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.
Ancient Times
“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.
Milk Fertilizer
“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.
Greetings and thank you for this article. I happen to have A LOT of extra milk every week. So much that it goes bad or unused or thrown out which is wasteful. Can the milk be utilized for plants even if the outcomes are the same as any other fertilizer?
Milk contains organic (chemists definition) chemicals that will be consumed by microbes. No harm using it.
Is milk a good source for calcium for plants. I see pepper growers saying it is good and peppers use a lot of calcium. Wondering what your thoughts are on that.
First question, do peppers need a lot of calcium? Probably not a lot more than other plants.
Milk does contain some calcium, but most of it is water. If you are buying it for calcium it is a very expensive fertilizer. Solid calcium sources are quite cheap. If you have some free milk or it has gone bad – use it. But don’t buy milk because your soil test shows a deficiency of calcium.
I have a few things to point out. I admire the sentiment of “but does it make sense?” For teaching other to strain out the garbage, but you’re applying it too narrowly. For example, when you ask if it makes sense that the ancients would waste good milk, you’re making several harmful assumptions, 1) that resources were always rare, and 2) all milk was always good, 3) accidents didn’t happen, 4) that using one resource to improve another was out of the question. You said it yourself, whole milk or cheese, it’s all the same to the bacteria, so spoiled milk fits easily within this theory.
Secondly, your use of the “does it make sense” against milk sugars being harmful to pests. You need to remember a few species of bacteria and all mammals are just about the only organisms able to easily digest milk, and the thing that makes most animals and most adult mammals intolerant to milk is lactose, a milk sugar. You made the claim that either we were lied to or milk sugar is vastly different from plant sugars. Two chemicals can be similar chemically yet behave very differently. Take glucose and cellulose for example. Glucose is a sugar, cellulose is a hard to digest natural polymer- yet cellulose is literally just a bunch of glucose molecules linked together. Or take cellulose and lignin, lignin is much more rigid and hard to digest that cellulose, yet is literally just several cellulose molecules formed together.
You have also made a strong financial case against using milk as a method of improving the soil, but you left out that milk is meant as a short term solution, a rapid-release source of nitrogen, and if the fed plants get composted, the nitrogen is recycled and what was a short term solution, has begun yielding long-term results.
You used “than” where “then” was appropriate.
Some valid points, but I don’t agree with “Two chemicals can be similar chemically yet behave very differently. Take glucose and cellulose for example. ” Glucose and cellulose might look similar structurally, but they are not similar chemically – hence the fact that they react differently.
I don’t go out of my way to use milk as a fertilizer what I do is rinse out my milk jug or if I happen to have a little bit of milk that has soured I will add water to that out shake it up I will let it sit on the counter overnight to build up a little microbial activity (cap on) and then I will fill it all the way up with water the next day and I will add it to my plants and it doesn’t seem to bother them at all….. I don’t stress out about it!
what is milk soil
no idea.
I have seen the use of milk in growing giant pumpkins by splitting a stem feeding a rag through it and soaking it in a container of milk. if memory serves it was in a wonderful film, “Rise of the Giants,” about competitive pumpkin growing.
I wouldn’t recommend milk as fertiliser, unless you’re ready to use lime stone to keep the soil pH around 7. Sour milk is fairly acid, which is bad for tomatoes.
Sour milk is not a strong acid so it will have limited effect on soil pH. More importantly, soil pH is a function of the rock in the soil. It is usually difficult to change the pH of alkaline soil. Pure rain contains carbonic acid and has a ph of 5.5.
A troll just stole my comment, here it is again: of course milk is no substitute for well tended soil but if you have a few ounces of no longer fresh milk, is it all right to water and feed a withering from the midday heat old rose with a dilution of milk and water?
Maybe you did not wait long enough to have the comment approved?
There is nothing wrong with spreading old milk in the garden – it is just not a great fertilizer.
Reading this article it seems you agree with a lot of the MEN article and only make nagging points that antagonize and do not disprove MEN. Of course plants can tell the difference between different types of food and nutrients. Im sorry you feel they have no preference. Soft bodied insects cannot digest the sugars/enzymes/polysaccharides of milk. There are people who do not do so well with it either. Drenching the soil will help. Ever heard of cal mag deficiency? Its more or less for the microbes in a living soil. You absolutely can use more or less in a ratio as we all know plants will only take what they need the difference between milk and nitrogen being milk will not cause nutrient lock out. Not until recent decades was milk so expensive. At one point in time people never thought to till the ground where they threw out their half eaten rotted food until they saw the vegetation around growing like crazy. The worst part of this article is that you didnt prove any counterpoints. Digress.
Actually I disagree with much of the MEN article.
Plants can’t tell the difference between different types of food. The food, provided this is large organic molecules must be decomposed into nutrients before the plant can use them. Once decomposed into nitrate for example – plants can’t tell where it came from. This is basic chemistry.
Yes you can use more or less – I never said you can’t. But you can’t expect the same results with more and with less. That is basic physics.
Do you have a reference to support the idea that in olden days milk was cheap and it was used in fields?
Hi I milk a cow for our family and have to dump close to 3 gallons of milk a day from once a day milking. I skim the cream off the top and dump what is left. I was worried about hurting my trees so I started dumping it right down the drain.
Milk is expensive but it is also abundant. The problem is inflation and taxes translated into labor costs and land that makes it so costly.
Also people prioritize convenience and ease over freshness and quality.
What I found was that the leaves on my small fruit trees and a redbud were browning.
While i agree with most of what you said, but it’s true that you provide vaery little or no convincing evidences of your argument. I want to add a few points…
1. You talk about Physic and Chemistry, it’s good and it was correct. But you might forget the Biology there. The microbe ecosystem in the soil do increase the nutrient absorption of the plant. And milk is an excellent medium to culture microbe. But again, it totally depends on the established soil.
2. The main drawback of the milk as fertilizer in my opinion, is that they contain too much nutrient value, but those have very little value and unbalance to plants. Milk is a great source of carbonhydrate (mostly lactose and lipid – which is useless for plant) and protein. There’re many other sources of protein that Is cheaper and can be used for fertilizer, fish for example.
3. Too much nutrient in the milk as i mention, promote growth of bacteria which in turn, alter the pH of the soil dramatically. There’s only ~3.5% protein, but ~5% lactose in cow milk. When any form of glucose is used by microbes, it produces acid. The weak acid nature of the metabolite acid is irrelevant because its the pH that counted, not the acid per se, and they produced in mass. Note that in microbiology innoculating medium, we use about 1-2% carbonhydrate at most for optimal growth of bacteria. Also, the pH change happens overnight to couple of days, but take weeks to month to put it back to normal. At the mean time, it will inhibit absorption of metal ions, especially K+ and Fe+.
In short, while provide very little and unbalance source of nitrogen, milk may do more harm than good because they are rich in nutrient content, most of it is either useless for plant, or require extensive metabolism process from microbes to transform into something usable by plant
I assume a little milk may help to improve microbiological ecosystem of the soils thus improve plant’s nutrient absorption. Any excessive use will be counter productive.
But i do look for a way to use milk (waste) as fertilizer.
-I typed this on the phone when on my way to work, please excuse any annoying incorrect grammar in my cmt-
I appreciate that you say “If you have expired milk that you can’t use in any other way, add it to the garden”. That seems balanced. I wouldn’t look to it as a substitution for better fertilizer or soil amendment sources. I haven’t read a whole lot about the value of adding milk to my garden other than the Mother Earth News (MEN) article and the referenced study but I came to the conclusion that if I had milk unfit for human consumption I would dilute it and spray it on my yards and gardens (not indoor housplants for fear of sour milk smell) instead of wasting it or pouring it down the drain.
I have used diluted milk as a foliar spray on plants with mildew, especially of the squash family, and found that it appears to be helpful especially if started at the first signs of mildew. My hope is that using it as a soil drench will act prophylactically in some way or another and lessen or eliminate the mildew altogether. That will be my experiment 😉
One of the points from the MNE article that I too pondered was “sugars are a poison to soft bodied insects as they do not have a pancreas to process the sugars. This also explains why insects will leave healthy, high brix level plants alone, as they contain more sugars than the stressed and sickly ones.” I don’t know much about insect anatomy or their ability to assimilate this or that and I don’t have the science or references to prove or support this but I’m inclined to think that healthy, properly fed and properly watered plants are stress, disease and insect resistant much in the same way that healthy, properly fed and happy humans have stronger immune systems and hence are better defended from viruses and perhaps parasites and illness in general.
Milk does seem to control mildew when sprayed on leaves. I doubt it will have any effect sprayed on the soil.
Re: “This also explains why insects will leave healthy, high brix level plants alone, as they contain more sugars than the stressed and sickly ones.”” I think that the reason healthy plants are left alone is because they have the chemical resources to make natural pesticides that keep insects away. Stressed plants will be less able to do this. It is also possible that stressed plants give off chemical signals that insects can detect.
You have an impressive amount of patience, Robert.
The anecdote that got me rinsing out milk bags (Canadian, obviously) for my garden was how ”the cows stampeded to the grass where I dumped excess milk”… worked back Dude’s ratios and my front yard needs about 1/3 cup of milk.
If there is any effect, perhaps it is bacterial, but not from the native bacteria in raw milk.
Dude was big on brix of grass going from 1 to 30, thanks to raw milk!! Having used refractometers, I know they measure soluble sugars and any insoluble bits totally screw up readings. One would think soluble sugars going up by a factor of 30 may have <> osmotic effect on the plant?
As for milk, it rates up there with STP in engine oil— probably no effect, but the owner gets a good feeling above/beyond the cost/effort.
Brix – that is another big boondoggle that i need to write about one day.