If the worm castings are mostly vermicompost, the price should reflect the smaller concentration of castings contained in it. Vermicompost is an excellent amendment to your garden soil, but it will require more fertilizer applications at planting time and/or be replenished quickly.
Think of vermicompost more as a soil enhancement, adding tilth to your planting medium rather than as a fertilizer, although it does contain microbes and has fertilizer value. Vermicompost should be relatively inexpensive and probably should be bought locally, since the shipping costs can be prohibitive (especially if it's wet).
Worm castings with residual vermicompost material
A lot of the worm castings fertilizer you find for sale have a high concentration of worm castings but with a substantial amount of organic material (vermicompost) remaining in the product. This higher concentration of worm castings makes for better fertilizer than the vermicompost above, but a premium should not be charged for this fertilizer since it isn't "pure" and requires higher application rates.
Again, the shipping of un-concentrated worm castings that also contains large amounts of vermicompost is not cost effective. Before purchasing an unknown source of worm castings, it's probably wise to get a small amount or sample before getting large amounts only to find it isn't straight castings. Click here for a test to do for judging whether a worm castings product contains vermicompost.
Pure Worm Castings
Fertilizers that are pure castings are the most concentrated. The process to reach this level of purity is longer and more expensive than with simply vermicompsting organic material to a certain point and bagging it up. The application rates are lower for pure worm castings and the price should reflect this concentration.
Certified Organic and grown organically
Whether the pure worm castings being sold are certified organic or not is another consideration. Some castings are grown organically but lack the "certification". They may or may not conform with the standards set. You should not pay a premium for castings that do not have organic certification.
The certification process for fertilizers is difficult to achieve and maintain. This certification provides a user the assurance they are applying a truly "organic" product free of harmful toxins and poisons and that it conforms to standards set by organic certification boards.
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