Archive for March, 2011

I find one of the most ignored aspects of gardening is pH – how acid or alkaline your soil is. Apart from having an effect on plant nutrition, soil pH can also contribute to plant health by making plants more or less susceptible to disease.

Western Australia is a real mix of pH. Many coastal suburbs have underlying limestone which makes maintaining a neutral pH an almost impossible task. Even when you do manage to get it down to a reasonable figure there is still this everlasting source of alkalinity underneath which means you can never really rest! However, in other areas of Perth and out in the wheatbelt, soil pH can be quite acidic.

The scale for soil pH goes from 0 to 14 with 7 being neutral. Most plants like to be around 6.5 but some, such as camellias and azaleas prefer acid soil. WA native plants are adapted to a wide range of soil types and pH, usually on the acid side but some, such as Banksia ashbyi do well in more alkaline soils. The pH scale is logarithmic (like the Richter scale used to measure earthquakes) which means that a pH of 6 is ten times more acidic than a pH of 7. This is part of the reason why it can be so hard to move from a pH of 8 say, down to 6 – there is a factor of 100 involved!

One of the most common problems around Perth is yellowing of young plant growth. Where soil is alkaline this is often iron chlorosis – the high pH prevents the plant from taking up iron. While it can be fixed by using iron chelate, usually as a foliar spray, the better option is to decrease the soil pH by using sulphur or iron sulphate. However, many plants won’t respond well to foliar iron and will simply burn if the yellowing is too bad. Plants deficient in iron as a result of high pH are also often deficient in copper, manganese and zinc because their uptake is also impaired under those conditions. This means it can be useful to use a trace element mix as a foliar spray instead.

I guess this is where one of the great lessons in growing plants comes in.

Just because a plant is looking deficient in something doesn’t mean that nutrient is in short supply in the soil! More on that later.

We’ve just seen that high pH can make some nutrients unavailable to plants. It turns them into a form the plant can’t take up. They are still there though. For other nutrients, acidity does a similar thing. Acidity can also kill plant roots. A pH of 4 will kill rose roots. And its not hard, or uncommon to get a soil pH that low. Especially at depth – say 15-30 cm – which is where a lot of the plant roots are. In many cases this is the result of years of fertilising. Almost all fertilisers in common use, turn soil acidic – uptake of potassium (K+) for example, causes the plant to release hydrogen (H+) ions back into the soil to maintain electrical neutrality. Hydrogen ions are acidic. Plants also take up nitrogen as nitrate (NO3-) so when ammonium (NH4+) is added to the soil it has to be converted to nitrate. That process also releases hydrogen ions.

So when you check your soil pH, make sure you do it at depth, not just on the surface.

The best way to check your soil pH is with one of those kits that have the powder and liquid. Cheap meters are not very good and pH meters also require a fair bit of maintenance and calibration. A reasonable pH meter these days will set you back about $200 whereas for most purposes the powder and liquid kits http://www.inoculo.com.au are quite adequate for only a fraction of that price (about $30). Some nurseries stock them, also Ag supply shops like Elders etc.

OK, so what other conditions can make plants deficient in nutrients even when their supply is OK?

The first one is cold weather. When root activity drops, so does plant uptake of nutrients. Phosphorus is the most obvious one – you often see plants turning purple in cooler weather. That can be phosphorus deficiency. And piling on more phosphorus may not help, what you need is warm weather – or warm soil at least!

Then there’s root damage – no roots, no uptake. Quite simply really. And root damage can happen as a result of a wide ranges of causes – root rot ie diseases such as dieback, low pH, high salts (too much fertiliser or again soil drying out), toxicity eg herbicide, tannins/phenols in eg fresh pine or eucalypt bark or even nematodes (microscopic worms that infect plant roots).

Dry soil? Plant nutrients must be in solution for uptake so if soil is dry nothing gets into the plant. Another reason to watch for patches of non wetting soil – and they may not always be visible. With the current 2-3 day a week watering regime there are lots of soils out there that are damp on the top 5-10 cm then progressively drier below.

And then of course there are interactions between elements in the soil. So too much of one can suppress the uptake of another.

Bottom line – next time your plant looks hungry, have a good think about why that might be. It may not be short of food, it might just not be able to get to it!


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Its been an interesting week. And the first semi quiet weekend in a while – hence my return. As usual the working week provides fodder for my posts.

There have been two quite disparate instances this week of people making, or trying out recommendations for controlling disease using natural remedies. Don’t get me wrong, I am all for using “natural remedies” but the reality is that in most situations they don’t work. Using beneficial microbes like Trichoderma, Bacillus subtilis,and Streptomyces etc is mostly a waste of time when you already have a problem. They can be of some use, however, to keep things in balance when there are no real problems – so more as a preventative than a curative. And I gather the latest evidence on Trichoderma is that it needs to be local provenance to work. So that pretty much cuts out buying it in a packet as an option. We do have more luck on the insect front however. Predatory mites for example generally work if you don’t kill them off with the chemicals you use. And provided you don’t put them into an heavily infested environment first off. The other thing that helps is a relatively constant environment. The conditions that predators require to breed well are about 25ºC and humid. Too hot or too dry and they suffer and their prey have an advantage and will often gain control. They work especially well in greenhouse situations and where there is a relatively dense canopy so they can move from plant to plant. And of course Bacillus thuringiensis (Dipel) does work well for killing caterpillars though the pressures of diamond back moth and Heliothis in the warm weather when you’re trying to grow broccoli in Perth can be a bit much for it.

There are some fairly common chemicals sold to home gardeners that are pretty hard on non-target species. For example imidacloprid or Confidor® has a bad reputation for harming bees. And then there are the chemicals approved under organics that are pretty toxic in themselves – eg Neem and rotenone. The latter has been associated with much higher risk of Parkinsons disease (Rotenone, Paraquat and Parkinson’s Disease). Conversely there are modern chemicals such as the Strobilurin group that are quite soft on beneficial insects and are also approved under NASAA accreditation (the organic certification scheme).

On a slightly different tack, I was at a forum this week where comments were made again and again about the need to look at things holistically. Too many studies are done with only one end in mind. One good example of this is the drive to save water which often results in practices that leach nutrients (two or three times a week watering on sands) or use more power (as in water-efficient washing machines that take 2-3 times as long to do a wash and then release water so toxic you couldn’t possible water anything with it). Organics is another discipline where a wider view needs to be taken sometimes. There have been studies that look at the bang for your buck you get from growing organically. In most cases, when you try to relate the inputs required against what you get out of it, it fares poorly. And in a world where we need to become more efficient in growing our food its not always the best solution. Yes its true that food may have a higher nutrient density but if yields are uncertain and even the nutritional status of the food is under question then is it the answer to feeding the world? That is purely a point to ponder. Don’t take me as being anti-organic. At the same forum there was a talk about Peak Phosphorus- you know – like Peak Oil. It was suggested we might want to breed and grow rice with a lower phosphorus content. But you’d need to ask what ramifications that might that have for human nutrition?

So a fairly philosophical post this one! But I think its worth acknowledging that natural is not always best and it doesn’t always work. Its a nice thought though.

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