Soil water

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Soil water:
(1) supporting meniscus
(2) soil particles
(3) air bubbles in capillary water
(4) capillary water
(5) adsorption
water (6) groundwater or backwater

Soil water is all the water present in the soil with the exception of the crystal water of the soil minerals .

The soil water is determined gravimetrically (weighing) by drying the soil at 105 ° C. The water of crystallization remaining after drying is added to the solid soil mass.

In the pore space of the soil, a distinction is made between freely moving water ( seepage water ), retained water held in the pores against gravity ( capillary water , adsorption water ), and backwater that is prevented from seeping away by a retention horizon above 1.3 m depth .

Soil water

Water is an essential part of soils. Only the water-containing soil is able to chemically weather, provide nutrients in dissolved form to the plant roots and produce organic matter. Cultivation measures can influence the water balance within a natural framework and adapt it to the needs of the cultivated plants . Because the soil water participates in the water cycle in the landscape, effects on the groundwater and surface water must be taken into account.


Precipitation that falls on the ground either runs off as surface water or seeps away. Part of the seepage water is held by the soil against gravity as adhesive water . Adhesive water surrounds the soil particles as microscopically thin shells ( adsorption water ) and fills the network of finer pores in the soil ( capillary water ). The amount of water that a soil can hold against gravity is called field capacity . The seepage water reaches the groundwater sooner or later, primarily through the system of coarser pores . All pores in the groundwater are constantly filled with water. If the seepage is hindered by water-retaining layers at a shallow depth, backwater forms . Plants cover their water needs from the retained water or the capillary rising groundwater or backwater. However, they can only use the part of the adhesive water that their roots can tear away from the soil with their suction forces. This share is called the usable field capacity (nFK). It includes the soil water in the central pores (0.0002–0.01 mm Ø) and the slowly draining coarse pores (0.01–0.05 mm Ø). The portion bound in the fine pores (Ø less than 0.0002 mm), which is no longer available to plants for crops, is called dead water . The water content of the soil at which all the water available to plants has been used up and the plants begin to dry up is called the permanent wilting point . It is a characteristic of every soil. The water that is available to the cultivated plants as usable field capacity in the root area can be specified in mm or l / m² , as with precipitation .

Water movement in the ground

Soil water cycle

If it rains on dry soil , the soil can initially saturate up to its field capacity . Requirement is,

  • that the soil surface is protected from the impact of raindrops by vegetation or mulch to such an extent that the pores on the soil surface are not silted up and thus sealed ,
  • that water that has penetrated the soil is not prevented from seeping away by damaged soil layers (e.g. impermeable plow floors ).

Suitable soil cultivation measures help ensure that precipitation can actually seep away. When the surface becomes silted up and compacted, rainwater that runs off the surface is lost to the plants and can lead to soil destruction through erosion and flooding in the floodplains. When the root space is filled with water, water seeps into deeper layers and then contributes to the formation of new groundwater. In the event of heavy rainfall, part of the rainwater can seep away through continuous macropores , i.e. tunnels of soil animals, root tubes or cracks in the soil, before the soil storage is filled. However, this macropore flow usually ends at a depth that can still be reached by the plant roots. A continuous network of pores of sufficient size ( pore continuity ) is a prerequisite for the water pathability of soils . Abrupt pore size change, as it occurs e.g. B. occurs between roughly worked crumb and grown subsoil, hinders the seepage. This change can lead to temporary overtaking and inability to drive after precipitation. The same phenomenon occurs when fine-pored layers (e.g. clay ) lie on top of coarse- pored layers (e.g. gravel ) because the pore system of the clay layer , which is characterized by medium pores , does not connect to the coarse pore system of the gravel layer.

Water movement in the saturated state

Especially in cultivation technology, water permeability in the saturated state plays a decisive role in assessing the necessity, the type and the chances of success of a drainage as well as the dimensioning of the drainage distances and depths. Soils with a high proportion of coarse pores, such as B. sandy soils, well-structured mineral soils with high earthworm population and little decomposed peat soils.

Water movement in the unsaturated state

The movement of water in the unsaturated state is slower the drier the soil, i.e. H. the smaller the diameter of the water-filled pores becomes. In loess soils, seepage water moves below the root space in the central pores and the slowly draining coarse pores by a few decimetres to one meter in the direction of the groundwater when the water balance is positive. If the soil dries out, the capillary pores can suck in water from more humid or water-filled soil layers against gravity. The speed and maximum height of the capillary water ascent are primarily dependent on the grain size composition of the soil. Water quantities of 5 l / m² per day rise in uncompacted silt soils up to 85 cm, in coarse sand and clay soils only approx. 20–30 cm above the water table. The prerequisite for this is pore continuity.

Water balance

The water content of the soil is constantly changing. On the income side are the precipitation, under special conditions slope and groundwater influx. On the expenditure side are runoff, seepage , evaporation and evaporation . The soil acts as a buffer in this water balance equation. He can absorb and store surpluses on the income side (about the amount of the NFC) and thus compensate for deficits. In the course of winter, the soils generally become saturated until they reach field capacity . With the warming up and the onset of plant growth in spring, the soil begins to dry out from above. The plants draw from the water supply in the soil. Water deficiency symptoms appear long before the wilting point is reached. In irrigation farms is therefore usually already with the irrigation started when the nFK has fallen below 70%. The less the precipitation falling during the vegetation period (in terms of quantity and distribution) can cover the plants' water requirements, the more the usable field capacity determines the yield.

Regulation of the water balance

A considerable proportion of the cultivated land in Central Europe was only usable through amelioration measures . The first priority was to regulate the water conditions. Today it is no longer necessary to intensify agricultural use of the remaining wetlands for food security . Conservation concerns have priority on these areas. Existing drainage systems must, however, be maintained and, if necessary, replaced if the soil's productivity is to be maintained.

Soil wetness

Soil wetness can be caused either by groundwater or by backwater . To eliminate ground moisture is the water table lowered. In waterlogged soils ( Pseudogley ) which aims amelioration from thereon to derive the backwaters and increase water permeability and water storage capacity of the bluff body. A prerequisite for any water drainage is the presence of a sufficient receiving water (natural waters, ditches).

Pipe drainage

It is the common method for draining groundwater-soaked gley soils. Their goal is to lower the water table to a depth that allows optimal plant growth. For arable land this depth is around 80–120 cm, for grassland around 40–80 cm. The desired lowering of the groundwater level is achieved by means of a drainage depth that is adapted to the permeability of the soil and the corresponding drainage distance. In the case of permeable soils, a drainage depth of 1.0–1.2 m is usual, with less permeable soils of 0.8–1.0 m. If the drainage depth is shallow, the drainage must be tighter in order to achieve the same effect as in the case of deeper drainage pipes. The groundwater level arches between the drainage lines (suckers), the higher the less water-permeable the soil is. The higher the bluff body, the closer the drainage distance must be. In waterlogged soils, the impermeable layers are often so high that sufficient water drainage would require uneconomically narrow drainage distances. In these cases, and in soils with poor water permeability, a combined drainage is carried out, i.e. H. Pipe drainage in combination with a pipe-less mole drainage or with deep loosening .

Pipeless drainage

With this drainage , a loosening coulter with a pressing head is pulled through the soil and forms a pipe (earth drain). The depth of these tubes is about 60 cm, their distance about 2 m. A sufficiently plastic, clay-rich soil is a prerequisite for the formation and maintenance of the tube. The functionality of the tubes is limited even under these circumstances.


This means the mechanical breaking up of dense and water-retaining layers of the deeper subsoil from a depth of approx. 40 cm. Single and multi-armed devices with rigid or movable loosening shares are used for loosening measures. Common working depths of 70–80 cm require enormous traction . In order to stabilize the loosened soil , deep loosening is often combined with amelioration liming. Because of the unfavorable cost: benefit ratio, deep loosening is rarely carried out today .

See also


  • Udo Quentin, Johannes G. Schwerdtle: Drainage in agriculture. 1st edition. DLG Verlag, Frankfurt am Main 2013, ISBN 978-3-7690-2029-8 .

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