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CalPhos Recipe

Let me first outline the steps to make CalPhos, below, using chicken eggshells. You can also use other calcium sources like seashells or bones.

  1. Pan fry eggshells (or seashells, or bones) until they are 50% brown/black.
  2. Grind or crush the eggshells, to achieve maximum surface contact.
  3. Put the eggshells into a jar or large bottle and pour in vinegar. The ratio of eggshells to vinegar is 1:5, volume wise. You will see a lot of bubbles appearing – that is an exothermic reaction in progress.
  4. Wait for the bubbles to subside, then seal up the jar or bottle.
  5. Ferment for 20 days.
  6. Filter out the eggshells.

The steps above are just a rough guideline. For more details of making CalPhos and how to use it, do refer to this excellent article from The Unconventional Farmer.

Now that I’ve got that out of the way, let us step back for a moment and consider why we do the above. Some of us might be thinking, having to “ferment” the eggshells CalPhos for a few weeks is too cumbersome. Why not just dump crushed eggshells straight into the soil?

For sure, dumping crushed eggshells into the soil could work, by relying on the soil microbes to break down the parts, but this takes a long time. If we want a faster process, then we will have to accelerate it ourselves to make the calcium and phosphor immediately accessible to the plants.

Next, let us look into the chemistry of the process of making CalPhos.

The Chemistry Behind The Process

Our goal is to make calcium and phosphate compounds that are soluble in water so that plants can happily absorb them through their roots.

The chicken eggshell is made up of 95-97% calcium carbonate (CaCO3). The remainder being calcium phosphate, magnesium carbonate and proteins.

Calcium carbonate is insoluble in water and cannot be absorbed by plants. It can however, be broken down into soluble compounds such as calcium oxide (CaO), either by heating up calcium carbonate, or by dunking calcium carbonate into acid.

Let us explore these two methods of obtaining soluble calcium compound that is accessible to plants.

Method 1: Extraction by heat

The process of applying heat to calcium carbonate will release carbon dioxide (CO2), to form calcium oxide. Calcium oxide is soluble in water, but it is also unstable if left alone and will revert back to calcium carbonate when it fuses with carbon dioxide in the air. To keep the soluble compound stable, one can add water or vinegar (acetic acid). In fact, I would recommend that the eggshells are grinded first before applying heat; this way, we can maximize the preservation of calcium oxide.

Calcium oxide mixes with water to form calcium hydroxide (CaOH). All hydroxides are soluble in water. And like all hydroxides, its pH level is alkaline.

Mixing calcium oxide with acetic acid (CH3CO2H), on the other hand, will yield calcium acetate. It, too, is water soluble. Generally speaking, calcium acetate is acidic, and is known for its use as a neutralizer of fluoride in water.

Method 2: Extraction by reaction with acid

If eggshells were to be combined with vinegar sans heating, calcium acetate would be formed, just as it is formed when vinegar is combined with calcium hydroxide or calcium oxide. The minor difference being that the former reaction will produce carbon dioxide.

So, what about the Phosphate?

What about it? If you have that question in mind, you are on the right track! The eggshell also consists of calcium phosphate – a mostly-insoluble compound. That’s right! Most phosphates are insoluble in water, but they are soluble in acids, including vinegar. More importantly, heating phosphates will yield pyrophosphates. Pyrophosphates exhibit the highest solubilities among the phosphate compounds.

What this suggests, is that dunking eggshells into vinegar without prior heating will yield a pretty decent calcium acetate solution but the availability of phosphor to plants will be lower. This is in comparison to an acetate solution whose eggshells were heated prior to adding vinegar.

Bottom line

Heating up the eggshells is necessary unless your plants do not require immediate access to phosphor.

Here are the key points to take home from this article:

  1. Do you need to reduce soil acidity?
  2. Do your plants need immediate access to phosphor?

If you answered “yes” to 1, and “no” to 2, then heat up the eggshells and mix with water to get an alkaline solution. Your plants will obtain immediate access to calcium, but not phosphor.

If you answered “no” to 1, and “yes” to 2, then heat up the eggshells, mix with vinegar and let them “ferment” for a few weeks before use. Your plants will obtain immediate access to both calcium and phosphor. Don’t forget — the acetate solution is acidic and should be diluted appropriately before applying to plants.

So, figure out what you want to achieve. Your plants’ needs will decide which method to choose from.

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