Synthetic commercial fertilizer that you buy from the stores lasts for years.
Most manufacturers will label their products with a use by date. Typically they will recommend using it up within three to eight years, depending on the type of fertilizer and brand.
Solid fertilizer pellets tend to last longer than liquid formulations. Pure mineral fertilizers, like powdered rock phosphate, also lasts a long time - about five years. - because they are quite chemically stable and decay slowly.
If you are going to use a fertilizer that is well past its expiry date, it will probably still able to supply nutrients to your plants well enough.
However, like all chemical products, its efficacy will decline over time. This isn't unique to chemical fertilizers - it even applies to unsealed organic compost, which will reduces in volume and microbe content after one year.
Another problem with using long-expired plant fertilizer is that the minerals inside it might begin to crystallize. This is common when the mixture has been exposed to low temperatures.
Crystallized liquid fertilizer is difficult to dispense. You could dissolve the crystals by adding warm water, but because this changes the original concentration of the minerals, it becomes hard to judge how much you should be giving your plant.
There are quite a few reasons why it might be easier to get a fresh product.
Organic fertilizers, like the kind you can make at home using scraps, are made from biological materials. These lose their efficacy much quicker than synthetic chemical fertilizers.
That's not to say that organic fertilizers become toxic to your plant over time, simply that they may not be able to provide your plant with the nutrients they need.
Organic fertilizer however could pose a risk to your plant's health if it attracts mold and other pathogens while in storage.
Because organic fertilizers are more unstable, it is sensible to make them in small batches rather than in large volumes. Aim to make up an amount that will last you about three months.
Some commercial biological fertilizer can be used for up to two years.
To lengthen the shelf-life of your organic or homemade fertilizer, you should store it in an air tight container like a tupperware box in a cool, dry, and dark place. It is better to keep the organic fertilizer inside an opaque container to protect it from light.
If you store your organic fertilizer well, it can last up to a year. Bear in mind though that it is probably most effective at feeding your plant within the first three months of making it.
It is especially important to properly store organic fertilizer containing beneficial bacteria These are living materials that need the right conditions to thrive. Temperature fluctuations and moisture can destroy them, rendering the formula much less effective.
Always check the condition of the fertilizer before you apply it to the soil. If it is developing mold, throw it away immediately.
Although organic fertilizer needs a little more care when it comes to prolonging their shelf-life, they have huge advantages in other ways. 100 percent bio-based formulas are safer to keep around pets and it is more difficult to over-feed your plant with them.
Should I use expired fertilizer?
To sum up, chemical plant fertilizers that have passed their expiry date will not be chemically toxic to your plants.
However, expired chemicals fertilizers can damage your plants because it is easy to add to much of it to the soil. This is because once they are stored for long periods, they tend to crystallize and become highly concentrated.
Organic fertilizers are rarely as potent as synthetic mixes. However, it is advisable to use it up within a year of making it. It will lose its efficacy much more quickly than synthetic brands.
In either case, it's a much safer bet to just buy a new fertilizer.