Archive for July, 2012

Since I’m having to stay quiet at the moment due to a dodgy back you have the pleasure of two posts in one week!

I get asked a lot about what sort of clay and how much to use in sands.  This is not an easy question to answer.  Clays as we know them are seldom pure minerals and therefore all clays tend to be different and so will react differently.  Bentonite (which is partly composed of montmorillonite) from one source will not be the same as that from another.  The bentonite mined in WA is calcium bentonite.  Most of the other bentonite sold in WA for sealing dams is sodium bentonite from interstate or overseas.

Clays are also dynamic and will respond differently depending on the environment into which they are put.

Water retention

Kaolins are only a 1:1 layer lattice and so do not hold much water.  Montmorillonites are a 2:1 lattice and can absorb large quantities of water. Depending on how these clay minerals were formed they may have calcium or sodium in the internal lattice. Sodium montmorillonite can hold 15-20 times its weight in water whereas calcium montmorillonite holds only 1-5 times its weight.  This is why sodium bentonite is used for sealing dams.

But, if you put a sodium bentonite clay into an environment where there is lots of calcium and magnesium, over time the sodium in the lattice will be replaced by the other cations.  That means the swelling ability of the bentonite will decrease.  Calcium bentonite is a more stable clay than sodium bentonite.

In one paper I came across, a comparison of water retention curves obtained for two natural bentonites, one predominantly calcium and magnesium and the other mainly sodium, showed the water retention capacity of the first bentonite to be greater, although the difference between them became smaller towards low suctions.  In another paper, retention and release of water were greatest in bentonite followed by illite and then kaolinite. As suction increased, the difference in water retention became less. At 10 centibars, bentonite retained 244 per cent more water than kaolinite, whereas at 15 bars, it was merely 9 per cent.

Water repellency

Trials in Western Australia showed no difference between the sodium or the calcium version of clays in their long term impact on water repellency but kaolinite seemed to improve water repellency better than smectites at the rates used (up to 1.6% by weight)

Cation exchange capacity

If your main purpose in using clay is to increase the retention of fertiliser then the CEC of the clay is relevant.  And in that case almost anything is better than kaolin or spongelite.  But a good cation exchange capacity is only relevant to cations (positively charged ions).  So if you apply all your nitrogen as nitrate then it won’t reduce leaching.  But don’t think you can apply all your nitrogen as ammonium either because that could be toxic to the plant.  Ammonium must be converted to nitrate by soil microbes before it is taken up by the plant.  Nitrate is the preferred form of nitrogen by most plants but is not held well by soils and moves freely in soil water.

So what does this mean for amending my soil?

That is the $64M question!  Firstly clays aren’t the only means of amending soil – there are organic materials as well and they tend to have much higher CEC’s than clays.  And they also usually contain nutrients.  BUT in our coarse sands, trying to amends soils with only organic matter is difficult. Keeping the organic matter content above 2% is nigh on impossible because it turns over and burns off so quickly.  There is a sort of cap on the amount of organic matter that can be held in soils that relates to the amount of clay in a soil.  So to increase organic matter you need to also increase clay content.

Organic matter can be quite water repellent so adding clay also sorts that problem out.

Water holding capacity is the area I think where there is the greatest lack of information.  But the soil with the greatest water holding capacity is not necessarily what we are after.  If we are growing plants, we need that water to be readily transferrable to the plant and that is where the real lack of information is.  Clays hold lots of water but don’t necessarily give it up easily.  And in our coarse sands, for water to be plant available, the soil needs to be much closer to field capacity than is traditionally accepted – because they ARE coarse and hydraulic conductivity is extremely low.  The bottom line is that we lack meaningful soil moisture retention curves for our situation.   The other thing to think about is that a soil that holds more water will also hold more nutrients in the root zone even if its not held by the organic matter or by the clay.  Sure, if you get a downpour it will be washed away but there is capacity there for nutrients to be held just in the soil solution and not bound by other mechanisms.

Working in this area is difficult.  Nothing is one dimensional.  We are dealing with complex systems.  Create a better soil environment for your plant and it may respond by growing better.  And by doing that it may NEED more water and nutrients.  Add clay to your soil and not only may it hold onto nutrients better but in the case of phosphorus it may not give it all back – at least in the first year or so anyway.  So adding clay may change the way you need to fertilise as well as the way you need to water.

Confused?  I can assure you, you are not alone!

Addendum:  Since someone asked the question.  It is possible to calculate the water holding capacity of a soil from the range of particle sizes in it.  But that doesn’t tell you anything about how tightly that water is bound up by the soil and hence how plant available it is.


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Lots of people think that all you have to do to grow in pots is grab some of the backyard soil and chuck it in a pot and Hey Presto!  But growing in pots is way different to growing in soil in the ground.

Confined space:  pots have sides and a bottom.  This restricts root growth and decreases the volume of media from which water and nutrients can be extracted.  Temperature fluctuations may be far greater in a pot.

Having a restricted root area in a pot is not necessarily a bad thing. Much better to have that ornamental Ficus in a pot than in the garden where it will seek out water leaking from any drainage/septic system and quickly grow towards and into it and clog it up.  And if you water frequently and feed well the top will grow and do so quite happily despite looking out of proportion.  So plants in pots will need more frequent watering than those in the garden.  If you have particular reason for wanting to keep plants long term in a pot then look at some of the more innovative root or air pruning designs such as rocket pots   or anti spiral pots .

The size and shape of the pot also plays a big part in how well (or not) the pot drains.  Tall thin pots drain better than wide shallow ones.  While we are on drainage,  NEVER sit keep your pot in a saucer filled with water.  You will get a perched water table (a soggy bottom) into which no roots will want to venture.  Zero aeration, reduced forms of nutrients which can be toxic and a high probability of root rot.  Plus if that drained water is never thrown away what you are essentially doing is recycled hydroponics without periodic leaching (throwing away the wastewater).  Some of the stuff that leaches out of the bottom of pots is unwanted and unhealthy as far as the plant is concerned.  Yes there may be nutrients in it but there’s also other nasty stuff.

Invariably, media for pots needs to be better aerated than soil.  As far as potting media goes you generally get what you pay for.  There is an Australian standard for retail potting mix.  Look for it as it means quality.  The pH will be good, it will be well formulated, well aerated and not toxic. Cheap potting mixes may be made with ingredients like sedge peat that look nice and black but are way too fine and just totally unsuitable for pots.  Cheap potting mixes have no guarantee of a suitable pH, may carry disease, be salty, devoid of any nutrients or if they contain relatively fresh wood chips or sawdust, can actually drawdown nutrients and take them away from the plant.  Mixes made from uncomposted manures and woody materials will be very dynamic, with large swings in pH shrinking in volume over time decreasing aeration and becoming easily waterlogged.  Your plants deserve better!

Potting mixes are dynamic.  Over time they will degrade and that is partly why you need to re-pot every once in a while.  Ingredients like pine bark will deteriorate and shed fine particles that can silt up the sides and base of the pot, decreasing drainage and aeration.

A few pointers on pots

  • Watch for pots with tops that taper in at the top – think ahead to when the root ball has filled the pot and you need to get it out to repot – and have to break the pot up to do so!
  • When you buy pots have a look at the drainage holes in the bottom.  Often they are inadequate – don’t be afraid to drill more.  And watch if the base of the pot is completely flat – think about how any excess water will get out – either buy a different pot or recognise you will have to place it on some sort of support that will permit it to drain.
  • Don’t over pot.  Putting a small plant in a big pot doesn’t do the plant any good.  You will have a large volume of potting mix that will be unexplored, go sour, be cold and generally yuk.  Much better to pot in a smaller pot and then repot later.
  • Never fill a pot to the brim with mix.  Leave a couple of cm at least, to act as a temporary reservoir when you water.
  • Think about the stability of your pot and the mix in it – Perth’s howling Easterlies are one good reason to have more sand than polystyrene in your pots!
  •  Black pots tend to last longer than coloured pots, especially light ones.  They do get hotter but because no light can penetrate, the roots explore more volume.

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