Soil pH – whats it all about?

The pH of your soil is important as it gives you an idea what will and won’t grow in your soil.  But what is pH anyways?

A little about water
Water is an amazing substance – without it life would not exist on our planet.   One of its amazing properties is its ability to dissolve other compounds making them available for living things to absorb into their cells.   The reason water can do this is because of the structure of the water building block – its molecule.

Amazing waterfall in Iceland...water is the source of all life!

Amazing waterfall in Iceland…water is the source of all life!

Everything on our planet is made up of different types of atoms bonded in combinations to each other to form molecules.  Earth of these atoms has an electrical ‘field’ which holds the molecules together.  The water molecule is made up of an oxygen (O) atom bonded to two hydrogen (H) atoms.   The famous H2O molecule!   What is important in the water molecule is that the electrical field is not evenly spread – it has a slightly negative end and a slightly positive end.   Chemists call it a “polar” molecule.   This property makes it able to dissolve other molecules, especially those we call ‘salts’.   These ‘salts’ are what living things need to build cells and have normal growth.

Acids and Alkalis
In nature all water has some level of dissolved salts.   Some of those salts release into the water a positively charged atomic particle called the hydrogen ion (H+).  This hydrogen ion is the active item of what we call acids.   Acids can be found in car batteries, lemons and saliva.

Some other salts release into water a negatively charged atomic particle called hydroxyl ion (OH-).  This hydroxyl ion is the active item of what we call alkalis or bases.  Alkalis can be found in ammonia, bleach and lye.

Amazingly (for the non-chemists amongst us) if an acid and a alkali ‘find’ each other dissolved in water that bond together to form….WATER!   In the process that take the H+ and the OH- out of solution.  They neutralize each other!  This comes important when we are trying to fix the pH in a soil.

So what is pH then?
pH is a term used by science to describe the amount of acid ions (H+) in a solution.  The pH scale goes from 1 to 14.   The lower end of the scale is very strong acids (like car battery acid) and the upper end of the scale is very strong alkalis (like bleach).  Right in the middle (at pH 7) is the neutral point – and the place pure water would fall.

The pH scale - borrowed from

The pH scale – borrowed from

Soil and pH
Soil is made up of a soup of chemical compounds – some that easily dissolve into water found in the soil.  Some of those will want to make the soil water acidic while others will want to make the soil water alkaline.  Most of these neutralize each other, but the remaining soil water will remain either slightly acidic or slightly alkaline.

Most soils fall between pH 5.5 and pH 7.  As a result most plants can happily survive in that range – although some prefer slightly more on the acidic side and other on the alkali side.

The important thing to know is that the pH of the soil is the key to what chemicals are available for plants to absorb.  A change in the pH means a change in the chemicals that the plants can take up.

Elements available for plants - from the University of NSW.

Elements available for plants – from the University of NSW.

 The Aluminum (ok Aussies – Aluminium!) problem
One of the problems with very acidic soils – those with pH levels less than 5 – is that Aluminum ions in the soil become very available for plants – so much so that plants can die from aluminum toxicity.

So…still to come in this blog series in the future
* Testing your soil pH
* How to change your soils pH

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About Gaz

I'm an Australian transplanted to rural Maine where I live on a small property with my wife and two youngest children. Life about family, work and trying the make the planet a better place for everyone.
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