Archive for the ‘Irrigation’ 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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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.

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Just a quick one – firstly, observations in my own garden last weekend.  Wherever there was a layer of organic matter over the ground there was bone dry soil underneath nice wet organic matter.  Not much point in that being wet though when the  soil which is where the roots are! Now, I don’t mulch as a general rule, so I’m talking whatever lands there from trees around the place, and breaks down over time.  Fine textured organic matter.   Conversely, wherever there was soil only and no layer on top, the rain of the past week or so had wetted the soil up to a depth of a few inches. 

I also received some progress reports on the mulch work done at Murdoch TAFE over the last summer.  They quite definitely show that coarse mulch is the way to go and no more than 50 mm thick if you still want soil to be wetted in the root zone (bear in mind that work was in summer with supplemental irrigation).

We are doing some work outside of Perth (north by a few hours actually) in some quite clayey soils with a range of soil moisture monitoring gear.  We’re getting quite high moisture levels in the soil but the shape of the graphs and the way the probes react to irrigations are telling us that much of that water is not plant available, the bulk of it simply sits  there making the soil feel moist but not helping the plant much at all!

And in case you haven’t been tracking rainfall and you have just been blindly following the watering days regime, perhaps you wouldn’t like to know that in May I had 210 mm rain at home compared to 21 mm in June.  So if you didn’t water in June  as per instructions, you may be in some degree of trouble especially if you’re growing things with shallower root systems eg veges or if you have new plantings in the garden.


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I’ve been away for a while.  Mostly because work is busy but also because I haven’t had much new to report.  But in the last couple of weeks I managed to get some more data on some recent mulch trials and have written it up and presented it to a group of people at a meeting.  The most interesting thing to come out of it was that you CANNOT rely on winter rains to wet up the soil profile.  Even after 600 mm of rain over winter, at 20-30 cm depth the soil was still dry!  And of course the thicker the mulch layer the worse the problem.

The other things that’s apparent is soils are highly variable.  Lots of preferred pathways exist in soil so, in these trial for example, there was a high degree of variability between the three replicate treatments in all cases.  When a significant rain event occured (33mm) the response under the unmulched soil was less than under the other treatments – probably because of runoff due to non-wetting.

While some of the mulches maintained the moisture content in the soil below over summer most were so dry that it really didn’t matter!  They were drought stricken.  This raises an interesting point.  What is the point of amending soil to hold more moisture when there isn’t any!  Certainly, in discussions with someone that conducted trial on a range of soil amendments recently, under the Water Corporation watering regime there was absolutely no difference between treatments. Drought is drought!  If plants are water stressed it doesn’t matter a damn about the improved water holding capacity of the soil if there’s no water in it they can’t access it anyway!  And in fact the situation could be made worse because clay, while holding more water, holds much of it a higher soil moisture tension (ie its harder for plants to extract it).

So this whole issue is complex.  But the main thing to think about is whether or not the soil profile is wet up throughout the root zone over winter.  And due to preferred pathways that exist in soil, you will need to dig down in more than one place to find out!  If the soil largely remains dry in the root zone (20-30 cm) at the end of winter then you need to think about how you apply mulch, especially if you are a) overhead watering and b) only watering 2-3 times a week.

I would dearly love to do some research on this topic but alas funding for “home garden” type issues doesn’t exist so we are reliant on the bits and pieces done here and there, often by institutions like TAFE.  And extrapolations from the commercial stuff done by eg DAFWA.

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Often when I’m out at the local hardware I see people fossicking around for a replacement lawn sprinkler.  I am sorely tempted to tell them that they shouldn’t just buy any old sprinkler, there is a reason that irrigation systems are designed and having water applied evenly over a lawn is one of them.  The performance of sprinklers and pop-ups varies greatly.  Even at one given pressure, different brands have widely varying specifications in terms of how much water they put out over a given time period (ie how many mm of precipitation) and over what area (usually circular but not always).  I see specs from around 9 mm an hour right up to 60 mm an hour.  This translates to a six-fold difference in watering time!

Superimpose on that, the pressure your system is operating at and the output can vary from the stated specs quite widely.  Even scheme water with mains pressure, can vary.  With bore or dam water you really need to do some serious measuring to determine your flow rates.  Lastly, how many sprinklers you’ve put on a station will also affect the pressure they run at and therefore their output.  You just can’t add sprinklers to a station and expect that not to affect the output of each sprinkler.  Add another row and you may end up with dead patches because the sprinkler patterns no longer overlap (they are usually designed to overlap by 50%).

Another variable to think about when you water is wind.  The howling easterlies in the morning can have quite an impact on whether you end up watering your lawn or your neighbours!  Or the street.  Maybe water earlier, or after the winds have died down.

And lastly, please get out and watch your sprinklers working once in a while.  If they have filters, these can get blocked and that will affect the output.  Some nozzles have a habit of turning themselves around, occasionally helped by playful dogs, so when you think you are watering your lawn you may be watering something else – like the verandah!  Lawnmowers and dogs are both good at wrecking sprinkler heads.  So often when I’m out running early in the morning I am greeted by gushing sprinklers – and these may last for weeks on end.  I feel like popping a note in the letterbox but of course am never carrying anything on me!

Lastly, I was shocked to find out most people never touch their controllers to alter the settings from day to day or season to season!  OK, maybe I’m unique :-), I accept that but I certainly alter mine almost on a daily basis depending on the temperature and whether or not its rained.  If you have 4mm or more of rain in a day you probably don’t need to water unless we’re talking veges or other plants that are growing rapidly.  Four mm is commonly regarded as the minimum effective rainfall – so anything less that that you may as well ignore.

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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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Perth is a city with variable water quality. There are places in Perth with great water – I am lucky enough to live in one of those places. The EC of my water is good (<500 mg/L or an EC of about 0.8 dSm-1) and it doesn’t have iron or calcium problems. Its just acidic and eats out valve seats in taps. But there are other places, some nearby, that have quite salty water. And over near the coast also there is sea water encroachment into a lot of the shallow aquifers (where people have their bores) which means their water is getting progressively salty -twice what mine is or more.

Other areas of Perth, notably coastal and northern suburbs have calcium in their water which means they may get white deposits on the foliage after a period of overhead watering. This is mostly cosmetic but also can also affect photosynthesis. The EC of the water is often good.

Many areas around Perth have iron in their water – were the water has that ‘rotten egg smell’. The quality of the water is usually good but the iron oxidises and can block up drippers if not removed. The easiest way to remove iron is by exposing the water to air so it can oxidise and drop out – use a water feature or some sort or simply spray water into the air before you tank it or distribute it to your garden. There are filters but they are a more expensive option.
It is useful to know the pH of your water – not that you’d often want to treat it but it can give you an idea of whether you may need to lime your garden over time. Using fertilisers generally depresses pH so having acid water as well may make that worse. Conversely, if your water is alkaline and you use fertilisers you may not have to lime at all!

If your water is salty then that will cause problems with any overhead watering. As the salty water evaporates the residual salt burns the leaves. Over time, with sprinkler irrigation the salt may accumulate in the root zone and that can also cause problems. If you can get through summer then winter rains may do the required leaching, otherwise you may periodically have to do a heavier irrigation to wash the salts down out of the root zone. Commercial growers generally work on a 10-20% ‘leaching factor’.

You can get your water tested simply at a Pool shop, they can do pH and EC for you and that’s often all the infomration you need.  If you suspect you have bigger issues, then the Chemcentre  is one place that will comprehensively test it for you.

There are a heaps of good references out there if you can to know more about water quality. The first is probably The Perth Groundwater Atlas which tells you what your water quality is likely to be for a particular location and how deep it is to the water table.

The Department of Agriculture and Food WA has a number of publications about water. Water quality in home gardens, gives good information on you can grow according to your water quality.

Other publications are:

The latest version of the Australian Drinking Water Guidelines are here and here.

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