Soil Porosity Information – Learn What Makes Soil Porous

By: Darcy Larum, Landscape Designer

When researching plant needs, it is frequently suggested that you plant in rich, well-draining soil. These instructions very rarely go into detail about what exactly constitutes as “rich and well draining.” When we consider our soil quality, we usually focus on the texture of the solid particles. For example, are they sandy, loamy or clay-like? However, it is the spaces between these soil particles, the voids or pores, which most often determine the quality of the soil itself. So what makes soil porous? Click here for soil porosity information.

Soil Porosity Information

Soil porosity, or soil pore space, are the small voids between particles of soil. In heathy soil, these pores are large and plentiful enough to retain the water, oxygen and nutrients that plants need to absorb through their roots. Soil porosity usually falls into one of three categories: micro-pores, macro-pores or bio-pores.

These three categories describe the size of the pores and help us understand the soil’s permeability and water holding capacity. For example, water and nutrients in macro-pores will be lost to gravity more quickly, while the very small spaces of micro-pores are not as affected by gravity and retain water and nutrients longer.

Soil porosity is affected by soil particle texture, soil structure, soil compaction and quantity of organic material. Soil with fine texture is able to hold more water than soil with coarse texture. For example, silt and clay soils have a finer texture and sub-micro porosity; therefore, they are able to retain more water than coarse, sandy soils, which have larger macro-pores.

Both finely textured soils with micro-pores and coarse soil with macro-pores may also contain large voids known as bio-pores. Bio-pores are the spaces between soil particles created by earthworms, other insects or decaying plant roots. These more sizeable voids can increase the rate at which water and nutrients permeate the soil.

What Makes Soil Porous?

While the small micro-pores of clay soil can retain water and nutrients longer than sandy soil, the pores themselves are often too small for the plant roots to be able to properly absorb them. Oxygen, which is another important element needed in soil pores for proper plant growth, may also have a hard time permeating clay soils. In addition, compacted soils have decreased pore space to hold the necessary water, oxygen and nutrients needed for developing plants.

This makes knowing how to get porous soil in the garden important if you want healthier plant growth. So how can we create healthy porous soil if we find ourselves with clay-like or compacted soil? Usually, this is as simple as thoroughly mixing in organic material such as peat moss or garden gypsum to increase soil porosity.

When mixed into clay soil, for instance, garden gypsum or other loosening organic materials can open up the pore space between soil particles, unlocking the water and nutrients that had become trapped in the small micro-pores and allowing oxygen to penetrate the soil.

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The Dirt on Dirt – Raised Gardens

Soil is the basic foundation for any garden, which is why we have multiple articles in our ‘Dirt on Dirt’ series. After several questions from gardeners asking us what is the best soil to use in raised beds, we’ve decided to add an article that covers the ins and outs of creating soil for raised bed gardens. Taking the time to get the soil right in your raised bed will make the rest of your gardening season much easier…after all, happy roots are the path to happy plants.

It is tempting to think of raised beds as really large containers, but that isn’t quite the right mindset. Raised beds are a hybrid between a landscape bed and a large container and thus the soil used in raised beds is a hybrid as well. You can’t just use the soil in your garden which will compact too much for plant roots to be happy. However, potting soil is too light and fluffy for this purpose. So what exactly should you be using?

Low Hanging Fruit

If you don’t have the time or the inclination to create your own raised bed garden soil, you can buy bagged soil formulated for raised beds. It’s a legitimate option, especially if you are creating fairly limited raised beds. However, if you are installing multiple beds or a large raised bed bagged soil will get expensive. Look for soil mixes that are specifically mixed for raised beds. For many of us, mixing our own soil will be a better option.

Mixing your Own Soil - The Components

While it would be comforting to have a specific all-purpose recipe that is the only one to use, real life rarely gives us such cut and dry instructions – at least when it comes to plants. I am going to give you two different recipes to consider. They have similarities – the ingredients in the soil mixture are quite similar. However, the proportions and some of the details differ a bit. The recipes are similar enough that comparing the two recipes to each other and to the components you can find locally will hopefully reassure you that despite some variation, you are in the ballpark. Mixing soil has a lot more in common with making soup, which is a very forgiving form of cooking, versus baking angel food cake where ingredient proportions must be exactly right. So don’t worry about being exact.

Garden Soil, Mineral Soil, Topsoil – They’re all the Same Thing

Garden soil is simply the native soil in your garden. Native soil comes in an extremely wide range and includes everything from very sandy soil, to dense clay, to the lovely loam soils that some are fortunate to have – and every kind of soil in between. Even if your soil is less than ideal, it can be a good idea to incorporate native soil in your raised beds. Native soil will include minerals and nutrients, as well as earthworms, insects, microfungi, and bacteria that occur naturally in your soil. While we might tend to think of fungi, bacteria, and insects as problems to solve, they are usually a great addition to your soil. All of these “critters” are the main reason to include your own garden soil into raised beds when possible. If your soil is terrible, you may want to buy premium garden soil rather than incorporating your native soil.

Well-Aged Compost

Compost is organic matter that has decomposed and transformed into a soil-like substance that is great for growing plants. You can buy bagged compost or create your own, both are great choices. However, creating enough compost to establish raised garden soil may be tough for the average gardener. Fortunately, many communities have municipal compost operations that use yard waste, old Christmas trees, leaves, etc… to create compost. These locations can be great options to find inexpensive, bulk compost to use in your raised beds. It isn’t unusual for the compost to be free or low cost. The very nature of compost is that it will continue to decompose over time, so you will need to add additional compost regularly to your raised beds. The good news is that adding compost also adds nutrients that your plants will use to power growth and flowering.

High-Quality Peat Moss

Peat moss is used to add structure and water holding capacity to your raised beds. Oddly it both promotes great drainage and holds water that plant roots can access as needed. Peat moss has a rather wide quality range and using the best quality peat moss you can afford is recommended.

There are lots of different types of peat moss sold in garden centers and box stores. You can tell the quality of the peat moss by two different parameters: fiber length and dust content. Look for a high fiber content with long strands of peat fiber. The color should be tan to light brown. You do NOT want very fine peat that has a dust-like texture. The color of this type of peat moss is dark brown to black. This type of peat moss is inexpensive but breaks down in the soil within a year and doesn’t add any structure to the soil mix.

The higher quality peat mosses are compressed in bales and usually expand 2-3 times their volume once they are unpackaged and have absorbed moisture.

Some alternates to peat moss are composted bark (preferably made from hardwoods), Coir which is coconut fiber and rice hulls. While rice hulls are pretty much on size, it is best if you choose coarser versions of composted bark and coir. Like peat moss, larger fibers will take longer to break down and will give more porosity to your soil.

Additional Amendments

Mix in a slow or controlled release plant food that is appropriate for the plants you are growing in your boxes. This type of fertilizer will provide nutrition for a set number of weeks or months. There are many options including organic versions (those formulated for veggies) or other formulations developed for flowering plants.

If you are mixing soil for crops that develop edible roots (such as onions, potatoes, carrots, and chives) adding perlite to the mix will allow for better aeration and root penetration. We have found that many root crops like carrots grow straighter and fill out more evenly along the root if they are grown in a porous soil.

The John Gaydos Raised Bed Soil Mix

John is the original Proven Winners employee and is an incredibly well-versed gardener. He currently has 11 raised beds—eight of the beds are 4’ by 6’ by 12” tall and are used for veggies. The other three beds are 2’ by 12’ by 18” tall and are used for garlic, onions, and cut flowers.

John has nice loam soil in his garden. Yes, I know the rest of us are jealous. John’s raised garden bed mix is about 1/3 native soil, 1/3 well-aged compost, and 1/3 peat moss measured by volume. Plus plant food, and for the root veggies perlite is added.

Laura LeBoutillier’s Raised Bed Soil Mix

Laura at Garden Answer uses a different mix than John. She has, as she has calls it, “crappy soil.” Rather than using her native soil, she buys premium topsoil in bulk when she creates raised beds. She mixes that with high-quality compost as well as composted manure. Her ratios are about 60% topsoil, 30% well-aged compost, and 10% composted manure.

There are many ways to create soil for your raised beds, but the main components included in the mix are reasonably similar. You may need to tinker with your mix for several years to get the best performance from your plants. As the compost continues to decompose, you will need to regularly incorporate additional compost. If you have several raised beds it might make sense to use a three-year rotation where each year you add compost to 1/3 of your beds. That way you don’t have to tackle that large project all at once.

Mixing the Components

If you are creating a brand new raised bed and are using your native soil as a component, once the walls of the bed are in place begin by using a shovel or spade to loosen the soil in the box. This will help encourage good drainage in the raised bed.

From there simply add layers of your ingredients in the box, incorporating slow or controlled release plant food with each layer. Once the box is full, turn the soil over with a shovel until the ingredients are well blended. You may want to have the soil mounded a bit higher than the sides of your bed since the soil will settle. Finish up by gently and slowly watering in the soil until it is moist and leave the bed to sit overnight, or longer. You can begin planting as early as the next morning.

Extension Education in Travis County

The Austin area is home to three ecoregions that have very different types of soil the Edwards Plateau, the Blackland Prairies, and the Post Oak Savannah Floodplains. All of them are somewhat alkaline, have challenging clay issues, and are low in organic matter. We’ll describe each region to help you identify which one you are gardening in and give you some tips for success. Here is a link to Soil Survey of Travis County Texas if you want to deep dive even further to see the variability in the Austin area soils.

Edwards Plateau Description and Gardening Strategies

The Edwards Plateau is characterized by thin soils on top of exposed limestone

I-35 helps to physically separate the western Edwards Plateau from the eastern Blackland Prairies region here in Austin. This is the reason that garden center employees and the Travis County Master Gardeners ask you which side of the freeway you live on!

You know you’re on the Edwards Plateau if you can observe the following:

  • Exposed Limestone
  • Soil contains large amounts of crumbled limestone, referred to as calcareous rubble
  • Clay soil prevalent

The biggest challenge is not having enough soil. Sometimes builders will bring in soil for new housing developments, but it’s usually not very much and sometimes can be from poor materials like sandy clay. Your best bet is to prioritize what you want to grow and think of your landscape as planting islands. Planting islands allow you to concentrate on soil improvement or find affordable ways to purchase more soil.

Slow Down Runoff

The thin soils and elevation changes contained on the Edwards Plateau make this area prone to runoff. Evaluate your gardening site for ways to slow down the runoff. You can do this with physical barriers, tiers, raised beds, and retaining areas like rain gardens. Utilize materials like larger stones and coarsely ground mulch for transition zones to help prevent decomposed granite or soil mixes from washing away. Summer and Winter cover crops also help stabilize things and keep bare soil in place. An added benefit is that roots help to break down limestone. The clumping and fibrous nature of native prairie grass roots are also good plant choices to help break down limestone.

Add Organic Matter for Soil Health

All clay soils are deficient in organic matter, and especially so on the Edwards Plateau. But the trick is not to go overboard. Your garden soil should contain at least 30% by volume of real soil. This mineralized content is vital for plant health. If you need to purchase soil, make sure that it is not a soilless mixture like a potting mix. Look for words like “top soil” and ask to see the ingredients listing.

Improve the soil that you have with one to two inches of compost added to the topsoil each spring and fall. You’ll also want to retain soil moisture by using three inches of mulch. These thin soil layers leave plants susceptible to summer drought when the root zone can quickly deplete the soils limited moisture reserves. The mulch will eventually break down and become compost, so you may need to add more each planting season. When applying mulch, be sure to give your plants some space by allowing a gap between the mulch and the main stems of the plant.

The Blackland Prairies Description and Gardening Strategies

The black and reddish clays of Central Texas are deep, dense soils that pack tightly into a heavy mass. They absorb water very slowly and have little interior air space making it a tough go for plant roots trying to get a foothold. The good news is that clay soils tend to retain moisture once they’ve absorbed it and don’t easily wash away.

Blackland Prairies can be recognized by the following:

  • Wet soil is extremely difficult to remove from tools
  • Soil holds it shape in your hand, feels like pottery clay
  • Soil profile is very deep, solid rock rarely found

The biggest challenges for these soils are mud and dried areas resembling concrete. They tend to shrink and swell when exposed to moisture, forming wide cracks and heaving objects like fence posts and patios. You can improve the soil over the long term, but if in a hurry, you can garden on top of the clay utilizing raised beds or berms. A common mistake made with clay soils is creating impervious planting holes. This is caused from digging out the hole and filling it with compost or peat moss instead of the soil that came out of it. Amending soil in this way while installing plants will create mini swimming pool holes that retain excess water and girdle roots.

Don’t Add Sand to Clay Soils!

For many people, it seems like you can solve clay soil problems by adding sand or other aggregates. Don’t do this! Clay will bind the larger particles together, and when dry, will form something resembling concrete, mortar, or adobe. There is one material that can be used, and that’s expanded shale. Expanded shale is mined from special types of clay, then heated in kilns until it becomes a porous ceramic aggregate. It doesn’t bind with the clay and it’s porosity helps with aeration and moisture retention. The manufacturing process adds cost, so it is usually more expensive than other soil amendments.

Create Healthy Soil with Organic Matter

The best way to improve clay soil is to add organic matter. There are two primary ways to do this: cover crops and compost.

Cover crops are usually some sort of leafy or grassy forage plant like clover or beans that are meant to be left and decomposed by soil microorganisms. You can turn the crop into the soil, or use the “cut and drop” method to leave it on top as mulch. Common winter cover crops for Central Texas are Fava (sometimes called Windsor or Broad) beans, cereal or elbon rye, wheat, any brassica (like mustard or turnips,) and crimson clover (which needs to be inoculated with rhizobium.) In summer, use crops like hairy vetch, cow or cream peas, and black-eyed peas. An added benefit of growing the peas is that you can harvest the crop before tilling the residue into the soil.

Compost is the best way to improve clay soils, and can be the most cost effective since you can make it yourself. Maintain soils with two to three inches of compost each spring and fall by working it in lightly on top (we call this “scratching it in.”) Mulching is also a good idea to hold in soil moisture, with the added bonus that soil microbes eventually convert it to compost.

The Post Oak Savannah Floodplains Description and Gardening Strategies

Along the river bottoms across the area and the broad plains to the southeast of Austin the soils are often deep sandy loams. They absorb water well, drain well, but don’t hold soil moisture. These soils, like all soils in our area, are usually low in organic matter content and have an alkaline pH.

The characteristics of Post Oak Savannah soils are as follows:

  • Terrain is relatively flat and is mostly east of I-35, commonly found along river and creek bottoms where water moves along broad channels
  • Soil is loose and drains extremely fast and has difficulty retaining moisture
  • Clay hardpan promotes flooding
  • Soil will not hold a shape in your hand and mounds quickly flatten, very few visible clods

The biggest challenge in this ecoregion is flooding. These soils are typically sitting on top of a hard layer of impervious clay, which quickly saturates and floods. Flooding usually results in the soil being lifted and moved. Keep this in mind when designing your landscape and consider using structures like planting boxes or retaining walls to hold soil in place and reduce erosion. Utilize ornamental grasses or other perennial plants with fibrous root system to collect and hold silt after downpours.

Loam soil from the Colorado River bottom

Moisture Management is Key

The frustrating part of gardening on the Post Oak Savannah is that when it’s not flooding it’s bone dry! Proper moisture management plays a key role for successful gardening, and the practices are slightly different than what is recommended for the other two ecoregions. Mulching helps a great deal, but be careful to monitor the soil underneath for excessive dryness. Drip irrigation under the mulch makes sure that plants are delivered the moisture they need. If you must do overhead watering, make sure to pull back the mulch so that the water gets to the roots, then rake it back in place once the water is turned off. Don’t depend on natural rainfall to do the job since it may not permeate the mulch layer.

Keep Adding Compost

The sandy soils of the Post Oak Savannah are usually deficient in organic matter because it floods away or is quickly consumed by microorganisms. As a result, use a heavy hand when adding compost in the spring and fall (up to four inches) and monitor the rest of the year to see if more should be added. No matter what, retain a ratio of at least 30% soil to compost.

Strategies for all Three Areas

If you want to be successful at growing things in Austin area soils, some soil improvement is needed. The two key things most often needed are compost to improve the quality of soil and more usable soil depth to support a strong, extensive and resilient root system. A soil depth of 6” is minimal for lawn grasses but more is better. Turf on shallower soils will be weak and require constant watering just to keep it alive. Flowers and vegetables also need 6-12” inches of soil to do well. Building up raised planting beds can turn a shallow rocky soil or one with an impervious clay layer into a beautiful garden spot.

Keep in mind that plants native to this area or areas with similar soil conditions do not require as much soil preparation to thrive here.

Our central Texas soils are usually high in pH levels making them well suited to our western native plants and ill adapted to acid loving plants of the southeast like azaleas and blueberries. Our soils typically have high levels of phosphorus and potassium but may need some additions of nutrients for optimum plant growth. The best way to tell what your soil needs is to have it tested before you apply fertilizers. Instructions on how to take a sample, where to send it, and cost can be found at Texas AgriLife Extension Service Soil, Water and Forage Testing Laboratory Department of Soil and Crop Sciences.

Cover crops can also be utilized to add organic matter and keep soil in place. Here is a link for more information on common cover crops.

No Green Thumbs, Just Great Soil

It doesn’t matter what color you think your thumb is. Everyone can be successful once you’ve identified what type of soil you have and adopt practices to maintain soil health.

Additional Resources for Soils and Composting

Fertilizer Calculator (once you have your soil test results)

Consider Repotting

Sometimes even the best potting soil mixtures can get packed down, losing their aeration qualities by getting compacted. Every time you water a plant, the soil settles down a little more.

Without any natural disturbances to mix things up (like the worms we mentioned earlier), the soil just stays settled.

This can be the time to repot, and give your plant a whole container of fresh soil mix. A good potting blend will already have a nice balance of drainage materials mixed in, or you can add some extra bark or vermiculite to keep things really light.

While you are doing this, it can’t hurt to give your houseplants a little more leg room and go for a bigger pot. If they are still doing fine in their current container, feel free to just freshen up the soil and put them right back in.

Biodiversity: Conserving Biodiversity in Agroecosystems

Ecosystem Engineers: Soil Structure and Hydraulic Properties

When water infiltrates into the soil, it is either stored for later use by plants or released to recharge aquifers. Soil water status and the balance among these important functions are made possible by soil porosity and the effect of gravity. Porosity is a very complex and dynamic soil attribute that has both a textural and a structural component. Textural porosity is a function of mineral particle size, whereas structural porosity is formed by either physical (e.g., creation of cracks in drying soil) or biological processes, such as bioturbation and burrowing. The latter is what allows vertical and horizontal water flows and plant provisioning.

The literature provides ample illustration of the importance of soil invertebrates and plant roots for the formation of macro- and meso-porosities ( Lavelle and Spain, 2001 ). Subtle balances among soil-dwelling invertebrates that compact and aggregate soil particles and those that decompact and disaggregate these structures determines the size, shape, and organization of the pore space. This balance, which is strongly influenced by plant roots, seems to optimize the permeation of water into the soil by infiltration (in pores>100 μm), plant provisioning from pores of a size between 0.02 and 6 μm, whereas the transfer of water to deep aquifers and rivers occurs through the largest macropores ( Figure 8 ).

Figure 8 . Different kinds of water according to the size of pores within which it is retained.

Soil aeration and water dynamics, therefore, largely depend on the activities of ecosystem engineers, which themselves are dependent on a biodiverse ecosystem. Losses of biodiversity may lead to imbalances in the dynamics of pore formation, including, for example, by invasive compacting earthworms (Pontoscolex corethrurus) causing severe soil compaction ( Chauvel et al., 1999 ) or the uncontrolled multiplication of surface structures by ants Camponotus punctulatus, as in abandoned rice fields in Northeast Argentina ( Folgarait, 1998 Figure 9 ). It is evident, therefore, that the diverse activities of soil invertebrate ecosystem engineers are of the utmost importance in agricultural ecosystems, especially those that do not use mechanical tillage.

Figure 9 . Uncontrolled multiplication of mounds created by Camponotus punctulatus in Northeast Argentina.

Watch the video: Porosity

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