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The Poop on Manure
Why use it?
Farmers and gardeners have been using manure for centuries to add organic matter to their soil. Over time, as organic matter breaks down in soil, it becomes depleted. The mineral soil that is left over becomes less able to support abundant microbial life, so by “manuring” such a field, the farmer is able to integrate organic matter into the soil and re-start or feed that microbial life. The microorganisms and invertebrates that live in soil break down minerals and organic matter into forms that are accessible to plants. So if you have a healthy soil biomass, you can grow healthy crops without the use of chemicals.
Animal manure is a bi-product of farming, and there’s quite a lot of it about. Cattle in the US alone produce an estimated two billion tons of manure each year. In some traditional farming systems, livestock are kept primarily because they produce manure. Depending on where you live, you may be able to find farms or horse stables that are anxious to give the stuff away because their animals produce so much of it. But as we shall see, not all manures are created equally.
Fresh vs. “Mature” - how to store it
Using manure that has been freshly dug from the barn, coop, or paddock poses problems. Depending on the kind, it may be very high in ammonia, or contain so much nitrogen that it will burn the roots and stems of any plant it comes in contact with. It might also be full of weed seeds. Fresh manure may also contain pathogens from the animal’s gut. Storing manure allows it to mature in the same way that compost does—bacterial action causes a buildup of heat that will, ideally, kill weed seeds and other pathogens.
If you order a delivery of manure and it smells upon arrival, it is unfinished, and needs to be stored (composted) before use.
Stored manure is often referred to as “rotted manure.” It has no unpleasant smell, and its texture has changed since it was produced. Rotted manure is a fantastic soil amendment. Ideally it will retain some of its natural nitrogen, but not to the extent that it causes burning or excessive foliar growth in your crops. In some farming systems manure is stored for a year in a pile with steep, compacted sides. This helps to retain some of its inherent nitrogen (as does covering the pile with tarps), because rain is prevented from leaching out the nitrogen. But some moisture is needed in the pile to encourage microbial action, so you don’t want it do dry out entirely. If it does, you’ll want to give it a good soaking before use.
Fresh manure can be spread over a growing area in the fall, and incorporated into the soil in the late winter prior to planting. By the time you are planting, it should have no unpleasant odour. Certified organic farmers are prohibited from spreading fresh manure for at least 90 days before harvesting crops intended for human consumption. For crops that come in direct contact with the soil, the minimum time period is 120 days. These regulations are useful to the home gardener, to indicate how seriously this is taken.
How to use it
Rotted manure can be spread on the surface of the soil or tilled into the soil. Many organic growers prefer a “no-dig” method in which manure and other soil amendments are added to the soil in layers, always on the surface. This encourages sub-soil microbes and creatures like earthworms to feed on the material at the surface, and drag it down into the sub-soil. Tilling works, too, but may be disruptive to sub-soil life. Because the texture of rotted manure is relatively fluffy, compared to soil minerals, most of it is going to remain near the surface, even when tilled.
The amount that you choose to incorporate should be relative to the area in question, and the existing fertility and structure of your soil. A farmer with depleted, dusty soil, for instance, might want to apply manure at a rate of 40 tons per acre. She might apply half that amount if the existing soil is thought to moderately fertile. In subsequent years, 10-20 tons applied every other year would maintain adequate fertility.
Remember that it is possible to over-apply organic matter of any kind. Soil wants to maintain an ideal balance (loam) of soil particles (sand, silt, and clay) with organic matter. And while there are a few types of hardy plants that will thrive in pure manure, it’s more useful to think of manure as organic matter—as a general soil amendment to promote microbial action.
Types of manure
Animals digest their food in different ways. And different animals eat different sorts of food, so it’s no surprise that the end product will vary from creature to creature. So after your manure’s “maturity,” it’s type is a critical consideration.
All birds have relatively high metabolisms and body temperatures. One of the best qualities of chicken manure is that few, if any, weed seeds can survive passing through the gut of a chicken. The manure of all poultry (turkeys, pigeons…) is a combination of feces and urine, and it’s extremely high in nitrogen. Fresh chicken manure is far too strong for direct application, so it should be fully composted. In small quantities, it makes a good additive to your compost pile, combining well with high carbon matter like lawn clippings and leaves. As a fertilizer, mature chicken manure has an NPK rating of 1/1.5/0.5.
This may be the most balanced of all manures for organic growing because of the nature of cows’ stomachs. Cows can digest the cellulose that makes up so much of the bulk of the plants they eat, so their manure breaks down very easily. It also tends to be moist, which helps in the composting stage, and it’s not strong in terms of nutrients. Composted steer manure typically has an NPK rating of 0.8/0.5/0.5.
Horse manure is abundantly available and well balanced. Horses digest their food less thoroughly than cows, so their manure is richer in organic matter. It is, however, more likely to contain viable weed seeds. Horse manure often contains bedding and straw soaked with nitrogen-rich urine, which is of particular value to growers. Expect an NPK rating of 0.5/0.3/0.4.
Pig manure should be composted to the point that it has little or no smell before use in the home garden. It is noxious as a fresh product. Be cautious of manure produced from intensive meat production facilities, as it may be high in copper. Organically farmed pig manure is an excellent amendment with an NPK averaging 0.6/0.4/0.3.
Sheep & Llama
These animals spend a great deal of time outdoors leaving their droppings in the field. But any that can be collected is very valuable as garden manure. Like cows, these animals digest their food well. Their potassium rich fertilizer has an NPK rating of 0.4/0.3/0.8.
Rabbit pellets are high in nitrogen and phosphorus. Some of the literature suggests that if the pellets are kept dry, they can be used fresh, simply scattered around plants like pelleted plant food. This should be done with some degree of caution, as the pellets can be soaked with ammonia-rich urine. In a food growing system, it’s probably safer to compost rabbit pellets before use. Its NPK rating is 2/1.4/0.6.
This product can be purchased in bags, or is sometimes available in bulk amounts. It is the residual waste of the mushroom growing industry, and is usually comprised of a mix of straw, horse manure, dried blood, chalk, and other ingredients that have been thoroughly composted. If you can find organic mushroom manure, it’s an outstanding soil amendment with an NPK of 0.7/0.3/0.3. Mushroom manure that is not specifically listed as organic may contain traces of pesticide residues used to control fungus gnats.
Not all manure exits the gut of an animal, of course. Green manure is a general name for cover crops that are grown with the intent of tilling under. They may add carbon, or other nutrients to the soil, and improve structure and drainage. Legumes are planted to fix nitrogen in the soil through a symbiotic relationship with bacteria. The uses of green manure are broad enough to justify their own article.
Slurry & Tea
In some larger farm systems, fresh dairy cow manure (and other types) is collected in concrete basins, and mixed with water to form “slurry,” which is then machine spread on fields prior to tilling or planting. On a much smaller scale, compost and manure tea can be made for the home garden or small farm. This involves steeping organic matter in water, straining it, and using the resulting tea as a foliar or crop fertilizer. Because the decomposition of the organic matter is anaerobic, it can result in a seriously stinky final product. But it is cheap to produce, and very effective.
Try filling a bucket 1/3 full of aged manure (or ½ full of finished compost), and top it up with water and a tight fitting lid. Let this steep for about one week, and then strain the tea into another container. The tea should be diluted one part tea to two parts water before use.
While the thought of using human waste in food production may be offensively unpalatable to Western growers, it’s worth considering that such waste is far too valuable in some parts of the world to waste by simply polluting rivers. Human manure can, indeed, be composted and spread for crop production. Human urine is extremely high in nitrogen, and has its uses in the garden as well. If you are compelled to experiment with human manure, it’s probably worth doing some thorough research beforehand. Many websites and several books have been dedicated to the subject. The use of human manure is banned under organic certification for farms producing food for human consumption.
Problems with manure
Some organic food proponents are vehemently opposed to using animal manures of any kind. Scientific evidence appears to show that it is possible for some plants to accumulate antibiotics from soil amended with animal manures that contain these drugs.¹ It should be pointed out that manure might be available from organic farms where no antibiotics are used, or that keeping your own chickens organically is one way to address this concern.
Pathogens such as E. coli and salmonella can exist in manure, and have been known to cause outbreaks in human populations due to improper use. By composting manure, its interior temperature can be raised to 55-60°C (130-149°F) for two weeks or longer, thus killing these pathogens and rendering the manure safe to use. Properly composted manure is considered safe for use in organic farming systems, and by the regulatory bodies of the CFIA and USDA.
The other major issue is with the introduction of weed seeds, but as we’ve seen above, some manures are less likely than others to contain them. And if correctly composted, many of these seeds can be sterilized by achieving high temperatures through bacteriological action.
There will be some growers who simply can’t be sold on the concept of using manure to grow food, and fair enough. By using cover crops to create green manure, nearly all the benefits of using manure can be achieved—namely the introduction of nitrogen and organic matter into your soil to improve soil fertility and structure