Archive for August, 2011

This blog is a bit of a soil chemistry lesson. I hope its not too painful!

On the Swan Coastal Plain we live mostly on sand. Sands have almost no ability to hang on to nutrients. Most of the fertiliser you put on today is washed away by your watering tomorrow unless it is slowly soluble or controlled release. We can measure the ability of a soil or potting mix to hang onto nutrients – that is its cation exchange capacity or CEC. CEC is usually measured in me/100 g and the CEC of our sands is generally around 3. Compare that to a loamy soil which is has a CEC of 5-15 or a clay which is over 30.

But you can add things to a soil to help this. Clay minerals such as zeolite which has a CEC or between 120-145, bentonite (70-120), attapulgite (25), vermiculite (120-150) and spongelite (1-10) are some clays sold for amending soil. Many of these are quite absorbent and so they are used for other purposes such as spillage kits and, kitty litter. However just because they absorb and hold water, don’t think they give it back up easily. Plants actually find it hard to extract water from clays and so, although a clay soil may seem damp, don’t think that that remaining water is all available to the plant. It is also possible that the release of nutrients back to the plant is also not exactly easy! That issue is seen commonly in the form of phosphorus adsorption where clays hang onto up to 90% of the phosphorus applied to the detriment of the plant.

Claying sandy soils for managing water repellence increases the CEC of the surface layers by a small amount depending on type and amount of clay added. Usually, CEC is increased by less than 1 me/100 g.

Some people promote the idea of managing cation ratios, claiming ideal ratios for Ca:Mg or Ca:K. for optimum plant nutrition. This is open to debate, a more critical factor is whether the net amount of Ca or K in the soil is enough in its own right, for plant growth.

Organic matter has a very high CEC. It is dependent on pH (ie how acid or alkaline the soil is). Humus has a CEC of about 200 at pH 8 but only 120 at pH 5. Spagnum peat moss is about 100-250 but cocopeat is only around 50. Adding organic matter to soil does increase the CEC of a soil but not as much as you’d think, especially in sands because much of the organic matter is burnt off over the course of the growing season. Its also very hard to add meaningful amounts of organic matter to soils without running into nutrient leaching problems and potentially polluting the groundwater, paradoxically. Potassium from composts is very readily leached and surprisingly much of the phosphorus is also water soluble and therefore also easily leached. Nitrogen is little better!

Potting mixes, especially those that are based on composted bark, generally have quite a reasonable CEC of about 200 me/L (note the different unit) and maybe up to 300. The easiest way to increase CEC in potting mixes is to add something like zeolite. The new CEC can be calculated mathematically using the proportions of each material (if you are so inclined). Much is said about the ability of zeolite to fertilise. Zeolite is NOT a slow release fertiliser. Its ability to break down and release nutrients is of little real value to a plant trying to grow at any reasonable pace. But what it can do is hang on nutrients added to a potting mix or soil so they aren’t watered away so quickly.

Manipulating the ingredients in a potting mix or soil is one way to help your plants grow better by allowing them to access nutrients for longer. Another thing you can do is use controlled release fertilisers but more on that at another time.


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Surprisingly, the answer is NOT potassium! This is one of the great myths of gardening. But how many times do you see recommendations for applying potassium to plants to induce flowering. Those recommendations are a bit like the ones that say you need phosphorus for root growth. Equally fallacious. The bottom line is that you need all nutrients for all bits of the plant and all functions. Certainly you need all nutrients to be in balance and in most cases you need phosphorus in about 10-20% of the amount of nitrogen and potassium. And that goes for spuds too! And generally speaking you need nitrogen and potassium in similar amounts give or take a bit. Where it gets tricky is when you incorporate soil type into the equations. Sands, like Perth you don’t need to worry about but clay soils adsorb phosphorus and hang onto it so the plant can’t get it. So where you have clay soils you often have to feed phosphorus on the basis that the soil will take up 90% of it and then 10% is left for the plant to access. Of course when you feed phosphorus repeatedly over time this changes. Eventually the ‘bucket’ is full and plants can access all you put on. But in Perth, on our sands, this isn’t the case so you can throw away all those recommendations for blood and bone and manure that all assume the soil is going to grab and hang onto most of the phosphorus you apply and use only a fraction of the suggested amount.

So getting back to what does make your plants flower and whether potassium is indeed important.

Plants oversupplied in nitrogen will be lush and sappy and prone to disease. And certainly they may not flower – simply because they aren’t in balance. Plants oversupplied with potassium often have brittle stems. When they are potassium deficient they may have weak cell walls, then may not elongate well – but that has nothing to do with flowering. Everything to do with nutrition, plant health and overall growth, but nothing to do with flowering.

What does trigger flowering? For many plants, it’s actually changes in daylength. For many of our native plants this is true, they flower in response to lengthening days (well actually shortening nights). Why do you get chrysies for Mothers Day? Because the trigger for them to flower is shortening daylength (or longer nights). Sometime in February the daylength falls below a certain value (about 13 hours) and triggers a response in them that makes them shift into flowering mode and they flower so many weeks later. Geraldton wax also requires a certain daylength to trigger flowers. In Israel, where they grow them for potplant production, they black them out to shorten the day artificially and induce flowering.

It doesn’t take much to switch some plants onto, or off flowering. Zygocactus are such plants – they flower in response to lengthening nights. If you place them in a situation where they get light at night, such as on a verandah or near a street light, that can be enough to interfere with their physiology and prevent flowering.

So nurserymen can manipulate flowering by manipulating daylength. In actual fact though, its NIGHT length. Flowering can be manipulated sometimes by only lighting plants for a short period in the middle of the night – night break lighting. For other species it works best at one end of the day or the other – day length extension.

There is another complicating factor. Well a few actually. Temperature is the other biggie. Some plants need a particular temperature regime at the same time. They may not flower unless temperatures are below or above certain levels. Or it may be that they need a certain difference between the day and the night temperature. And of course some plants don’t give a damn about daylength OR temperature. In some plants we can replace a chilling requirement with chemicals – gibberellic acid is commonly used to replace chilling in those plants which respond to it.

Finally some plants need a certain number of nodes (leaves) before they will flower. So even though the conditions are right they won’t flower if the required number of nodes isn’t there.

So it all gets very complicated. And plants come in all combinations and variations of the above. You will see words like facultative (is more amenable to a range of conditions) and obligate (an absolute requirement).

An interesting byproduct of this can be some strange symptoms when things aren’t going right! If you’ve ever seen leaves coming out of flowers – or even whole new little plants coming out of the centre of a carnation or a rose flower, that is often due to weather conditions triggering physiological processes that switch plants from flowering to vegetative. Its called phyllody. It can be caused by other things – herbicide damage, eriophyid mites, mycoplasma, viruses but usually its simply strange weather conditions.

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