Soil Porosity Is Vital For Plant Growth. Without It Plants Die, Soils Degrade & Soils Become Waterlogged. But Too Much Porosity Leads to Dry Soils That Cannot Support Plants. So How Do We Ensure Adequate Soil Porosity?

  Soil Porosity isn’t Soil Porosity Is Determined by Particle Size something that most gardeners dwell on too much. Thats a pity as it can make the difference between good crops and crops that die due to being waterlogged or too dry. Yes, poor soil porosity can result in problems that are at opposite ends of the spectrum. Ensuring soils have sufficient porosity isn’t difficult and the results will be worth the few minutes it’ll take to read this post.  So let’s start with some basic questions. To explain what porosity is and why it matters I’ll use some simple analogies, but please forgive me if I occasionally use a few scientific terms.  
Soil Porosity Is Largely Determined by Particle Size
Soil Porosity Is Largely Determined by Particle Size
Yes, poor soil porosity can result in problems that are at opposite ends of the spectrum. Ensuring soils have sufficient porosity isn’t difficult and the results will be worth the few minutes it’ll take to read this post.  So let’s start with some basic questions. To explain what porosity is and why it matters I’ll use some simple analogies, but please forgive me if I occasionally use a few scientific terms.  

What is Soil Porosity?

The simplest way I have of describing soil porosity is to suggest you think about a sponge. Its full of holes that can contain air or water. It’s porous. The measure of porosity is the volume of holes (voids) in the sponge or soil. It’s a measure of “empty” spaces really and we normally measure it as a percentage.  But of course they aren’t really empty spaces as they contain water or air. The important thing to note here is that the spaces must be linked together. The air and water must be able to flow in and out of the soil or sponge. Now think about a block of packing material. Often they are made os polystyrene or similar and are full of holes. It makes them lighter than if they were solid and makes the material more resilient if the box its in gets dropped. But the big difference here is that often the holes are not joined up. If you pour water on the packaging not doesn’t run through. It’s not like a sponge or soil. It’s a solid with discrete holes in it. So it’s not porous. If you are chocolate lover you might have eaten an Aero.  It’s a solid bar of chocolate full of bubbles.  But the bubbles don’t interlink.  So it’s not porous. Water can’t flow from bubble to bubble. This is important as I’ll explain as we delve deeper into porous spoils!  

What Makes Soils Porous?

We know that soils are made of a mix of soil particles that range from grit and sand down to fine clays. And of course they are a few other things mixed in such as organic matter (decaying and decayed vegetation). Between these particles there are gaps, the voids or spaces mentioned earlier. They vary inside depending on the size of the soil particles. If you’ve ever taken part in one of those guess the number of beans in a jar competition you’ll know that if you put large beans in a jar the spaces between the beans are big. Now imagine going down to the beach and putting sand in the jar. The air spaces will be much smaller. And if you put clay in the jar the air spaces are very small. But in each case there will be air spaces. And whether it’s beans, sand or clay water could trickle through the jar and it would take a lot off water to fill this spaces.  

What Makes Soils Impervious to Water?

Have you ever made a clay pot? If you have you know you wet the clay, pound it to make it workable and then shape the pot.  If you’ve done this and poured water in the pot you know the water just sits there. It doesn’t soak through. The pot is impervious. It isn’t porous. And soil is the same. If you have wet soil and keep walking on it, or even trying to rotavate it, it smears and starts to become impervious. It’s one the the common gardening mistakes gardeners often make. There’s more about that if you follow this Gardening Errors link. And if you have clay soil it is extremely easy to smear the soil. The technical term of soil that has been mistreated in this way is soil compaction. Farmers can get soil compaction in their fields. It can result from using heavy machinery such as tractors when the soil is too wet. You’ll also see soil compaction in field gateways. It can be caused by tractors or livestock. A cow weighs half a ton and is walking on relatively small feet. That puts a huge pressure on the soil (if you’ve ever had a cow stand on you’ll foot you’ll understand what I’m talking about), and in wet weather that can cause soil compaction. And soil compaction decreases soil porosity.  

Why is Soil Porosity Important?

When a soil is porous the rain drains through it, roots grow into the voids and the soil microfauna (especially worms, bacteria and fungi) thrive. The soil can retain some water in the soil spaces and there is also a healthy amount of air in the soil. Without water and air in the soil the plants can’t survive. They need a good balance of air and water in the soil and good soil porosity provides that. Interestingly air and water work together. As water enters the soil it displaces the air and takes its place. And as water drains through the soil it pulls air back into the voids. Soil draining through the porous soil then feeds the water aquifers and springs. Without a porous soil we wouldn’t be able to tap into subterranean water to fill our taps.  

Soil Shrinkage and Expansion

Where a soil is mainly clay it can expand when it gets wet and shrink when it gets dry. You can see the shrinkage in clay soils as the soil cracks open. Sometimes the cracks are big enough to put your hand in to. And it can be devastating in garden as these soils are dry, seeds don’t germinate and plants die of drought. It’s equally as important to the construction industry as it affects buildings where subsidence and “heave” can cause severe structural damage.  

How to Improve Soil Porosity

Soil Compaction and Soil Porosity

Firstly don’t compact your soil. Don’t walk on it or try to cultivate it when its too wet. Sandy soils are a bit more forgiving but clay soils will compact and set like rock. And they certainly will not be porous. This is one of the reasons I like No Dig Gardening. And though I’m not the biggest fan of raised beds, in most cases they do keep your feet off the soil. If you have to work on wet soils spread your weight a bit. Standing on a scaffold board or similar will help. You might compact the top centimetre or so but that’s not as bad as compacting much deeper down. The good news is that soils can be improved given a bit of effort and time.  

Organic Matter and Soil Porosity

The best way I know is to add organic matter. It can be home made compost, farm yard manure, stable manure or similar. Just spread it on the soil and the worms will work it into the soil for you. Some people like to dig it in or oven rotavate it in, but there’s no need unless you like hard work. The worms will do the work for you.  

Worms and Soil Porosity

The advantage of getting the worms to do the work is that they will work it into the top few inches and increase the porosity and fertility of that soil. And being worms they will drag some organic matter deep into the soil. And when they borrow deep they form channels in the soil which aids drainage and hence soil porosity.  

Plant Roots That Aid Soil Porosity

Like worms roots form channels deep into the soil. In some cases roots will penetrate several metres, though most don’t go that deep. However, when the plants die the roots decay, add their organic matter to the soil, and more importantly, leave channels behind. These channels increase porosity. We can aid this process by not digging roots out. Instead just cut the stems at ground level and leave the roots to decay. Of course there are certain caveats to this strategy. Don’t for example leave the roots of nettles. docks, dandelions etc in the soil. They will just reshoot and be a problem. But crops such as peas, beans, lettuce etc can be cut at ground level and left. They will benefit the soil. If you aren’t sure which crops you can leave it’s mainly the ones where you crop them from above ground level. Clearly root veg are underground so need digging or pulling. But lots of other crops can be left too improve the soil.  

Adding Sand or Gravel to Increase Porosity

I often see advice on adding sand or gravel to a soil to increase porosity. In theory it might work but think about the volumes needed. If you take a barrow of sand and add it the soil, it will form a permeable layer and not affect the rest of the soil UNLESS you mix in in very well. But when it is mixed in each grain of sand will be surrounded by the original soil and the amount of porosity is barely changed. It’s only when you have say, 10% of sand really well mixed (homogeneously mixed) that a few sand particles will sit next to one another and start to improve porosity. Try to visualise it like this. You drop a football into a bag of clay. Will it increase porosity or will the clay just surround the football? Now add more footballs and see how many you need to start to see several footballs touching one another. Is there pore space between them or has it filled with clay? Even with a lot of footballs in the clay the porosity doesn’t improve a lot. Now imagine doing that with a mix of clay and sand. Once you have a really good mix of all sizes of material then its possible for pore space to be suitable for water to flow through the mix. Just adding the odd barrow of sand will not make a significant difference.  

Does Soil Pore Space Affect More than Drainage?

The simple answer is yes. It also affects fertility. Large soil particles create more pore space but the surface area is smaller than where you have smaller particles. So a sandy soil has higher porosity than a clay one BUT it has less particle surface area. Surface area is important for chemical reactions to take place. The more surface area the better from this perspective. That’s why clay soils are generally more fertile than sandy soils. So the best garden soils are loams where there’s a mix of particle sizes and a balance between porosity and fertility.  

Now the Science Bit

The Six Soil Pore Types

I’ve spoken about pores as if all pores are equal. They aren’t. There are six types of soil pore and each has its own characteristics.

Soil Macropores

Macropores are very large voids in the soil such as those formed when clay cracks. They hold little water unless waterlogged because the soil particles are to far apart for capillary action to hold water between the particles.

Soil Mesopores

These will hold water when etc soil is at field capacity (which is the amount of water a soil can hold after its drained). The water is held by capillary attraction but it isn’t so strong as to stop the plants obtaining water from the soil.  

Soil Micropores

These pores are so small that they don’t drain due to gravity as the capillary attraction is stronger than gravity. But the plants can still suck water out of the pore spaces.  

Soil Ultrapores

These are very small pore spaces but bacteria can inhabit them. Their size is 0.1-5 μm  

Soil Crytopores

These pores are so small not even microorganisms can get into them. That’s small! What water exists in these pores is unavailable to plants.  

Soil Hydraulic Conductivity

This is the measure by which water can move through the soil. It depends on porosity, compaction and the degree of saturation.  

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