A lot of folks swear by the pooey goodness of tea made with worm castings or bat guano. Is there a difference in the nutrient value of worm poo versus bat droppings? That all depends. The secret lies in what they are eating. What goes in will at some point come out, and diet has a great deal to do with whether the composted manure in question will be good for nitrogen, potassium or phosphorus boosting. Forage animals will all have manures that are nitrogen rich, but worms and bats have a more varied diet.

Commercial worm castings will all be higher in nitrogen content than potassium and phosphorous. It is most likely to contain absolutely nothing but nitrogen with an NPK analysis of just 1-0-0 on the bag. The reason for this is that it is cheaper for these operations to feed the worms paper and cardboard waste, which is a byproduct of trees. This is great for top growth and in a mild enough format that mixing it 1 part to 3 parts of potting mix will give you excellent results for both house plants and food plants. However, there is nothing here for root and flower production or improving flavor and aroma, not to mention raising the immunity system.

Compare the label of any kind of bat guano and its easy to see that they have a better overall analysis with at least something being present for N, P and K. But the strength seems to be concentrated in the middle of the analysis for all types of guano. N being the nitrogen content and bats not being foragers, this makes perfect sense. They feed on fruits and insects and try finding a bat farm somewhere. Worms are easier to keep in captivity – they can’t fly away and they eat whatever they’re fed.

Bats are more selective in accommodations and choice of diet. Most bat guano comes from caves that have long established population. These caves will be located in mountainous areas and most likely a tropical climate. Bat guano is high in phosphorus – the middle number in the analysis, or the P quotient of NPK. Phosphorus feeds bloom and root development. This makes it easy to see why you want bat quano for the flower to fruit stage of a grow. Some types also give your plants a good source of nitrogen at the same time. Look at the possibilities:

  • Indonesian Bat Guano (0.5-12-0.2)
  • Jamaican Bat Guano (1-10-0.2)
  • Mexican Bat Guano (10-2-1)
  • Peruvian Seabird Guano (10-10-2)

Obviously, that last one isn’t a bat at all, but its guano and gives you the closest thing to a balanced analysis in the realm of droppings and castings. It’s feeding all parts of your plants at once, though the potassium availability is a bit low. The Mexican variety will push lots of vegetation as opposed to promoting fruit and flower, while the stuff coming from Jamaica and Indonesia gives you the biggest boost in bud production.

If you’ve got lots of plants planned for your garden, the price of imported guano might not fit your budget. Of course if you’re lucky enough to know of an abandoned old house or two with bats in the attic, you might have a minimal source of free local guano. Then again, you can also produce your own worm castings very inexpensively, and insure that it has N, P and K by feeding the worms the right things. Lower their nitrogen intake and up their phosphorus and potassium with a selective diet.

Phosphorus Rich Potassium Rich Nitrogen Rich
  • Parsley
  • Potatoes
  • Carrots
  • Sweet Peppers
  • Radishes
  • Soybeans
  • Comfrey
  • Bananas
  • Potatoes
  • Spinach
  • Late Squash
  • Avacados
  • White Beans
  • Kidney Beans
  • Lettuce
  • Broccoli
  • Peas
  • Cabbage
  • Greens
  • Chard
  • Paper

Tomatoes are also high in phosphorus, but they are acidic and don’t belong in your worms’ diet. Radishes might also be a bit acidic, but in small amounts now and then should be fine to feed them.

As far as getting an analysis of your casting production’s NPK, you could take some samples to your local Extension Office in the USA or try a good soil test kit available from garden centers and grow shops everywhere. Naturally, the test results from the Extension lab will be more accurate, but the one you can do at home is faster. Expense wise, both are highly affordable. Extension tests run about $10 and the RapiTest Soil Test Kit runs under $20 – plus gives you a tool you can use a number of times.

There is one other plus to using worm castings. It has lots of humus in it. While this isn’t a nutrient, it is something that plants need and an excellent soil builder that does wonders for indoor garden container growing too when mixed with potting medium. It’s all natural and unless you fed your worms things that weren’t certified organic, homemade worm castings are purely organic too.

Tammy Clayton

Content crafter and Senior Editor at Garden Culture Magazine
Tammy has been immersed in the world of plants and growing since her first job as an assistant weeder at the tender age of 8. Heavily influenced by a former life as a landscape designer and nursery owner, she swears good looking plants follow her home. (Some feel she's got a perennial obsession, others say the problem is tomato plants.)

If you don't find her at the desk - check the gardens. When not writing and weeding, she enjoys a good book, painting junk furniture, and blending the harvest of heirloom tomatoes and chiles into salsas.