When it comes to soil, space matters. The gaps between soil particles and aggregates provide channels for aeration, hydration and living space for organisms.

Soil porosity is a measure of this space between the particles. It is usually measured in cm^3 per cm^3. This is the volume of pores in cubic centimetres per cubic centimetre of total soil volume.

Porosity can vary massively between soil types. Take a clay for example, in this soil the pores are very small and not very well connected. When it rains the water takes some time to drain through the soil. This leads to features such as mottling – the green of reduced iron oxides speckled with patches of red oxidised iron – as water is not evenly distributed. Another issue with tight packed, non-porous clays is the difficulty roots have to penetrate through. It takes a water-loving plant with a shallow and efficient rooting system to thrive in clay. A sandy soil on the other hand has large and well connected pores for water, gas, plants and animals to travel through easily.

Soil pores can be full of either air or water or a mixture of the two. The water-filled pore space (WFPS) is a useful measure of how much of your soil’s pore volume is full of water. Even very dry soils retain some residual moisture, which clings to the particles in a film as a result of surface tension. This slick particle surface is the ideal living space for micro-organisms. As a soil drains, it becomes more and more difficult to pull more water out of it as dry particles hold on to the water very tightly. This is why plants start to struggle in a very dry soil, and why we water them.

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The gases that make up the atmosphere in the soil pores is quite different from the air that we breathe. There is far more carbon dioxide for example, thanks to the respiration efforts of millions of aerobic micro-organisms. There is also a little less oxygen, and sometimes there are higher amounts of nitrous oxide, methane and ammonia gas than in the Earth’s atmosphere.

These gases, after being produced by micro-organisms, have various ways of getting out of the soil. The key transport pathway is diffusion – simple passing of the gas through air or water according to a concentration gradient. The gas will always travel from a region of high concentration to a region of low concentration – hence the carbon dioxide making its way towards the surface where the concentration is much less.

Another way that gases travel is through convection. This is bulk flow that is driven by a difference in temperature or pressure from the pores to the surface. On a very windy day, pressure in the soil might end up higher than at the surface, as air is pushed away by the wind, and so, thanks to this gradient, the soil gas will travel out seeking equilibrium. In wet soils, gases that do not readily dissolve can accumulate in large bubbles, which are forced out of the soil by gradients in pressure or temperature. This process is known as ebullition.

Soil gases also travel through plants, particularly some reeds and sedges that have tissues specifically adapted to help them survive in wet soils where oxygen is limited.

So for transport of gases, soil porosity is very important, primarily in determining the rate of transport through diffusion. The amount of water in the soil pores is also a strong control on whether a gas produced at depth will make it to the surface and when that will happen.

In the next article, I will discuss some gas forming processes and talk a little bit more about what happens to a gas trapped in the soil by high amounts of water and/or low levels of pore space/connectivity.

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