The pH scale measures acidity or alkalinity, ranging from 0 to 14, with 7.0 being neutral. Substances below 7.0 are acidic, and those above 7.0 are alkaline. Horse manure’s pH is highly variable but usually falls within a slightly alkaline range. This fluctuation is influenced by factors like the horse’s diet and the type of bedding used, which determines how the manure should be used as a soil amendment.
The Baseline pH of Fresh Horse Manure
Fresh horse manure, without added bedding, is typically neutral or slightly alkaline, exhibiting a pH between 7.0 and 8.0. This initial alkalinity is primarily due to the high concentration of nitrogen compounds, a natural byproduct of the horse’s digestive system. Nitrogen is largely present in the urine as urea.
Microorganisms begin to break down the urea immediately after excretion. This process, called ammonification, converts urea into highly alkaline ammonia gas. The release of this ammonia drives the pH value up, resulting in a slightly basic reading for the fresh waste. While the feces itself is near neutral (6.5 to 7.0), mixing with alkaline urine quickly raises the overall pH of the material.
Key Factors That Influence Manure pH
The pH of a horse manure mixture is significantly altered by external inputs like the animal’s diet and the bedding material used in the stall. These factors can pull the mixture’s pH in either an acidic or alkaline direction. Higher protein content in feed results in more nitrogen, leading to greater production of alkaline ammonia. For instance, a diet rich in high-protein feeds like alfalfa hay increases nitrogen content, pushing the manure mixture toward a higher, more alkaline pH.
Bedding material is often the most influential factor determining the overall pH of stall waste. Naturally acidic materials like pine shavings or sawdust can lower the mixture’s pH, sometimes counteracting the manure’s inherent alkalinity. Conversely, bedding treated with hydrated lime, sometimes used for odor control, dramatically increases alkalinity, potentially resulting in a pH as high as 8.4. Straw is a more neutral bedding option, and its effect on pH is less pronounced than that of wood products or lime.
How Composting Alters pH Stability
Composting introduces chemical changes that stabilize the horse manure’s pH, making it safer for gardens. Initially, the rapid microbial breakdown of nitrogen compounds causes a sharp, temporary spike in alkalinity. This releases large amounts of ammonia gas, sometimes causing the pH to rise above 8.0.
As composting continues, high temperatures and microbial activity drive off volatile ammonia. Simultaneously, the decomposition of complex organic matter produces organic acids, such as humic and fulvic acids. These acids gradually neutralize the mixture’s alkalinity. Properly finished compost typically settles into a stable, near-neutral pH range, often between 6.5 and 7.5. This stable pH is why composted manure is preferred over raw manure for gardening, as it eliminates extreme alkalinity that can harm plants.
Using Horse Manure pH to Benefit Your Soil
Understanding the pH of horse manure is essential for using it effectively as a soil amendment. Since aged or composted manure is generally pH-neutral, it improves soil structure and nutrient content without significantly altering the existing soil pH. This makes it a versatile choice for most gardening applications. The stable pH of finished compost also buffers the soil, preventing sharp shifts in acidity or alkalinity.
Applying fresh, highly alkaline horse manure, especially with lime-treated bedding, can be problematic for plants that prefer a slightly acidic environment. This raw, alkaline material may cause nutrient deficiencies by reducing the availability of micronutrients like iron and zinc. A simple soil pH test kit can check the final pH of your manure or compost before widespread application. This proactive step ensures the amendment supports the specific pH needs of your plants.