Using Manure in the Garden: What's Safe and What's Not? How and when to use manure as fertilizer.

As a gardener and as a livestock owner I was told for years that certain manures didn't need to be composted or aged before being used in the garden. I blindly followed this advice and didn't really dig deeper. Turns out a lot of that advice was wrong and could have caused me to make my family sick. As someone who now grows for the public I learned much more about this topic as I went through food safety courses, but this is a question that gets brought up a lot in different gardening and homesteading groups and there is a lot of misinformation out there. So, let's clear some things up. 

Alpaca, llama, goat, and sheep manures are all excellent for the garden and tend not to “burn” when they’re used before being composted because they don’t contain as much nitrogen or salts as, say, chicken manure. Horse manure can be high in weed seed because their digestive system does not work as thoroughly as some animals. Poultry manure is high in ammonia and salts and will burn if over-applied, especially if fresh.

This is why we are often told some manures are "safe" to be used without composting and that cattle, horse, and chicken manure has to be aged first. Yes, some herbivore manures may be safe for the plants without composting first, but the problem isn’t always the nutrients or the manures effect on the plants. The danger is that un-composted or un-aged manure from any herbivore, other than rabbits, can contain human pathogens, like bacteria, salmonella, and e. coli. Fresh manure has the potential of transmitting these pathogens that can then be taken up into plant tissue and then consumed by us.

The food the animal has eaten, the manure’s moisture content, the amount of bedding mixed in, and how long the manure has been sitting are all factors to be considered before using it in your garden beds.

Aging the manure from herbivores will help with making it safer to use and is faster than waiting for it to fully compost. Incorporating garden soil into the manure and then aging the mixture for at least 120 days helps soil microorganisms clear out pathogens and reduce weed seeds by about 60%. Composting is safer, but if the manures has been sitting in a pile for a few months waiting to be used and has been in contact with native soil, it should be okay.

If you’re unsure if it’s aged and you want to incorporate it right away, you can use it to create new beds. When we're filling new raised beds, we should be using that top 6” layer for the growing plants and filling it with good compost, worm castings, some potting soil, etc. If you want to use manure to help fill the new bed, build up a base of fresh manure in the bottom and then cover it with a 6-inch layer of topsoil and compost for planting into. The fresh manure will heat up the bed as it starts to compost underneath, which helps the seeds in the top layer germinate. You just don’t want to plant anything that is going to reach down below that 6 inches, like root veggies, because you risk direct contact with the manure before it’s done composting. This is a really good way to get a jump on the garden season if the manure hasn’t been aged yet or a good way to fill a raised bed in the fall for planting in the spring.  You can use this same method with aged manure, if you have access to it, it just won’t heat the bed up.

None of this is necessary with rabbit manure. Rabbit manure contains four times more nutrients than cow or horse manure, is twice as nutrient rich as chicken manure, and it doesn’t need to be composted to use as fertilizer. So, your neighbors bunny may be your new best friend for the garden (so long as they're not in the garden eating it!).

And, even though there are many groups that say guinea pig manure is just as safe, there is a risk that the guinea pig manure can harbor giardia, which can persist in the garden soil for several months and is transmissible to humans. So, the safest practice would be to treat guinea pig manure the same way you do any other herbivore.

I hate being bearer of bad news when people say certain manures don’t need to be composted. But it’s really more a matter of food safety rather than the affect it has on the plants. We can be safe and enhance our garden soil at the same time with herbivore manures, it just needs to be properly handled.

Your Friend in the Garden,

Additional Resources:

Animal-Manure-in-the-Garden.pdf (wsu.edu)

Bunny honey: Using rabbit manure as a fertilizer - MSU Extension

Colorado Master Gardener Training (colostate.edu)

Focal Point Friday: Manure in the Garden

Focal Point Friday: Follow-up to Sunflowers, Compost and Manure