Posts Tagged ‘Manures’

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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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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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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