity is at an appropriate level. One common problem of over-liming is lime-induced chlorosis (iron is involved in photosynthesis and deficiency results in yellowing of leaves). See also lime, active.