Manure is a natural fertilizer that can transform garden soil by improving its structure and boosting fertility. Gardeners have relied on this organic material for centuries to introduce beneficial microbes, increase water retention, and provide a slow-release source of plant nutrients. Using manure successfully requires understanding its properties and following crucial safety steps to protect both plants and health.
Choosing Appropriate Manure Sources
The source of the manure significantly impacts its nutrient concentration and potential risks. Manure from herbivores, such as cows, horses, chickens, and rabbits, is generally suitable for gardening, but their potency varies widely. Chicken and rabbit droppings are considered “hot,” containing a higher percentage of nitrogen compared to milder cow or horse manure. Milder manures can be applied more liberally, while the concentrated nature of poultry manure makes composting necessary to prevent plant roots from burning.
A contamination concern for gardeners is the presence of persistent herbicides, which can pass through an animal’s digestive system. These herbicides, used to control broadleaf weeds in hay fields or pastures, remain active in the manure and can severely damage sensitive crops like tomatoes, beans, and potatoes. Gardeners should always inquire about the feed source and herbicide history before accepting a manure donation.
Manure from carnivores and omnivores, including cats, dogs, and humans, should never be used in vegetable gardens. This waste carries a high risk of transmitting human pathogens and parasites, which are difficult to eliminate through home composting methods. Dog waste also often has a high nitrogen and salt content, which can easily “burn” plants and create nutrient imbalances in the soil.
Understanding the Need for Aging and Composting
Fresh manure should not be applied directly to an active garden because it contains high levels of ammonium, a form of soluble nitrogen that can burn plant tissues. This material also harbors viable weed seeds and potentially harmful pathogens. The process of curing or treating the manure is necessary to stabilize its nutrients and reduce these hazards.
“Aged” manure is material left to break down passively in a pile, typically for six to twelve months. This aging allows volatile ammonia to dissipate and reduces the total volume, but it does not guarantee the elimination of pathogens or weed seeds. The risk of pathogens remains because the pile’s internal temperature is not consistently monitored or maintained.
Composting involves managing the manure pile to accelerate decomposition under controlled conditions. Proper composting requires the pile to reach a high temperature, ideally between 131°F and 160°F, for a sustained period, often several weeks. Regular turning is necessary to ensure all the material is exposed to these high temperatures, effectively killing most weed seeds and human pathogens.
Methods for Applying Manure in the Garden
The best way to apply aged or composted manure is to incorporate it into the soil during the off-season. Applying manure in the fall allows the material to integrate into the soil structure over the winter months, minimizing the risk of nutrient runoff. For vegetable gardens, a thin layer, about one-half to one inch thick, should be spread across the surface and lightly tilled or dug into the top six to eight inches of soil.
Timing is especially important for food safety, as a waiting period must pass between manure application and harvesting edible crops. For vegetables where the edible part touches the soil, such as root crops or leafy greens, a minimum of 120 days should elapse after application. For crops that do not contact the soil, like corn or trellised tomatoes, the waiting period is slightly shorter, at least 90 days.
Aged manure can be used as a side dressing or top dressing for established plants during the growing season. A light application around the base of heavy feeders like corn or squash provides a gradual nutrient boost. The manure should be kept a few inches away from the plant stems or trunks to prevent potential burning.
Gardeners can also create a liquid feed known as “manure tea” to provide a quick nutrient delivery to plants. This is made by steeping one part aged or composted manure in about five parts of water for one to three days. The resulting liquid extract contains dissolved nutrients that are readily available for plant uptake. Before application, the liquid should be diluted with water until it resembles the color of weak iced tea to prevent foliar or root burn.