Horse manure has long been recognized as a valuable resource for enriching agricultural land and garden beds. Its high organic matter content improves soil structure, aeration, and water retention capacity, making it a popular choice among growers. However, understanding its chemical properties is necessary before application. A frequent question concerns its acidity or alkalinity, specifically how its inherent chemistry might influence the existing pH balance of garden soil. This inquiry requires a closer look at the manure’s composition, both in its fresh state and after processing.
The Typical pH Profile of Fresh Manure
Fresh horse manure is not acidic; it is typically considered neutral to slightly alkaline. The pH range for fresh equine waste generally falls between 6.5 and 7.5, with some reports extending this slightly higher to 7.8. This near-neutral measurement is a direct consequence of the horse’s digestive system.
The hindgut of the horse is home to a microbial population that thrives in a near-neutral environment to efficiently ferment fibrous material. The manure exiting the digestive tract naturally maintains this pH balance to support these necessary microorganisms. The specific diet of the horse also plays a role in the final measurement of the manure. High-grain rations, for instance, can sometimes lead to the production of slightly more acidic manure. Despite these minor variations, fresh horse manure establishes a baseline of neutrality or mild alkalinity before any other materials are introduced.
How Bedding and Decomposition Alter pH
The pH of the final stable product is rarely just the pH of the fresh manure itself, as it is heavily influenced by the inclusion of bedding material and the subsequent process of decomposition. Bedding materials like wood shavings, sawdust, and straw are often mixed with the waste, introducing their own chemical properties. Wood products, such as pine shavings, can be slightly acidic, which may temporarily lower the overall pH of the stable waste mixture.
The decomposition process, whether through simple aging or active composting, then becomes the dominant factor in shifting the pH. As the organic material breaks down, two major chemical changes occur. First, the breakdown of nitrogenous compounds, particularly urea from the horse’s urine, leads to the formation of ammonia. Ammonia is an alkaline compound, and its presence in the early stages of composting will push the pH of the material upward.
Secondly, the composting process involves the breakdown of organic acids, which also contributes to a rising pH as the material matures. Through active, high-temperature composting, the final product generally stabilizes in a neutral to slightly alkaline range, often registering between pH 7.0 and 8.0. However, if hydrated lime is used in the stalls to control odor, the resulting compost can become highly alkaline, sometimes reaching a pH of 8.4.
Practical Impact on Soil Acidity
The practical impact of adding horse manure to a garden bed is largely determined by its final, composted pH and the rate of application. Because fully composted horse manure is typically near-neutral, it is generally considered a balanced amendment that will not drastically upset the existing pH of the soil. This makes it a safe choice for most garden applications where the goal is to improve soil structure and nutrient content without significant chemical alteration.
In soils that are already slightly acidic, the mild alkalinity of well-composted horse manure can provide a beneficial liming effect. The introduction of calcium and magnesium helps to neutralize soil acidity, promoting an ideal environment for many common garden plants. This gentle buffering capacity is unlike highly acidic amendments, such as peat moss, or highly alkaline ones, like agricultural lime, which are intended for major pH adjustments.
For gardeners, the most actionable advice is to use only fully composted manure, which has stabilized both its nutrient profile and its pH. Fresh manure is high in ammonia, which can burn plant roots, and its high carbon-to-nitrogen ratio can temporarily cause a nitrogen deficit in the soil as it attempts to decompose. Before application, testing the existing soil pH is the best way to ensure the amendment is appropriate for the garden.
Even with composted material, heavy, long-term application should be monitored. Excessive use of alkaline manure over many seasons, especially if it was mixed with stable lime, can push soil pH too high. An excessively high pH can reduce the availability of certain micronutrients, such as iron, zinc, and manganese, leading to deficiencies in susceptible plants. Using composted horse manure as a regular, moderate amendment is the best approach to gain its many benefits without causing unwanted long-term changes to soil acidity.