Do stored fertilizers and inseticides deteriorate?
By Composto
United Kingdom 
I have never seen a "best before" or "use by" date on any fertlizers or insecticides or weedkillers so can one assume that they don't deteriorate? What happens if they are granular and the box has been opened and air is present?
3 Aug, 2009
Fertilisers do deteriorate when in contact with the moisture in the air. For this same reason I do not buy seed compost from garden centres that store it in the open.
Chemical pesticides should not deteriorate significantly but 'organic' insecticides can. I tried using an organic insecticide on the greenfly and whitefly on my 'special' plants in the greenhouse. Using the same bottle the following year I noticed that it was burning the foliage of the plants. I think that the fatty soaps in the spray had broken down into an acid! The ornamental plant greenhouse is now one area where I use chemical insecticides.
4 Aug, 2009