Chemical side of worm casting

I do realize that the chemical and microbial content of each and every worm farm would be different and I am sure everyone accepts that, but as a general guideline what is the norm for worm casting. I myself have had a few bad experiences, like planting a pot plant into pure worm castings with great expectations as I thought this was the best stuff in the whole wide world, the plant did not do well at all, even with potting soil 50:50 mixture no fireworks. I am sure you have heard many such stories. My point being please points us with a general rule of thumb to follow using our worm farm harvests. Thanks” ~ Kevin Elphinstone

Hi Kevin,

There is so much that I can talk about with this topic that I will dedicate this weeks and next weeks newsletter to your question. We will focus more on the chemical side of worm cast this week.

First of all, I have mentioned before that NPK (Nitrogen, Phosphate and Potassium) values is not important in wormcast. In fact, compared to normal fertilizers it is very low, but it does not matter. NPK value is a scientific measurement of the readily soluble forms of NPK in chemical fertilizers. These can be directly absorbed by plants once it is used in the soil. However in worm casting, the NPK are locked in the cast, and is released into
the plants slowly as micro-organisms break it down. This is much better for plants, as it takes time for plants to uptake nutrients rather than all at once.

Having said this, the NPK value in wormcast is much higher than soil, and I will explain its importance later.

Let’s start by understanding what kind of nutrients there are in soil, and how the deficiency of each particular mineral will affect plants. I will list out the minerals one by one, and explain the plant symptoms when there is a lack of this mineral. The following is taken from Virginia Tech publications on hydroponic plants:

Nitrogen – Entire plant is light green in color; lower leaves are yellow; growth is stunted.

Phosphorous – Entire plant is bluish-green, often developing a red or purplish cast; lower leaves may be yellow, drying to a greenish-brown to black color; growth may be stunted.

Potassium – Leaves have a papery appearance; dead areas along the edges of the leaves; growth is stunted.

Magnesium – Lower leaves turn yellow along the tips and margin
and between the veins; the lower leaves wilt.

Calcium – Young stems and new leaves die.

Zinc – Leaf tissue between the veins is lighter in color; yellowed; papery in appearance.

Iron – Leaf tissue appears yellow, while the veins remain green.

Copper – Leaf edges appear dark green or blue; leaf edges curl upward; young leaves permanently wilt.

Sulfur – Young leaves turn pale green, while the older leaves
remain green; plant is stunted and spindly.

Manganese – Growth is stunted; lower leaves have a checkered pattern of yellow and green.

Molybdenum – Leaves are stunted, pale green, and malformed.

Boron – Young leaves are scorched at tips and margins.

Now you can guess why worm cast is great to use in plants, because they do contain at least some of each of the minerals listed above. Of course there are some more than others, but the plants don’t use all of them in the same amounts. Take Molybdenum for example, it converts nitrate (NO3) into amino acids (building blocks for plant  protein) and it is used within the plant more than once. Too much molybdenum will be toxic to plants.

Just to give you an idea, here is an example of the percentages and composition of nutrients and minerals in vermicompost:

• Organic Carbon 20.43 – 30.31 %
• Nitrogen 1.80 – 2.05 %
• Phosphorus 1.32 – 1.93 %
• Potassium 1.28 – 1.50 %
• Carbon : Nitrogen 14-15 : 1 %
• Calcium 3.0 – 4.5 %
• Magnesium 0.4 – 0.7 %
• Sodium 0.02 – 0.30 %
• Sulphur Traces to 0.40 %
• Iron 0.3 – 0.7 %
• Zinc 0.028 – 0.036 %
• Manganese Traces to 0.40 %
• Copper 0.0027 – 0.0123 %
• Boron 0.0034 – 0.0075 %
• Aluminium Traces to 0.071 %
• Cobalt, Molybdenum Present in available form

The above is to give you an idea only, you are correct in noting that all vermicompost is different because the composition does truly depend on the type of feed you give to the worms. But from the above table you can see which kind of mineral and nutrient is more abundant within wormcast.

However I know all these figures makes no sense by themselves. So how does vermicompost compare with normal garden soil?

Other than Aluminium and Magnesium (which is about the same in both) and iron (which is less in vermicompost), the values of all the other nutrients/minerals are significantly much higher (with Nitrates up to 9 times higher) in vermicompost than garden soil.

This poses a problem in its electro-conductivity, because this is also much higher than garden soil which means there is more salts in vermicompost. When there is too much salt in soil, it sucks out water from the plant roots resulting in the ‘burning’ of plants. Although there isn’t enough salts in vermicompost to do that (it is much more common in chemical fertilizers), using too much wormcast at any one time can stunt plant growth.

It has been studied what the optimum ratio of wormcast to potting soil is. Researchers potted plants starting in different ratios of cast to soil, and found the plants with the optimum growth were planted in a cast:soil ratio of 1:4, or 75% soil. However they have shown that even 10% of wormcast shows significant difference in plant growth.

Once the percentage of wormcast is over 40%, plant growth performance is stunted and may even appear worse off than having no wormcast at all. So your use of 50% wormcast is definitely too much!

You will need to use a less amount of worm cast, and hopefully you will start to see your plants flourish!

Kenneth

c/o WormMan Newsletter http://wormman.org/?p=162