The worm castings I added on top of the soil seems...

Some dissolves, but most gets eaten by the soil microbes. Also, as you water and the castings break down into tiny bits, they slowly sink down between the grains of soil. I apply new castings every 4-6 weeks. They are natures wonder food for plants and can not burn or injure your plants in any way, even if growing in pure casting and they have been part of my soil mix for the last 30 odd years and I swear by them. They are also great for making "worm casting tea", that your plants will love too. Just put about a half a pound/250 grams in a 5 gallon/20 litre bucket and fill with water. Let it soak for 12-24 hours, stirring once in a while then let it settle so that all the "bits" sink and either scoop out the liquid or pour it out trying to leave all the bits/sludge behind. Mix the "worm casting juice" in a 1:3 ratio with water and give to your plants. Use all tea within 24-36 hours after the steeping and decanting process, otherwise it will start fermenting. The "sludge" at the bottom of the bucket can also be poured around any plant without causing any problems. To make a cleaner tea, you can put the castings in an old sock and make something like a tea bag, but you will have to squeeze the "tea bag" periodically to release the goodness from the castings into the water. Doing it with a sock prevents the sludge at the bottom of the bucket, but the other way of doing it is easier and you don't have to find an old sock without holes in the wrong places, or empty the sock when finished. Either way, the resulting worm tea is the same. If making the worm casting tea, it can be applied more frequently, every 2 weeks seems about right, but then again, just putting the castings around the top of the pot every 4-6 weeks to a depth of about an inch/2.5cm is probably the easiest method of all. Hope this helps,........... Organoman.