Do you use manure in your garden? Perhaps you’re not sure if it’s safe? This common garden input has many advantages but also risks to consider. It’s worth doing a little ‘digging’ into the manure you might use! Not just the end product. But ALL the inputs into the animal during its life. An animal’s manure reflects what they eat, their medical care and overall health.
If you’re aiming to grow an organic food garden, it’s important to weigh up which manure to use. Plus consider the benefits vs potential contaminants and how to apply manure safely. Ultimately, the choices we make about our inputs may affect the food we grow and eat.
What are the Most Commonly Used Manures?
The waste product from cows, chickens, horses, sheep and alpacas are the most popular poops for gardeners! These vary in nutrient value and age. Fresh (hot) manures include horse and chicken. However, aged (cold) manure types include cow and sheep.
- Chicken and Horse Manure. These are both high in nitrogen (N). They are best used to improve soil where you want to encourage strong leaf growth. Chicken manure can contain reasonably high levels of phosphorus (P). Read the label if buying a bagged product. Some plants like some Australian natives are P sensitive and prefer low phosphorus soil amendments.
- Cow Manure. Lower in nutrients but composted cow manure is gentle around plants. It’s unlikely to burn or stunt growth of tender roots or seedlings.
- Sheep Manure. Contains potassium (K) which can be beneficial for flowering plants.
Manures may be fresh, partially decomposed or well aged. Depending on your source, they can also contain bedding material like hay or sawdust.
What are the Benefits of Animal Manures?
There are many advantages to applying composted manures as a soil amendment. Manures:
- Improve soil structure and texture with organic matter.
- Add soil microorganisms and attract earthworms.
- Improve water-holding capacity.
- Increase soil carbon.
- Add major nutrients like nitrogen, phosphorus and potassium to enhance soil fertility.
- Are a renewable resource that’s easily available at low cost.
- Recycle waste to feed the soil.
- Are a natural soil amendment rather than a synthetic fertiliser.
There are lots of pros to using manures but it’s also worth considering the cons. Then we’ll look at how to safely use manure in your garden.
Contaminants and Risks with Manures
There are definitely some disadvantages and considerations to be aware of before you use manures.
Genetically Modified Organisms
Factory-farmed chicken or feedlot cattle eating genetically modified grains produce manure containing GMO (herbicide) residues. A clinical report on the use of GMO-containing food products in children states: “An unfortunate consequence of the increasingly heavy use of herbicides late in the growing season on herbicide-tolerant corn and soybeans is that measurable quantities of glyphosate and other herbicides, termed “residues,” remain present in GMO grains at harvest. The World Health Organization’s International Agency on Research for Cancer has determined that glyphosate, a herbicide widely used in producing GMO food crops, is a probable human carcinogen.” A cancer-causing agent in animal and human food!
Although the organic industry has strict guidelines on allowable inputs, there is still a loophole. At least in Australia, in NASAA certified organic manure or fertiliser products. Conventional chicken manure from birds who have been fed GMO grains, are still allowed in these products! In their guidelines, they use the “one step back assessment” for deciding what is acceptable. Consequently, manure is an ‘allowable input’ in certified organic garden products. Why? Because it comes from a non-GMO chicken (one step back) from what the chicken ate (GMO grains). So, the assessment process allows manufacturers to then work the system to produce ‘organically certified’ products. Even though, two steps back in the food chain, these chickens consumed GMO grains. Indeed, gardeners need full transparency about the entire food chain so we can make informed decisions.
This is another big concern. Horses, sheep, chickens and cattle have regular vet medications (including growth promoters, steroids, hormones, worming treatments and antibiotics)1 that end up in their manure. Plants can take up these meds via their roots. Vet drugs have been found in edible plants consumed by humans.2 These bioactive chemicals are harmful to humans, animals, sensitive soil microorganisms and worms.
A concerning peer reviewed study5 notes “Earthworms appear to be sensitive to parasiticides, whereas plants appear to be sensitive to many of the antimicrobial groups and the macrocyclic lactones. Not surprisingly, the antimicrobial compounds are most toxic to soil microbes.” Antimicrobial substances harm and kill soil life by limiting the growth of beneficial bacteria and fungi. This has the flow on effect of reducing the ability of plants to take up nutrients. This study also confirms “antibacterials are toxic to soil microbes and could reduce a soil system’s capability to degrade other contaminants, such as pesticides.”
Additionally, soil amendments like ‘Blood and Bone’ or bone meal may also contain potential drug contaminants like antibiotics.
Furthermore, in another study3, veterinary drugs were found in soil layers at depths of 20-40 cm and 40-60 cm to a greater extent than closer to the surface 0-20 cm depth. The results indicated that “veterinary drugs accumulate easily and persist in the deeper soil.”
Yet another concern is vet medications leaching and ending up in waterways via surface water runoff or soil erosion creating environmental risks.4
Many animals feed on pastures or hay crops contaminated with persistent broadleaf herbicides. Roundup (Glyphosate) has been found to be incredibly toxic. It is sprayed on the crops animals eat or embedded in some GM Roundup Ready crop seeds. The residues can last in the soil for many years. They leach into soil with rain, dew and irrigation.
A University of Florida publication ‘Herbicide Residues in Manure, Compost or Hay‘ explains: “manure contamination can occur if the animal has been fed forage treated with aminopyralid or other closely related herbicides, such as clopyralid or picloram. Because aminopyralid is absorbed into plant leaves and sequestered for the leaves’ lifetime, the herbicide residue will be present. This is the case even if the grass is cut, dried, and baled as hay. When this forage is fed to livestock, the leaf tissues are broken down and the herbicide is released within the digestive tract of the animal, then excreted in manure.” Farmers regularly spray weeds in pasture crops that are then baled for hay or mulch. A major concern.
Also, manure contaminated with chemical residues, can result in herbicide injury to plants. Not to mention impacting animal health. Residual herbicide poisons can continue being active even after the animal’s manure has been composted! Herbicides negatively impact microorganisms in the soil especially worms.
Have you ever noticed twisted, deformed or distorted leaves appear suddenly on your plants? Wondered what the cause was? The damage could possibly have been unintentional herbicide residue contamination. Sudden plant death can also be a clue. Have you noticed strange symptoms on your edible plants that don’t appear to be pest or disease related? If so, perhaps err on the side of caution. Avoid eating the fruit, vegetable or edible portion of the plant.
Heavy metals in manure come from a variety of sources. These include grass fodder chemically sprayed with pesticides, herbicides and fertilizers. Animals may also drink contaminated water that bioaccumulates.
Manures from animals like horses and sheep eating grass may contain weed seeds. If you are buying bags from roadside farms, unwanted weeds may germinate in your garden!
Fresh manures often contain bacteria like E. coli that can contaminate food crops when in direct contact. Be sure to wash edibles before eating even if they’re homegrown!
Fresh chicken manure is very strong. High in ammonia, it can burn young seedlings and roots if applied directly. Nevertheless, it’s a great soil conditioner when used correctly.
Avoid using pet or pig manure in your edible garden or compost. These manures commonly contain parasites that can infect people.
Organic “Safety” Standards
Commercial bagged certified organic products have to follow strict safety guidelines. They can be a convenient way to add small quantities to your soil. Safety recommendations include wearing gloves and a mask. Allowable inputs do vary considerably. According to the Australian Organic standards, organic livestock production prohibits the use of synthetic chemicals, pesticides, fertilisers, or genetically modified organisms (GMOs) on the land or within production, including in animal feed. “Organic livestock must be fed 100% certified organic feed, with an exception for trace minerals and vitamins sometimes required to meet the animal’s nutritional requirements.”
However, according to the Agricultural Marketing Service of the USDA, many certified organic products allow manure inputs from livestock raised in confined industrial-scale feedlots and animals fed GM feed.
Another issue for organic product manufacturers is there is not enough manure available from animals raised as ‘certified organic’ to provide sufficient inputs. So, factory farmed animal products including manures can end up in our gardens. Our regulatory authorities definitely have room for improvement. Especially if ‘certified organic’ products are going to truly be safe, free from genetically engineered ingredients or harmful contaminants.
Food for thought!
Best Ways to Apply Manures
When Using Manures Do Your Due Diligence
Firstly, apply the 90-120-day rule.* Aim to work fresh animal manure into the soil ahead of time when preparing for planting. Avoid applying directly to food crops.
- Apply manure at least 120 days before harvesting the edible part of the plant that has contact with the soil. e.g. squash family and leafy greens.
- Or 90 days prior to harvesting all other food crops with the exception of fruit trees and sweet corn where the crops don’t touch the soil. [* Source: USDA]