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Posts Tagged ‘nitrogen’

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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Sorry this has been a long time coming.  I think we will have a look at nitrogen since it’s the most problematic nutrient in our sands.  Here today, gone tomorrow!

Nitrogen is mostly taken up by plants in the nitrate form so any other form of nitrogen has to be converted.  There is evidence that plants can directly take up organic nitrogen.

Common ways of applying nitrogen

1) In chicken manure (urea) or compost (ammonium and nitrate)

2) Ready made fertilisers eg NPK Blue, Nitrophoska, citrus/rose/whatever type of fertiliser (usually ammonium nitrate)

3) Controlled release fertilisers eg Osmocote (potassium nitrate, ammonium nitrate), Nutricote (potassium nitrate, ammonium nitrate).

4) IBDU/Ureaform (urea)

5) In liquid fertilisers such as Aquasol (urea) or Thrive (urea)

6) In fish emulsion (organic N, often supplemental urea)

You may hear comments about urea being harmful in winter.  This is because it requires conversion to ammonium (by an enzyme in the soil called urease) and then soil bacteria convert the ammonium to nitrate (nitrification) when it can be used by the plant.  In cold weather, soil bacteria slow down and the build-up of ammonium can cause damage to plants.  I have never seen this in Perth except when people toss on heaps of chicken manure.

Most of the nitrogen in poultry litter is readily available. Between 6%–30% is in the form of ammonia which will be lost to the atmosphere unless incorporated, the rest of the nitrogen will be lost within about 6 weeks unless taken up by the plant.  Even nitrogen applied in compost will easily leach.

The best organic sources of nitrogen are blood or chicken feathers – both about 12% N.

In Perth’s sands there isn’t much to hold onto anything.  If you add clay minerals they help retain more ammonium but nitrate tends to leach regardless.  The conversion of ammonium to nitrate occurs rapidly, especially in warm weather – within 24 hours.

Slow or controlled release fertiliser technology is great.  Products like IBDU or ureaform give slow release of nitrogen over about a three month period.  Other slow and controlled release products that contain phosphorus and potassium are all good but often can release too slowly for things like veges which have high growth rates.  This is where liquid feeding can be helpful but remember to only apply enough to saturate the root ball.  In the case of new seedlings that may only be a few mL per plant.  Anything you apply beyond that area is wasted.

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Fertilisers are a bit of a black box for most people.  Walk down any hardware or garden centre aisle and be confronted with an endless array of products all designed for different plants and situations.  But is it really that complicated?

Plants all need the same elements – nitrogen (N) and potassium (K) in the greatest amounts and often in about the same quantities give or take.  Phosphorus is only required in about 10-20% of the amount of N and K. Next come magnesium and calcium and then a whole array of others – sulphur and trace elements.  While plants might take up nutrients in differing amounts it often doesn’t matter what ratio they are in the soil, they will take up what they need.  Or in the case of nitrogen often more – that is called luxury consumption.  Unlike humans, plants don’t get fat though they just get overly leafy, sappy and prone to pests and diseases.

What about the type of fertiliser?  Are organics better than chemical fertilisers?  What about slow release or controlled release fertilisers?  And liquid versus granulated?

Liquid fertilisers – those that you buy as a powder or liquid and dilute with water are the ultimate in instantly available and quick acting.  Unfortunately in sandy soils the next time you irrigate, or if it rains, they will all be gone.  They are good for seedlings that have a small root ball because you can place it just where its needed and you can apply as little as you need.  So for a typical 6-8 pack type seedling you might only give each plant 50mL max but you might do that every 2, 3 or 4 days in the first 2-3 weeks.  No point in fertilising the whole bed, most will be totally wasted.  Just the plant.

Granulated fertilisers like NPK Blue are good when the plants get slightly bigger.  Sprinkle around the canopy area and do every 1-2 weeks for veges.

Sheep or other animal manures are also good but can be relatively high in phosphorus and contrary to popular belief, a lot of the N, P and K in them is water soluble and therefore instantly available and liable to be leached.  Animal manures may have to be aged to avoid burning from ammonia and they may carry weed seeds.

Slow release fertilisers are great for plants that don’t need to be pushed and are long lived.  So most garden plants, fruit trees if you wish, pot plants etc.  They are available in many formulations including low phosphorus for natives.  So anything from 3-4 month to 8-9 and there are even tablets that last 12 months.  The way these all work can vary.  Some are plastic coated and rely on the slow breakdown of that coating to work.  For others the fertilisers are embedded in a slowly soluble matrix.  Temperature ultimately controls the rate of release and for most the time frame on the label is worked out at about 21ºC.  In our hot summers it will be much quicker.  The disadvantage of these types of fertiliser is the rate of release may be too slow for some quick growing crops but otherwise they are excellent.

It is possible to get single element slow release fertilisers.  The most common available to the home gardener is nitrogen.

More on plant nutrition next time.

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If you’re in the business of trying to grow anything, the weather we’re having isn’t making it easy! And even if you’re not a commercial grower, the twists and turns of the weather lately makes it really hard to get the best from your plants.

I downloaded the weather data from Medina Research station for last December to today. The daily evaporation for that period ranged from 1.5 mm for the day up to 11.4 mm! That’s a ten fold difference. And I bet the settings on your irrigation controller haven’t changed in that whole time! When you sum up the evaporation over a week it doesn’t look quite as variable – 44.1 mm, 44.2, 57.9, 63.4, 54.2, 66.2 and 58.4 – which is only about a 50 % variation. Actually last week would have been less because I haven’t subtracted off the rain we had, which at home was 42 mm but for Medina was about 30mm. So you really need to take that off the weekly evaporation. Incidentally, from a commercial point of view we don’t regaard anything under 4 mm as being effective ie its not regarded as beng enoguh to do any good.

Anyway, what I’m trying to say is that unless you vary your irrigation you are either going to run into trouble with plants drying out, or you are overwatering like mad to cater for the worst case scenario, wasting water and probably washing away most of the fertiliser you put on into the bargain.

Commercial growers vary their water on a daily basis if they are doing their job properly. And at this time of year, depending on their crop they may be watering anything up to four times a day in amounts that will add up to the total daily evaporation – plus or minus depending on crop and stage of growth. Now, no home gardener can be bothered with that, plus if you have to abide by the two or three time a week watering edict, you simply can’t water every day unless you hand water. And in sand that’s a problem because it does not hold water and actively growing plants will dry the root zone out in well under a day and get stressed or even die – particularly if they are small seedlings.

What can you do to help this? Amend your soil with clay and organic matter. Be wary of the source – you get what you pay for! Don’t run the risk of getting dieback with your clay and remember that compost has quite a lot of phosphorus in it relative to nitrogen. Much of this is explained in previous posts.

The other thing you can do is mulch. But remember to use coarse chunky and COMPOSTED material (again to prevent importing disease and weed seeds) so any applied water runs straight through or you will end up with wet mulch and dry plants. Keep mulch away from the collar of the plants. Mulch does a good job of moderating soil temperatures and the trials we have been running at Murdoch TAFE show that if you start with a nice wet soil profile, mulch can do quite a good job of helping to maintain the soil moisture. Don’t layer the mulch on too thick though, keep it in proportion to the size of the plants it is around. And don’t add the same amount to it each year, just maintain the thickness you need.

Still though, if you are having trouble with plants not thriving in the garden, the best thing you can do is get out with a spade and dig. You may be surprised. I have been. Beds I thought should have been well wet have shown dry patches and even been dry about 25 cm down – well within the root zone of any plants.

Always beware of newly planted plants. Inferior potting mixes (you get what you pay for) and simply the type of mix, may mean the root ball of your plant dries out before the soil around it.

In my experience many plant problems come down to the basics and on our sands its odds on its watering or lack of nitrogen and those two are inextricably tied together. Too much watering means you wash all your nitrogen away. Assuming you had enough in the first place – nitrogen is the one nutrient always in short supply on our sands, and manures and composts never have enough of it while at the same time they are overloaded with phosphorus.

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