Archive for the ‘Potting mixes’ Category

Plants can be stressed and wilting when there is still quite a lot of moisture around in the soil.  The reason for this is partly because they can’t “pull” hard enough to get it out – and how much pulling power they have is partly dependent on species.  Many WA natives can pull pretty hard!  Most European plants can’t pull very hard at all.

Then there’s the soil itself.  Depending on what the soil or potting mix is made of, plants may have  a hard time getting anything useful from it.  Some clays cling onto their moisture pretty tightly.  Other materials may be coarse and have very little capillary action or hydraulic conductivity so the water is trapped and plant roots can’t get to it.

From way back in the archives this great little experiment was done (in Australia in 1979).

A range of soil and potting mix components were put into plant pots.  Either alone or mixed.  Marigolds were planted into the pots and grown, fertilised and watered as normal.  Then at a point in time watering stopped and the researchers recorded how long it took the plants to wilt (they used three stages of wilting – first signs, then all leaves wilted and finally death!  They also then measured how much water was left in the soil/potting mix.

This is what happened.


Days to first leaves wilting

Days to all leaved wilted

Available water holding capacity (vol %)

Unavailable water (vol %)

Coarse sand







7.1 34.7 6.6
Peat moss 5.7 7.3 50.2 6.1
Sandy loam 6.5 7.8 30.8 4.5
Pinebark 10.1 10.4 38.5 13.7
Sawdust 10.0 11.6 51.9 12.3
Poppy straw 8.1 12.0 50.8 12.3

The table show that while peat moss holds one of the highest amounts of water plants in it wilt faster.  Why?  Well peat moss loses a lot of water by evaporation.  Poppy straw, scoria and coarse sand lose a bit less while sawdust and especially pinebark resist water loss through evaporation.

Then there is transpiration ie the plants use of water.  Again there was rapid loss of water from peat moss caused by plant transpiration whereas pinebark had the highest resistance to water loss from transpiration. Plants do not transpire as rapidly in coarse sand, scoria and brown coal as they do in peat moss. Sandy loam also limits plant transpiration possibly because of its poor aeration and drainage properties in a pot.  Where plant transpiration is limited you can expect plant growth to be limited.  So in peat moss you can achieve very high plant growth rates but only if the water can be kept up.

I have only given the results for the straight soil/potting mix components.  The results for some mixes are in the original paper (Water relations of nursery potting media) and the performance of mix can actually be calculated from the relative amount of each in a mix.

So what is the bottom line of this?  Available water holding capacity is easily calculated in a lab.  But it ignores the plant factor.




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I’ve already been accused of making things too complicated this week.  But it seems its worth reading labels and even safety data sheets when it comes to fertiliser.  It all started with two empty pails of Osmocote I had at home. Bought from Bunnings. Casually looking at the label of the All Purpose Landscape mix I noted 2.2% magnesium and 0.2% iron.  Then skipping over to the Native Gardens mix I saw 0.2% magnesium and 2% iron!  A typo maybe?  Maybe the two were mixed up?  The Safety Data Sheet for the native mix from Bunnings says less than 1% each of iron chelate, iron sulphate, magnesium sulphate and magnesium oxide.  That doesn’t really compute and its not that helpful. Oh well lets move on. Lets look at the commercial stuff.  Osmocote Pro low P 8-9 month has 1.8% Mg and 0.2% iron. The normal landscape 8-9 month has 1.2 and 0.33 as iron chelate.  Seems reasonable.

Now check the SDS of both commercial products.  Well the first line is interesting!  The commerical products are both based on ammonium nitrate as the N source.

Have a look at the Bunnings versions.  Instead of being 30-60% ammonium nitrate they are both 30-60% urea!  Less than 10% ammonium nitrate.  Now I don’t know about you but I don’t know any 8-9 month slow release urea formulations – usually more like 4 months.  So while both are called Osmocote there are fundamental differences between the commercial and the Bunnings formulations.

Does it matter?  Probably not that much on the nitrogen side of things.  The Bunnings versions of each say to reapply every 6 months so a bit shorter than the 8-9 month commercial ones.

The magnesium is a worry though.  Magnesium is sort of between a trace and  a macro element and leaf tissue concentrations generally run at around 10-20% that of N and K.  I can’t see 0.2% supplying a plants needs.  And as I noted previously the magnesium content in Nutricote is similarly abysmal.  Perhaps that’s why Epsom salts (magnesium sulphate) gets such a good rap!

And a final reminder about iron. If you’re using any potting mix with wood waste or pine bark in it, they adsorb iron.  We have always added something like 500-1000g/cubic metre of pinebark potting mix.  You might get away with as little as 200g for other wood based materials.  But make sure it is iron – combine chelate and sulphate if you wish but don’t try to do it with mixed trace elements – you will end up with boron toxicity.


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A few weeks ago I posted on the stupidity of controlled element formulations.  I can now add to that post as I have received some more information about a few other fertilisers.  The one that’s really been interesting is Troforte™.  I specifically looked at Troforte™ Native.  The really interesting thing is how much higher than all other products is the iron concentration.  The magnesium is not bad either, nearly three times higher than the Macracote™ Grey  range but not as high as Baileys Native or some of the other Polyon™ formulations.

I am will to bet that the superior performance of Troforte™ observed by many has more to do with the amount of iron in it than any added microbes!  A dangerous statement I know but until someone provides me with proper experimental evidence to the contrary I think I will stick by that comment.  Now iron is cheap as chips to buy so you could try adding more iron to any other controlled release product, especially if using it in a wood based mix – and maybe a bit of magnesium too depending on the analysis.  Or you can just dig a little deeper in your pocket and pay the extra for Troforte™.

The disparity in nutrient contents between fertilisers makes it very hard to make a fair comparison between products. Invariably even the N, P and K don’t match up let alone getting down to trace elements.  This is why I am always highly skeptical when someone says one product is better than another.  Seldom are you comparing apples with apples.

Troforte™ is also the only product on my spreadsheet to state a calcium content.  That doesn’t mean the rest don’t have any but its not stated, not even on the SDS sheet and I did ask for complete analyses.  A few other products also don’t state copper, molybdenum or boron contents although their SDS sheets do show those elements to be in there.  Now some plants do require boron in reasonable amounts – carnations, cauliflower, apples and strawberries, for example, so if there truly wasn’t any in there, that would prossibly cause a problem.  Many of these products are imported from America.  A part of me wonders if the lack of boron might be because some irrigation water over there is high in boron and the line between deficiency and toxicity is quite fine.

I don’t have the money to go and analyse all these products but it would be nice to know what really is their analysis.  And why do manufacturers have to be so cagey about what’s in their products.  Not all, but some.  I have encountered this before while working in the Department.  We were compiling a fertiliser spreadsheet that required inputting the analyses of all the fertilisers.  Some resellers were really helpful and quite upfront.  Others were not!

Plant nutrition is not rocket science. There are no secrets.  But I suppose that means there would be no marketing edge for any company wouldn’t it? 🙂



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I’ve recently had cause to look at controlled release formulations. We’ve been having unsatisfactory results with a couple. Both commercially available products. I looked at the NPK and trace element contents and found one had a N:K ratio of 4!!!! Obviously made for high leaching situations but totally unsatisfactory for anything needing cell wall strength, branching ability, disease and pest resistance. The other one has a reasonable N : K ratio but negligible magnesium and iron so no wonder the plants weren’t happy. Especially in a wood based potting mix which will fix lots of iron.

So what gives? We are considering shandying the first one with a slow release K only product. But why produce something with absolutely woeful amounts of both Mg and iron? This is not a matter of low, its woeful.

Moral of the story is it never hurts to look closely at the analysis of what you are using. The problem mightn’t be you!

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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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If you’re in the business of trying to grow anything, the weather we’re having isn’t making it easy! And even if you’re not a commercial grower, the twists and turns of the weather lately makes it really hard to get the best from your plants.

I downloaded the weather data from Medina Research station for last December to today. The daily evaporation for that period ranged from 1.5 mm for the day up to 11.4 mm! That’s a ten fold difference. And I bet the settings on your irrigation controller haven’t changed in that whole time! When you sum up the evaporation over a week it doesn’t look quite as variable – 44.1 mm, 44.2, 57.9, 63.4, 54.2, 66.2 and 58.4 – which is only about a 50 % variation. Actually last week would have been less because I haven’t subtracted off the rain we had, which at home was 42 mm but for Medina was about 30mm. So you really need to take that off the weekly evaporation. Incidentally, from a commercial point of view we don’t regaard anything under 4 mm as being effective ie its not regarded as beng enoguh to do any good.

Anyway, what I’m trying to say is that unless you vary your irrigation you are either going to run into trouble with plants drying out, or you are overwatering like mad to cater for the worst case scenario, wasting water and probably washing away most of the fertiliser you put on into the bargain.

Commercial growers vary their water on a daily basis if they are doing their job properly. And at this time of year, depending on their crop they may be watering anything up to four times a day in amounts that will add up to the total daily evaporation – plus or minus depending on crop and stage of growth. Now, no home gardener can be bothered with that, plus if you have to abide by the two or three time a week watering edict, you simply can’t water every day unless you hand water. And in sand that’s a problem because it does not hold water and actively growing plants will dry the root zone out in well under a day and get stressed or even die – particularly if they are small seedlings.

What can you do to help this? Amend your soil with clay and organic matter. Be wary of the source – you get what you pay for! Don’t run the risk of getting dieback with your clay and remember that compost has quite a lot of phosphorus in it relative to nitrogen. Much of this is explained in previous posts.

The other thing you can do is mulch. But remember to use coarse chunky and COMPOSTED material (again to prevent importing disease and weed seeds) so any applied water runs straight through or you will end up with wet mulch and dry plants. Keep mulch away from the collar of the plants. Mulch does a good job of moderating soil temperatures and the trials we have been running at Murdoch TAFE show that if you start with a nice wet soil profile, mulch can do quite a good job of helping to maintain the soil moisture. Don’t layer the mulch on too thick though, keep it in proportion to the size of the plants it is around. And don’t add the same amount to it each year, just maintain the thickness you need.

Still though, if you are having trouble with plants not thriving in the garden, the best thing you can do is get out with a spade and dig. You may be surprised. I have been. Beds I thought should have been well wet have shown dry patches and even been dry about 25 cm down – well within the root zone of any plants.

Always beware of newly planted plants. Inferior potting mixes (you get what you pay for) and simply the type of mix, may mean the root ball of your plant dries out before the soil around it.

In my experience many plant problems come down to the basics and on our sands its odds on its watering or lack of nitrogen and those two are inextricably tied together. Too much watering means you wash all your nitrogen away. Assuming you had enough in the first place – nitrogen is the one nutrient always in short supply on our sands, and manures and composts never have enough of it while at the same time they are overloaded with phosphorus.

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OK, green side up is a good start! And take it out of the pot too. Don’t laugh it has happened!

Seriously though how should you plant something to ensure it has the best chance of survival?

Choosing the right plant is a good start. For the situation that is. If a plant needs full sun then planting it under a tree is a good way to get a leggy, lopsided plant. Likewise if it needs shade, then come summer it will likely get burnt. Also be aware of what situation the plant was in when you bought it. If it was under shadecloth then be careful about putting it out in full sun in the middle of summer. Plants need acclimatising and that takes a few weeks of gradually moving it and adapting it to its new environment.

Having a healthy plant to start with is also important. That means buying your plant from an accredited nursery look for the NIASA logo. This helps ensure you are not bringing disease home. Nurseries with this logo follow strict guidelines for hygiene such as keeping plants away from contact with any possibly infected soil ( so plants must not be directly on the ground). Water sources must be from a source that will not carry disease (such as a bore) or it must be treated to ensure it is free from disease (eg dam water). Nurseries also should not use fungicides that will mask signs of root diseases such as Phytophthora (dieback) because as soon as you get them home and they run out of their fungicidal protection they will likely succumb and die, infecting your soil and garden in the process.

Look out for cheap plants. You generally get what you pay for. Throw-away lines are usually root bound and although they may grow initially they will soon strangle themselves and die. Don’t think that slashing and ripping roots apart solves the problem – it doesn’t. About the only thing those plants are good for is growing on long enough to take cuttings from and then discarding.

When buying larger plants that appear to have plenty of room for the root ball it is still possible for problems to exist sight unseen. If that plant was root bound in the tube and then potted on once or twice, then that root bound portion of the root ball still exists and will still give problems down the track. Another reason from buying from a reputable accredited nursery. Consider buying tubestock where its easy to look at the root ball and see what condition it is in. Small plants are capable of growing fast and will often catch up to larger ones – possibly because they do develop much better root systems in the ground if put into the right environment.

When you get your plant home, make sure it is well watered if you are not planting it straight away. And don’t YOU keep it until it is rootbound before planting.

When you get to planting time make sure you excavate a hole bigger than the plant requires and as you fill the dirt in around the root ball tamp it firmly with your fingers at the base of the hole and back fill. A good watering and you’re done! NEVER fill the hole and tamp it all from the top. You need the ground to be uncompacted at soil level for good moisture penetration.

Should you add other stuff into the planting hole? Well I seldom do. But then I plant mostly natives that come from an environment that is pretty devoid of anything. And my soil is foothills soil so I have a bit of clay around. You certainly may need to use something to ensure the soil is wetting well and evenly. There is a bit of controversy about the value and longevity of some wetting agents (in another one of my blogs) so I’ll leave that up to you. A bit of clay works well and is longer lasting. Just mix it into the profile. Composts and other sources of organic matter can be useful for several reasons. They provide some nutrients but beware of those that are fresh because they may contain ammonium or high levels of phosphorus, both of which can be harmful. A well-made compost should smell earthy and not generate heat if left in a pile. Most composts also have a low nitrogen to phosphorus ratio which means your plants will still require additional nitrogen. Contrary to popular belief, the nutrients in composts and manures are largely water soluble and can leach quickly if you overwater.

However, given that most potting mixes are largely bark/sand/sawdust etc, then incorporating some potting mix/compost like materials around the planting hole can be useful. It provides a gradient for the roots so they don’t suddenly come from the highly organic potting mix out into a hostile gutless sand – an environment where there is a relatively high water-holding capacity into one which dries out very quickly. The trap being that you may be watering the root ball and thinking it is moist when under the ground, the roots are getting out into sand and being subjected to drought conditions.

Should you put fertiliser in the base of the planting hole. My belief is NO. In our sands, the only way fertiliser goes is down so with no roots below, it will be wasted. I prefer to see fertiliser applied closer to the surface. Controlled release fertiliser should be applied (dibbled) below the soil surface to minimise fluctuation in temperature which will see its release rate go sky high.

Seasol and other seaweed products. These contain small amounts of cytokinins which help promote root growth but just remember most are NOT fertilisers, you will need to use fertiliser as well.

Now what if you’re planting a larger tree. The trick is to not plant too deep. The flare of the roots should be visible at the surface. If you buy a tree and cannot see the root flare at the base at the potting mix surface when it is in the pot, you must clear away that excess mix first in order to find the correct level. There is an excellent series of powerpoints with the finer details of planting large trees at this website.

Mulching? If you want to mulch, make sure your irrigation will penetrate below the mulch. Preferably driplines should be below the mulch. Use a coarse mulch, don’t have it too deep, it needs to be in proportion to the plant and keep it away from the base of the plant so you don’t induce collar rots. There are articles on mulching elsewhere in this blog. Meanwhile, there are some trials going on as we speak that I hope will give us some insight into what happens down in the root ball under various depths of mulch, especially with respect to soil moisture. More on that later.

Finally, remember that your plants were probably watered in the nursery once or twice a day so coming into your garden and subjecting them immediately to lesser regimes may not be a good idea. Again you will need to acclimatise. Give your plants time to get out into the soil and develop a bigger root system before you start restricting water to them.

Many highly organic potting mixes, if they dry out can be hard to rewet and many also dry out quite quickly because they have been designed for the environment in a pot, not in the ground. The way water behaves in a pot is quite different to the way it behaves in the ground. This is why its not a good idea to use soil as a potting mix. Drainage and water movement is quite different. You can get a perched water table at the base of a pot, pots also heat up more than the soil and depending on the shape of the pot there can be big variations in capillary movement which mean a pot may or may not drain well. This is why low, wide pots or trays drain poorly compared to taller slimmer pots.

Wow! I think that’s about it, this has been a long one but there is a lot to cover. I hope you find it useful.

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