I’ve already been accused of making things too complicated this week. But it seems its worth reading labels and even safety data sheets when it comes to fertiliser. It all started with two empty pails of Osmocote I had at home. Bought from Bunnings. Casually looking at the label of the All Purpose Landscape mix I noted 2.2% magnesium and 0.2% iron. Then skipping over to the Native Gardens mix I saw 0.2% magnesium and 2% iron! A typo maybe? Maybe the two were mixed up? The Safety Data Sheet for the native mix from Bunnings says less than 1% each of iron chelate, iron sulphate, magnesium sulphate and magnesium oxide. That doesn’t really compute and its not that helpful. Oh well lets move on. Lets look at the commercial stuff. Osmocote Pro low P 8-9 month has 1.8% Mg and 0.2% iron. The normal landscape 8-9 month has 1.2 and 0.33 as iron chelate. Seems reasonable.
Now check the SDS of both commercial products. Well the first line is interesting! The commerical products are both based on ammonium nitrate as the N source.
Have a look at the Bunnings versions. Instead of being 30-60% ammonium nitrate they are both 30-60% urea! Less than 10% ammonium nitrate. Now I don’t know about you but I don’t know any 8-9 month slow release urea formulations – usually more like 4 months. So while both are called Osmocote there are fundamental differences between the commercial and the Bunnings formulations.
Does it matter? Probably not that much on the nitrogen side of things. The Bunnings versions of each say to reapply every 6 months so a bit shorter than the 8-9 month commercial ones.
The magnesium is a worry though. Magnesium is sort of between a trace and a macro element and leaf tissue concentrations generally run at around 10-20% that of N and K. I can’t see 0.2% supplying a plants needs. And as I noted previously the magnesium content in Nutricote is similarly abysmal. Perhaps that’s why Epsom salts (magnesium sulphate) gets such a good rap!
And a final reminder about iron. If you’re using any potting mix with wood waste or pine bark in it, they adsorb iron. We have always added something like 500-1000g/cubic metre of pinebark potting mix. You might get away with as little as 200g for other wood based materials. But make sure it is iron – combine chelate and sulphate if you wish but don’t try to do it with mixed trace elements – you will end up with boron toxicity.
Gardening WA style 