Fluvisol (alluvial soil)

A fluvisol (fluvizem, alluvial soil) forms from sediments that a river or stream deposits during floods on its floodplain. It typically has a layered profile and humus content that decreases irregularly with depth, because the sediments were deposited gradually. In the international WRB classification it corresponds to Fluvisols.

How to recognise it in the BPEJ code

Under Decree No. 227/2018 Coll., fluvisols are indicated in the BPEJ code by main soil units 55 to 59:

• 55: light fluvisols on light floodplain deposits, often over sand and gravel
• 56: medium-textured fluvisols on deep floodplain deposits
• 57: heavy and very heavy fluvisols
• 58 and 59: gleyic and gleyed fluvisols with unfavourable moisture conditions

In wetter floodplains fluvisols grade into gleysols, which are shaped by a high groundwater table. Unit 70 covers wide floodplains, unit 71 narrow floodplains within 50 m of the stream, which according to the decree often suffer from flooding.

What it means for a buyer

Agriculturally these are often very valuable soils: under Decree No. 48/2011 Coll. all codes with unit 56 fall into protection class I and units 57 and 58 mostly into class II. For a garden this is good soil.

For a house, though, alluvial soil is a signal: the plot lies in a floodplain, land the watercourse has flooded in the past. Check the flood zone map, the Q100 flood line and the groundwater level, which tends to be close to the surface in floodplains, especially with gleyic fluvisols and gleysols.

Watch out for

Alluvial soil does not automatically mean a designated flood zone. The stream may be regulated or the floodplain may lie outside the designated area. The extent of a flood zone is set by the water authority on the proposal of the stream manager; in built-up areas and development areas it also defines an active zone. What counts is this map and the flood history of the place, not the BPEJ code itself.