Water- that weird substances

Water has many weird properties apart from being one of the few substances which expands when it freezes so ice floats and pipes burst.

Surface tension

It has an affinity for some substances – particularly most soil and a total hatred of others – like waxy leaves.

When we pour a bit of water onto soil we might expect that is would just flow straight down through the soil and keep on going. But is does not – it so loves soil that is will hang onto the soil particles. This is more than lucky as nothing would grow otherwise and there would be no life on earth.

But is does give us a bit of a problem if we want a bed with a nice uniform Goldilocks moisture level. If we just add the exact amount of water to get that Goldilocks level it will just stay put and not move so we end up with some bits of the soil very wet while other bits are dry.

We need to find a way of getting that uniform moisture level spread over all the soil by studying how water moves through the soil.

Gravity

One way is gravity – which seems to work well on most days (other then Friday nights). If we put enough water onto the soil it will become so saturated that it will overcome waters love of soil and will flow downwards. Faster if there are any cracks in the soil.

Hydraulic or pressure flow

But if there is some restriction to this downward flow, say from a bit of plastic film or clay then pressure will build up and the water will flow sideways. This is an important mechanism in Wicking beds spreading the flow over the base.

If the soil is drier above this wet layer the water will wick upwards by surface tension but there is always a gradient wetter at the base – drier at higher levels and at some distance it will just stop.

Pic water mover downwards, sideways then upwards.

Wicking is a slow action so it takes some time for the water to wet the soil – this is important to understand in operating a Gbiota Wicking Bed as that lower wet layer must be held wet for long enough for this upwards Wicking to occur.

The resistance to hydraulic flow in soil can be quite high so the water may not move that far. That is why we need a pipe to distribute the water along the base of the bed and a relatively porous layer on the bottom for sideways movement.

Osmosis

There is more to come – water enters the root zone by a process called osmosis in which water will always move from a weak solution to a stronger one. Typically the solution in the roots is stronger than in the soil so water is sucked into the roots.

Of course if the solution in the soil is stronger than in the plant – water will be sucked out of the plant – and it will die (so stop peeing on the lawn and those yellow spots will go away).

Osmosis is very important in Gbiota beds as this is how the tiny microbes we want in our gut enter the roots from the soil and into the plant and then into our gut.

Diurnal flow

Water does not get to the top of tall trees by wicking or osmosis but by another mechanism. As water evaporates from the leaves it will create tension in the water. We think of water as a liquid but one of its weird properties is that it has tensile strength just like a steel rope so water is literally pulled up by the energy of the sun to great heights.

At night the sun generally goes down (does in my place anyway) so this powerful evaporation stops and the water will flow back into the root system and soil. But is will not necessarily go the same way it came up so water may be sucked up from one wet spot then flow back to another drier area.

This day and night cycling is called diurnal flow.

Brownian movement

One more to go. If you have a wet area in the soil then water will evaporate from the surface of the liquid water forming saturated water vapour in the air. Water vapour molecules are active little beasts buzzing around all over the place and if they happen to find some nice friendly soil particles (hydrophilic) they will condense and wet that bit of soil while the molecules that have just condensed are replaced by fresh water molecules from the liquid water surface.

This is the way water can move from a wet area to a dry area – just water molecules bombing around at random.

This is the way many so called Wicking Beds with coarse gravel work – they are not Wicking beds at all as there is no significant Wicking action with gravel. They should really be called Brownian beds (after Dr Brown who was really into little molecules dashing about at random) or to be posh Pedesis beds (why do scientists keep on inventing weird words normal people have never heard of?).

 Epiphytic plants

 Don’t dismiss this mechanism as some scientific oddity. Some plant, the epiphytic plants, just let there roots hang out in the air, some unsuspecting water molecules just happens to be passing bye and sees this nice friendly root which being hydrophilic looks a very attractive spot for the water molecule to take a bit of a rest. So it just settled down but is immediately picked up by the root to give the plant a bit of a drink.

This can be a significant source of water for plants living in the dank undergrowth of forests with a high humidity. Plant that naturally grow there (like orchids) get a lot of their moisture from the air and have a rather pathetic soil root system so they quickly die if you try and grow them in a dry hot desert air.

But don’t think this is just a feature of the forests. Deserts may be hot in the day but can get very cold at night so the humidity may be high – but may be not high enough to cause condensation but certainly high enough for the specially adapted desert plant to catch the water molecules by their hydrophilic surfaces.

The ability of plants to take water from the atmosphere is an important consideration. I recommend growers consider using plants on the edges of Gbiota beds to reduce evaporation from the beds and increase local humidity.

 Inertia

The great challenge facing humanity today is how to grow food of the needed quality in the times of climate extremes of floods and droughts. Normally in growing plants water is moving very slowly so we don’t have to think about inertia.

But water is heavy and when we get these torrential rains then the inertia of water becomes a serious issue. Many people (and I admit myself) has built there growing area with proper drainage thinking that this would protect them from these torrential rains.

But inertia has different ideas and if there is sufficient gradient up stream the water will pick up so much inertia it will just barge ahead – even uphill – to flood growing areas that may reasonably the thought flood proof.

It is just a fact that we have to go through a difficult learning period learning to deal with the effects of climate change.

Now we have the theory we can look at how Gbiota Wicking Beds work.