By Susanne von Rosenberg, UC Master Gardener of Napa County
This is the time of year when we start to prepare our soil beds for spring planting. Because Napa County has large rural areas and many households keep backyard chickens, many of us have access to free manure. Used properly, manure can be an excellent soil amendment and organic fertilizer, increasing soil fertility and tilth. But how do you make sure that you are using it properly and what concerns should you have?
Animal manures typically contain the poop and urine of animals, as well as bedding material, and often undigested or partially digested food that has passed through the animals' digestive tract. They may also contain feathers or hair. Risks associated with using animal manure include food-borne illness, excess nitrogen “burning” plants, high salt levels, herbicide residues and excess carbon.
To prevent food-borne illness, use only manures from herbivores. Do not use manure from pigs, dogs or cats; they may carry parasites that can infect humans. Even manure from herbivores (including cows, horses, goats, chickens and rabbits) can be contaminated with E. coli, Salmonella and Campylobacter bacteria, and Giardia or Cryptosporidium protozoa.
Many commercially available manures are pasteurized to kill soil-borne pathogens. If you have a source of raw manure, it's best to compost it before using. Your compost pile should reach a temperature of at least 140°F for several weeks, and you need to turn the pile several times to make sure all of the manure has composted.
Another benefit of composting manure is that it stabilizes the nutrients, enabling a long, slow release. Aging manure is not the same as composting it. Aging manure creates conditions that cause pathogens to decrease over time due to changes in moisture, temperature and nutrient availability. But pathogens are not actively killed by aging. Aging keeps them from reproducing, so populations slowly decline.
If you do want to use uncomposted manure in food gardens, then you should turn it into the soil at least 120 days before you plant crops that grow in or near the soil (such as root vegetables, but also plants such as strawberries) and 90 days before you plant trellised crops such as tomatoes or pole beans. Even if you've composted the manure, it's best to wait 90 days after you add it to the soil to plant food crops in that bed.
Of course, you can always use uncomposted manure in flower beds as long as you don't plan to eat the flowers. Be extra careful in washing vegetables from any garden beds containing uncomposted manure. People who are more susceptible to food-borne illness (including pregnant women, small children and people with compromised immune systems) should not eat uncooked vegetables from these areas.
Most manures are relatively low in available nitrogen. As with most organic fertilizers, the nitrogen in manure typically requires bacterial action to make the nitrogen available to plants. Uncomposted chicken manure is an exception. It can contain excess nitrogen (high ammonia levels) that can “burn” plants, causing leaf tips or entire leaves to dry out and die. In extreme cases excess ammonia can kill your plants. Overapplying chemical fertilizer can have the same result. Composting reduces the ammonia content so it will no longer burn plants.
On the other hand, if your manure contains a lot of bedding material, you may end up with too much carbon. The ideal ratio of carbon to nitrogen is about 30 to 1. If your composted manure exceeds this ratio, it may immobilize nitrogen in the soil and deprive nitrogen-loving plants (most annuals) of the nitrogen they need. However, high-carbon composted manure would still make a great mulch.
Finally, manures can be contaminated with herbicides such as those used on lawns, turfgrass, pasture and hay crops. Herbicide residues can remain active for a long time, even after passing through an animal's digestive system and, in some cases, even after manure is composted. These residues can cause poor germination and kill seedlings or cause new leaves to become twisted and malformed. Sensitive plants include most vegetable crops and grapes.
The residues do eventually break down and lose activity over time, particularly as they are exposed to microbes, heat and moisture. If you're getting manure from someone else, ask if the animals are eating hay or pasture treated with herbicides. If you're using manure from your own animals, avoid feeding them hay from herbicide-treated fields and avoid using herbicides on their pasture. If you purchase manure, try to get assurance that it contains no herbicide residue.
Be aware that some manures, especially cow or steer manure, can be high in salts. If you have used cow or steer manure, rotate it with other sources to give the salts a chance to leach out of the soil over time.
Napa Library Talks: First Thursday of each month. Register to get Zoom link. Thursday, February 4: Soil is the Solution: Healing the Earth One Yard at a Time.
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