If you are confused about what fertilizer should you use then nothing is better than well decomposed dry cow dung manure. Use one handful per plant.  

Also known as Black Gold, is very popular among gardeners as it is not only a good source of NPK but it also act as a "Bio-fertilizer" which increases microbial activity. 

They are made of dry fallen leaves, if you want to increase vegetative growth of your plant them you can add leaf compost. Avoid adding them during flowering phase of the plant. 

Another good source of nitrogen required for vegetative growth of plants specially foliage houseplants is compost made from used tea leaves. 

Apart from being a good source of nitrogen, they are also helpful in control fungal and bacterial diseases in plants. You can use them in preparing potting mix. 

If your plant is suffering from Calcium deficiency, mainly it happens in tomato plants, then you can add used eggshells to fulfill plant calcium needs. 

Instead of throwing away banana peels you can add them to your flowering plants to increase flowering in them, as they are good source of potassium. 

Just like banana peel, onion peels are also good source of potassium, calcium and magnesium that are good for increase flowering in plants.