“How much of the worm castings do I use on the garden?” ~ Conrad Parker
That’s a great question, and something many people likely wonder about. It is often assumed that vermicompost/castings are essentially the same thing as regular thermophilic (hot) compost, when in fact they are quite different.
Scientific research has clearly shown that only a small of quantity of castings needs to be used in order to stimulate significant additional growth in plants. This will likely come as good news for anyone who has wondered why castings tend to be so expensive in comparison to other composts and mixed soil products.
Multiple research studies conducted at Ohio State University have clearly demonstrated this. For example, Atiyeh et al (2001) examined the growth promoting effects of worm castings (vermicompost) on tomato plants.
The researchers used a standard commercial growth medium (MetroMix 360) and substituted in various percentages of castings (made from pig manure): 0%, 5%, 10%, 25%, 50%, and 100%. Growth of the seedlings was then monitored for 31 days. It was found that even at the 5% level, the castings had a significant positive impact on the growth of the tomato plants!
What’s interesting is that growth of the plants was actually hampered at the 100% substitution level! So clearly, not only are large amounts not needed, but they may in fact have a detrimental effect on your plants.
Another interesting example can be found in Casting Call (Volume 7; #1, p.3). A letter from Jack Chambers (Sonoma Valley Worm Farm) to the editor of the newsletter provides an intriguing look at the value of even small quantities of castings. Chambers recounts the experience of a client who purchased a cubic yard of castings to help stimulate the growth of young grape vines.
According to this person (who plants vinyards for a living), he simply added a cup (8 ounces) of castings in the hole before adding each young plant. Unfortunately he ran out of castings with two rows left to plant but decided to simply plant the rest anyway. The planting took place in March, and apparently by June all the plants with castings added had grown as much as is typically expected by the end of the first year! Unfortunately, those planted without castings were at normal growth levels – and the difference was very obvious (thus angering the owners of the vineyard and embarrassing Chambers’ customer).
Getting back to your question, Conrad…
When using castings in the garden, I’d recommend taking a conservative approach to help ensure that your supply of the material can benefit as many plants a possible.
Start by planting seeds in tiny pots or peat pellets – you can add a small pinch of castings to help stimulate germination and growth of the young seedling. Once your plants are ready to be planted in the garden, simply dig a small hole for each and add in one or two spoonfuls of castings before adding the young plant.
In the case of larger (older) plants, add a cupful of castings in your planting hole. This should greatly assist in root development, disease suppression, and overall growth stimulation.
Another approach you may want to try is ‘in situ’ worm composting
– i.e. creating a worm bed within the garden itself. This way the plants can directly access the castings as they are produced, and you are saved the hassle of harvesting your own castings or purchasing them.
Simply dig some trenches between your planting rows (preferable before you start planting) and add a mixture of bedding (shredded cardboard, straw, fall leaves etc) with lots of food waste or aged manure. Let this mixture sit for a few days (or more) then simply add your composting worms.
You can then continue to add more waste materials over time as they are processed by the worms. If you place planks of wood over your trenches you can effectively create a walkway for yourself helping to protect your worm beds from drying out (and excesses solar heating).