Module 1 – Sand

  Course

Sand

No Sand – No Civilisation

Sand is a benign substance that resides in the back of our minds. For most of us, it holds no more day-to-day interest than the rocks and roads – and the other physical aspects of our environment. We know that it exists but it hardly warrants a thought.

But, as Vince Beiser reminds us in his book – “The World in a Grain” – there is no civilisation without sand.

(If you have a spare hour to burn in the quest for sand knowledge, here’s a video in which Vince Beiser explains why that’s the case.)

Of all of the things that we can do with sand, none is more worthy than to grow food with it. And of all of the ways we can grow food, nowhere will we find a better media in which to grow it.

With that, it’s time to learn more about the most cost-effective growing media available – anywhere – so let’s begin with…

Sand is a substance that most people don’t often think about or pay attention to. It’s usually just a part of the physical environment, like rocks and roads, but it’s an essential part of civilization. Sand is made up of mineral and rock particles and typically ranges from 0.06 mm to 2 mm in size, which is finer than gravel, but coarser than silt or dust. It’s the most commonly available substance on the surface of the planet and silica quartz sand is the most commonly available type of sand, which is the type that is used in iAVs.

Vince Beiser wrote the book “The World in a Grain” in which he reminds us of the important role of sand in the development of civilizations. He emphasizes the idea that sand is essential for a civilization to be able to exist and function properly.

Coarse sand is beneficial to plants and microbes in the rhizosphere because it increases nutrient availability. The larger pore spaces of coarse sand allow for more oxygen to be available, which helps with root respiration and microbial activity. This increased oxygen availability also helps with the breakdown of organic matter, allowing for better nutrient uptake by plants. Additionally, coarse sand has a higher water-holding capacity than finer sands or soils, meaning that it can help retain moisture around plant roots even during dry periods.

Coarse sand is beneficial to plants and microbes in the rhizosphere because it increases nutrient availability. The larger pore spaces of coarse sand allow for more oxygen to be available, which helps with root respiration and microbial activity. This increased oxygen availability also helps with the breakdown of organic matter, allowing for better nutrient uptake by plants. Additionally, heterotrophs such as bacteria can use rhizophagy (the process of consuming other organisms) or siderophores (iron-binding molecules produced by some microorganisms) to obtain nutrients from their environment when they are not readily available in the soil. These processes help create a healthy soil ecosystem that supports plant growth while also providing food sources for other forms of life in the rhizosphere such as earthworms or nematodes.

Sand is a great media to grow food in because it is one of the most readily available substances on the planet and it can provide the best environment for plants to thrive. The USDA-funded trial at Caroline State University confirmed the commercial viability of sand as a growing medium, showing that it is the most productive, resilient and sustainable food production method ever devised. It can even offer the difference between life and death for those suffering from poverty.

What is Sand?

Back in ‘Horticulture’s Best Kept Secret’, we discovered that sand:

  • is a particle size as much as a substance
  • usually comprises mineral and rock particles
  • ranges in size from 0.06 mm to 2 mm (0.002–0.08 inches)
  • is finer than gravel and coarser than silt/dust.

Sand is the most commonly available substance on the surface of the planet…and silica quartz sand is the most commonly available sand. And that’s the sand we want for use in iAVs.

Desert Sand – Namibia
Silica Quartz sand – under the microscope.
Why Sand?

Sand is far more available than any soil type – much less good organic soil.

Sand can be managed to provide the ideal plant-growing environment…both physical and microbiological.

You can systematise your food production when you use sand – providing the ideal mix of drainage and water retention – and intermittent irrigation – so that your plants have all the water and nutrients that they need – on time!

Sand is the most cost-effective filtration method ever…available since before recorded history…and used to capture the solid wastes from the fish and convert them to plant-available nutrients.

Sand is 100% recyclable…it can be washed and sieved and re-used…forever!

Sand can be stored…dry and pest-free…and get bio-activated quickly and as required.

When you use sand, your garden grows both biologically – and financially.

Washed and graded sand is a permanent asset that does not require replacement – and only very modest maintenance. Given the global demand for sand (upon which our lives depend so much), the price of sand can only continue to rise.

Why Sand? Sand is used for growing plants because it is much more abundant than other types of soil such as organic soil. Additionally, sand is easily shaped and manipulated to create the ideal space for plants to thrive by providing the correct physical and microbiological environment.

Sand creates an environment where drainage and water retention is balanced, giving the plants the ideal amount of water and nutrients in an efficient amount of time. This is important for food production

Sand is also special because it can be used over and over again. It can be washed and sieved and reused, meaning it will never run out. This makes it cost effective, as well as a permanent asset that does not require a lot of maintenance. Sand is really important to us because it is in high demand, so the price of sand will only continue to rise over time.

Sand + organics + oxygen + water + microbes + light = SOIL

if you add sand, organic material, oxygen, water, microbes, and light together in the right combination, then the result is soil. Sand helps to improve drainage and water retention in the soil, organic material helps to add nutrients, oxygen is necessary for the microbes to work properly, water is needed for growth, the microbes help break down organic material and create new nutrients, and light is needed by the plants for photosynthesis. All of these things together form a complete, healthy soil

Unfortunately, not all sand is suitable for iAVs.

iAVs-Suitable Sand

Sand that is to be used for sandgardening should be:

  • Inert
  • Free of silt and clay
  • Able to drain effectively

When we speak of ‘inert’ we mean that the sand should not produce a chemical reaction when in contact with water.   Sand that contains carbonates, for example, will create issues with the pH of the water in the system…leading to nutrient deficiencies.

The sand that best satisfies these criteria is crystalline (sharp) quartz – Silicon dioxide – known more commonly as silica sand.

Granite – and rounded sand – will work, too.

Sandstone and beach sand will usually contain substances that make them less suitable for sandgardening purposes.  Similarly, flat and flaky sand is to be avoided.

Sand that contains silt and clay may harbour harmful pathogens or pollutants and will impede drainage.

Not every type of sand can be used for iAVs, also known as sandgardening. The sand needs to be inert, meaning it will not create a chemical reaction when it comes in contact with water. It should also be free of silt and clay, and be able to effectively drain. The ideal sand for sandgardening is crystalline quartz, but granite and rounded sand can also work. Sandstone and beach sand are usually not suitable, and sand that is flat or flaky should be avoided

The sand that is best to use for a garden is a type of sand called silica sand. Silica sand is made up of a crystal called silicon dioxide, which is usually in a sharp form. This type of sand is best for gardening because it creates the perfect balance with water retention and drainage. It is also recyclable, so it can be used many times over and stored away safely.

Sandstone and beach sand may contain different compounds which make it unsuitable to use when creating a sand garden. These substances could have a negative effect on the pH levels and drainage of the sand, which makes them unsuitable.

 

Sand Particle Size Range

 

Sand that is not of the correct particle size range will also present drainage problems.

As we learned earlier, sand ranges in size from 0.02mm to 2mm (0.0008–0.08 inches). The ideal sand will comprise the following particle sizes:

  • 40% by volume in the size range between 1 to 2 mm.
  • 40% by volume in the size range of 0.5 to 1 mm
  • 20% by volume in the size range of 0.25 to 0.5 mm
  • A small amount larger than 2 mm is okay but virtually nothing less than 0.25 mm is recommended.

 

Sand particles can come in a variety of sizes, starting from 0.02mm up to 2mm (0.0008–0.08 inches). The ideal sand for drainage purposes should be made up of 40% particles between 1 to 2 mm, 40% between 0.5 to 1 mm, 20% between 0.25 to 0.5 mm, and a small amount larger than 2mm. It is generally not recommended to have particles smaller than 0.25mm.

Rarely, if ever, will you encounter a sand vendor who actually knows the particle sizes of their sand mixes so the only worthwhile use for the above information is to impress on you that the range provides optimal drainage – neither too slow nor too fast.

the importance of having a variety of sand particle sizes that provide good drainage – not too slow and not too fast – as either can cause problems. Too slow of drainage can prevent the bed from emptying completely, while too fast of drainage will cause the sand to not retain enough moisture, reducing the effectiveness of the sand as a biofilter for the soil. It is better to choose a range of sizes that will provide good drainage than to pick something that does not work.

If the sand drains too slowly, it will not allow sufficient time for the bed to empty properly. For reasons that we’ll explain later, sand beds must drain completely – and too great a proportion of fine particles will inhibit this.

The amount of time it takes for water to drain out of the sand bed is an important factor. If the sand drains too slowly, there won’t be enough time for the water to be completely emptied out of the sand bed. This could be a problem because it can lead to plants being stressed, reducing the effectiveness of the biofilter – There will also be less microorganisms, which can limit the effectiveness of the sand bed.

In an ideal situation, sand beds should be able to empty completely after water is passed through. If the sand contains too many fine particles, this drainage process will take too long, and the sand bed will not be able to empty properly.

The sand beds, which are used to filter water and support plants, need to drain completely. However, if there is too much sand with very small particles, it will restrict the sand bed’s ability to empty out the water completely. Therefore, it is important to be careful not to use too much of this fine sand, or the sand bed will inhibit its function

If the range of soil particles in the sand beds is dominated by larger particles (coarse material), it will drain too quickly. This is a problem because it won’t allow enough time for enough water to be absorbed and reach the plants. If there’s not enough water, the plants won’t have enough to stay healthy. Additionally, the limited amount of water available could prevent beneficial soil microbes from growing and thriving in the soil, which would reduce the health of the soil (called the rhizosphere). Finally, organic matter wouldn’t be able to stay on the surface of the soil, but instead would sink to the lower parts of the sand bed.

When a sand bed has too much coarse material, the organic matter can sink into the lower sections, becoming mixed with the sand

Sand that contains too much fine material is not suitable.

If the range comprises too great a proportion of coarse material, it will drain far too quickly – and that’s a problem because:

  • There may be insufficient time to saturate the sand beds.
  • The sand will not retain sufficient moisture between irrigation events – resulting in stressed plants.
  • It will reduce the effectiveness of the sand bed as a biofilter…limiting the number and variety of soil microbes…and negatively impacting the rhizosphere.
  • Organic matter will penetrate the lower sections of the sand bed instead of being trapped on the surface.

If you had to make a choice between having sand that is too fine – and that which is too coarse – it’s better to go for the coarse option because you can add other particle sizes to the existing sand to slow the drainage rate to something more suitable. The best option, of course, is to get it right the first time.

Sand Availability

From time to time, we hear people say that silica quartz sand is not available in their area.

I can’t remember when we failed to locate a source of sand – or that it took much longer than 15 minutes of googling to find it.  Anywhere!

And while it’s invariably deeply satisfying for the person for whom we’ve done this little trick, we are no longer available for the task.

People also periodically say that sand is available but it’s far too expensive. That’s a different story – and a highly subjective one.

Silica quartz sand is a type of sand, so people ask if it is available in their area. Usually it is easy to find with a quick search online and takes no more than 15 minutes. People who have requested information about sand availability have found this useful. However, the availability of sand at an acceptable cost can be a difficult choice since this is a more subjective thing – what may be expensive for one person may be a good price for another.

Sand Cost

The two factors that most influence the cost of sand are its purity (absence of silt, clay, and other contaminants) – and how far it must travel from its source.

At the top end of the purity and price spectrum is filter sand.  It will be free of silt and clay and of a predetermined sand particle size range.  It will come in bags for ease of handling.

This is likely the most convenient – and most expensive – way for us to acquire sand.

At the other end of the sand spectrum, we have the stuff that’s straight out of the river, quarry or sandpit.

This is likely to be the cheapest option because it hasn’t been cleaned or separated into the particle size range that we’re seeking.

The viability of general-purpose sand really comes down to luck – and how ‘hands on’ you wish to become in obtaining your sandgardening media.

We can screen or separate the particle sizes…and wash the silt and clay out of it…but, even assuming we’ve got enough of the right material left to justify the cost of the raw material (and its transport), we’re going to be up for a lot of hard work.

If you want the highest quality sand, it usually comes in bags and is more expensive because it has been cleaned and sorted into the desired particle size range. On the other hand, sand from rivers, quarries, and sandpits is usually much cheaper as it has not been cleaned or separated into the desired size.

When it comes to using general-purpose sand for sandgardening, things can depend a lot on luck. You can sift and sort through the sand to make sure it has the right particle sizes, and you can also try to wash out any silt or clay in it. Even if you have to buy a lot of sand and pay to transport it, you may have to work very hard to get the sand that is suitable

 The Happy Medium – or Media

Somewhere in between the costly filtration product…and the run of the mill general purpose stuff…is bulk sand that has been washed and graded for specific purposes – like construction, top dressing of golf courses and artificial wetlands.

This sand may not be quite as clean as filter sand…and it might not have the same precise particle size range…but it will be much less expensive – often being only marginally more costly than general-purpose sand.

The “Happy Medium” is a type of sand that falls in between the expensive filtration product and the lower quality general purpose sand. This sand has been “washed and graded” for certain uses like construction, golf course top dressing, and artificial wetlands. It won’t be as clean or have the same particle size range as the filter sand, but it will be much less expensive and usually only slightly more expensive than the general purpose sand.

 Where to Obtain Bulk Sand

Our preferred sources of sand include:

  • Quarries – where they manufacture sand.
  • Specialist sand vendors – Google is your friend (so we’re told).
  • Cement or cinder block makers – ask them to tell you where they buy their sand.
  • Retail landscape suppliers.

 

Some good places to find bulk sand include quarries that make sand, specialist sand vendors, cement and cinder block makers, and retail landscape suppliers. Do not try to get sand from beaches, riverbanks, or industrial farming regions, as these places can contain pollutants and other unwanted substances.

Sand Sources to Avoid

The places we don’t recommend include beaches – because the sand almost always contains calcium carbonate – and that will, upon contact with water, produce high pH levels…about which we’ll say more later.

It is not recommended to use sand from beaches because it often contains calcium carbonate which can react with water and cause the pH level of the water to become very high.

The high pH levels caused by the calcium carbonate in beach sand can be detrimental to plants, fish and microbes. It can cause an imbalance in the water chemistry which could lead to stunted growth of aquatic life or even death. Additionally, it may also reduce oxygen levels and increase toxicity due to increased ammonia concentrations.

The other places to avoid are riverbanks in urban areas – where the sand is frequently full of pollutants…including pharmaceuticals, chemicals, heavy metals, sewage, and even nuclear waste…and in industrial farming regions where the issue will be synthetic herbicides and pesticides – and chemical fertilisers – and manure. Lots of manure!.

Riverbanks in cities should not be used as a source of sand because they often contain a lot of pollutants, such as pharmaceuticals, chemicals, heavy metals, sewage and in some cases, nuclear waste. Areas that are near industrial farms are also best avoided, since the sand there may contain synthetic herbicides and pesticides, chemical fertilisers and manure. These substances can have a negative effect on your sand gardening project and should be avoided.

Rivers (and river sand) often contain farm chemicals and livestock wastes. Urban rivers are no better.

We always need to remember that our sandgarden is about producing clean fresh food and that should drive what we put into the system.

So, if you suspect that a sand sample contains potentially harmful contaminants, get a materials testing laboratory to confirm that it is OK before you use it.

Not all sand vendors are equal in terms of the purity and grading of their product so unless you’re buying sand that is certified as to its content and particle size range (like filter sand), your only option is to test it.

A square metre of iAVs growing space – at the recommended depth of 350mm – will contain over 300 litres of sand – weighing upwards of 450kg.

So, it’s something we need to get right – the first time. Fortunately, that won’t be difficult if we test the sand before committing to large quantities.

The other thing is that you only need to source the sand for a particular sandgarden once. In human timeframes, sand lasts forever.

Testing sand – for our purposes – is simple and costs next to nothing – and we’ll show you how to do it right now!

Creating a sandgarden means that the goal is to produce clean and fresh food. This means that whatever is going into the sandgarden — such as sand, farm chemicals, and livestock wastes — must be of good quality and suitable for growing food. It’s important to confirm the quality of sand before using it in a sandgarden.

It is important to go to a materials testing laboratory to be sure the sand sample we are using is not contaminated with any potentially harmful contaminants. Not all sand vendors sell the same quality of sand, and if the sand is not certified for its content and particle size range, like filter sand, it is best to have it tested to make sure it is safe to use.

you only need to find the type of sand that you will use for your sandgarden once. This type of sand will not change or run out over a short period of time, like other materials might. In other words, it will remain the same for a very long time, as long as humans are around.

Sandgarden is like a special kind of garden that you can make with sand. You need to find the right type of sand and then use it to build your own unique garden. The good thing about this type of sand is that it will stay the same for a very long time, so once you have found it, you don’t have to worry about finding more!

Testing Sand

Confirming that a particular sample of sand is suitable for use in an iAVs, is quick and easy and involves a series of simple field tests:

  • Carbonate
  • Turbidity
  • Differential Settling
  • Pore Space Volume
  • Hydraulic Conductivity
  • Water Retention
  • pH

Testing Sand involves running different tests on a sample of sand to see if it is suitable for use in an iAVs (Ideal Automatic Visualization System). Carbonate tests measure the amount of carbonates dissolved in the sand. Turbidity tests measure the amount of sediment in the sand. Differential Settling tests measure the different size and weight particles in the sand and how quickly they sink under water. Pore Space Volume tests measure the air pockets between the sand particles. Hydraulic Conductivity tests measure how easily water flows through the sand. Water Retention tests measure how much water is held onto by the sand. Lastly, the pH test measures the amount of acidity in the sand.

To a 5 year old, I would explain that an iAVs sandgarden is like a special kind of sandbox. It has different kinds of sand in it and we can use tests to make sure the sand is just right for our garden. The tests measure things like how much air there is between the grains of sand, how easily water flows through it, and if there are any acids or other substances in the sand that could be bad for our plants.

To do these tests, you’ll need:

  • A couple of 1-litre measuring jugs.
  • A clear bottle or a jar with a lid.
  • A freshwater pH test kit that measures 4.0 – 9.0
  • Vinegar.
  • Water
  • Pen and paper – to record the results.
  • A simple percolation tester – that we’ll show you how to make for pennies.
  • A bucket.

….and let’s not forget…..some sand.

For the tests you will need several things: two jugs that each hold one liter of liquid, a clear bottle or jar with a lid, a freshwater pH test kit that can measure between 4.0 and 9.0, vinegar, water, a pen and paper to record the results, a simple percolation tester (which the paper will show how to make cheaply), a bucket, and lastly, some sand.

Vinegar Test

When we talk about the need for the sand to be inert, we mean that it is not chemically reactive.  That is, the pH of water should not change when it comes into contact with the sand.

The Vinegar Test is a way to check if sand is chemically inert. It is a simple test that involves mixing a small amount of vinegar and water with some sand to see if the pH of the solution changes.

When carbonates are present in the sand, they react with the vinegar and produce bubbles of carbon dioxide gas. This is because when an acid (vinegar) comes into contact with a base (carbonate), it causes a chemical reaction that releases CO2 gas. The presence of these bubbles indicates that there are reactive compounds in the sand which could affect its performance as part of an iAVs Sandgarden.

Why is pH important?

The pH of water in an iAVs impacts the availability of nutrients. Operating in the range of 6.4 (plus or minus 0.4) ensures that the full spectrum of essential nutrients is available to the plants.

The pH is important because it affects the availability of nutrients for the plants in the iAVs. If the pH of the water does not stay within the optimal range of 6.4 plus or minus 0.4, then certain essential nutrients may not be available for the plants to use. If the substances in the sand elevate the pH of the water above this range, then the plants will not have access to these nutrients.

The pH of the water affects the availability of nutrients for plants in an iAVs sandgarden. If it is outside a range of 6.4 plus or minus 0.4, then certain essential nutrients may not be available to them – such as nitrogen, phosphorus and potassium which are necessary for healthy growth and development. A high pH can lock out these important elements while a low pH can cause iron deficiency in plants due to its reduced solubility at lower levels.

 At this level, essential elements such as nitrogen, phosphorus and potassium are available in their most soluble form which allows them to be taken up by plant roots more easily than at higher or lower levels. Additionally, iron is also more readily available at a pH of 6.4 due to its increased solubility compared with other levels outside this range.

The optimal pH for soil microbes in an iAVs sandgarden is slightly higher than that of the plants, at around 6.8 to 7.2. This range provides the best balance of nutrients and minerals for microbial activity which helps to break down organic matter into usable forms such as nitrogen and phosphorus, making them available to plants in the garden. Additionally, this range also allows beneficial bacteria and fungi populations to thrive which can help protect against disease-causing organisms by competing with them for resources or producing antibiotics that inhibit their growth.

Yes, the rhizosphere can change the pH of an iAVs sandgarden to increase nutrient availability. The rhizosphere is a region around plant roots where microbial activity is increased due to their presence and secretions. This area has a slightly higher pH than that of the surrounding soil which helps make essential nutrients such as nitrogen, phosphorus and potassium more available for plants to absorb. Additionally, this also increases iron solubility which can help prevent deficiencies in plants growing in these conditions.

Yes, siderophores can affect the pH of an iAVs sandgarden. Siderophores are molecules produced by certain bacteria and fungi which help them to absorb iron from their environment. These molecules have a slightly acidic nature which can lower the pH of the surrounding soil or water, making it easier for these organisms to take up this essential nutrient. This process is beneficial for plants as well since it increases iron solubility in these conditions, helping prevent deficiencies in plants growing there.

heterotrophs can affect the pH of an iAVs sandgarden. Heterotrophs are organisms that rely on organic matter for their energy and nutrients. As they break down this material, they release acids which can lower the pH of the surrounding soil or water making it easier for them to absorb essential elements such as nitrogen and phosphorus from these sources. This process is beneficial for plants as well since it increases nutrient availability in these conditions, helping them grow healthy and strong.

Ammonia-oxidizing bacteria (AOB) thrive at a pH of 6.5 to 8.0 in an iAVs sandgarden. This range provides the best balance of nutrients and minerals for these organisms which helps them break down ammonia into nitrite, making it available to plants as a source of nitrogen for healthy growth and development. Additionally, this range also allows beneficial AOB populations to thrive which can help protect against disease-causing organisms by competing with them for resources or producing antibiotics that inhibit their growth.

Nitrite-oxidizing bacteria (NOB) thrive at a pH of 6.5 to 8.0 in an iAVs sandgarden. This range provides the best balance of nutrients and minerals for these organisms which helps them break down nitrite into nitrate, making it available to plants as a source of nitrogen for healthy growth and development. Additionally, this range also allows beneficial NOB populations to thrive which can help protect against disease-causing organisms by competing with them for resources or producing antibiotics that inhibit their growth.

Yes, oxygen can increase the population levels and efficiency of all the microbes mentioned in this paper. Oxygen is essential for microbial activity as it helps them to break down organic matter into usable forms such as nitrogen and phosphorus which plants need to grow healthy and strong. Additionally, oxygen also increases beneficial bacteria and fungi populations by providing an environment where they can thrive while inhibiting disease-causing organisms due to their increased competition for resources or production of antibiotics that inhibit their growth

Fish waste can oxidize or mineralize on the surface of an iAVs sandgarden. Oxidation is a process where organic matter is broken down into simpler forms such as carbon dioxide and water, while mineralization involves the conversion of these substances into inorganic compounds which are then available for plants to absorb and use. Both processes help to make essential nutrients such as nitrogen, phosphorus and potassium more available for healthy growth and development in these conditions.

Conversely, the presence of substances in the sand that elevate the pH of the water above the optimum range will mean that certain nutrients are unavailable to the plants.

The most likely influence on the pH of water that comes into contact with sand is that of carbonates.

Sand that contains carbonates is not inert.

Interestingly, the presence of carbonates can be most easily established with plain vinegar.

To conduct the vinegar test, place some sand in the jar lid – and pour some vinegar on it.

To show you how sand containing carbonates behaves, in the presence of vinegar, we collected a sample from a local beach.

The vigorous bubbling evident in this sand tells us that it contains carbonate – is not inert – and is not, therefore, suitable for use in iAVs.

Turbidity

Turbidity is cloudiness in the water that would indicate the presence of large numbers of individual silt or clay particles that would otherwise be invisible to the naked eye.

The turbidity test is also easy and requires nothing more than a glass jar or drink bottle with a lid.

Half-fill the jar with sand and then top it up with water.

Shake vigorously for 5 – 10 seconds and then place the jar on a bench to allow the contents to settle.

This sample suggests the presence of clay. This would be confirmed if the water remained cloudy for longer than a few minutes.
This sample suggests the presence of clay. This would be confirmed if the water remained cloudy for longer than a few minutes.
Differential Settling

Our next test – to establish the proportionate volume of silt or clay in our sample – is the easiest test of all. Simply leave the jar and its contents undisturbed for several hours.

Through a process known as differential settling, we can determine the proportionate fractions of silt or clay in the sand.

Once the sand has settled, any silt will show as a dark line on top of the sand….as evidenced in the photo below.

DSCN7590
The black line on the surface of the sand is silt. The floating black layer is organic matter.

Clay, which has the smallest particles, will show as a pale layer above the silt…when it eventually settles.

This sample is free of silt and clay. Note the small amount of powdered sand on the surface. The black lines in this photo are refracted light – not silt.

Having confirmed that our sand is relatively free of silt and clay, our focus shifts to particle size and pore space volume…or porosity.

Pore Space Volume

The pore space volume refers to the amount of space (for water or air) that exists between the sand particles.

To measure the pore space volume, take one of the one-litre graduated jugs and place a litre of sand into it. Fill the second measuring jug with one litre of water.

The pore space volume is the amount of space that exists between the individual particles of sand – either for air, or for water. It is determined by looking at the amount of water that can fit between the gaps in the sand particles. This information is used to measure characteristics such as particle size and porosity.

Then, using the measuring jug, add water to the sand – noting the amount of water that you are adding – until the sand is saturated – or the water is just level with the surface of the sand.

Dry sand of the correct particle size range will have a pore space volume of around 25% – 30%.

The pore space volume is a measure of the amount of space between the particles of sand. A pore space volume reading of 25% – 30% would indicate that the sand consists of the right sized particles, as this would be an ideal amount of space for water, oxygen, and microbial life to exist in the sand.

If the pore space volume was much lower, that would indicate that the sand contained fine particles – and, depending on their volume, that might restrict drainage.

A higher pore space volume reading indicates that the sample contains coarse material – and depending on its volume, that may allow for the water to drain too quickly.

Another value of the pore space volume test, however, is to demonstrate that sand is not a solid – and that it has space for water, oxygen and the microbial life that powers iAVs.

Another thing to note is that sand will continue to settle the first few times that it is flooded and drained.  Pore space volume will diminish initially and then stabilise.   The importance of this is that, if your pore space volume is marginal at the outset, drainage could be negatively impacted in future flood and drain events.

Hydraulic Conductivity

The flood and drain cycle is important to iAVs because it provides the moist environment required by the plants and the microbial life that underpins iAVs. The drain cycle is equally important because it drives the gaseous exchange – the charge of oxygen-rich air that is also needed by plants and microbes.

Hydraulic conductivity is a term used to describe the ease with which a fluid – in our case water – can move through pore spaces in various media.  Of course, when we speak about media – in an iAVs context – we’re talking about sand.

The measurement of hydraulic conductivity – also known as the percolation rate – simply confirms the accuracy of the pore volume space.

Measuring hydraulic conductivity requires the use of a DIY device called a percolation rate tester.

Water Retention

Water retention refers to the amount of water that remains in the sand after the sand bed has drained.

Dry sand might have the appearance of a solid mass but, as we observed during the pore space test, it’s actually about 25% – 30% – by volume – of air.

It’s also important to understand that a quantity of water will remain in the sand following the drain cycle – and it remains there (available to the plants) until the next flood cycle.

The water that remains is bound to the surface of the sand particles by hydrostatic tension.  We’re guessing that amount would be about 5% of the pore space volume – depending on the particle size and shape.

Note: It will be necessary to top up the fish tank after the sand beds have been flooded for the first time.

After the first flood and drain cycle, the exact amount of water retained in the sand will vary according to the elapsed time from the last flood cycle.

For example, about 5% of the water that was pumped may be retained after the first flood cycle in the morning – after a break of 8 hours or more – whereas only 1% will be retained immediately after subsequent flood cycles through the day (approximately two hours apart).

Note: These numbers are not prescriptive – they may vary from situation to situation, We use them to illustrate the dynamic nature of the relationship between sand and water in the system.

To establish the water retention of a sand sample repeat the hydraulic conductivity test several times….measuring the amount of water that drains from the sand after each flood cycle.

pH

You complete the assessment or your sand sample with a pH test and we’ll learn how to do that in the next section on “Water”.

Reconciling the need for iAVs-suitable sand with your budget may be tricky but there are opportunities for those who want to make the effort. With that, we conclude this section on sand.

Key Points
  • No Sand – No Civilisation!
  • Of all of sand’s uses, none is more worthy than using it to grow food.
  • Sand is an appreciating asset.
  • There is no more cost-effective media anywhere.
  • The only certain way to know if a sand sample is suitable for iAVs is to test it.
  • Sand comes with a lifetime warranty.
  • The further you transport and process sand, the more expensive it becomes.

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