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Growing in Perth's sandy soils is a huge challenge, and according to the results of a recent customer survey, it is one of the biggest frustrations faced by many of our customers.
The sandy soils here in Western Australia are ancient, and are devoid of nutrients and organic material. As far as a growing medium, sand provides for good drainage - but that's about it!
There is good news and bad news about having sand as a starting point in your garden.
The good news is sand is actually easier to work with than heavy clay soils. Sand CAN be improved to create a healthy loam, and CAN be developed into a wonderful growing medium.
The bad news is that there is not ONE magic silver bullet, and it may take time and/or money to get to a point where your garden will thrive. Here we will look at some options which will be helpful:-
1. Soil Waterholding ability.
Comparitively speaking, sand particles are quite large (microscopically). They have a smooth surface, and combined, have a small surface area.
Clay particles are on the opposite end of the spectrum - they are tiny - and combined have a huge surface area compared to sand.
(To explain: Imagine a basket ball is a grain of sand. Think about its outer surface area.
Now think about a number of golf balls - and how many would be required to take up the space of one basketball - I haven't done it - but what do you think? 20 - 30 golf balls? Now take the outer surface area of one golf ball, then multiply that by 20 or 30 times - and you can see that comparitively, the number of golf balls that take up the space of one basketball have a greater combined surface area.)
Soil particles all have tiny micropores on their surface which fill with water. Sand particles have larger pores, but but due to the smaller surface area, cannot hold as much water.
Because of this 'surface area' phenomena, clay soils hold between three and six times the amount of water that the same volume of sandy soil holds.
2. Organic Material and Humus.
Much of Perth's sandy soil contains less than 1% of organic matter.
Organic matter is made up of plant and animal residues in various stages of decomposition. The final stage - and most long lasting is humus, which is the residues of micro-organism activity, and is the most stable and long lasting form of organic matter; lasting thousands of years. All forms of organic material (decomposing to humus) are important additions to soil to feed micro-organisms. It is these creatures in their activity and life cycle which make nutrients in the organic matter available to plants. Under a microscope, humus is like a porous sponge. This sponge like structure holds onto water and nutrients, making them available to plants as required, and helps prevent leaching of nutrients.
Organic matter also hugely improves soil structure, allowing air and water to penentrate, and soil roots to grow into voids created around pieces of organic material.
How much organic matter to have in your soil is a matter of contention. It depends on what you are growing, other management practises in place (eg. mulching and irrigation) and even seasonality. But one thing is sure - the ideal amount is LOTS more than 1%!
3. Soil Structure.
A range of particle sizes is ideal for plant roots to grow. Lots of nooks and crannies created by big particles, with gaps in between them filled with small particles, creates pockets of air and water that plants need to thrive.
Too many big particles (Sand) - there is too much space, so water flows straight through. Too many small particles (Clay), and there is not enough space, so compaction and crusting happens.
Ideal soil has a range of particle sizes, and is generally referred to as 'loam'.
4. Cation Exchange.
Without getting too technical here, we all know what happens when we rub a balloon to generate static electricity and hold it near someones head, right? Hair is attracted to the balloon and it stands on end. This has all to do with electrical charge and the negative and positive attraction forces.
Amazingly, the nutrients in soil and plant roots have a very similar relationship.
Clay and humus (due to their electrical 'charge') hold onto nutrients in a way that sand simply cannot. Plant roots are able to remove these nutrients - slowly, and as required - from clay and humus particles. This process is called cation (Cat-iron) exchange, and you want soils to have a high cation exchange capacity (CEC) - otherwise nutrients applied to your garden will leach through with water, and won't be available to plants long term.
If you are interested in Chemistry, this is a fascinating field and there is much to be read about cation exchange and how it works.
If you are not interested in Chemistry, then take it from the scientists that clay and humus is good stuff to have in your soil.
So now you can see what sand lacks, and why it's not ideal to garden in. So what is the solution? As we told you - unfortunately there is not one quick, simple magic bullet. (If someone is telling you there is - be cautious.) However there ARE strategies to improve your soil, and do so dramatically.
1. Add Clay.
With those two simple words, I could continue writing for hours. There are several options now available to Perth gardeners for clay improvers to add to sandy soil. Each manufacturer will claim various benefits - and like most things in gardening - each product will have it's pro's and con's. However every garden is different, every gardener is different - and different things will work for different people. In fact a combination of things is usually the way things work in my experience.
Types of Clay available.
Bentonite Clay has been around for years and has been promoted by Perth gardening leaders for decades. Yes, decades. Amazing that there are still so many people who aren't aware of it, and it's benefits.
There are two types of bentonite available - sodium bentonite and calcium bentonite. Both will do the job of holding moisture in soil, however we tend to recommend calcium bentonite as calcium adds beneficial things to the soil, sodium (salt) not so much. There are other differences, but I don't feel they are particularly relevent to home gardeners.
Bentonite has many industrial uses. Used as a dam liner (the white clay you see around dams on many farms), in blasting and drilling, and even as a food additive!
The bentonite clay we sell is calcium bentonite, from a large reserve in Watheroo, about 250kms north east of Perth. This bentonite is one of the active ingredients in our product, Sand Remedy. There is a range of other ingredients we add to enhance Sand Remedy's functionality; and to provide a wider range of trace elements.. It is designed to create a soil environment where soil organisms thrive, thus increasing the health and vigor of your plants. Sand Remedy is Certified Organic, and independent testing has enabled certification under the Smart Watermark Approved scheme - and in WA this is the Watercorp's Waterwise endorsement. .
The other form of clay promoted in Perth is Kaolinite clay. This is a more common form of clay, and we use it in our mixes and sell it separately as "Cassies Clay". Cassies Clay is also Certified Organic and has achieved approval under the Smart Watermark scheme/Waterwise endorsement.
Bentonite Vs. Kaolinite
There is much discussion on social media & online forums about one type of clay being superior to another - in our opinion BOTH types work, and have their place. I would again say that every product has it's pro's and con's - and different things work better for different gardens, and gardening methods.
One thing that does not seem do be in dispute is application rates. If you see in gardening books that clay content of soil should be around 20 - 40% - this refers to kaolinite clays. If you were to use bentonite at that ratio, you would end up with soil not unlike concrete!
Bentonite should be incorporated at a ratio between 1 - 5%. We always suggest adding a smaller quantity initially then adding more if required. Start at around 500gm - 1kg per square metre - incorporated into the top 200-300mm of soil. You will know when you have reached an adequate mixture in your own garden by observation and checking soil moisture regularly. In our trials this is certainly sufficient to have effect. Don't be tempted to over apply - people that have run into problems using bentonite have usually used too much.
Kaolinite clay needs to be added at somewhere between 5 - 15kgs per square metre (depending on other soil improvement practices). If you're incorporating compost or biochar - potentially less clay is required. In more sandy soils; you'll need the higher end of the application spectrum. Don't be tempted to skimp at these rates - remember it is a once off application, so do it properly!
Bentonite needs to be incorporated into soil when dry; once it's wet it tends to stick to itself. Kaolinite clay is easier to work into any soil, another reason why it is favoured by some.
Please note - adding clay of any kind will improve your soil's water holding ability. But it WILL NOT stop soil from drying out. (Remember - no magic, silver bullets?) What it DOES, is make soil much easier to re-wet. If your soil is VERY dry you will need to add a lot of water to re-wet it. Remember clay holds up to six times the amount of water that sand does? So it will take A LOT of water before it is once again moist.
But once it is re-wet, providing you water regularly (and don't forget to mulch) you will notice it really holds onto moisture in between waterings; meaning overall you use much less water. And more importantly, water is always available to plants, not just once a week or twice a week on your allotted watering days! This promotes stronger and healthier plant growth.
2. Add Organic Matter.
To create a healthy, living soil you need to add organic matter. This can be in the form of animal manures, compost, straw, etc. It needs to be breaking down in order to feed soil microbes, which make the nutrients contained in the organic material available to your plants.
The quality of these nutrients is very important if you are growing food crops. If you aren't providing a particular nutrient to the soil - there is no way the plant can provide it to you in turn. This is why we recommend a range of materials to add to your garden. Making and using your own compost is terrific - but make sure you add lots of good stuff to your compost, or else add different things to your soil every now and again. Trace elements are extremely important - but they are only required in very small amounts. Adding rock dust once or twice (maximum) a year to the soil is sufficient.
Everyone with sandy soil complains that organic material 'vanishes'. Typically, 70% is used within a year (being eaten by microbes) and a further 50% of that remaining 30% is used the following year.
Stick with your efforts! Adding organic matter will build humus levels in your soil over time which will result in better plant growth, better soil structure and more disease resistant plants. The organic gardener's mantra is "feed the soil" - because this is what really feeds the plants.
Over time, (between 3 - 5 years) you will notice you need to add less material, or add material less often, as the humus levels build up. You will notice the depth of decent soil improves from a few centimetres to 20 - 30cms or so, and the soil will be full of worms and hold much more moisture.
The other option we have now is biochar. We sell Charlie Charcoal (biochar) - which acts like humus in the soil; it makes a habitat for beneficial soil microbes but they don't consume it - so it stays in the soil permanently. Charlie Charcoal is 98% pure carbon - so it is very stable, making it a once off application. Due to its honeycomb like structure, it is also very beneficial for water AND nutrient holding - we use it in our mixes at Green Life and really love it! It is Certified Organic, and has Smart Watermark/Waterwise endorsement. It works brilliantly with clay - they are perfectly complimentary in the way they combine to hold nutrients and improve soil structure.
Nobody said having a great garden would be cheap. Of course there are many things you can do to save money (eg. make your own compost) - providing you have the time. But surely even if it costs financially - there are worse vices to have than a hobby that gets you outdoors, keeping fit, and growing food for your family.
I have heard Peter Cundall say "If you can garden in Perth you can garden anywhere in the world." - so stand tall, sandgropers. Remember - no magic, silver bullets. But lots of rewards down the track. And that's the real magic. Enjoy the journey.
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