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Why?
It is simply because worm castings contain a mixture of nutrients and life useful to organic gardening and farming. Simply put, worm castings contain nutrients needed by your plants. Its kind of like the nutrition you get from your diet.
And worm castings are one of the most nutritious foods your plants will get. Not to mention they contain beneficial life like fungi and bacteria which plants also need. The type of worm castings you will get from your compost will also vary, however, depending on the type of food you feed your worms. For example, if you feed your worms food mainly coming from paper products like corrugated cardboards, newspaper strips and the like, you will get worm castings with a lot of fungi.
This type of worm castings is good for trees and shrubs.
If you feed your worms food which consists mainly of food leftovers and other materials from your kitchen like grape, orange and apple peelings, you will get worm castings abundant with bacteria life. This type of casting is best used for flowers or vegetables. If you feed your worms with a mix of both kitchen waste and paper products, you may get a more balanced type of worm castings from your compost.
This may be used on trees, shrubs, vegetables and flowers. If you are saving on space, you can do this but its usually better to have one compost fed by mainly paper products and one fed by mainly kitchen or food products. Now you can either get worm castings by placing an ample amount of worms on your garden or you can buy worm castings. There are a lot of sellers out there offering ready packed worm castings you can use for your garden or you potted plants.
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From: (vermiculture) http://allorganicsusa.com/why-worm-castings-are-important-in-organic-gardening/