Soil - Water Processes

The Basics of Soil - Water Dynamics:

It is common saying "The grass is greener on the other side of the fence"Well, that is not absolutely true. The grass is greener where it gets water! But whether or not nice green grass or any vegetation for that matter, depends not only on the water, it depends a lot on the soil, and more specifically, on soil water interactions.

'Hydrological Cycle' describes the continuous cycling of water from the atmosphere to and through the earth and back to the atmosphere. This continuous cycling of water is a vital part of life on earth, and 'soil-water processes' play a key role in it.

But what does soil water processes mean?
Well, when it is raining or there is irrigation, water arrives on the soil surface and then a number of things can happen to that water. To begin with, water will enter the soil by infiltration or stay on the surface and form puddles if it cannot infiltrate. If the soil is on hilly terrain, that water will runoff. Once water has entered the soil, it will spread out and move in multiple directions. This is known as redistribution, and involves percolation (downward movement) and capillarity (this is sideways and upward movement). An important soil-water process  is the function of storage which makes water available for uptake by plant roots and evaporation. Finally, the water that gets into the soil, but is not stored there continues downward as drainage.

Why are these soil water-processes so important?
Plants need water and they also need air and nutrients, water movement through soil is what allows there to be air exchange in the soil too. And it is water that delivers the nutrients which plants need and get mostly by their roots. Soil is the sponge of the Earth, and it needs to be able to both absorb and release water to reduce flooding and also make water available when needed. And Soil is also the filter of the Earth. It purifies water that may contain all kinds of nasties. The soil-water processes I introduced you to are key to all these valuable functions of soil; And that's why they are so important!

How these processes happen in a particular soil is greatly affected by the composition of the soil?
Soil is a mixture of minerals, and organic matter. It may also contain air, water, various organisms, and other substances. Soils connect the atmosphere, biosphere, chemosphere, hydrosphere and lithosphere. Which are  the air above it, the organisms living on top and in it, the chemical substances in it, the rain falling on and flowing through it, and the rocks beneath it. The mixture of minerals in soil consists of three things: sand, silt and clay. Sand is the biggest particle, silt is already much smaller and clay is really, really small - the smallest mineral particle you find in a soil.

Two terms related to the composition of the soil  and which affect soil water processes are 'texture' and 'structure'.

The combination of sand, silt and clay particles is called the soil 'texture'.

The soil 'structure' refers to the arrangement of the soil particles and the pore spaces between them.

Because of all the different particles in the soil, the plant roots growing in there and the activity of soil organisms, soils are very heterogeneous – which means not the same, or not uniform -  by nature.

Both soil texture and structure play an essential role in regulating how much water infiltrates into the soil, how much will run off, how much will be stored and available to plants, and how much will flow towards groundwater.

How much room is available for water to be stored in the soil is known as the soil 'porosity'. And how easily water can move through the soil is known as the soil 'water permeability'. Both of these are also determined by soil texture and structure and to some degree the characteristics of the water itself. For most plants it is important that soil is not too dry, containing no water or too wet, containing mostly or only water. A balanced combination of water and air is absolutely needed for plant growth and food production. So, all these soil and water processes and interactions are pretty important!

It's important to mention that these soil-water processes don't always work the way we expect. They can get disrupted by nature, and also by how we humans manage the land. When this happens by loss of wettability or compaction or soil sealing for example, then there is increased risk of soil degradation. Soil-water processes are vital for soil to function and provide life supporting services;

There are natural and man-made disruptor of these processes that reduce soil function and lead to other problems; Soil water processes can and need to be sustain-ably managed.

Risks to good soil water dynamics:

How and why soil water processes may not always function the way humans expect?

It is explain by the hydrological cycle which describes the movement and storage of water between the air, the soil, the plants and the open water The amount of water remains more or less the same over time, but where that water is depends on a wide range of factors, most of them related to the climate in a particular location. The water cycle is important for ecosystems, life and certainly us human beings and soils have an essential role in it.

Soils store and filter water, and healthy soils are very important to transport water by infiltration, redistribution, percolation and drainage, to ensure plants can have enough water and air to sustain ecosystems and food production.

A dry climate with little rain results in a desert where no or only few plants can grow, there life is hard for people as well. On the other hand, in a wet climate extreme amounts of rainfall may cause the soil to be so full of water it will start to overflow and can then form a mud stream, as you can see here. Soil water processes can be disrupted by nature. For example if the climate is too dry or too wet. But soil water processes can also be disrupted by how we humans manage the land.

So, what kind of natural and human induced things disturb these soil water processes?

• First of all there is loss of soil wettability, which can happen due to soil crusting or the development of soil water repellency. Soil crusting is the formation of a thin layer of dense soil.
• Soil crusts often look like a platy structure, such as in this picture. Soil crusts often form by the impact of raindrops falling on the soil. There are also biological crusts, formed by for example bacteria. Soil crusts make it more difficult for water to enter and wet a soil. 
• Soil water repellency is when the soil will not accept water because waxy coatings have built up - kind of like a raincoat! It develops due to the many organic organisms and substances that are in and on the soil. Water repellency can develop in both cultivated soils and after fires,  which along with burning things cause organic substances to melt and coat the soil. If you would put a drop of water on a water repellent soil it will simply stay there on top of the soil.
• Another soil water process disruptor is soil compaction which can happen when certain soils are put under pressure, like here by a tractor that is used to work on the land.
• Soil sealing can be caused by natural causes as well, such as the lava flow or by humans covering the soil with pavement and buildings. With soil sealing water cannot infiltrate at all and simply collects on the surface or runs off. Because the soil is covered it will not be eroded by wind or water, but all the water that cannot infiltrate can become a problem and unless it is consciously collected, is not available for plants or we humans.

Soil crusting, soil water repellency and soil compaction reduce soil function, and lead to two things.

• First rain water and irrigation cannot infiltrate easily anymore. This leads to ponding of water on the surface when the land is flat, or to increased runoff when the soil is on a slope.
• Second, when water can’t get in, it will be harder for plants to grow which leads to poor quality vegetation and even to bare soil.

What other kinds of things can happen when soil-water processes are disrupted?

Well, if the soil is no longer protected by plants because they cannot root there, the wind can take massive amounts of soil with it. For example In Iceland the erosion of wind is much bigger than the erosion by water.

Here you see a dust storm where vegetation has disappeared while in the front of the picture the vegetation still protects the soil from erosion. And if water cannot properly infiltrate in the soil it may lead to erosion by water.

Here you can see the result of water erosion. A deep gully has formed on land otherwise used for grazing. If nothing is done to prevent further erosion the gully will become larger and larger, while less land for grazing will remain. Erosion can become so bad it removes all fertile soil and a barren landscape is left behind. Plants not growing and water not infiltrating will have a further effect. Plants normally intercept rain water when it falls and soils filter and store that water, but if this is no longer possible, a tremendous amount of the water from a rain storm will run off as soon as it has fallen on the ground. With large rain storms increased run off can lead to floods. Floods may occur especially in hilly areas as all water collects at the lowest point in the landscape, which is where the river flows.

Often towns and cities are located near these rivers, and get flooded as well.

Soil water processes may be disrupted by various natural and human induced causes. As a result of the growing global population, the soil degradation caused by human impacts has increased as well.

Notwithstanding the large global land area affected by soil degradation, there are a lot of soil management options that help to save and restore the soil function and ensure soil water processes function properly again.

The good news is that we can learn about these soil-water process disruptor and troublemakers – both the natural ones and the ones we humans are responsible for; and there are sustainable soil management actions we can take to prevent and/or correct them - and their associated problems.

If we all do this, our soil can continue to function well and support life for many generations to come.

To recapitulate:

• Soil water processes can be disrupted by natural causes and by how we humans manage the land.
• Soil water processes can and need to be sustain-ably managed to provide the life supporting services we all depend on.
• We can learn about soil disruptor and the sustainable soil management actions that can prevent and correct them.