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.




I have to apologise for an error in yesterday’s post.  You may have been more confused than necessary, I missed one graph out and put two in of another one.  So now if you look at the second graph its different.  And correct!

This post is a bit longer and more technical than usual.  I originally wrote it for a different audience and this is my attempt at making it user friendly for most people.  Like many things in nature it’s complex.

Healthy soils teem with microbes. Inoculating soil with with microbes to boost crop production is all the rage at the moment with companies sprouting – well, like microbes! Usually they are being sold to the commercial grower but more and more they are also being marketed into the home garden market where cost is less of an issue (ie home gardeners don’t have to make a profit).

Whether you can parachute in foreign organisms and expect them to live and prosper is really open to debate.  In most studies it doesn’t happen.  And in sandy soils with virtually no organic matter there is nothing there for them to feed on so it is even less likely.  Sure you can add composts and manures but these don’t tend to hang around much in sands, they burn off very quickly unless some clay is also added.  And large amounts of compost and manure provide phosphorus far in excess of plant needs which means it leaches down into the groundwater, potentially getting into waterways and causing algal blooms.

I have seen some papers showing how mycorrhizae have increased plant growth, even without fertiliser being added.  Those papers carefully omitted to cite a soil analysis!  One thing to bear in mind – I certainly don’t dispute that some substances can increase plant growth but they can’t do it forever without, at some stage, the food supply having to be replenished.  What you are doing is mining the soil.  It might work once or twice but then you run out of food.  Plants can’t feed on fresh air!  OK I can hear you saying they can fix nitrogen from the air – yes – but not phosphorus or potassium!

Mycorrhizae may alter root architecture – ie the way roots branch and proliferate.  That can make plants take up nutrients more efficiently by exploring more of the soil.  So you might see a growth spurt – but again, that nutrient will have to be replenished if that response is to be repeated and maintained.

So lets look at microbes.

A changing population

Populations of fungi are not constant. They change frequently in response to a whole range of factors. Even those associated with a single plant, change with the growth stage of that plant. Figure 1 shows the relative amounts of different fungal species over time for a pea plant. In the vegetative stage, Fusarium is most common but diminishes over time in contrast to Heliotales which increases as the plant matures.

The relative amounts of each fungus also change with fertility (Figure 2) and crop health (Figure 3).

                     Vegetative                                                    Flowering





Figure 1.  Change in microbial populations with growth stage in the rhizosphere of pea. (Note the rhizosphere is that thin coating of soil left on the roots after the rest of the soil is shaken off).

OM rates

Figure 2  Relative amounts of fungal species in pea roots grown with either nil or three levels of organic fertiliser (OF)


Figure 3.  Relative abundance of fungal species associated with the rhizosphere of healthy and diseased pea plants.

You will note that in all the examples above there is a mixture of ‘good’ (e.g. Glomus sp.) and ‘bad’ species of fungi (e.g. Olpidium sp.).  This is normal and disease only occurs when this balance is disrupted in favour of the pathogen and other conditions in the environment and host are right for infection and disease development.

Mycorrhizae are one group of fungi known to have beneficial effects on plant growth in some circumstances, but this is highly variable. The term mycorrhiza covers a large number of genera. Glomus species are some of the most prevalent. Examples of their effects on plant growth are provided below.

Highly specific effects

Example – Three mycorrhizal species were studied on basil.  None affected plant phosphorus level.  Only one significantly affected plant growth and increased the amount of one essential oil produced while the other two increased the amount of a second oil and decreased that of a third.

Microbes can also have adverse effects such as growth suppression

Example ‑ A trial on onion and plantago showed varying effects of mycorrhizae on plant nutrient levels.  The authors expected the mycorrhizae to increase host nutrient levels, however in some cases they reduced them.

Example – Twenty-three different mycorrhizal strains were evaluated for their symbiotic response with Piper longum (long pepper).  Almost all resulted in increased plant growth, biomass and nutrient content (nitrogen and potassium) over the control, however six species depressed growth.

It’s Complicated!

Example – A comparison of mycorrhizal versus non-mycorrhizal roots showed phosphorus uptake doubled and was independent how much was in the soil. There was no additional benefit of the mycorrhizae on plant growth other than that due to increased P uptake.

Example – The effects of a mycorrhiza on growth and photosynthesis of cucumber were studied using different rates of nutrient supply, phosphorus ratio and different forms of nitrogen.


Plants inoculated and given full-strength nutrient solution showed a 19 per cent reduction in total biomass compared with mycorrhizal (AM) plants (Figure 5).

The highest percentage of mycorrhizal infection in cucumber was found at the low P treatment, however a 90 per cent reduction in total nutrient supply almost totally counteracted the potential positive impact of a low concentration of P on mycorrhizal infection.

The extent of mycorrhizal infection in cucumber was correlated with a low root P concentration, which agrees with other studies that plant P status influences mycorrhizal infection.

Not all plants are mycorrhizal

Some plants do not form relationships with mycorrhizae even when inoculated. Brassicas, beetroot and spinach are among those.

Local species usually prevail

This is probably the most important consideration of all.  Any microbe placed in a field situation will face competition from local species and may eventually be displaced.

Example.  A Turkish study took 70 soil samples from 25 different plant varieties grown in local fields.  Arbuscular mycorrhizal fungi (AMF) were found in 59 soil samples; 58 of these were identified as Glomus and one as Gigaspora.

The effect of the local Glomus sp. was compared to a commercial preparation on tomato and cucumber plants.

The local Glomus species increased cucumber and tomato plant growth, but the commercial mix did nothing.  The local Glomus species colonised plant roots at almost twice the rate of the commercial one.

The relationships are very specific

Each plant species, and even variety, tends to have a preference for certain mycorrhizal species. It is difficult to manipulate relationships and so the effect of inoculating a soil with mycorrhizae depends on many factors.  If there are already successful mycorrhizal associations present, those existing association may be stronger and the introduced species may fail to displace those already present, or it may displace a proportion of the existing associations.  Inoculation with mycorrhizae does not necessarily mean more root colonisation in terms of either numbers or species.

And lastly – Beware trial results!

Many research results, when you read the fine print, are from work that has trialled mycorrhizae under unnatural situations, whether using sterile media or in a laboratory. Those results are not often reproducible in field situations.  They work because the introduced (foreign) microbes have no competition.  A better approach and one more likely to succeed, would be to use local species, bulk them up and inoculate back.

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.

Because manure is organic and natural it must be good right?

Well not always.  The use of raw animal manures is actually banned to a large extent under most food safety programs (and that includes organic produce) due to the risk of pathogens that can infect humans such as Salmonella, Listeria and E coli.  They are totally banned from applying to any leafy veges and have limitations in terms of days before harvest for other crops.

Despite what most people think, the nutrient content of most animal manures is largely water soluble and in sandy soils doesn’t hang around for long.  Given most people also apply most of the manure at planting time when the plants are smallest and have the least extensive root system, all this results in is ground water pollution!

Animal manures also have quite a low nitrogen to phosphorus ratio which means you end up putting on a heap of manure simply to get enough nitrogen – which means the system is well overloaded with phosphorus – more leaching!  The reason manure gets such a favourable rap in most places is because if you have a large clay component to your soil, some of the phosphorus binds to that clay and becomes unavailable.  That has the effect of bringing the nitrogen ratio up to reasonable levels.  That doesn’t happen in our sands.

The ability of the clay to bind to phosphorus diminishes over time.  Depending on how much manure and fertiliser you apply, eventually those sites are filled up, the clay can’t hold any more and any more phosphorus you apply will leach in the same way as nitrate.  A soil survey done a few years back showed that most gardens in the older suburbs of Perth are in that category and don’t need phosphorus to be applied – probably for the next 10-20 years!

The nitrogen in raw animal manures is mostly ammonium to start with –that is toxic to plants in large amounts – most plants need to convert ammonium to nitrate to take up the nitrogen.  While ammonium is more readily bound to cation exchange sites in soil, in our sands it’s also very readily converted to nitrate which is highly leachable (more groundwater pollution).

Average composition of stored animal manures at 40 to 60% moisture.

One tonne (1000 kg) of manure contains
  Nitrogen (kg) Phosphorus (kg) Potassium (kg)
Range Average Range Average Range Average
Horse 7 to 12 9 5 to 9 7 4 to 13 5
Cow 8 to 11 9 5 to 8 6 4 to 13 7
Sheep 5 to 14 9 4 to 10 8 5 to 7 6
Pig 6 to 12 9 5 to 8 7 4 to 10 7
Fowl 8 to 26 18 6 to 20 13 4 to 12 7
Note: To convert these figures to percentages, divide by 10.

Chicken manure breeds flies – in particular, stable fly which is a biting fly that attacks horses.  These flies will also breed in vegetable residue if it is applied and buried too thickly.  Other types of manure breed flies but to a lesser extent.  The flies are attracted largely by the ammonium but also by other compounds in the manure.

Many manures can bring in in large amounts of weed seeds – weeds that may not exist in your area or your garden until you buy in manure from elsewhere.

So how to use manure responsibly? 

  • Composting is a great start, it gets rid of the harmful pathogens.
  • Apply in amounts that are relative to the size of the plants and only in the drip zone.  Don’t assume that one application will last.
  • Amend your  soil with clay so it can better hang on to nutrients.
  • Dilute manure with other sources of organic matter that are lower in phosphorus such as greenwaste materials (also composted to avoid bringing in disease and weed seeds to your garden.

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.

There are many myths about potassium. The biggest one being you need it for flowering.  WRONG!  You need potassium no more or less than any other nutrient for flowering.  Sure, some commercial growers might change the ratio of nitrogen to potassium as the plant moves from vegetative to fruiting but it has nothing to do with the formation of flowers and fruit, it’s more about flavour.  Too much nitrogen can produce fruit that is big but watery, tasteless and low in sugar and other flavour components

Potassium is important in that it helps plants tolerate stressors such as cold/hot temperatures, drought, and pests.  It is a catalyst for many plant enzymes and helps regulate water use in the plant by affecting the opening and closing of the stomata in the leaves and water movement in and out of cells.

Potassium is a funny nutrient in some respects because we often see little response to it in trials.  But palms and other plants that clump, may respond to it by increased clumping and branching.  We’ve seen that response in some native plants as well, such as Stirlingia.  Too much potassium can make stems very brittle so they snap easily.

Signs of potassium deficiency can be quite dramatic and also species specific.  Sweet corn gets a sort of burning – a band around the leaf margins which is dried out and dead looking.  Carnations also get necrotic spotting at the tips of older leaves.  Hoyas end up with necrotic spots all around the leaf edges.  The symptoms are always on the older leaves because potassium is mobile and will move to the place of greatest need – we call that a sink and is often the fruit or flowers, or at the very least a growing point.

Potassium is supplied in many products as potassium nitrate – 36-38% potassium and 12-13% nitrate depending on formulation and purity.  Potassium sulphate can also be used but is more acidifying – an effect we often wish to avoid in our soils.  The cheapest source of all is muriate of potash or potassium chloride which we don’t really recommend at all because of the high chloride (salt) content.

Good organic forms of potassium are wood ash, dried seaweed and blood and bone, though the phosphorus content of blood and bone is rather high relative to the nitrogen (N) and potassium (K) and plants fed with blood and bone will need both N and K supplemented.

Potassium is readily leachable in our sands but is held on slightly better than nitrate in the presence of clay or compost as it is a positively charged ion.  Nevertheless, we see high rates of leaching of potassium from newly applied compost.