Milk

Milk in the garden? Some say it can be used as a fungicide or fertilizer, while others praise milk’s ability to acidify the soil. Who is right? Are these myths or useful tools? Let’s find out!

Milk and sunflowers (Pixabay)

According to Mother Earth News, a Nebraska farmer, David Wetzel, worked with a local County Extension agent, a soil specialist, a weed researcher, and an entomologist for 10 years to study the effects of milk on plant and soil health. According to his experience, plant production increased, soil porosity doubled, microorganism populations increased, grasshoppers abandoned his pastures, his pasture grass contained more nutrients and sugars (brix levels), and even his cows were healthier and produced more milk. At first glance, all that falls under the Too Good To Be True category, but is it? Can spraying milk on your garden plants really make things that much better?

Before we learn what milk can and cannot do in the garden, you need to know that Wetzel’s ‘research’ was never published, and that the real data showed absolutely no correlation between milk and any of the benefits listed. You can read an excellent summary of this at Garden Myths. After winnowing through several real studies, I learned that milk can be useful in the garden in some cases, but not all.

Milk as fertilizer

Anything added to the compost pile or the soil will ultimately be broken down by microorganisms into its elemental parts. While milk is the perfect food for baby mammals, it may or may not be good for plants. Milk contains amino acids, enzymes, fats, minerals, proteins, salts, sugars, and vitamins. Those minerals include calcium, chloride, citrate (an ester of citric acid), magnesium, phosphate, potassium, and sodium. Most of those minerals are useful to plant health. 

The protein in milk contains an average of 0.5% nitrogen, another useful plant nutrient, but in a very small supply. So, yes, milk can be used as a fertilizer, but not a very good one. By the time you added enough milk to get a reasonable response, your garden would smell like a rotten dairy.

Milk as soil amendment

Milk can add calcium to the soil, improve porosity, and acidify the soil. That’s what they say, anyway. The fact is, milk is 90% water, which does nothing to improve soil structure or porosity. It also contains very little in the way of organic matter.

Milk starts out with a pH of 6.4 to 6.8, which is great for plants. When milk starts to curdle, it has a pH closer to 5.0, which is acidic. Putting practically anything with a pH of 5.0 in your soil will alter its pH temporarily. As expensive as milk is, there are far more affordable, effective methods, but it won’t hurt.

Oleander aphid on milkweed (Kate Russell)

Milk as insecticide
​
It is said that if you spray soft-bodied insects, such as aphids, with milk, they will weaken and die because they do not have a pancreas with which to digest the milk sugars. Huh. That just doesn’t sound right to me. I mean, soft-bodied sap-sucking insects live on sap. Sap contains a lot of sugar. Insects may not have a pancreas but they sure know how to process sugar!

Some researchers believe that aphids may be deterred by foliar sprays of milk. It is not yet known why or how the milk spray does this. It may be that aphids simply do not like the milk film left on the plants, or it may be that microscopic pathogens that grow on the milk are a threat to aphids. We don’t know.

Milk as disease treatment 

Foliar sprays of milk on wheat and grape, squash, melon, and pumpkin leaves are said to prevent fungal and bacterial diseases, such as powdery mildew and leaf black spot. There is, however, no research that demonstrates milk can control black spot on roses or other ornamental plants. Most of the studies that support this claim were performed in greenhouses, rather than outdoors, which makes a big difference. Just because something works in a greenhouse, a highly controlled environment, does not mean it will work in the field.

Powdery mildew on butternut squash leaf (Kate Russell)

Generally speaking, milk or whey, applied before exposure to powdery mildew does reduce disease incidence. This may be because the benign microorganisms that start growing on the milk make it more difficult for powdery mildew organisms to take hold. Another theory is that the fatty acids contained in milk have antifungal properties. It may be for another reason altogether. We don’t know yet. Note that none of these studies succeeded using nonfat milk. 

Research has also shown that foliar sprays of milk are effective in treating viral diseases, particularly tobacco mosaic and other mosaic diseases on barley, beans, beets, celery, peas, spinach, sunflowers, tomatoes, and zucchini. It is believed that the milk may either deactivate the viruses, or physically isolate them, but no one knows for sure. The milk spray may also prevent aphid attack, thereby reducing the number of aphid-borne viral diseases.

Problems with milk in the garden

The main problem associated with milk in the garden is that there has been very little scientific research conducted. Most of the information available is anecdotal, at best. Besides being expensive, there are three problems you should keep in mind before deciding to use milk in the garden:

1. Rotten milk smells bad.
2. Dried skim milk, in particular, has been shown to induce, rather than prevent, Alternaria leaf spot, black rot, and soft rot on cruciferous crops.
3. Benign fungal organisms that grow on the milk can make your plants unattractive.

How to apply milk

If you decide to use milk to prevent disease, it is recommended that whole milk be sprayed on soil prior to planting and again when insects appear or just prior to when powdery mildew and similar diseases are expected to occur. You can also dump sour milk into your compost pile or around acid-loving plants, such as blueberries.

So, can milk help improve your plant and soil health? Yes, and no. Used as a preventative antifungal, antiviral, antibacterial treatment, milk may reduce disease incidence by 50% to 70% on some plant species.