Archive for the ‘Manures’ Category

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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Because manure is organic and natural it must be good right?

Well not always.  The use of raw animal manures is actually banned to a large extent under most food safety programs (and that includes organic produce) due to the risk of pathogens that can infect humans such as Salmonella, Listeria and E coli.  They are totally banned from applying to any leafy veges and have limitations in terms of days before harvest for other crops.

Despite what most people think, the nutrient content of most animal manures is largely water soluble and in sandy soils doesn’t hang around for long.  Given most people also apply most of the manure at planting time when the plants are smallest and have the least extensive root system, all this results in is ground water pollution!

Animal manures also have quite a low nitrogen to phosphorus ratio which means you end up putting on a heap of manure simply to get enough nitrogen – which means the system is well overloaded with phosphorus – more leaching!  The reason manure gets such a favourable rap in most places is because if you have a large clay component to your soil, some of the phosphorus binds to that clay and becomes unavailable.  That has the effect of bringing the nitrogen ratio up to reasonable levels.  That doesn’t happen in our sands.

The ability of the clay to bind to phosphorus diminishes over time.  Depending on how much manure and fertiliser you apply, eventually those sites are filled up, the clay can’t hold any more and any more phosphorus you apply will leach in the same way as nitrate.  A soil survey done a few years back showed that most gardens in the older suburbs of Perth are in that category and don’t need phosphorus to be applied – probably for the next 10-20 years!

The nitrogen in raw animal manures is mostly ammonium to start with –that is toxic to plants in large amounts – most plants need to convert ammonium to nitrate to take up the nitrogen.  While ammonium is more readily bound to cation exchange sites in soil, in our sands it’s also very readily converted to nitrate which is highly leachable (more groundwater pollution).

Average composition of stored animal manures at 40 to 60% moisture.

One tonne (1000 kg) of manure contains
  Nitrogen (kg) Phosphorus (kg) Potassium (kg)
Range Average Range Average Range Average
Horse 7 to 12 9 5 to 9 7 4 to 13 5
Cow 8 to 11 9 5 to 8 6 4 to 13 7
Sheep 5 to 14 9 4 to 10 8 5 to 7 6
Pig 6 to 12 9 5 to 8 7 4 to 10 7
Fowl 8 to 26 18 6 to 20 13 4 to 12 7
Note: To convert these figures to percentages, divide by 10.

Chicken manure breeds flies – in particular, stable fly which is a biting fly that attacks horses.  These flies will also breed in vegetable residue if it is applied and buried too thickly.  Other types of manure breed flies but to a lesser extent.  The flies are attracted largely by the ammonium but also by other compounds in the manure.

Many manures can bring in in large amounts of weed seeds – weeds that may not exist in your area or your garden until you buy in manure from elsewhere.

So how to use manure responsibly? 

  • Composting is a great start, it gets rid of the harmful pathogens.
  • Apply in amounts that are relative to the size of the plants and only in the drip zone.  Don’t assume that one application will last.
  • Amend your  soil with clay so it can better hang on to nutrients.
  • Dilute manure with other sources of organic matter that are lower in phosphorus such as greenwaste materials (also composted to avoid bringing in disease and weed seeds to your garden.

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