Saturday, 23 January 2016

Using egg shells in the compost heap.

This is an old post but I accidentally deleted the other one.  Don't want to loose any of my pearls of wisdom so I have reposted it. 

As the soil is not full of undecomposed egg shells from thousands of years of not decomposing and the probability that reptile and dinosaur eggs are of a similar composition, I find it very difficult to imagine that the egg shell is not recycled within the soil.

Egg shell is mainly made up of calcium carbonate with a little calcium phosphate. Both calcium and phosphorous are plant nutrients. These are inorganic compounds that will not decompose. On the inside of the egg you will get membranes and possibly egg residues that are made out of protein and which are rich in nitrogen. If you wash the eggs you wash away the nitrogen. Cooking the egg shells will denature the proteins and hopefully they will remain caked to the inside of the shell. Egg shell will only break down relatively quickly if you crush it up  in a mortar with a pestle or use a blender. I crush them with the back of the shovel as I turn the compost. They will eventually break down into pieces that are difficult to see especially when covered in compost juice. They are a valuable addition to the compost in my view.

Really it comes down to what we mean by decomposition. The communition of material such as egg shell by soil animals like earth worms is a major contribution to the recycling of materials. I would argue that this is an integral component of the decomposition process. Increasing the surface area of materials such as egg shells means that they can be more easily processed by microorganisms. Fungi secrete oxalic acid from their cell surfaces and this degrades the egg shell into calcium oxalate. The formation of calcium oxalate crystals provides a reservoir of calcium in the ecosystem which can be made available for plants to use. The ability of oxalate to bind divalent cations permits detoxification of copper, particularly evident in wood preserved with copper salts.  So this is might make the use of treated decking wood for raised beds a little safer.   Oxalate plays a unique role in lignocellulose degradation by wood-rotting fungi, as an  agent initiating decay.  A vital process for someone like me who likes to add lots of woody material to the soil. 

While egg shell, lime and chalk are all mainly calcium carbonate and not organic molecules, they will add calcium to the soil as they are broken into smaller and smaller pieces.  Technically they do not decompose like organic matter.  I suppose that they weather in a similar way to other minerals in the soil.  Calcium is a nutrient for plants and they need to have a constant supply dissolved in the soil water around the roots. 

Calcium is a nutrient for most if not all living organisms and in a soluble form would be incorporated into the cells of many micro organisms and higher life forms so I cannot believe that egg shells would not be exploited for their calcium content. The calcium carbonate within the egg shell would be slowly dissolved in carbonic acid produced from carbon dioxide; acids produced by microbes; acids produced by plants and atmospheric pollution.

There is, however evidence of recalcitrant calcium from organic sources such as in limestone and chalk and it could be a component of complexed humus molecules. Indeed, teeth and bone composed primarily of calcium phosphate can take a very long time to decompose.   However, bone meal is used as a fertiliser.

Whether particular people use it or not, bone meal is a fertiliser that contains calcium phosphate which decomposes relatively slowly – not unlike egg shells containing calcium carbonate, but bone meal must decompose or it would not be a fertiliser.

Whether we compost egg shells or some way else dispose of them, they will break down, albeit relatively slowly, and add nutrients to where ever they have been added to the soil.

Why give those nutrients away to someone else when they could be used in your own soil?

What else would you do with them anyway?

To judge a substrate on how long it takes for it to break down seems to be a very arbitrary way to select items for the compost. However, to base compost making on the proportions of carbon and nitrogen within micro organisms seems to be quite arbitrary as well. Deciding on what is green and what is brown not to mention whether the ratio is volume or weight seems a little ridiculous particularly when considering the rate of decomposition of different components – such as egg shell. I have found that the proportions for the C:N ratios are ratios of the dry weight of carbon and nitrogen. Suberin, lignin, hemicellulose and cellulose all decompose at different rates and some may remain in the soil for thousands of years before they are decomposed.

Whatever the proportions of nitrogen and carbon that are added to a compost heap is irrelevant to microbes.  They seek out what they need and actively import the compounds that they need to survive regardless of what we do. 

If compost substrates are not decomposing or decomposing very slowly, micro organisms will not be using nitrogen or using it very slowly.  So is there a problem with nitrogen being scavenged from surrounding soil?   

So carefully working out your ratio of brown to green, whether by weight or volume, while some of that brown will not be of any use in producing nutrients for plants, seems pointless to me. However, the brown will be essential to increasing the potential CEC. Compost and other organic amendments to soil, such as egg shell, do a lot more things than just add nutrients.

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