Aerating Your Soil with Perlite

One of the biggest problems we have with soil in Australia is one of compaction.  Agricultural soil becomes compacted through over-tilling, over-cropping and use of machinery.  Garden soil becomes compacted from the building process and then us walking on it when wet.  Soil in a natural bush environment also becomes compacted as a result of various processes, including water saturation.

If soil is compacted, then there are a reduced number of pore spaces available for water and air infiltration.  The soil will drain very slowly.  It may become anaerobic.  There will be little root penetration.

In short, a compacted soil is not great for growing plants in!


Generally, the solution to compaction is to break up the compacted layer and then prevent it from becoming compacted in future.

In agriculture, this might involve machinery being use on the same track only.  In your garden, you might stick to the paths where possible, not work garden beds when wet and use planks of wood etc to try and more evenly distribute your weight if you can't avoid getting in there.

Improving the structure of the soil with generous applications of organic matter and reducing tilling to allow aggregates to form, will improve your soil and help prevent compaction, as well as adding in organic carbon and increasing the soils water and nutrient holding capacity.

Another way to do this, with a more immediate result, is to add perlite.

Perlite is a naturally occurring volcanic rock, actually a natural glass, which is a light grey, almost transparent, to a glossy black.  Because it's a glass, it is chemically inert and has a neutral pH.

It is commercially heated until it pops in a manner similar to popcorn.  The expanded perlite is white, can be four to twenty times larger, and very light weight - a 100l bag of perlite is surprisingly light.

Perlite (courtesy of US Govt, via WikiCommons)

So much so, that the first time I bought it I prepared myself to lift this heavy weight, completely overdid it and unbalanced myself.  I left the store that day carrying a baby in one arm, my bag over that shoulder, a 100l bag of perlite over the other shoulder and holding a toddler's hand.  That's how light it is :)

There are different grades and sizes of perlite produced for different applications - plasters, filters, insulation etc.

In horticultural applications, perlite is utilised because:

• it is pH neutral, sterile and of course weed free, meaning it can be used in the soil, in containers, in greenhouses, in hydroponics or aquaponics...  the list can go on :)
• it's structure means it cannot be compacted and is a long term soil amendment;
• it is lightweight, making it ideal for potting mixes;
• it has many surface pores which can hold nutrients and up to 10x the particles weight in water, releasing these to the plant roots on demand;
• the surface pores also give a greater area for microbial activity to take place;
• the irregular size of the particles mean that they create pore spaces between them, allowing air, water and plant roots access.

Production nurseries commonly use a mix containing mainly perlite for rooting cuttings, because the potting media therefore allows easy access to developing roots.

I like to use it in garden beds that are heavy in clay, because it prevents compaction, improves drainage, air movement and root penetration.  I also use it in raised beds, with organic soil and compost, because perlite is a more cost effective way to fill the beds (I dislike the idea of using builders waste or bricks in the bottom of a bed).  In potting mixes, it improves root development.  In all situations, it holds nutrients, reducing the effects of leaching from irrigation and heavy rain, as well as holding water and making my garden more drought tolerant.

There's a local source close to me in Dandenong, who can do prices for bulk purchases, but you can generally buy it from hydroponics stores, gardening centres (the big sheds especially, though usually in smaller bags) and on occasion aquarium supply stores.  There's also these guys in Heatherton, Vic and another that has outlets throughout Vic (NB: they sell 20kg bags apparently - not sure how much that is in litres!).

Now I've sold it to you, I should make you aware of what might be a big negative for perlite for some.  

It is a non-renewable resource that is mined.

I'm not going to go into the politics of this, but will simply say that I use perlite with little guilt because my current understanding is that there is very little environmental impact of it's mining and, although non-renewable, it is stable in form and therefore an almost permanent amendment to the soil.

For your info...

The biggest producers are known to be the US and Greece, with several other countries also mining supplies.  The US of course provides details of their perlite mineral resources and state:

"Perlite mining generally takes place in remote areas, and its environmental impact is not severe. The mineral fines, overburden, and reject ore produced during ore mining and processing are used to reclaim the mined-out areas, and, therefore, little waste remains. Airborne dust is captured by baghouses, and there is practically no runoff that contributes to water pollution."

The Idaho Minerals website has this to add:

"The global perlite industry actively mines a total of less than 8 square km (3 sq miles)—about 1% of the total perlite deposits available worldwide. Perlite mining has limited environmental impact. There is little overburden to manage, minimal waste, and no chemicals are needed to process the ore. Once mined, perlite does not create by-products during expanding and processing, as only the physical form is altered."

Further info:

http://www.perlite.com/Sustainability_Fact_Sheet.pdf

http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0004/238180/Perlite.pdf

And to show that there are occasional adverse effects of perlite mining: 

http://news.google.com/newspapers?nid=1370&dat=19960728&id=-WUVAAAAIBAJ&sjid=0goEAAAAIBAJ&pg=5092,4394077