What is the best manure for the garden? This question is routinely debated on social media providing a wide range of opinions. Some say chicken manure is great but others say it’s too hot and will burn plants. Horse manure can be used almost fresh, but cow manure needs to be composted. Sheep manure is higher in nutrients and sheep digest their food more fully – is that important? Some swear by rabbit manure.
Many of these discussions consist of a lot of opinions, but they’re very weak on facts. In this post I’ll look at the science and try to determine which manure is best.
You Have To Define “Best”
Online discussions about manure suffer from two main problems. They lack facts which makes it hard to reach any conclusion, but more importantly, they never define the word “best”.
Does best mean high nutrients? Lower cost? Easier to apply? More eco-friendly? Less likely to contain herbicides or antibiotic? Is the best manure the one you can get with the shortest drive so that you save time and cause less pollution?
All of these are valid definitions of best, but without agreeing on something, you can’t reach an intelligent conclusion.
Do You Want Manure Tartare or Well Done
You can use manure in one of three forms; fresh, aged or composted. Fresh manure is stuff that is less than a month old. If you let manure sit around for a while it becomes aged. It has not composted yet and you can clearly see the individual components in it, but it is no longer as hot as fresh manure.
Composted manure has gone through a hot composting process and now looks like healthy black soil.
Farms and large gardens might use manure fresh or more likely aged, but this is not readily available to most home gardeners, so I will ignore this type of manure in this post. Instead I’ll focus on hot composted manure.
The Manure Industry
The gardener needs to be aware of the fact that there are few if any regulations about naming manure in most localities. Words like “manure”, “aged manure”, “rotted manure”, and “composted” don’t really mean very much. You certainly can’t compare products based on the descriptions used.
When a supplier was asked about the difference between bagged cow and sheep manure, they replied, “do you really think we manage two piles?”
Manure Factors to be Considered
To compare the benefits of different manures it is important to understand the factors or properties that impact a garden.
- Soil Improvement
- NPK of manure
- Herbicide content
- Antibiotic content
- Degree of digestion by the animal
- Weed seeds
There are two main reasons for adding manure to the garden; organic matter and nutrients.
Many soils have been over worked by agriculture or damaged by construction and they now have low levels of organic matter. Adding organic matter improves soil structure, increases its water holding capacity, increases its nutrient holding capacity and increases the microbe population.
The second reason for using manure is to add nutrients to the soil. They take two different forms. The inorganic component, which people call salts, is what plants can use. These include nitrate, phosphate, calcium, potassium, magnesium and a range of micronutrients. The second form consists of organic nutrients which are not available to plants. Slowly over time, as decomposition continues, these organic nutrients are converted to the inorganic form so plants can use them.
Organic Matter in Manure
The most important reason for adding manure to the garden is to increase the level of organic matter but discussions about the best manure never seem to mention differences in organic matter.
In fact, I could not even find values for the organic matter of different manures. The closest I found are values for the dry matter content, which should be fairly close to the organic matter level. Chicken has the highest value, probably because a lot of their manure consists of bedding material.
So which manure is best for organic matter? We have almost no data to compare manures and commercial products don’t usually provide this kind of information and yet this is one of the main reasons for buying manure. I don’t think the gardener has a way to evaluate manures for this property.
NPK of Manure
Fresh manure contains a fair amount of inorganic nutrients, but by the time it’s composted it consists of mostly organic nutrients.
Try to find the NPK of composted manure. I couldn’t find a reliable source for this information. So how is it that discussions on social media always compare the nutrient levels of manure from various animals? The NPK values of fresh manure are readily available and this is what most people use to compare manures. But such comparisons are only valid if you use fresh manure.
I looked at a variety of commercial bagged products that contained both the words “manure” and “compost”. One had an NPK of 1-1-1 and all of the others had an NPK of 0.5-0.5-0.5 or 0.3-0.3-0.3. I did not find a single product that had a different ratio. Many products have no NPK listed on the package or in the online information and none of the bulk products provided the NPK.
The NPK value for any particular type of manure can vary wildly because it depends on the type of animal, type of feed, age of animal, degree of composting and amount of urine collected. The biggest factor is the diet since 50% to 90% of what is eaten, ends up in the manure. So a grass fed horse will produce different manure than one fed on alfalfa and grain. Manufacturers of composted manure don’t want to print new bags with each batch of product, so they settle on a value that will be close to the final value.
The important point here is that if you buy composted manure, it all has about the same NPK. The fact that fresh cow might have a better NPK than fresh horse is immaterial.
What if you start with fresh manure and hot compost it yourself? Your starting material will depend on the source, but a lot of the nitrogen is lost during the composting process. Even P and K are lost. Hot manures have a low C:N ratio and to compost properly you need to add more carbon, which has the effect of reducing the relative nitrogen level. I suspect that the end product for all types of manure won’t be much different than 0.5-0.5-0.5.
Rabbit manure may be the exception since it does not need to be composted before being used. But one product I looked at was $40 for two pounds. I bought 12 yards of horse manure (hundreds of pounds) for about $60 and I can get bagged composted manure for about $4 for 35 pounds.
Herbicide Content in Manure
Herbicides are used to control weeds in forage (animal feed) and then this plant material is fed to animals. Some herbicides can survive the whole process including composting, resulting in manure that harms plants, especially seedlings.
It is important that manure is herbicide free, making this one of the most important selection criteria. How do you know it is herbicide free? Most certified manure has been tested for herbicides but a lot of commercial manure is not certified.
Antibiotic Content in Manure
All farm animals may contain administered antibiotics and none of these animals completely digest the antibiotics so it ends up in their manure. Manure from organically raised animals should be free of antibiotics.
Antibiotics affect microbe populations, but they don’t seem to inhibit the composting process. The high heat of hot composting does degrade antibiotics. Cold composting is much less effective.
Fresh manure and cold composted manure (ie aged) will contain larger amounts of antibiotics. Hot composted manure has much lower levels, but it still contains some.
Do plants absorb antibiotics from the soil?
The answer is a clear yes and it seems as if higher concentrations in soil result in higher concentrations in plants. You might think that organically certified vegetables would be antibiotic-free but you’d be wrong. Organic farms do use manure from animals that have been given antibiotics.
The level of antibiotics found in vegetables is very low, and should not be a concern. But keeping antibiotic levels low in your soil is a valid goal.
Pathogens in Manure
“Animal manures contain a wide range of bacteria, viruses and protozoa and some of these are known for their adverse effects on people. Bacteria, that are known human pathogens, include certain strains of E. coli, including 0157:H7, Salmonella spp., Listeria, Streptococcus spp., Campylobacter, Clostridium spp. Protozoa include Giardia and Cryptosporidium. Most viruses found in animal manure do not infect humans.”
The type and the amount of pathogen varies a lot. For example E. coli 0157:H7, a major worldwide food-borne pathogen known to cause life-threatening conditions, is most common in cattle but has also been found in the manure of other mammals. Camplyobacter is common in poultry manure but can be found in all species. If you are using manure you have a potential exposure to pathogens – no fresh manure is free of them.
Most pathogens die on their own within a year and many die sooner. At higher temperatures, their life expectancy is reduced to weeks or even days. Hot composting at 55ºC (130°F) for 2 weeks will kill almost all pathogens.
For those still considering the use of compost teas, “Adding sugar or molasses materials during the steeping process has been reported to increase the incidence of pathogens in the compost tea. ”
Hot composted manure, especially certified manure, should be free of pathogens.
Degree of Digestion by the Animal
I see claims that the degree of digestion is an important factor. Some say ruminant manure such as cattle, sheep and goat, is best because it goes through a more efficient digestive system. Horses, for example, have a less efficiency system and a lot of what goes in, comes out, undigested.
If you take plant material and compost it – taking the animal our of the loop – you get good compost. This seems to indicate that the degree of digestion is not an important factor. The type of food eaten by the animal is probably more important.
Manure is rarely just manure. It is almost always mixed with some type of bedding material. The amount of this, and the type are also important, and most of this is undigested.
I don’t think the degree of digestion is an important factor.
Availability of Manure
From an environmental point of view and from a cost point of view, the best manure is the one you can get easily. There really is no point driving out of your way to get a manure that is marginally better than the one close at hand. For many gardeners this means getting bagged manure at the local grocery store or nursery.
Free manure is great. Less cost for you, and you are recycling a waste product.
Some manure has higher levels of weed seed than others, but this is a non-issue in a properly mulched garden. Hot composting also kills most weed seed.
Hose manure can have higher levels of weed seed, and sheep has lower levels than even cow.
Which Manure is Best?
Lets return for a moment to the assumption I made early on, namely that we only need to consider composted manure. If you decide to use fresh or aged manure, then there might be some minor differences to consider. Sheep manure is produced in small nuggets that are easier to handle than big cow pies. Fresh manure does have different NPK values depending on the animal source but these vary a great deal so unless you measure the NPK you won’t know what it is. Using fresh manure in the garden sooner will add more nutrients and organic matter to the soil than if you use a hot composting process.
The risk with fresh manure are the pathogens. Personally, I think that risk is quite low, but authorities need to be more cautious and they recommend composting.
If we look at the above criteria for composted manure it becomes obvious that many online discussions don’t reach the right conclusion. It’s not about NPK values since most composted manure has the same value. The hotness is also not an issue since composting eliminates this. Herbicides and antibiotics are an issue in any type of animal manure and weed seed is not an issue.
Assuming you want to reduce the risk of pathogens, you are left with using composted manure, and then there really is only one parameter left to consider; availability.
Some brands may be better than others, but the gardener has no way to evaluate and compare one product to another. Online advice that says sheep manure works great is of zero value.
The best option is to compost your own plant refuse and use that. If that is not enough compost, than buy locally. Don’t ship a special bag of alpaca manure across the country because someone online said it was better. Once composted, it’s all black shit!
Selecting the Best Soil
The following posts will help you select the best soil: