Dave is right, it could be re-tempered, but it's not quite that simple. The process of tempering is to remove some of the hardness, hence the term 'temper'.

To do this successfully, and assuming it is a good quality fork, (i.e.one manufactured from medium to high carbon steel), you would have first to nomalise the whole fork by heating it to cherry red and allowing it to cool.(don't forget to remove the wooden handle. The steel will now be in it's annealed (soft)state and you will be able to straighten the prong with relative ease.

You would then need to heat it to cherry red and quench it in water. This will make it very hard indeed, but will also make it very brittle. Any attempt to remove a root with this will result in prongs breaking off.

You would then need to temper it by first cleaing it with emery cloth until it is shiny. (if you don't do this you will not be able to see the tempering colours).

You would then need to heat it uniformly; you should notice the shiny steel starting to change colour to light straw, through brown, then various shades of blue to purple. When the correct colour is reached for the degree of temper you require, the fork should then be quenched again. It will now be ready for use.

I am guessing that the colour at which you would need to quench would be dark brown, but you could check this by refering to a good metalworking reference book. A wood chisel which needs very little hardness removed would be tempered to pale straw, while a spring would be blue.

Too much trouble? You're damn right, but I just thought you might be interested to know just how much trouble.

I'm sure other methods posted here will do it well enough for digging the garden.