If you are a gardener who has been on any social media platform in the past five years, you will have seen them: extolling the virtues of using milk as a magical ingredient to transform the health of your houseplants. Now, you don鈥檛 have to be a botanist like me to think this might not be the best idea. But science is all about trying to stay as objective as possible, so I decided to comb through the evidence to challenge my preconceptions.
Given how many times I have seen this idea circulate online, I have yet to see a single meme cite a study to back it up. Trawling through the academic databases I soon found out why. There really don鈥檛 seem to be any well-designed scientific experiments published on the subject. Occasionally, you see a reference to this being an 鈥渁ncient practice鈥�, but a search of historical sources also failed to turn up anything.
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In fact, the earliest reference I could find to this idea was from 2002, when Nebraska steel industry executive turned dairy farmer David Wetzel on fields and noticed the grass seemed healthier. If 2002 is 鈥渁ncient鈥�, I really do feel old, but it was in these articles that I saw the only reference to solid data on the subject. It was in quotes from University of Nebraska scientist Terry Gompert on an experiment he ran. , but it is plausible that milk could be a good plant food.
In fact, milk is a rich source of protein, which is built using nitrogen 鈥� a key plant nutrient needed for healthy leaves. It also contains phosphorus, essential for root growth, as well as calcium and a range of other minerals too. However, by and large, this is really the case for any organic material, plant or animal. Once decomposed by bacteria and fungi, the minerals are returned to the soil for plants to take them up again in a never-ending cycle. So is milk somehow superior to conventional fertilisers?
Skimmed milk, as apparently tested by Gompert, contains about 3.1 per cent protein, . That makes a solution containing 0.5 per cent nitrogen when used undiluted. Standard liquid feeds, by comparison, around 15 per cent nitrogen. But the nitrogen in milk is only available to plants after decomposition by bacteria and fungi in soil. Not only are these microbes not often present in potting mix for houseplants, but when they are, they also produce an almighty odour.
Finally, let鈥檚 look at cost. The store-bought houseplant food on my desk works out to about 拢0.17 per litre when diluted. Meanwhile, the milk in my fridge cost 拢1.06 per litre. This makes milk six times as expensive, while delivering fewer nutrients, in a less accessible form, with a more than pungent aroma.
James Wong is a botanist and science writer, with a particular interest in food crops, conservation and the environment. Trained at the Royal Botanic Gardens, Kew, in London, he shares his tiny flat with more than 500 houseplants. You can follow him on X and Instagram @botanygeek
For other projects visit newscientist.com/maker.
