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The poop on gardening with manure continues

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Posted: Sunday, July 18, 2010 4:07 am | Updated: 1:01 pm, Wed Dec 26, 2012.

FAIRBANKS — This topic started last week when I talked about discovering, to my horror, that gardening writer Ann Lovejoy believes that aged horse manure is terrible for the garden.

She says the wormers used can be toxic to plants, herbicides on the horse feed pass through the animal’s system and are deposited on the ground along with the poop, and it causes a magnesium overload on the soil. Anchorage gardening writer Jeff Lowenfels seconded Lovejoy’s objections to horse droppings and added his own — the risk of being contaminated by the pathogen E. coli.

However, Organic Gardening continues to support the use of aged horse manure. So, totally confused, I contacted Michele He’bert of our local Cooperative Extension Office. We exchanged a flurry of emails, which I excerpt for you below. The order has been changed a bit so that the thread of the conversation makes more sense to an outside reader.

Me: “Is E. coli really an issue for composted horse manure?”

Michele: “No, it is not an issue for composted horse manure because the high temperatures kill the pathogens.”

Me: “Is E. coli an issue for any composted manures?”

Michele: “No, It is wonderful stuff!”

Me: “What about worms and dewormers?”

Michele: “It sounds like wormers are not an issue. Here is link:”

Me: “Will horse dewormers kill red worms? The most common wormer used is known by the brand name Ivermectin® made by the Merial Company. Merial’s research shows that the active chemicals in Ivermectin® are deactivated when manure is exposed to sunlight. Equine studies show that 95 percent of the active chemicals in Ivermectin® are deactivated in the horse before being passed in the feces. Leading experts in vermicomposting believe that the concentration of Ivermectin® in the horse manure is not high enough to seriously injure Eisenia.”

Me: “Is composted horse manure dangerous to plants due to herbicides?”

Michele: “Herbicides would not be an issue.”

Me: “What about magnesium?”

Michele: “Horse manure has approximately 2.8 pounds of magnesium per ton of horse manure. The average magnesium rate needed per acre for plants is 50 pounds per acre. So, you would have to add 25 tons of manure per acre to get to adequate magnesium levels. Magnesium is need by plant for photosynthesis and is a secondary nutrient.

I would totally recommend composted horse manure, but as with all things, there is a point at which you can use too much. So in a way, I agree with Lovejoy and Lowenfels you can use too much … because of too much N and K. (Note: N is nitrogen and K is potassium.)

I guess the caution is not to grow your vegetables in just composted horse manure by itself! In reality, as I am sure you are aware, using too much horse manure gives you too much N and your plants will be so happy, fruiting will not happen.

We could do the math on application rates, but a general rule would be no more than 30% composted horse manure.”

So, many thanks to Michele for her wisdom. In the end it comes down to what it always does with conflicting views: you have to make your own decision. I find Michele’s information the most persuasive, and fully intend to continue using horse manure and asking for it as a birthday gift.

One final caution: while most consider the manures of chickens, cows, ducks, horses, pigs, rabbits and sheep to be a great addition to the garden, not so for dog or cat poop. No one thinks they should be used in plot or compost. Their waste carries diseases that can be harmful to children, and while certain types of composting can destroy the pathogens, most home gardeners don’t have the ability to follow the exacting directions.

Even if you could follow the directions, it is a disgusting enterprise. Long time readers may remember the year I participated in a local program to compost dog waste. At that time I was between dogs, so I ended up collecting the remains from dogs owned by my friends---convincing those who may have been giving me the benefit of the doubt that I had gone over the edge.

The process was very specific and laborious and even after tests showed no pathogens, I was advised to use it only in areas where non-edibles were grown. However, even if I had been able to use it, making it was so tedious and literally nauseating that even though I now have a source of my own, I have never been tempted to turn Alistair the Irish wolfhound’s poop into garden gold.

Linden Staciokas has gardened in the Interior for more than two decades. Send gardening questions to her at


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