October 27, 2010
Ideally for short term crops (vegetables and leafy greens) I would suggest you add 1/3 vol of compost. For a 10 L pot it would be about 3 to 3.5 L. This should last the duration of the crop.
You can use the excess soil in new pots.
In case you cannot remove top soil then you can dig small holes and drop the compost. Only could also dissolve the compost in water (good vermicompost should completely dissolve in water) and water the plants.