Danger of Horse Manure
Here in north Los Angeles County we have practically unlimited amounts of composted horse manure available for free on Craig'slist. It seemed like a good deal to me so I filled up my truck and headed home with a load.
Most discussions regarding the negative aspects of horse manure revolve around the problems associated with weed seeds and pathogens that may be present in the material. After top dressing my vegetable garden with a couple inches of the stuff I began to wonder about the chemical composition.
I mixed a large sample of manure with distilled water, I inserted my pH meter into the water and got a reading of 10.1. Thinking that the meter must be out of whack I checked my tap water and then some vinegar for calibration reference. The meter was working properly.
After a little research I learned that stables frequently add lime to horse manure for odor control. This will result in manure that is so alkaline that it could be disastrous to add to a vegetable garden, especially here in the west where soil is frequently quite alkaline to begin with. Adding limed manure could raise the soil pH significantly for many years, causing severe nutrient deficiencies in the plants.
I have removed all of the manure top dressing from my gardens. Now I need to decide whether I should try to amend this pile with acid or just throw it all out in the garbage.
Here is a link that might be useful: Long Term Effect of a Single Application of Horse Manure