There has been a lot of chatter on the net recently about the magical benefits of milk as fertilizer in the garden. Most of the Magic is Imagined.
Is Milk a Good Fertilizer?
The idea of using milk as a fertilizer was started around the year 2000 when a steel industry executive turned dairyman by the name of Dave Wetzel started spreading his excess milk on agricultural land. There have now been several studies looking at this and they have all concluded that milk adds very little value and has no significant effect on plant growth. You can read the full story about how this myth started in How the Myth of Milk Fertilizer Started.
Is Milk Good for Plants?
Most of the information on the internet about the benefits of milk in soil say the same thing so I will use one source to keep it simple. The following quotes are from a Mother Earth News post called Milk as Soil Food.
“amino acids, proteins, enzymes and natural sugars that make milk a food for humans and animals are the same ingredients in nurturing healthy communities of microbes, fungi and beneficial bacteria in your compost and garden soil”
Absolutely true. Microbes will degrade the larger molecules in milk into basic nutrients which plants can then use as a food source. However, the same statement can be made about every living material; fruits, vegetables, plant waste, manure, compost, wood chips and even paper. All living or dead material that was recently living will do exactly the same thing for your garden.
The problem with milk is that it mostly water and the small remaining amount of beneficial organic matter has very little impact on soil, microbes or plants.
Heat Treated Milk is BAD!
“Raw milk is the best, as it hasn’t been exposed to heat that alters the components in milk”
The author clearly does not understand what happens once microbes work on the milk. They break the large molecules down into simple nutrients like nitrogen, and phosphate. Heat would actually speed up the process. Your organic source could be raw milk, heated milk or even cheese–it is all the same thing as far as your microbes and plants are concerned.
“Using milk on crops and soils is another ancient technique”
I can’t honestly argue against this statement from a scientific point of view, but does it seem likely to be true? Would people in ancient times, when resources were low and famines were frequent, dump good milk on the fields in the hope of a bigger crop? I kind of doubt it.
The ancient Greeks used it to feed animals and to make cheese.
Milk Reduces Powdery Mildew
There is some scientific evidence that milk does keep fungal spores of powdery mildew from germinating. However, it does not stop an existing infection so it does not cure the problem.
Sugars Poison Insects
“Milk sugars are a poison to soft bodied insects as they do not have a pancreas to process the sugars”
I can’t comment on the science behind this statement–need more time to research it. Does the statement make sense? All plants and animals contain sugars–they are vital for life. Insects eat plants and they eat other insects and larger animals. So we know they ingest sugars all the time and their bodies are able to handle the sugars in their diet. So either the sugars in milk are vastly different than sugars from other sources, or this statement is not true.
If soft-bodied insects can’t eat sugar than sugarcane would be free of aphids! A quick look on the net will show you examples of the sugarcane aphid–clearly it is not poisoned by sugars.
The “no pancreas” part may be true, but I doubt sugars are poisonous to soft-bodied insects.
“For the home gardener, the ratio can range from 100% milk to a 20% mixture with water, with no loss of benefits”.
Milk will act like a fertilizer. As stated above any organic matter added to the garden will be decomposed by microbes into plant nutrients–they are all fertilizers. The important question to ask, “Is it a good source of fertilizer”?
Have a close look at the quote. Milk is a fertilizer. You can use it straight out of the bottle or dilute it to 1/5 the strength, and it gives the same benefits. THAT’S AN AMAZING FERTILIZER! Imagine a fertilizer that you can dilute to 1/5 and still get the same amount of nitrogen from it. I wonder if you could dilute it 100 times and still get the same amount of nitrogen? Or 1,000 times?
Clearly this statement makes absolutely no sense. Never trust an article that contains such rubbish!
Nutrient Value of Milk
If milk is a fertilizer then it is natural to ask how good it is. In other words, how much nitrogen does it contain. We are most interested in nitrogen because it is the nutrient that is most likely deficient in soil.
It turns out that that milk contains 3.1% protein, and protein is about 1/6 nitrogen. So milk contains 0.5% nitrogen. Compare that to bagged fertilizer that is 10 – 40% nitrogen and other organic fertilizers that have about 2% nitrogen. Milk is a fertilizer, but it is a weak fertilizer.
Cost of Milk Fertilizer
Plants can’t tell the difference between nitrogen from milk, manure or commercial fertilizer–see my post What is Organic Fertilizer for more details. Because of this it is always valuable to figure out the cost of any fertilizer.
Around here 3 L of milk costs around $5 which is 15 g of N. So if you are using milk you are paying $330 for 1 Kg of nitrogen.
Commercial fertilizer is about $12 for 1 Kg of N (10Kg bag of a 10-10-10 is about $12).
How about manure? Manure is about 0.6% N (wet weight), and 1 cu ft of wet manure = 60 lbs = 27 Kg. A cubic yard is 27 cu ft, so a yard is 730 Kg or 4.4 Kg of N. At $15 a yard, a Kg of N in manure costs $3.40. Buying composted manure in bags would be more expensive, but not nearly as costly as milk.
Milk is mostly water and so it does very little to build soil structure. Commercial fertilizer also does not build soil structure, but manure does.
So you have a choice. Use milk at $330/Kg nitrogen and get no soil improvement, or use manure at $3.40/Kg nitrogen and improve soil structure. Note that I have not misplaced the decimal point here. Nitrogen from milk is 100 times more expensive than nitrogen from manure.
Using milk as a fertilizer makes no sense! Try using it to control powdery mildew, but other than that it has no value in the garden.
Should You Use Milk?
If you have some spoiled milk it is better in the garden or on a compost pile than down the drain. But don’t use food quality milk to fertilize gardens or houseplants.