Worm Composting Secrets

Preparing Worm Bedding

"Sir can you give me formula for preparing bedding. I have cow manure and wheat straw. In what % i shell mix to properly compost before use. Want to know c:N ratio in dry cow manure and wheat straw and what is mixing ratio in kg for one tone. Thankyou." ~ Fajr Sadiq

Hi Fajr,

That's an interesting question and one that requires some calculations. There are also a variety of variables that can come into play here - but for the sake of this response I will make some assumptions.

Cattle manure and wheat straw are indeed ideal composting materials, and you are correct in thinking that they will need to be composted prior to use as bedding. This will help to stabilize the materials somewhat and lower ammonia concentrations, creating a much more hospitable environment for the worms. Since you will be using it as bedding I would definitely recommend letting the materials compost for longer than if you were simply using it as food. The bedding acts as a safe refuge for the worms, so you will want it to be well aged.

I am glad you are dealing with dry cow manure - that definitely makes things a little more straight forward. Moisture content can totally alter our calculations, since water can make up the majority of the weight of liquid manures.

Even though we are dealing with "dry" materials (straw and manure), let's assume a moisture content of 15%  for both (since neither of these will be perfectly dry).

Here are some of our other assumptions:

A fairly standard %Nitrogen for cattle manure is roughly 3% with a C:N in the range of 15:1. Feel free to use different values if you happen to know the exact values for your manure. It's important to mention that I am assuming there is no bedding (eg. straw) mixed in already.

As for straw, I've seen C:N ranges from about 70-130:1, so let's use 100:1, with a %N of 0.4%

The ideal C:N ratio for composting is 25-30:1 (although 20-40:1 in general will work fine), so we will assume 25:1 for our calculation.

Here is the equation we will use (adapted from "On-Farm Composting Handbook"; Ed. Robert Rynk; NRAES-54 1992):

C:N = 25 = (C in 1kg of manure) + S x (C in 1kg straw) / (N in 1kg manure) + S x (N in 1kg straw)

Where S = amount of straw neeeded per kg of manure.

For the sake of keeping this as short as possible, let's just jump right in (I'll be more than happy to show full calculation on the discussion forum if people request it.)

25 = 0.3825 + 0.340S / 0.0255 + 0.0034S
25(0.0255 + 0.0034S) = 0.3825 + 0.340S
S = 1kg straw

Therefore if we are assuming the values I gave above and on a dry weight (15% moisture content) basis, for every 1 kg of dry manure you use, you should use about 1 kg of straw.

So for every metric ton (1000kg), there should be 500kg of manure and 500kg of straw.

Of course, given our 15% moisture content we are also going to need to add water to the mix. Effective composting requires a moisture content of AT LEAST 40% (50-65% is better) - again we are talking about thermophilic composting prior to giving it to the worms (moisture content for worm composting can and should generally be higher).

How long you let it sit before using it as bedding definitely depends on how effective your composting process is. Will it be a static pile (ie. no turning or forced aeration)? If so it will take longer.

I would probably leave a static pile for at least a few months before using as bedding. If you were using it as food for your worms (ie layering it over the top of your bedding) you could get away with a MUCH shorter 'pre-composting' phase - potentially as little as a week or less (again under ideal conditions).

Anyway - I don't want this response to go too long. Perhaps I will continue this discussion in the next newsletter (definitely plenty more that could be discussed) or in the discussion forum.

Hope this helps!

Bentley Christie