Optimal growth of most culture plants is only possible if the oxygen availability during the growing season is sufficient. Strongly waterlogged soils with insufficient aeration are only suitable for litter meadows or as wetlands, in which well-adapted plant species predominate. The latter can withstand long periods having little oxygen in the root area and they mostly have little feed value. Most of our wetlands were drained during the last centuries in order to allow intensive usage. In Switzerland strongly waterlogged soils mostly exist as small areas interspersed among other soils.
The determination of the degree of waterlogging of soil is done by digging pits or taking samples by boring, measuring the ground water level in boreholes and also by noting the sorts of plants that grow on the terrain. Blue, grey and green soil hues caused by the presence of iron in reduced form indicate to the expert strongly gleyed soils. Where rustto grey-brown layers up to 50 cm depth are not present most often indicates a strong waterlogging; the ground type then is usually a Gleysol (consisting mainly of mineral material) or a Histosol (FAO, consisting mainly of organic material).
The degree of waterlogging is the most important soil feature in defining soil suitability. Because the degree of waterlogging can vary in such small distances, ground suitability should be assessed taking practical considerations into account at local or regional level by means of scale 1:5000 soil maps, or at least of scale 1:25,000. This point is well illustrated by three soils maps of the same area in the Argovian valley area of the Reuss:

1. soil suitability map scale 1:200,000: «slight ground wetness' (s. Atlas of Switzerland).
2. soil map scale 1:25,000: mostly «no waterlogging», sometimes «damp ground» or «slight ground wetness».
3. soil chart scale 1:5000: «no waterlogging» in ca. half the area, rest slight ground wetness and «damp ground».
   

Map units of the scale 1:200,000 with gleysol or histosol as the most frequent soil form contain as well slightly or non-waterlogged soil forms. The degree of water saturation of a map unit was judged based on the weighted mean (according to the frequency of the existing ground forms). ATTENTION: In reality, the degree of water saturation can vary within one map unit between «non-waterlogged» to «heavily waterlogged».