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With apologies to any organic chemists out there and high school science teachers, chemistry is not something that most people wish to discuss day to day. I mean, I Googled 'chemistry jokes' and this was the type of thing I came across…

Chemistry Joke 7:

Q: What do chemists call a benzene ring with iron atoms replacing the carbon atoms?

A: A ferrous wheel:

Fe - Fe
/ \
Fe Fe
\ /
Fe - Fe

Yes well…

Even the Simpsons managed to sneak in some chemistry in an early episode:

http://www.youtube.com/watch?v=hPym09LQfnc

Anyhow if you're still with me, I'm hoping you'll soon have a better understanding of pH in your soil and maybe even think about regularly testing your own soil one day.

Back in 1909 in Denmark at the Calsberg Laboratory the focus (believe it or not) wasn't on beer. A chemist called Peder Sorensen had discovered the importance of 'potential hydrogen' and thus pH was born. I'm assuming he kept his findings in his awkwardly named Peder file.

I like to think Peder relaxed after his great discovery and had a quiet beer.

Mr Sorensen produced a logarithmic scale which starts at 0.0 and goes to 14.0.

Zero is the most acidic, while 14.0 is the most alkaline.

All importantly 7.0 is neutral and pretty much where we aim for with soil for our veggie patch.

Unlike a temperature or volume scale like most of us are used to, each step up or down on this scale is actually a ten fold increase.

To illustrate the power of this let's say your soil pH is 6.0. It means it's 10 times more acidic than a pH of 7.0 but 100 times more acidic than a pH of 8.0!

So the take home message is your pH chart doesn't work like the volume on an amplifier - small numerical changes are actually BIG!

In your soil, the pH has a huge influence on the availability of nutrients to your precious vegetables. It seems crazy, but can keep pouring on phosphorus, but if you're soil is too acidic, your plants can't access it. You can see by looking at the below chart that a sweet spot is between 6.5 and 7.0.

Testing your soil's pH.

There are a couple of kits available in nurseries and hardware suppliers that cost less than $30 and will last for years. In the kit you'll find a colour card, some liquid and some powder.

Take a sample of your soil and put it on a small plate. Add the liquid to make a paste, then dust over with the powder. Almost immediately you'll see the soil turn a colour that you can match on the colour wheel to discover your soil's pH. As I'm doing a lot of pH testing for clients, my own garden and testing the Backyard Harvest bio-compost product, I've invested in a professional lab model which can be calibrated for ongoing accuracy. This was several hundred dollars, so it may pay to see if cheaper hand held models are available.

How to change your soil's pH.

Generally soils in productive gardens will need to be made more alkaline, however there is an exception in the case of blueberries and if you're growing ornamentals then hydrangeas and azaleas also prefer acidic soil.

To raise soil pH (make more alkaline) we need to add calcium in the form of dolomite (calcium magnesium carbonate).

To lower soil pH (make more acidic) use elemental sulphur. This stuff forms with the water in the soil and produces sulphuric acid which lowers the pH.

Having said that don't forget to add compost, compost, compost! Good quality compost has a neutral pH of around 7.0 Adding compost helps to bring either acidic or alkaline soil back to more veggie-friendly conditions by helping to make nutrients available to plant roots.

Different pH for different veggies?

As a broad principle, green leaf based veggies (spinach, lettuce, cabbage, kale etc.) prefer a pH in the more alkaline range, say 7.0 to 7.5. Vegetables where the focus is on eating the fruit (tomatoes, capsicum, cucumbers, pumpkin) prefer a more slightly acidic soil around 6.0 to 6.8. This helps to explain why tomatoes and pumpkins happily pop up in compost heaps where conditions are typically more acidic.

Good luck!