Why is old garden fertiliser wet?

I sometimes use a bit of artificial fertiliser in my garden, as well as my home-made compost. I don’t use much, so the pack bought one year may last for a couple of years.

If it is kept for a couple of years, the pack becomes wet. It is as if it were sprayed with water, so that the outside packaging is damp, and curls up. The contents are caked together and moist, like potter’s clay.

This is not due to the storage place. it is a large building which is well-built and watertight. Nothing else stored there shows any sign of damp - only the fertiliser. So, I believe that the dampness is not caused by a wet environment. The fertiliser becomes wet in some way, and that makes the package wet.

I assume it is some chemical reaction. The current batch of wet fertiliser is a normal NPK mix - with Nitrogen, Potassium and Phosphorus. (Please convert to the USAian term or spelling for those chemicals, if it causes confusion to Amerenglish speakers.)

Two queries -

  1. Why does this happen?
  2. i still use it on the assumption that it still has the same chemical composition - but am I poisoning my plants unwittingly?

It’s Hygroscopic.
I don’t think this has much of an effect on it’s potency.

Granular fertilizers are hygroscopic. Store it in a sealed container and it won’t turn into a slurry or a rock like mass if it dries out again. I store it in plastic bucket with lids that bag can fit in.

Not just hygroscopic, but deliquescent. Able to keep extracting water from the atmosphere even after covering itself with water. That’s what it takes to get the bag wet.

There you go. You win able to take it to the pinnacle while others not so far.:stuck_out_tongue:

Thank you for your very useful comments.

Beowulf says “I don’t think this has much of an effect on it’s potency.”

I take it that the only effect of hygroscopy is to dilute it with water, in an inconvenient way. It would be diluted anyway from moisture in the soil. Just confirm this, are you confident that the fertiliser retains its benefit and has not been chemically altered other than dilution?

check with the manufacturer. a simple compound like sodium hydroxide (NaOH) is deliquesent but will not be affected. just dry it on an autoclave. but some compounds and mixes may lose their intended properties if they absorb too much. example: gunpowder and some other propellant powders for firearms are hygoscopic and that will likely ruin them.

a artificial fertilizer may no longer be homogeneous. some compound will move into and leak out of the bag. the rock might not have all the elements in intended quantities in a chunk that you break off. the rock might still have plant value though not in the intended fashion.