How to Safely Use Manure as a Garden Fertilizer
Animal manure has long been used as a fertilizer in gardens and on farm fields, creating nutrient-rich, moisture-retaining soil for your plants. See the best type of manure for your garden and the best time to apply—as well as a few things to consider first.
All manure is not created equal. Depending on the kind of animal, its age and health, its food, what kind of bedding is incorporated into the manure, and how the manure is collected and stored, the nutrients the manure contains can vary widely. Also whether the manure is fresh, composted, aged, or dried has a great impact on its nutrient values. To really know exactly what it contains, you would need to get the manure tested.
Different Types of Animal Manure
The most common sources of manure are cows, horses, sheep, pigs, goats, and poultry. (Other animal waste is not recommended as manure or fertilizer today.)
Here are some values commonly assigned to different animal manures:
- Fresh cow manure, sometimes called moo doo, is about 17% organic matter, offers .3% nitrogen, .2% phosphorus, .4% potassium, and is 83% moisture. To add .2 pounds of nitrogen to a 100 square foot garden patch you would need to add 75 pounds of cow manure without bedding or about three 5-gallon bucketfuls. Composted manure has even less nitrogen so you would need to add 200 pounds of it to have the same effect! Dried cow manure has much higher nutrient levels - 2% nitrogen, 2% phosphorus, and 2.4% potassium—so you could use much less, roughly 10 pounds per 100 square feet.
- Fresh sheep manure is about 32% organic matter, has .7% nitrogen, .3% phosphorus, and .9% potassium and is 66% moisture. To add .2 pounds of nitrogen to our 100 square foot garden we would need to add 40 pounds of manure with no bedding or 50 pounds with bedding included. Dried sheep manure is 4% nitrogen, 1.4% phosphorus, and 3.5% potassium so we'd need only 10 pounds per 100 square feet.
- Fresh chicken manure, sometimes called hen dressing, is 25-45% organic matter, has 1.1% nitrogen, .8% phosphorus, and .5% potassium, and is 55-75% moisture. Add 30 ponds of it, including bedding, to a 100 square foot bed to get .2 pounds of nitrogen. Composted it would have less nitrogen so you'd need 70 pounds. Because of its tendency towards alkalinity, poultry manure is unsuitable for lime-hating (ericaceous) plants, such as rhododendrons, azaleas, camellias, blueberries and heathers.
How do you like them apples?
- Fresh horse manure, sometimes called road apples, is 24% organic matter, has .7% nitrogen, .3% phosphorus, and .6% potassium, and is about 75% moisture. To add .2 pounds of nitrogen to the 100 square foot bed we'd need to incorporate 65 ponds of it with bedding included.
- Fresh rabbit droppings, AKA bunny honey, are 33% organic matter, offer 2.4% nitrogen, 1.4%phosphorus, .6% potassium, and only 43% moisture. As little as 10 pounds would add .2 pounds of nitrogen to the garden.
- Fresh llama manure, also called llama beans, offers 1.5% nitrogen, .2% phosphorus, and 1.1% potassium. It would 20 pounds to add .2 pounds of nitrogen to the 100 square foot patch.
Fresh vs Composted Manure
Fresh manure is less than 6 months old and has not been composted. Never use it to sidedress your plants! It has high enough nitrogen and ammonia content to burn them. The plants are not able to make use of all of the nitrogen and along with it potentially damaging them it is also a waste as most of it will be lost and can contaminate groundwater.
The USDA's National Organic Program (NOP) has specific rules about the use of manure in organic gardening. Raw manure must be applied no less than 120 days prior to harvesting leafy crops or those that come in contact with the soil such as lettuce, beets, carrots, and potatoes. For those that do not touch the soil, such as peppers or tomatoes, raw manure must not be spread less than 90 days before harvest.
For those like me who have a short growing season this means fresh manure must be spread in the fall. Fresh manure also can contain harmful pathogens including E. coli, salmonella, and listeria, along with lots of undigested weed seeds.
Bottom-line: Either till raw manure into the soil at least a season prior to planting. Or, the manure must be composted fully and "aged" before you add to the soil.
Composting the Manure
Composting manure will greatly reduce the risk of illness and can render the weed seeds incapable of germinating. If it doesn't have a lot of bedding such as straw, wood shavings, or sawdust mixed with it, add leaves, grass clippings, food waste, or newsprint to increase the ratio of carbon to nitrogen to between 25:1 and 40:1 for the best results. Make sure your pile reaches high temperatures (131 to 170 degrees Fahrenheit) for at least a week, then add limestone, turn, and aerate the pile and let it cure for 2 to 4 months before using to cut bacteria risks.
The nutrient values in the composted manure will be lower than in the fresh manure, but it will still supply your soils with organic matter, beneficial micro-organisms, and trace minerals including calcium, boron, magnesium, copper, sulphur, zinc, and iron. To learn more about safe use of manure in your garden check out this tipsheet.
Make sure your pile heats up enough to kill off dangerous pathogens.
And here's a post specifically about preparing the soil in fall for next year's garden. Fall is the best time to improve your soil!
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