Citrus in the Compost
You may have read somewhere (probably on the internet -- so it must be true) that putting citrus peel in a compost pile is a bad idea. I first heard this alleged no-no from a "master gardener" while my Rantings book was at the printer, thus it is not addressed therein. The above pic is from an article entitled 5 things you should NEVER include for making a good compost which confirms it is a bad practice.
So, I gave the claim a think and concluded that it is based either on (1) the acidity, (2) due to compounds in the citrus or (3) perhaps it is not valid. Let's address the second part first. Citruses belong to the Rue Family (Rutaceae) most of which are woody and contain oils that cause the plants to be strongly scented, the familiar citrus aroma. Some of the compounds can cause skin irritation. This led me to think that perhaps the "don't use" suggestion was therefore based on chemicals that might impede the decay process by inhibiting the composting organisms. This line of thinking also could be supported by the fact that some of the compounds found in citrus have been employed to make pesticides. However, I discounted this possibility based on the probable amount of citrus peel in a compost pile. Moreover, some of these compounds are volatile and would likely be greatly diminished by evaporation or off-gassing by the time the peels reach the pile. (Note that when I say peel I am referring to the entire outer covering, both the colored exterior and the spongy white pith beneath). The average person or household does not consume enough citrus for the percent composition in the compostable material to be high. Perhaps in a household that regularly fresh squeezes citrus for juice but, even then, I would be skeptical. The only situations where the quantity of citrus peel would present a problem are (1) at a citrus farm or (2) in vermiculture -- some worms do not like citrus,
As for the acidity being a problem, I did some research and testing. Turns out that citrus peel is less acidic than the flesh and lemons are more acidic than oranges, but there can be considerable variation in both -- dependent on species or variety. Some oranges are actually more acidic than some lemons, and we are eating the flesh or drinking the juice (at least most of it), not putting it in the compost. Again, I would be shocked to find a household where citrus peels makeup more than 5% of the compost mass. And, if so, I suspect their dentist has discussed enamel erosion with them. Next, I processed orange peel and measured the pH. (Why oranges? Because they are larger than lemons and we use more of them.) The mean reading from several measurements and three separate navel oranges was about 6. Now consider this, the organic compounds found in humus and compost (which I also measured) make them naturally acidic, with a pH of 5.5-6. Hello.
I am a serious composter and consume a fair amount of citrus. The citrus peel in my piles decompose just fine, and I have worms -- in my compost : ) The citrus warning is not true. Nor is the similar claim regarding coffee grounds -- that putting them in compost is bad since they will lower the pH. The pH of used coffee grounds is 6.7-6.9 (variable due to the water pH and the roast of the coffee -- dark roast tends to be less acidic). I have done the testing several times. Furthermore, the same percent of total mass argument applies to coffee grounds and it is ludicrous to suggest that adding something with a high pH (less acidic) to something with an equal or lower pH (not or more acidic) would lower the pH of the second something. I address 88 other gardening and plant myths in Chapter 4 of my book. I also debunk myths in my classes and at speaking engagements, yet the fiction persists. BTW The article cited above, like so many others on the internet, is pure crap. The people who push this malarky, which includes some master gardeners, obviously do not know (simply passing on false info) or they are lying. Please stop! It is hard enough to get people to recycle and compost without filling their heads with misinformation.
Finally, having citrus peel in your compost is an advantage since many critters that might forage the piles are dissuaded by the aromatic compounds.