iAVs is sand……and sand – at its most productive – is iAVs.
But not just any sand will do.
iAVs sand has 3 essential characteristics:
- It must be inert – that is it is not chemically reactive.
- It must be free of silt and clay.
- It must drain effectively.
The only safe way to know that we have sand that matches these three characteristics is to test it.
To that end, we give you the 5-Gallon Bucket Tests…..so-called because, for some of them, we use a 5-gallon plastic bucket.
The tests and measurements that we’ll be carrying out include:
- Differential Settling
- Pore Space Volume
- Hydraulic Conductivity
- Water Retention
To do these tests, you’ll need:
- A 5-gallon bucket. Actually, two such buckets would be handy.
- A measuring jug
- A clear bottle or a jar with a lid.
- Freshwater pH test kit that measures 4.0 – 9.0
- Pen and paper – to record the results.
….and let’s not forget…..some sand.
Take one of the 5-gallon buckets and drill sixteen 3/16” (4.5mm) holes in the side of the bucket – at the point where the side meets the bottom.
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.
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.
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, what we’d suggest for use in iAVs.
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.
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.
Clay, which has the smallest particles, will show as a pale layer above the silt…when it eventually settles.
Pore Space Volume
Having confirmed that our sand is relatively free of silt and clay, our focus shifts to particle size and 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 the bucket (or other container) without the holes and, using the measuring jug, put 4 US gallons (15 litres) of sand into the bucket.
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%.
If the pore space volume was much lower, that would indicate that the sand contained fine particles – and that might impede drainage.
The real 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 settle for 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 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 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 the plants and microbes.
And that brings us to the bucket test. We chose the 5-gallon bucket because it approximates the depth of a sand bed.
Take the bucket with the holes around its circumference and fill it with 4.5 US gallons (18 litres) of sand – leaving a bit of space for the water.
Measure out one US gallon of water into the second bucket (or other container).
We need to measure two things:
- The amount of time that takes for the water to enter the sand until it exits from the drain holes.
- The amount of water that drains from the sand.
To capture the water that drains from the bucket of sand (so that we might measure it), elevate it within a tub or tray using a house brick or paving tile.
Get ready to time the movement of water from when it hits the sand until it emerges from the drain holes. While the time is not critical, it should be reasonable accurate. You can count aloud if you find that easier.
When you’re ready, pour the water in.
We suggest that you repeat the hydraulic conductivity test 5 – 6 times.
The act of flooding and draining will settle the sand – largely within the first flooding – incrementally reducing the hydraulic conductivity with each subsequent event – until it stablises.
You may observe the level of the sand in the bucket dropping with each flooding. The exact amount of subsidence can be determined by measuring the distance from the top of the bucket to the surface of the sand.
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 in the range of 5% – 10% 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 immediatley 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.
You complete the assessment or your sand sample with a pH test.
You will have previously established that the sand is free of carbonates with the vinegar test, however, a pH test will determine if there are any other substances in the sand which will impact pH.
Conduct a pH test on the water that you propose to use in your iAVs – before it comes in contact with the sand you’re testing.
Now, mix some of that water with your sand sample.
Any movement of the pH of that water suggests that the sand contains a substance that is chemically reactive. At the very least, this signals the need for further investigation.
Not every one of the tests or measurements that we’ve proposed is essential – but they have been included to help you better appreciate that, when it comes to sand, there’s more going on than meets the eye – and to give you a better understanding of the medium.
Note: Water quality is a critical success factor in any food production regime. Conducting a full chemical analysis of the water (molecular and elemental) is essential. These tests can be undertaken, for a nominal fee, by your local country extension office. In the absence of such a service in your area, we suggest that you seek the assistance of your local university.
Interpreting the Test Results
This article has been all about how to undertake the tests that determine the suitability of sand for iAVs.
In the next article, we’ll test some sand and report on our results….and whether the sand fits the bill.
It goes without saying that, if you do your own tests, we’ll help you to interpret the outcomes. Simply post in the comments section if you’d like our help.