If you are making plans to amend garden soil with manure prior to spring planting, be aware of safety recommendations for manure. The key point is that fresh manure should not be applied to fruit and vegetable gardens during spring. If fresh manure is used, only apply it during fall.
Animal manures often contain microorganisms that are harmful to humans, such as Salmonella and E. coli. For this reason, the use of fresh manure is not recommended in vegetable gardens, especially during spring, where microorganisms could contaminate food as it is grown and harvested.
Composted or aged manure, which has been allowed to sit and begin to break down for at least one year, is a better choice for vegetable gardens. If fresh manure must be used, do not add it to garden soil within 120 days of the next harvest.
On vegetables with soil contact (carrots, beets, potatoes), fresh manure applications need to be made at least four months prior to harvest. On other fruit and vegetable crops, fresh manure applications should be made at least three months prior to harvest.
Horse, cow, sheep, or poultry manure is fine to use, but do not use swine manure because of the higher potential for contaminants. When manure is used as a garden soil amendment, always wash hands after working in the garden and wash all produce prior to eating it.
Other disadvantages of manure include potential for salt build-up in soil, weed seed introduction, and possible burning of roots and foliage due to high ammonia. Burning is more likely with fresh manure, especially poultry manure, which should not be applied to gardens in spring. Following this safety rule will reduce the risk of burning.
Repeated or heavy applications of manure can lead to a salt-build-up in soils which can harm plant growth. Salt content may be high in fresh manure but will decrease over time with exposure to rains and irrigation which leach salts. Feedlot manure is often high in salts since a salt additive may be used in the livestock diet.
To help avoid salt problems, limit applications to one inch of composted manure per year tilled six to eight inches deep in the soil. If manure will be used as an amendment on a regular basis, test the soil for salt content before adding large amounts. Avoid use of manure on soils that are already high in salts.
Weed seeds will be introduced into the garden through manure. If weeds are well managed in a garden, and not allowed to go to seed, this should not be a major issue. If manure is composted prior to use, don’t assume weed seed will have been killed by this process. Weed seeds will only be killed if all seeds in the pile have been subjected to compost temperatures above 145 degrees Fahrenheit.
In place of manure from a local farm, bagged and composted manure is now available at garden centers. While more costly, composted and bagged manure has fewer odors. It may be easier to haul and store, and the composting process used for commercially bagged manure may be more likely to kill weed seeds.
However, salts can become concentrated during composting as moisture is lost and volume is reduced. Many bagged manure products can still be high in salts. Also keep in mind the nitrogen in composted manure is in an organic form that will be slowly released to plants. This is also true of aged manure right off the farm.
Source: Colorado State University Extension CMG Garden Notes 242.
Kelly Feehan is a UNL extension educator-horticulture. She can be reached at (402) 563-4901 or by email at kfeehan2@unl.edu.
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