Archive for the ‘Microbes’ Category

Solid fertilisers should be easier to measure out but often aren’t. Some manufacturers put a handy measuring container in the packet but most don’t. Statements such as a handful to the square metre aren’t that useful when you consider the variation in the size of hands! Filling an everyday container, such as a cup, with your fertiliser and weighing it can be a useful guide.

• A teaspoon holds about 4g of fertiliser;
• A tablespoon holds about 16g;
• A match box holds about 25g;
• A cup holds about 250g.

It can be useful to have a rough guide to what your crop needs (see my post on “Crop removal or how do you know how much fertiliser to apply?”. Commercial lettuce crops generally get 2-300 kg/ha nitrogen. Things like cabbages which are much slower but also much bulkier get 5-700 kg/ha. Tomatoes are also in that ballpark. Then you have native plants which only use a fraction of that – say 80 kg/ha nitrogen for an adult Geraldton wax bush which is being picked heavily for its flowers and foliage.

Evaluating the nutrient value of a solid fertiliser is done in the same way as for liquid fertilisers. For example, something like CSPB’s garden fertiliser is 13.5% nitrogen (N), 1.9% phosphorus (P) and 8.0% potassium (K) with a range of other nutrients including trace elements. That means in every kilogram of the product there is 135g (13.5/100) x 1000 (g) = 135g of N. Using the same system we come up with 19g P and 8g K.

It does pay to check the bag to compare fertiliser products. If you are paying twice as much for a product with 5% N then its not as good value for money.

Products from many other countries are sold here. You may find American products that have analyses like 10-10-10 – that is because they express formulae as the oxide form . The N is OK, it’s the same but the P need to be multiplied by 0.44 and the K by 0.83 to be equivalent to the base element.

Always be wary of any product that has a really high figure in the middle (ie for P) check the label and the origin and its probably American.

These days fertilisers aren’t registered. That means almost anything can be packaged up and sold as fertiliser. Ideally it shouldn’t because there is an industry code of practice (which isn’t law yet) but I do see a constant flow of new products coming onto the market (coming and going). And the buzz these days is microbes and humates (humic acid). So companies will try and sell you something with almost no nutritional value but lots of other buzz words for a hugely inflated price!

Just bear in mind that for microbes to prevail in soil they need a food source which is carbon (organic matter). Put them in your sand and they won’t last 5 minutes! And if you put them into and environment that is already highly organic and has its own microbe population they may well get out-competed by those already in residence!

So is a high nutritional analysis everything? Not necessarily. If the fertiliser is a quick release one the higher the analysis the more likely you are to come to grief if you overdo it. Quick release fertilisers are designed to be applied every couple of weeks or monthly.

You can of course use slow release fertiliser like Osmocote™, Nutracote™, Macracote™ and so on. They are expensive, you pay for convenience but you only need to apply them every few months. And you may waste a lot less – the danger with quick release fertilisers is that you irrigate them away in the next few days. We monitor growers who fertigate (fertilise through the irrigation) and we see soil nitrate levels (nitrogen is highly mobile) plummet between fertiliser applications – going from 80 mg to 20 within, say 3-4 days.

Some cheap fertilisers may also contain things like muriate of potash – potassium chloride. Chloride is salty and you probably don’t want it. Better to go for potassium nitrate or even potassium sulphate for your potassium. Potassium sulphate will make your soil more acidic but the sulphur can be useful.

Fertilisers imported from overseas can also contain nasties like heavy metals (cadmium, lead, nickel). These are particular risks from China or India. There is random sampling of fertilisers on entry for these sorts of things so it shouldn’t be an issue but things can slip through occasionally. You also need to be aware that manures and composts can also contain toxic levels of heavy metals, microbes like E coli or even amoeba and they are largely unregulated unless you buy bagged product made to the Australian Standard. There are plenty of places where you can back up a trailer and buy – who knows what! Not exactly what you want if you are trying to produce healthy food on your block.

When to apply fertilisers?

Most people assume you should fertilise when you see activity but we only see what’s happening above ground. It’s the roots that take up fertiliser and its root activity you need. Its widely said that you shouldn’t fertilise in winter. But many natives have their active root growth in winter and are largely dormant in summer. Other deciduous species also take up nutrients during that time and store them in the plant frame for later redistribution and use in the plant. But when its really cold, nutrients ARE taken up more slowly and of course rain leaches fertiliser away from the root zone and it is wasted. So for this reason fertilising in autumn can be a good thing. Just remember that however you fertilise, plants need it to be dissolved in water to take up. No point in spreading fertiliser around the canopy of a plant that is watered from one dripper in one spot! If its watered using overhead retic or mini-sprinklers and the soil is uniformly wet all around – then fine.

Foliar fertilisers

Foliar feeding is largely a very expensive way of doing things. More often than not what you apply to the leaves gets washed off into the soil and feeds through the roots anyway. Only in very special cases is it worthwhile and that is mostly for commercial growers who can’t afford crop failures. Calcium is often fed in this way because its immobile in the plant and bouts of high humidity can prevent its uptake by halting the transpiration stream that carries it around. Immobile trace elements such as iron can also be foliar fed.

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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.

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