Gardening can take a toil on soil, as plants pull vital nutrients for growth and production of flowers, vegetables, and fruit. Another important factor to growing plants is the soil structure. You can address both soil structure and nutrient-deficiencies needed to maintain or even increase the ability to grow vigorous plants by adding manure. Manure is the waste products of animals and has many benefits. However, there are a number of cautions you need to be aware of before just dumping manure on your garden.
Manure increases soil organic matter, which can help improve soil structure. Manure also helps improve sandy soil’s ability to hold water and drainage in clay soil. It slowly releases nutrients into the soil and can promote beneficial soil organisms’ growth (Compost and soil organic matter: the more, the merrier?, Penn State Extension).
Acceptable types of manure for use in vegetable gardens include cow, horse, sheep, goat, llama, rabbit, and chicken/poultry. There are some additional precautions to take if you are going to use chicken/poultry manure that will be discussed later in this article. (Using chicken manure, UMN-Extension). Rabbit manure is a great source of manure, 'bunny honey'. Pig, dog, cat, and human waste should NEVER be used in gardens as they are more likely to contain parasites.
Use of manure in gardens does require precautions, especially where and when you use fresh or ‘raw’ manure. The biggest risk is that fresh manure may include bacteria and other pathogens that can cause diseases in humans such as e.Coli, Salmonella, Campylobacter bacteria, and others. You can’t determine if an animal may be carrying a pathogen by looking at them or their waste. For this reason, it is critical to not use fresh manure around vegetables as these pathogens can be taken up into vegetable plant tissue through the soil and water. Rabbit manure is the exception because of its pelletized form and low risk of pathogens.
If fresh manure is applied to areas where food is grown, nothing should be planted in that location for at least four (4) months for any food product whose edible portion has direct contact with the soil surface or soil particles. If the food product does not have direct contact with the soil surface or soil particles, then the timeframe is 90 days. (USDA Organic Tipsheet: Manure in Organic Production Systems). That means that you should not apply manure in the spring before planting unless you are only going to be planting late summer crops for fall harvest. (Safely Using Manure, UW-Extension, Using Manure in the Home Garden, UW-Extension). Early fall may be the best time for manure application.
As noted above rabbit manure is the exception. It may be used ‘fresh’ and has many benefits over other types of manure including having four times the nutrients of horse and cow manure and twice the amount of chicken manure.
The best manure to use has been composted, which when done properly, can kill any harmful pathogens, stabilize the nutrients, and lower salts that are present. Composting manure, along with any bedding material or other substances, involves regular turning, aeration, and making sure the compost reaches specific temperatures for specific amounts of time. According to the USDA Organic Tipsheet, depending on how the composting is occurring, the manure must reach
Temperatures between 131° F and 170° F and must be sustained for three days using an in-vessel or static aerated pile system.
Temperatures between 131° F and 170° F and must be sustained for 15 days using a windrow composting system, during which period the materials must be turned a minimum of five times, and this period must be followed by an adequate curing period.
Composting raw manure into manure that is safe to use may be difficult, but not impossible, to achieve by a home gardener. Achieving and maintaining the high temperatures is challenging in a home environment and turning and aerating the pile is a considerable commitment.
Another consideration when using manure is you don’t know the specific amounts of nutrients and micronutrients you are adding. It varies by the type of animal waste and any additional materials such as bedding that might be mixed into it. This is important because adding the wrong level of nutrients may produce less desirable effects. For example, if manure was added around tomato plants, the nitrogen might promote growth of the plant which may decrease the energy the plant puts into producing the tomatoes. Purchasing fertilizer in some instances might be a better approach as you can select the amounts of Nitrogen, Phosphorous, and Potassium (N-P-K) you add to specific areas of your garden.
Despite all these cautions, adding manure can be beneficial to the home garden by improving the soil structure, water holding capacity, and through the slow release of nutrients. With a little care, your plants will reap the benefits and grow and produce vigorously.
Photo Credit: University of Minnesota Extension (1,2)