Depending on the type of fertilizer that you’re using, it can be pretty hard to determine exactly when you have to stop using it and buy a new batch. In some cases, natural fertilizer can be kept in storage for more than a year without a problem, while in other cases, it has to be discarded much sooner to ensure that plants receive proper nutrients and that the old fertilizer doesn’t do more harm than good.
The #1 thing to consider is how you store your fertilizer. Even if the label on the bag says that it might last for a long time, you still have to be careful, as a greater amount of moisture and extreme temperature changes can lead your fertilizer to lose its effectiveness over time, or at least to become too clumpy to use effectively.
Of course, some fertilizer has a very long shelf life, so you don’t have to worry too much about it being affected even by poor storage conditions. This is the case with dry, organic fertilizer and mineral-based fertilizers that are either dry or liquid. Both these types are highly resilient and long lasting.
On the other hand, be careful when you store fertilizer that contains pesticide or liquid organic fertilizer with a shorter shelf life. These compounds are greatly affected by storage conditions, and can lose their effectiveness over time.