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. 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, suitable for use in iAVs.