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.




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.


So we have Manchurian pears against a south facing fence that look like – well half dead. The answer seems to be the soil! Well sorry, I bet its not. Afternoon sun, reflected heat and 2-3 day a week watering and who knows what else. It most likely ain’t the soil!

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? 🙂



A question about Manchurian pear trees on the weekend in the local newspaper. The reply was: Don’t add any more fertiliser because its locked up and the balance of the soil is wrong.

This sounds like a statement straight out of Albrecht (now well disproved in most circles). He preached it was all about balance – before they were familiar with the effects of pH.

The person even said they have others growing well but two aren’t and they are in a corner – near a fence, block of limestone? Reflected heat in a corner? Maybe they are getting half the irrigation of the rest from a sprinkler by virtue of the fact they are in a corner. If six are fine then the problem is an isolated patch of soil/microclimate or maybe, but unlikely the plant is the problem.

Switch to Growsafe mineral fertiliser. No kickbacks here? And a foliar fertiliser such as Turbotrace every two weeks. No kickbacks you say?

Are the plants potbound, how big are they? How long have they been in the ground? Anything been going on around them? Even next door over the fence, not necessarily within the owners place. These are all questions I would ask before I started recommending – wait – more fertiliser!

This is Perth. It’s a Manchurian pear. Is the soil wet through the profile? And what is the pH? I am willing to bet the problem is lack of water/non wetting soil, even building debris/chunks of limestone in a particular spot. No nutrients will be taken up if the soil is dry. End of story. As for microbes – without soil organic matter they will not survive. And if the soil is actually soil, with clay and organic matter, it will have its own microflora which will prevail. Providing they have water.

I have nothing against microbes but one thing no one ever considers is the nutrient profile of a fertiliser eg a slow release. They aren’t all the same. It is FAR MORE LIKELY the nutrient difference between fertilisers causes the differences, not the fact some have microbes in them. I have encountered a very good example recently where the fertiliser concerned was found to have negligible magnesium and iron in it. And we are talking a major brand.

Foliar fertilising is most often a waste of time except in very specific circumstances. Plants were designed to talk up fertiliser through their roots. If they aren’t, fix that problem first.

Having spent my life diagnosing plant problems I shudder when I see some of these gardening column questions and replies. I don’t know which is worse – the person writing in with the problem or the person answering it.

I often diagnose remotely. But at the least I ask for pics. And tests sometimes. And often you can start with the basics. Dig around the base. Check soil wetness. Look for chunks of limestone or building debris. Watch the sprinklers at work – is one blocked are they all watering properly? In my experience its most often the basics. And in Perth non wetting soil/lack of water is the biggie. Followed by pH especially in coastal areas. More in my other blog posts on all this sort of thing.

Incidentally I have no problem with Growsafe fertiiser, or Troforte for that matter but I don’t use either because I don’t see the need. I buy straight NPK either quick or controlled release.

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!

During summer, growers experience a lot of problems with tomatoes. This article deals with the effects of temperature on tomatoes – on pollination and fruit set and also on ripening.  I will deal with diseases in another post.

Tomatoes are affected by high temperatures in a number of ways. Some sensitive varieties are affected when average daily temperatures exceed 25°C, whereas more heat tolerant cultivars are not impacted until daytime (maximum) temperatures exceed 32°C. There are even some cultivars are able to set fruit at temperatures above 35°C.

Under marginal conditions fruit may set without adequate pollination but the internal fruit segments will contain few seeds and the tomato will be flat sided and puffy. Irregular pollination can also cause ‘cat facing’ (http://vric.ucdavis.edu/veg_info/catface.htm).

In general fruit set is adversely affected when temperatures fall below 10°C or rise above 27°C. Optimum temperature for fruit set is 18° to 24°C. Even moderate increases in mean daily temperature (from 28/22°C to 32/26°C day/night) result in a significant decrease in fruit set.

As a general rule, the 8 to 13 day period prior to flowering is the most critical phase. If the average maximum temperature in that time exceeds 29°C, pollination and fruit set are impacted. However as pointed out earlier, this does vary according to cultivar.

Why aren’t my tomatoes ripening?

In hot weather people expect fruit to ripen faster. But with tomatoes the optimum temperature for ripening is 21 to 24ºC. When temperatures exceed 29 to 32ºC, the ripening process slows significantly or even stops. At these temperatures, lycopene and carotene, the pigments giving the fruit their typical orange to red appearance cannot be produced and so the fruit stays green.

For tomatoes light has very little to do with ripening. Light is not needed for ripening and fruit exposed to direct sunlight can heat to levels that inhibit pigment synthesis (As explained above). Direct sun can also lead to sunburn. Do not remove leaves in an effort to ripen fruit. Also, soil fertility doesn’t play much of a role. High magnesium and low potassium can cause blotchy or uneven ripening or yellow shoulders. But slowness to ripen is generally not due to poor nutrition and adding more fertilizer won’t help.

You can remove fruit which are just showing the first colour changes (mature green), and store them at 21-24ºC in the dark, preferably in an enclosed space or in the presence of fruit that give off ethylene gas such as bananas. This may speed up the process by up to five days.

References and further reading