Manure has been used to fertilize farms and gardens for centuries. It is a prime source of slow-release nutrients and adds organic matter to the soil. As long as you use the proper methods, you can safely take advantage of its many benefits.

Benefits and Drawbacks of Manure

Adding manure is an excellent way to improve your soil. It promotes the growth of soil life such as earthworms and beneficial microbes, which in turn creates better tilth and soil structure. When you amend sandy soils with it, you’ll increase your soil’s water-holding capacity. Amending clay soils will increase drainage. As the soil life works, the rich humus left behind will create dark, spongy topsoil.

The soil life will make a majority of the nutrients available the first year, with the rest feeding the plants over the next one or two years. As time goes on, and nutrients build up in the soil, you will need to add manure on a less regular basis.

In fact, you need to be careful not to add too much manure to your garden. If you do, you may experience excessive vegetative growth, salt build up, nutrient runoff, and nitrate leaching. Proper application rates are essential when using manure.

Application techniques are also important, especially when using fresh manure. Incorrect methods can spread E. coli, Salmonella, Campylobacter bacteria, and Giardia or Cryptosporidium protozoa.

Types of Manure to Use

Typical manures used in the garden are from poultry, cows, sheep, and horses. Never use manure from cats, dogs, or pigs as they are more likely to spread parasites to humans.

Manures may or may not have bedding mixed in. The more bedding mixed in, the more diluted the nutrients are. You can also choose fresh or composted manure.

It’s important not to use manures that have been contaminated by herbicides. If broadleaf herbicides have been used on lawns, pastures, or hay that animals have eaten, their residues can pass through their digestive system to the manure and remain even after being composted. The herbicides will break down eventually, but they can cause damage in the meantime. If you use contaminated manure or compost, your crops will experience poor germination rates, seedlings will die, and vegetation will be twisted and malformed.

Fresh vs. Composted Manure

Fresh manure can cause more problems than it solves, so composted manure is generally recommended. However, you can use fresh manure if you take certain precautions.

Fresh manure contains high levels of ammonium, or soluble nitrogen, compared to composted manure. Poultry manure, specifically, will likely cause nitrogen burn in plants if it has not been composted. Fresh manure must be incorporated six to eight inches deep within 12 hours of application, or else the majority of the ammonium will be released into the atmosphere.

Fresh horse manure is notorious for containing weed seeds, which can lead to major weed problems in your garden.

Human pathogens are easily transmitted through fresh manure. E. coli, Salmonella, Campylobacter bacteria, and Giardia or Cryptosporidium protozoa can live in manure and contaminate crops. For this reason, organic standards dictate fresh manure must be applied 120 days before planting to give pathogens time to die off.

You can avoid these issues by using composted manure. To guarantee harmful pathogens have been killed, the compost pile must reach 131-140°F degrees for several weeks. The pile must be turned regularly to ensure all the manure had been exposed to hot enough temperatures.

Many commercial operations will pasteurize their compost to destroy pathogens, however, beneficial micro-organisms are also killed in the process.

Nutrient Availability

While you can estimate the nutrients available in manure, the only way to get an accurate analysis is testing it in a lab. When collecting your sample, be sure to collect several subsamples and mix them together to get the most accurate results.

If you are buying your manure commercially, you can request the nutrient analysis from the seller for nitrogen (N), phosphorus pentoxide (P2O5), and potassium oxide (K2O) content. The analysis will show you the nutrient content, but availability depends on the micro-organisms in your soil breaking the nutrients down into a form plants can use.

If you don’t want the expense of lab tests, you can approximate the nutrient content. For example, the Livestock Waste Facilities Handbook says horse manure with bedding will contain four pounds/ton of soluble, or readily available, N, and an additional 10 pounds/ton of organic N that still needs to be broken down by microbes. It will also contain about four pounds/ton of P2O5 and 14 pounds/ton of K20.

Composted poultry manure will contain about one pound/ton of soluble nitrogen, 16 pounds/ton of organic N, 39 pounds/ton of P2O5, and 23 pounds/ton of K2O. You can compare that to fresh poultry manure without bedding, which is estimated to contain 26 pounds/ton of soluble nitrogen, seven pounds/ton of organic nitrogen, 48 pounds/ton of P2O5, and 34 pounds/ton of K2O.

While fresh manure can contain too much N for the garden and burn plants, composted manure contains organic N which is not available to plants until microbes convert N to ammonium (NH4). This occurs over years.

Typically, 25-50 percent of the manure’s N will be available the first year, with less being available each subsequent year. In general, 70-80 percent of the phosphorous (P) and 80-90 percent of the potassium will be available the first year.

If you get manure that contains bedding, it is important to be aware of the carbon to N ratio (C : N) in the manure. If the ratio exceeds 30:1, the N will be tied up temporarily while it helps the carbon break down. If this is the case, you may need to add nitrogen fertilizer to your plants until the N in the manure is released.

How to Apply Manure to Your Garden

It is important to apply the correct amount of manure to your garden. If you don’t use enough, you’ll end up with nutrient deficiencies and poor yields. Too much can cause P run-off, excessive crop growth, and nitrate leaching.

It is recommended you test your soil and the manure you use to make sure your N and P levels are optimal. If you add manure every year, you risk building up excessive P levels. By testing your soil, you can make the decision to use a different kind of fertilizer to allow the P levels to come back down.

However, if you don’t want to put up the money for these tests, the following application rates are recommended by the Wisconsin Master Gardener Program:

  • For every 100 square feet, apply 75 pounds of cow manure without bedding, 95 pounds of cow manure with bedding, or 200 pounds of composted cow manure.

  • For every 100 square feet, apply 40 pounds of sheep manure without bedding or 50 pounds of sheep manure with bedding.

  • For every 100 square feet, apply 20 pounds of poultry manure without bedding, 30 pounds of poultry manure with bedding, or 70 pounds of composted poultry manure.

  • For every 100 square feet, apply 65 pounds of horse manure with bedding.

A five-gallon bucket holds roughly 25 pounds of manure or compost, so you can use it to measure instead of weighing your materials. For example, use a little less than one bucket full of poultry manure without bedding on a 10x10-foot garden, or a little less than three buckets full of composted poultry manure over the same space.

When using fresh manure, there are a few extra guidelines you should follow. The USDA National Organic Program rules state if the edible parts of vegetables have contact with the soil via rain or irrigation splash, the manure must be applied 120 days prior to harvest. If the edible portion is not in contact with the soil, then manure must be applied a minimum of 90 days prior to harvest. This prevents harmful pathogens, such as E. coli and Salmonella, from spreading. It is ideal to spread fresh manure in the fall so that it is ready for spring, or in spring to plant a fall garden.

Fresh manures are high in salts, so be sure to wait three to four weeks after applying fresh manure to plant seeds to avoid salt damage.

Fresh manure needs to be incorporated into the soil within 12 hours of application, or else the ammonium will be released into the atmosphere.

With the proper application rates, techniques, and safety precautions, manure can be an excellent way to provide nutrients and build up the soil in your garden.