Question: I found some old fertilizer in a bag. It has turned into concretelike lumps. Can I still use it?
Yes. All of the nutrients are still contained in the lumps, which will need to be crushed with a hammer before they can be applied. This caking of fertilizer is owed in part to the nature of the material and to poor storage. Synthetic fertilizers, both granular and powdered, are prone to caking.
Some fertilizers are more hygroscopic, or apt to absorb moisture, than others. For example, fertilizers containing calcium nitrate or ammonium nitrate are more hygroscopic than urea, ammonium sulfate, potassium chloride, calcium phosphate, potassium sulfate or calcium sulfate. To help prevent caking, some manufacturers apply a coating to granular ammonium nitrate and ammonium nitrate-based fertilizers.
To reduce the likelihood of this happening again, store fertilizer in its original bag in a protected dry location. Alternatively, place the bags in five-gallon buckets with tight-fitting lids. Never leave unused fertilizer in spreaders, because it will rapidly corrode any metal parts, quickly ruining the equipment. Finally, purchase only the amount that you intend to use in the near future to reduce the amount you have to store.
This post is excerpted from the September/October 2006 issue of Horticulture.
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