how_water_moves.htm  

How water moves through the soil

Colin Austin 13 Jan 2014

While the physical construction of the bed may not be so  important it is essential to follow certain rules to ensure the water moves in the desired way.  This can easily be achieved by following the advice later but it does help to understand the mechanics of how water moves through the soil. This may seem such a simple subject it is not worth discussing but it is far more complex than at first meets the eye. If you have no interest in the underlying theory behind wicking beds just skip this section.

Gravity

Let’s talk about the forces action of a drop of water. The first and most obvious force is gravity.  Most people would accept that this is a universal and constant force although I have noticed that on Friday nights the laws of gravity can appear to be suspended temporarily but by Saturday morning they are back to normal.

It would be easy to think that gravity would just keep on pulling a drop of water down all the time, and that is true but there are other forces which act on this drop of water which means it rarely goes straight down.

Hydraulic flow

Water is a viscous liquid, like honey or oil but the viscosity is very much lower.  In large channels the effect of viscosity is generally pretty negligible but as the flow channel gets smaller the effect of viscosity become more significant. Water flowing through a long pipe thin pipe, like a typical garden hose has significant resistance to flow from viscosity but when it gets to soil the flow channels can be extremely small so the resistance becomes very high.

Fill a conventional flower pot with sand and pour water onto the sand it will only take a short time for the water to flow through, but fill the pot with clay and it will seem to take for ever, this comes from the laws of flow, halving the size of the channel can increase the resistance by sixteen times (it is a bit more complicated than that but the message is clear small holes equals a lot of resistance). Clay has much finer pores sizes than sand so the resistance to flow is much higher.

Soil is rarely uniform, typically it gets more compacted the deeper we go and also it has many channels through the soil from old plants and disturbance.  When we pour water on the surface it may go through the top layer but then will meet higher resistance so will build up pressure.  This is the second force that affects how water moves through the soil – hydraulic pressure. Pressure acts equally in all direction so if the water meets resistance some water will start to move horizontally when it may find a fissure or crack in the soil then under hydraulic pressure it will flow down the crack.

The result is that some soil will remain dry while other areas will become saturated with water.

Surface tension

The third method that water moves through the soil is by surface tension.  Water is attracted to some soil surfaces so will spread from particle to particle.  The size of this surface tension force depends on the surface area, the bigger the area the greater the force so a soil with very fine particles with a high surface area will have a high surface tension affect and a coarse soil with large particles will have much lower forces.

Not all soils attract water, some repel water and are very difficult to wet out, this applies particularly to soils with oils in then so soils with gum leaves can be highly water repellent of or hydrophobic.

To grow plants we want the water to spread through the soil, in drip irrigation we apply the water very slowly so this wicking action has a chance to spread the water.  In flood irrigation we use exactly the opposite approach applying as fast as we can so it does not have a chance to soak deep into the ground and will spread by the hydraulic flow.

In a wicking bed with small particles water moves by surface tension or wicking.

Evaporation and condensation

If we looked at a jar with a lid partly filled with water the water level would remain the same.  If we took the lid off the water level would slowly drop as the water evaporates.  With the lid on the water is still evaporating but is recondensing and returning the water.  In the soil we have cavities, for examples if there are stones in sitting in a water reservoir the water will evaporated from the surface to form water vapour, if there is a layer of dry soil the water vapour will condense and be absorbed by the dry soil so there is a slow movement of water from the reservoir to the dry soil.

Osmotic pressure

Osmotic pressure is when water is trying to move from a weak solution to a concentrated solution.  It is the normal way for water to enter the root system but the concentration of the solution in the plants roots must be higher than in the surrounding soil.  If too much fertiliser is applied or the soil is saline water will be sucked out of the roots and the plants will wilt and eventually die.

Water tension

Water is extremely weird stuff.  The molecules have a great attraction for other water molecules; this is seen in the way water from drops. In a plant water evaporated from the leaves.  This literally pulls water from lower in the plant and is the basic way that plants pull water up to great heights like large trees.  Water tension is far greater than the other forces like osmosis and surface tension.

Diurnal movement by plants

During the day plants are continuously pulling the water out of the soil but at night evaporation stops so the water held by tension in the plants drops back down to the soil. But is does not necessarily follow the path is took on its upward journey so will pass water to other roots.  For example you may have a tree near a river, during the day the tree pull water up from the river but at night some water will be deposited on the other side of the tree and enter the soil away from the river.  This may in turn be picked up by other trees in a band away from the river which will in turn move some water even further away from the river.

The trees are all working together to move water.  That is why if you chop down trees near the river then trees further may may die.

Mycorrhizal fungi

Mycorrhizal fungi are one of the wonders of the natural world.  It can form vast underground webs stretching from plant to plant. They will transfer water from one plant to the next and will even transfer chemical signals for examples warning other plants of insect attacks.

You can have one deep rooted plant like a Senna Alata or a tomato with a deep root system and it will draw water up down in the soil and some of this water can be shared with other shallow rooted plants.