Scooping the Coop

How to use manure in the Southwestern veggie garden.

September 6, 2017

HomesteadIssue 26: September/October 2017

Using manure from your homestead to help grow the vegetables in your garden is the ultimate in recycling. It solves the problem of what to do with your homestead’s byproducts while helping to reduce the costs of producing your food and reducing the amount of waste that makes it to our landfills. Pima County’s recent amendment to Title 18 in the zoning code reduced some of the restrictions on raising chickens in our backyards, so you might be wondering how chicken manure stacks up to other fertilizers or manures, and how to make the best use of it.

Chicken, horse, and cow manures are most commonly used because they’re readily available and generally safe, but there are a lot of other critters living on our Baja Arizona landscapes, and each type of manure has its own risks and benefits. When aged or composted and used properly, manure can improve the structure and quality of your soil while providing some organic nutrition for your fruits and veggies. If used incorrectly, however, you could end up with burned plants or a nasty stomach bug. It takes patience, a little bit of planning, and a few easy, common sense measures to use most manures safely in your garden.

While it’s often thought of and used primarily as a soil builder rather than a complete fertilizer, manure does contribute slow-releasing nutrients that can energize your plants and the good microbes in your soil. The nutrients available from a manure compost will vary by the type of animal it came from, what it was fed, age of the manure, and method of composting or aging. We can estimate the nutrient contents in many of the more common manures, but things change as time passes and organic materials are broken down.

The best way to determine how much of your composted material you should use is to get the compost and your garden soil tested. Once you know the specific nutrient contents of your material and the state of your soil, you can figure out what your application rates should be. Your county extension office is likely to have a list of nearby soil testing laboratories, and they may also be able to help with rate recommendations once you have your results. If you have questions about the composting process, they can help with that, too.

There are a number of reasons why thorough composting is recommended before mixing manures into your soil. Six to 12 months of composting reduces bulk and odor, and breaks nitrogen down into less harmful forms that plants can absorb more easily without the risk of burning. It also kills most of the weed seeds and harmful microbes that can be present in fresh manure. This doesn’t mean that composted manure is completely safe; just that it’s a good deal safer than the fresh stuff. When you’re dealing with edibles you always want to err on the side of caution.

Wear rubber or vinyl gloves and boots that can be properly sanitized after you’re done. It’s best to spread manure well before planting crops, work it six to 10 inches into the soil, and water it thoroughly to leach out extra salts. Composted manure that isn’t being mixed into the soil immediately should be covered to help prevent nutrient loss while it waits.

A good time for many of us to plan this chore is just after the last fall harvest, but some of Baja Arizona enjoys a nearly year-round growing season, so a good rule to remember is to mix manure into the soil at least 120 days before harvesting crops that come into direct contact with the soil (carrots, greens), and 90 days before harvesting crops that can be kept off the soil surface (peppers, tomatoes). Most manures shouldn’t be applied to edible crops that are already growing, even as a side-dressing.

After the compost has been tilled into the soil, add a two- to three-inch layer of mulch over the bed to give a good buffer zone between the soil and your crops. Mulching is a great idea whether you’re using manure or not. It reduces exposure to soilborne pathogens from direct contact with the soil, and from splashing onto plants during storms or overhead watering. Mulching also aids in moisture retention, regulates soil temperature, and helps protect seeds and seedlings from predators. Trellises and grow cages can provide plants with extra support to keep fruit and branches up off of the ground wherever possible. (As always, wash your hands after working in the garden, and make sure to wash your fruits and veggies before putting them on your plate.)

For health reasons, don’t use manure from your dog or cat in the garden. Manure from herbivores is generally considered safe, but there are a lot of questions about the potential for carnivore manure to carry pathogens that transmit easily to humans. I’ve excluded pig manure from this discussion for the same reason, even though its use isn’t uncommon.

How do they stack up? Compared to synthetic fertilizers and some organic fertilizers, the nutrient contents of most manures are fairly low. It will take more pounds of composted manure than pounds of synthetic fertilizer to get the right amount of nitrogen to your garden, but manure releases nutrients more slowly, and builds better soil in the process.

Earthworm castings are the safest and easiest to use, and since a vermicomposting bin can be set up even in a studio apartment under the kitchen sink, this is an increasingly popular soil builder. The nutrients are extremely variable, depending on what the worms were fed to produce each batch of castings. This is the only manure on the list that you can use as a side dressing for plants that are already growing, but it can still burn the roots of sensitive seedlings. Use your castings to make a tea to get the biggest return for your efforts. (Check out Zotero Citlalcoatl’s three-part series “Grow Your Soil” at EdibleBajaArizona.com for the ultimate vermicomposting guide.)

Chicken manure is high in nitrogen, which is why it’s sometimes called “hot.” It’s normally weed and disease free, especially after composting, but using it fresh or using too much can easily burn your plants. This manure is more like a fertilizer than the others; it breaks down quickly and doesn’t last very long. On average, fresh chicken manure contains 1.6 to 4.3 percent nitrogen, 0.5 to 2 percent phosphorus, and 0.9 to 2 percent potassium. Generally, don’t add more than half a pound per square foot per year.

Cow manure is easy to purchase (already composted) at super low rates. It’s popular because the bovine multistomach processing does a great job of breaking down the bad stuff and infusing the manure with lots of good gut bacteria that benefit our soil organisms. Dairy cows have a diet different from cattle, so their patties are different, too. Dairy cow manure has around 0.5 to 2 percent nitrogen, 0.1 to 0.5 percent phosphorus, and 0.3 to 1.5 percent potassium. Steer manure has 0.5 to 1.5 percent nitrogen, 0.2 to 0.7 percent phosphorus, and 0.5 to 2 percent potassium. Don’t use more than one pound per square foot per year.

Horses don’t have the fantastic stomach setup that cows do. Their manure is more likely to contain some unpleasant seeds, so it should always be composted before adding it to your garden. Of course, if the horses are yours then you have a lot of influence on the quality of feed and thus the quality of their manure. Horse manure has about 0.5 to to 0.9 percent nitrogen, 0.1 to 0.15 percent phosphorus, and 0.2 to 0.6 percent potassium. It’s rich in nitrogen but tends to have lower levels of phosphorus and potassium, so it’s a better choice for leafy veggies than it is for the ones that we want to flower. Limit applications to 1¼ pounds per square foot per year.

Rabbit manure comes in small, convenient pellets that are also virtually odorless. The high phosphorus content is great for a boost to flowering, but also helps plants with resiliency and drought tolerance. Earthworms are pretty fond of this stuff, so you might consider starting a worm bin directly under the hutch for a jump start on the composting. Rabbit droppings are variable in nutrient content, but averages are around 2.4 to 3 percent nitrogen, 1.4 to 2.8 percent phosphorus, and 0.6 to 1.3 percent potassium.

Goat and sheep manures are favored by some for being nearly odorless, and they have lower nitrogen contents, so they’re less likely to burn your plants. They start out fairly dry, are easy to compost, and benefit the soil over a longer period of time. Diets, and thus nutrient contents, can vary, but averages are around 0.7 to 2 percent nitrogen (or up to 4 percent in pellets from holding pens), 0.2 to 0.6 percent phosphorus, and 0.7 to 2.8 percent potassium for sheep and goat manure. Use no more than ⅔ pound per square foot per year. ✜

Amy Belk is a garden writer and photographer, a certified arborist, and a certified nursery professional who has been learning from her garden for 16 years. She and her husband homestead on a little piece of the desert in the heart of Tucson.







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