First, Know This
All commercially sold fertilizer is analyzed for the stuff it brings to the soil, which is expressed with an NPK rating. For example, Miracle Grow's all purpose (non-organic) fertilizer has an NPK rating of 24-8-16. The first number represents the amount of nitrogen in the fertilizer; the second number, the phosphorus; and the third, the potassium.
Nitrogen helps leaves and stems develop and is ideal for leafy vegetables like cabbage, lettuce, kale, and collards. Too much nitrogen inhibits flower and fruit growth, however. Phosphorus helps develop root systems, flowers, and fruit. Potassium increases plant health, making the roots more hardy and helping the plant fight disease. It also helps develop fruit.
Manure also has NPK ratings, which you'll learn below, but know that because you're dealing with an organic product, actual numbers may vary.
Many people consider the gold standard in manure is horse manure. In the old days, horse manure was carefully collected and rarely was wasted. Even city families without horses swept the street in front of their home in order to collect this valuable stuff. Today, if you don't have horses of your own, you might be able to get some horse manure for free just by asking friends who own horses. Otherwise, you can purchase manure from nearby stables and truck it back to your house.
Horse manure, however, must be aged before it can be placed in the garden. As with most manures, applying it in the garden before it's sat around and decomposed will "burn" your plants, probably killing them. Fresh manure may also contain E.coli, making it a threat to your health.
If you're buying horse manure, only buy "aged manure." If you're getting the manure for free, you might have to take it while it's still "hot;" if that's the case, you'll need an out of the way corner to mound the manure until it ages. Just how long does it need to age? That depends upon how it's composted. (Is the pile turned or not? How hot is the weather? How large is the pile?) You'll know it's done when it's "crumbly, evenly textured, earthy-smelling, dark material that looks like a commercial potting soil mixture. It will probably take about one to three months for each pile to compost during the summer and about three to six months in the winter...[horse manure] compost should 'cure' for at least a couple of weeks before use," according to WSU Cooperative Extension.
Horse manure has a 1.5–1.0–1.5 N-P-K rating.
Caution: Recently, some gardeners have reported some horse manure as toxic. Apparently, horses who eat hay or grass sprayed with herbicides are producing manure that still has enough herbicide in it that - once it's applied to the garden - kills plants. So before you bring manure home, it pays to bring up this topic with the horse owner. (Learn more here.)
Now that many people are raising backyard hens, chicken manure is gaining in popularity. Chickens poop a lot. Why not use that poop to improve your garden? If you don't have chickens, some industrious homesteaders now sell the manure at farmer's markets and feed stores - or you can buy commercially bagged chicken manure at many gardening centers.
Again, chicken manure must be aged before you put it into your garden. Putting fresh manure in the soil may harm plants - and it might make you sick, too. Expert advice varies on how long to age chicken manure; some say 6 months, others say a year. I would err on the side of caution and age it for a year...although I have used 6 month old chicken manure in the garden, but only in autumn garden beds that will remain inactive until spring.
How to compost your own chicken's manure is a matter of preference. If you have enough land, you can just make a pile somewhere out of the way. But if you live in the suburbs, this can get pretty stinky. Dedicating a compost bin to chicken manure is a non-smelly, safe way to go.
Chicken manure rates at about 1.7-2.4-1.7. If it's mixed with bedding materials, it's more like 2.8-2.3-1.7
Turkey manure is about 1.4-1.0-0.9. Pigeon: 4.2-3-1.4. Duck: 5-23-17. All must be composted before being used in the garden.
As you may remember, last month we brought home a pet rabbit. He's a miniature, and not full grown, but oh my, is he a manure-making machine! I don't mind, though, because rabbit manure is a valuable fertilizer. And, unlike other manures, rabbit manure can go directly into the garden without aging. (That's because rabbits are herbivores.)
To give you an idea of how much manure one rabbit produces, our little bunny produces enough pellets to fertilize about 4 plants each week. (By the by, I figure that our rabbit more than pays for himself. I would be hard pressed to purchase as good an organic fertilizer for the cost of his feed.)
If you don't have rabbits (learn more here about raising them for meat on your homestead) but know someone else who does, they may be willing to give you pellets for free. Otherwise, you may begin to see rabbit manure sold by enterprising homesteaders at farmer's markets and feed stores.
Rabbit manure comes in at about 3.5-1.0–1.8.
Cow manure isn't as great as many other manures, but it's still good enough you may find it at gardening centers. If you know a local farmer, you may be able to buy cow manure from him, too. It does have to be aged for about a year, however, and is only 0.5-0.5-0.5.
Sheep and goats produce better manure for the garden, coming in at about 1.5–1.0–1.8. Because these animals are herbivores, their manure doesn't have to age before going into the garden.
Manures You Should Never Use
Cat and dog manure should never be used in the garden because they can contain parasites and pathogens that attack humans. Most modern sources recommend avoiding pig manure for the same reason.
Yes, that's right. Manure from humans can go into the garden, too. In fact, it's done all around the world; even here in the U.S., farmers often get "sludge" (dried up human poop) from local cities. I won't go into the details of safely using human manure, since I'm pretty sure most of us would only resort to it if we had no other fertilizers to use, but you can learn all about it in the The Humanure Handbook, a free .PDF book.
What About Weeds?
Manure may contain weed seeds, especially if it's not composted using a "hot system" (where the pile reaches at least 155 degrees F.). But according to Cornell University, even hot composted manure may contain persistent weed seeds, so it's smart to plan ahead and know how you're going to deal with weeds in the garden. (Two easy methods are laying cardboard down on top of the soil and using thick organic mulches.)
How to Use Manure in the Garden
For added safety, the University of Main's Cooperative Extension says manure should be added to the garden at least 120 days before harvesting crops that may touch the soil. This includes root crops and leafy greens. Otherwise, apply 90 days before harvest. Very often, gardeners and farmers apply manure in the fall, after harvest - but if you'll have a winter garden, you may have to rotate areas of your garden for active growing and for "active feeding" (or manure-ing) the soil.
To get the most from manure, it's best to dig it into the soil. The general recommendation is to use about 40 lbs. of manure per 100 square feet, dug down 6 to 9 inches. With herbivore manure, I'm not quite so exacting. For example, when I empty out the rabbit's manure tray (which is located directly under his wire-bottomed hutch), I simply dig a trench around a plant I want to fertilize, sprinkle his manure pellets in pretty liberally, cover, and then water well.
Always water deeply after applying any manure.
Also remember it's true you can have too much of a good thing. Over-applying manure of any type can actually throw the balance of nutrients in the soil way off, making it impossible to grow anything. To avoid this, most manures should be applied just once a year - usually in the fall or early spring. It's also wise to do a soil test once a year. You can buy inexpensive soil test kits at a gardening center - or online. For best results, conduct the test in the fall, before digging in any manure. The test will reveal if your soil is imbalanced - and what to apply to the garden to correct that imbalance.