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Fundamentals of Soil Water

Grassclippings - Soil WaterThe soil underneath your lawn is made up of many elements such as soil air, soil organic matter, soil nutrients, soil micro organisms, soil structures but most of all soil water.

Now I am sure that water is the last thing on your mind currently  but I was explaining the fundamentals of soil water to a client who had a soggy lawn and thought that it would be worth sharing it on this lawn advice blog.

The form water takes in the soil depends on a variety of factors, including the existing moisture content of the soil and the structure and texture of the soil.  Water may be divided into four groups:

RUN OFF WATER - When heavy rainfall occurs, the upper layers of a soil quickly become saturated with water.  Even pounding effects of heavy rain will tend to compact the soil and clog the pore spaces with fine particles.  Because of these effects, considerable amounts of the following rainfall will flow over the surface of the soil as run off water. This flow of water will always carry with it large amounts of suspended soil materials and it is of no use to plants, especially turf grasses. 

The presence of vegetation, leaf litter and sandy top dressings will help protect the soil surfaces against the pounding of raindrops.  All these materials will scatter the raindrops to a fine spray, thus having a less damaging effect on the soil surface.

GRAVITATIONAL WATER - Not all water that penetrates the soil is available to plants.  Gravitational water moves down through a moist soil in response to the pull of gravity, until it reaches the water table or underground streams.  It carries plant nutrients in solution.  Water tables may lie within a few feet of the soil surface or may be deep down.  Sometimes the water is held up by impermeable rock layers to form a perched water table.

Water that percolates through a soil is available to plants as it passes through the root zone, but then passes beyond the reach of the roots fairly quickly depending on the soil texture/content.  The rate is quite fast in sandy soils, with their large particles and large channels, but slow in clays where small pore spaces and large surface areas hold on to the water molecules.

Poorly drained soils are unsatisfactory for plant growth because the gravitational water occupies the larger pore spaces and excludes the entry of air.  In well-drained soils gravitational water causes the leaching of nutrients such as nitrates and potassium.  The presence of plant roots helps reduce these losses because they absorb some ions and they provide the organic matter that absorbs ions.

CAPILLARY WATER - As capillary water drains from a soil it leaves behind considerable amounts of water in moisture form, held to the soil surface and filling the smaller pores and channels.  It is the main source of moisture for plants as it is held loosely in the narrow pore spaces and can be readily absorbed. 

HYGROSCOPIC WATER - This water is very strongly held in the small pore spaces and forms a fine film around the particle surfaces of clays and humus.  The plant root pressure (suction power) is not powerful enough to attract the water molecules and so this water is mainly unavailable to the plants.

Available water is primarily capillary water.  It is high in clay soils and low in sandy soils.  Organic matter is more effective than clay at retaining available water.

Soil water types are not sharply separated, but form a continuous series from water that is not retained by the soil to water that is retained with considerable force or tension.  The type of water present depends on the kind of soil.

During rainfall, and shortly after, some gravitational water is available to plants, but only for a short time.  The first additions of capillary water are easily taken up, but it then becomes more difficult simply because the water lays closer to the soil surfaces and is more tightly held.  At this stage, capillary water merges with hygroscopic water. 

Evaporation processes, from the soil, lose much of the available water not taken up by plants.  Losses in this way are lower when surface layers have dried and capillary movements start to take place.  More moisture is lost by transpiration (evaporation from plant leaves) than from the soil because plant roots tap water supplies lower down in the soil profile.

One of the most important factors involved with water availability is root growth.  During periods of high transpiration rates, water may be taken up so rapidly that absorbing root hairs may be surrounded by relatively dry soil, from which more water cannot be absorbed.  The plant will require further water by irrigation or it will rely on roots growth extensions, which grow into new areas where moisture is available.

The force by which water is held in the soil can be measured by quantifying tension.  The tension is measured in bars, one bar equaling 1 atmosphere of pressure.  For example, gravitational water is held at less than 0.2 bars, capillary water at between 0.4 – 15 bars and hygroscopic water at 15+ bars.  The higher the figure, the more firmly the water is held. Roots can exert a tension of up to a15 bars, evaporation exerts a force of up to 30 bars and gravity exerts a tension 0.33 bars.

Surface tension – this is the attraction between water and soil particles as a result of the surface tension at the curved interface between air and water in the soil pores.

Capillary force – this is the attraction of water molecules to each other through hydrogen bonding.  Capillary force has a higher energy content than that holding Hygroscopic water - capillary water moves very slowly by film adjustment from areas of low soil moisture tension to areas of high moisture tension.

Osmotic force – this is due to the concentration of ions either in the soil solution or on the surface of the soil particles.  Osmotic pressure arises as a result of two unequal solutions, separated by a semi-permeable membrane.  Plant roots provide this membrane, which is very permeable to water but only partially permeable to ions.

Plant roots take up water from the film around the soil particles and to do this they must exert a force greater than that holding the water to the film.  This force, or suction, is known as the soil moisture tension and can be measured by a Tensiometer.  In an unsaturated soil, the soil water pressure is negative; in a saturated soil, the pressure is either zero or positive.

Soil moisture tension is related to the soil moisture content; the tension increasing as the soil moisture content decreases


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