Posts Tagged ‘potassium’

Experimentally we can measure the amount of nutrient a crop removes from the soil or from a nutrient solutions when it grows. That means we can calculate how much nutrient is used to produce a crop. Crop removal can be measured in a few different ways. Sometimes its done in hydroponics. Then its easy to calculate what nutrients are put into the system and what is taken out. You can harvest the crop, dry it and analyse it to see exactly what’s in the root system, the leaves, the actual crop and so on. Of course it will never balance exactly because there are inefficiencies in the system. Plants require energy to grow and some nutrients will be lost to the environment.

Another way of doing it is to grow a crop in its usual situation such as soil, and then go through the same process of measuring what is in the plant at the point of harvesting the crop. Either way you end up with a set of figures such as those in the table below.

Crop removal table

As with hydroponics there is a fudge factor. If you were to apply just those amounts of nutrients you’d not achieve similar yields. Nutrients are always lost the environment because plant roots don’t explore 100% of the soil, so many nutrients may be lost through leaching. Where you have clay soil, if the clay is highly adsorbent (ie you haven’t been applying phosphorus fertilisers or manure for several years), then much of the phosphorus you apply may be adsorbed by the clay and not readily available to the plant (not in the time frame of that crop anyway). Perth’s sands are generally below 1.5% clay so this is not an issue.
Crops vary in their ability to take up nutrients. Some are very efficient, others not so. Much of that is to do with the architecture of their root systems.

The figures above are per hectare so have to be related back to a per plant basis but you can see that there is great variation between the relative amounts of nitrogen, phosphorus and potassium that each crop uses. The figures can vary a lot depending on things like:
• climate – in countries with low light levels generally have lower yields and therefore lower crop removal figures
• crop variety, and
• time of year.

In our work, we find we may have to apply 30-50% more nutrients to a crop over winter than summer. Why? Rain and slower growth are the reasons. No matter how well you apply fertiliser one decent shower of rain will leach most of it away. And because winter is cooler, invariably the time the crop takes to grow is longer and those inefficiencies multiply.

So what is the fudge factor you have to apply to actually grow a crop? About double is not a bad average. Some crops you might get away with 40% more.

The other consideration is the amount of each nutrient that a plant can access. Each nutrient comes with its own set of problems. Nitrogen is highly leachable. In sand so is phosphorus and potassium. In clay soils things may slow down a bit for the latter two but nitrogen is still converted to nitrate within about 24 hours of application in Perth so the advantages of applying ammonium are not great.

Lets now they can relate some of this to the manures and composts you may use. I’ve used this table before.

Manure composition table

Using tomatoes as an example. If you need 297 kg N per hectare – lets say 30 g N per square metre, then that amounts to 3.3 kg of sheep manure (at about 40-50% moisture content) per square metre per crop. And if we double that for our inefficiency factor then we’re up to over 6 kg manure per square metre of ground.

But that amount of sheep manure contains almost the same amount of phosphorus and our tomato crop only needs just under 20% of that! What happens to the rest?

And what about potassium? Our crop needs more potassium than nitrogen so we will be short changed on that score.

You can see how easy it is to waste heaps of phosphorus and probably how much better your yields may be if you added a lot more nitrogen. And why you might run into disease problems and fruit quality issues due to lack of potassium.

We haven’t even considered yet is at what stage in its life cycle our tomato crops needs each of these nutrients. The figure below shows the pattern of nutrient uptake over the life of a tomato crop.

Crop removal Yara

OK, so sheep poo is not a good idea. What about using chook instead? Well, you will be slightly better off for the relative amount of nitrogen to phosphorus but you are even more short changed on potassium!

If you use half sheep and half chook , the ratio of N:P:K changes to 13.5:10.5:6.5. Not a lot of help – well over on P again and well under on K.

What is my message? Well if you’re growing veges organically using animal manures and compost, unless you are operating in a closed system, don’t kid yourself you are being environmentally friendly. You might be saving on food miles and pesticides but the Swan river isn’t going to thank you for all that phosphorus you are dumping in to the groundwater. And if you are using some sort of closed system, at some stage you are going to have to dump nutrient as the levels of phosphorus (and other plant exudates) become toxic – and where will you put it?


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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.

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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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