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Archive for March, 2014

So you’re standing in the aisle at Bunnings and you are being confronted with about 50 different fertilisers.
There are fertilisers for azaleas, citrus, veges, fruit trees…………
Then there are fertilisers you dispense through the garden hose, those you put a spoonful in a bucket of water, the controlled release, the powdered, the liquid, granular…………….
Oh and don’t forget the organic, the natural, the hydroponic………………….

Are you worn out yet? How do you make sense of all this?

First stop – the label. It should have something like a list of ingredients on it. It may go something like:
Nitrogen (as urea) 12%
Phosphorus (water soluble) 2%
Potassium (as sulphate) 8%

If it’s a liquid there should be something to indicate that those % are w/v (weight by volume), if its solid/dry/granular it will be w/w (weight for weight).

Next thing to look for is a rate – how do you dispense this product? One handful per square metre? One capful per bucket, one spoonful per 20 cm pot?

Lets look at the liquid feeds first – they are the most complicated. They will usually give you a measuring cap or a teaspoon and say use one of these in a 8L bucket of water (for example).

If they are really good they will tell you the volume (if it’s a liquid) or weight (if its powder) of the measuring cap or the teaspoon! If not you might have to weigh it yourself. To get an accurate result on your kitchen scales you might need to do say, 10 spoons and divide the answer by 10.

So lets suppose your powdered fertiliser comes with a spoon that hold 4 g product (when level) and you put that in 8L water. The analysis of your product is 22:4:15 (N:P:K) % w/w.

That means there is 22 g nitrogen in 100g of the powder. So one measuring spoon of the powder contains 22 x 4/100 g nitrogen = 0.88 g nitrogen or multiply by 1000 to get milligrams or mg: 880 mg N.

Put that spoon in 8L water and you get a solution that is 880/8 = 110 mg/L nitrogen.

You can repeat that for the P and K.

Take another liquid fertiliser and read the label. This time it says 10:3:6. And it says take the same sort of measuring spoon full of powder and put it in 5L water. The final solution will be 10 x 4/100 = 0.4 g N or 400 mg N/5L water = 80 mg/L N. So a bit weaker that the other one. Which is fine if its cheaper to buy but if its more expensive think again!

For liquids the sums are much the same except the analysis will be w/v so your 10 unit of N as in the last example will be 10g/100mL or 1g/L of product. If your measuring cap holds 20mL and you’re putting that into 5L water the sums are:
10 g/100mL means 10 x 20/100 = 2 g N or (2000 mg). In 5L water that’s 400 mg/L – quite a strong solution!

I recently went through this exercise from a supposed wonder product from the US – a liquid one at that and came out with something like 0.8 mg/L nitrogen as applied! It would have had to have been wonder product to do anything! Not only that liquids are often not very cost effective because you are shipping water around the countryside – very inefficient. Bear that in mind when you buy any ready-to-use product, typically those ones you attach to a garden hose and water on – they are terribly expensive for what you are getting in terms of chemical. They are convenient but you are paying for it big time!

It’s worth doing the sums. Kevin Handreck in his book Gardening Down Under did this exercise with about 20 products and the final nitrogen concentration went from 45 mg/L right up to 900mg/L! And I’m sure there wasn’t much correlation with price!

We’ll have a look at solid fertilisers next time.

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