Does Horse Manure Raise or Lower pH?

When it comes to gardening, either at a personal or commercial level, manure is well known to be a great source of fertilizer for the soil. Manure is not only a sustainable organic matter that helps to reduce waste, but it is also a great way to replenish nutrients in the soil that has degraded over time. How does manure, particularly horse manure, impact pH?

Horse manure is naturally alkaline and can slightly raise soil pH, depending on the amount you add to the soil. Like other kinds of manure, contains alkaline materials, including calcium and magnesium. These two substances are well adapted to receive hydrogen ions, making them bases on the pH scale.

Horse manure is a great additive to introduce to your farming and can benefit your plants in many ways if used correctly. Read on to learn more about how horse manure changes soil pH, how it rejuvenates soil’s nutrients, why you might not want to use it in some cases, and what concerns could occur from using too much manure.

Horse Manure and pH

Without delving too deeply into exactly what pH is and how it works, it is helpful to provide a quick overview of what pH is and how horse manure can impact pH. 

pH, in short, measures a chemical’s ability to receive hydrogen ions. Chemicals with a high capacity to receive hydrogen ions are called “bases”, while those that can’t receive too many hydrogen ions are called “acids”.

With this basic understanding of pH, it makes sense that horse manure is likely to raise soil pH.

There are five major chemicals found in manure: 

  • Carbon
  • Nitrogen
  • Phosphorus
  • Calcium
  • Magnesium

In short, because horse manure is horse fecal waste, extra nutrients that the horse’s digestive system hasn’t been able to process come out in their manure.

These chemicals, when added to the ground, mix with the nutrients and substances in the ground to change the makeup of that soil. For instance, when nitrogen is mixed with oxygen and water in the ground, it actually can form a small amount of nitric acid, making the soil more acidic.

This impact is quickly canceled by the other chemicals found in horse manure, however. Phosphorus is relatively neutral, meaning its impact on the soil’s pH is minute. 

Carbon, calcium, and magnesium, however, all have a high pH, meaning they are largely basic. Combined, their strength as a base is so high, that it actually neutralizes the acid nitrogen creates. While it won’t likely change your pH significantly, it can raise your soil’s pH levels.

For more information on pH, check out this article: 5 Reasons Why Soil PH is So Important.

In simple terms, horse manure is typically alkaline with pH ranging from 8-12, depending on the horse’s diet. In practice, working about 2 inches (5 cm) of horse manure (pH of 8.5) into the top 6 inches (15 cm) of slightly acidic soil (pH 6.0) can raise the overall pH by 0.5-1.0.

The effect can be so much higher if you increase the amount of horse manure.

Other Regenerative Effects

While increasing soil pH in itself is often a benefit horse manure has, there are many other benefits it can provide as well. Let’s look at some of them below:

It Contains Secondary Micronutrients

As mentioned previously, manure is largely just animal waste, so all of the nutrients the animal’s digestive system is not able to process are disposed of through the waste.

This means that, on top of the five major chemicals, manure might also contain other nutrients beneficial to plants, such as:

  • Potassium
  • Copper
  • Manganese
  • Zinc 

It Can Prevent the Soil From Drying Out

Adding manure to your garden can also help prevent the garden from drying out as quickly.

If the manure is somewhat moist, it can help add fluids to your garden. If manure is added to gardens that are directly in the sun or in areas of extreme heat, manure can act similar to mulch in that it will help to prevent your soil from drying up as quickly.

Manure also has great water-absorbing qualities. If manure is added to your garden and moistened, it can help your soil retain water for longer and can help provide extra moisture even after initial watering by acting like a sponge releasing water into the ground.

Manure Generates Heat as It Decomposes

Another great quality that manure brings, especially early in the growing season, is the fact that manure is somewhat warm. Of course, when it comes out of the horse it is very warm, but even after, manure can cause the soil to heat slightly as decomposers break down organic material. This can be helpful in thawing the soil at the beginning of the year.

More important, however, is that this warmth actually encourages fungi and bacteria in the ground to work harder at breaking down organic materials. This small amount of heat puts these microorganisms into “overdrive”, meaning new nutrients will be replenished into the soil even faster.

Arguments Against Using Manure

While manure has many positive benefits, it isn’t necessarily a “one-size-fits-all” solution to your gardening woes. Manure can have some negative impacts on your plant life if used incorrectly and shouldn’t be used in all cases. 

Sometimes Contaminated With Harmful Microorganisms

One such case, for example, is if you have incredibly delicate plant life. Manure, being animal waste, is prone to carry some diseases and parasites that can be bad for humans, animals, and plants. 

Some manure can carry diseases such as salmonella, E. coli, listeria, and even tapeworms. These diseases are bad enough by themselves, but if you were to expose incredibly fragile plant life to these diseases, there is a decent chance that they might become ill and die.

May Contain Toxic Levels of Chemicals

Another potential reason you might not want to use manure is because of the strength of some of the chemical components. 

In fresh manure, the amounts of nitrogen and ammonia can be so high that the roots of the plants underground are not able to absorb enough of the chemicals, get overwhelmed by the sheer quantity, and die. This is obviously especially bad in young plants and should be avoided whenever possible.

Can Be Bad for Plants That Thrive in Acidic Soil

You might not always want to use manure because it might change your pH too much. Some plants, such as blueberries and hydrangeas, need acidic soil in order to thrive. If you were to add too much manure and elevate the pH, you might cause the soil to become too neutral, or even basic, for these acid-reliant plants to survive. 

Dangers of Using Manure Incorrectly

While using manure is largely a sustainable practice that is good for the environment, there are some occasions in which using manure incorrectly can actually have catastrophic events for the environment. This is much less likely to occur in a home garden, but at a larger industrial level, this, unfortunately, is not uncommon.

The danger comes from using too much manure and mixing too much of it with water. When water hits the ground, it, of course, is absorbed by the plants and the soil. However, soil can only retain so much water. 

After a certain level, water drains through the ground into an aquifer beneath the ground. This process is called “groundwater recharge” and is the basic premise of how wells work.

However, if manure mixed with the water completely saturates the ground, this manure water can then run into the groundwater and local streams, causing major issues. Not only can this process cause harmful bacteria to drain into recreational and drinking sources for humans, but can also severely damage the ecosystem.

One such example of this is in Lake Erie. Due to industrial farming in the area, Lake Erie has increased levels of nutrients needed for cyanobacteria to grow, a type of harmful bacteria. This makes the lake in some areas turn green, preventing sunlight in and killing fish and plant life in the Lake. It is instances like this that suggest that safe manure practices are essential.

Final Thoughts

When it comes to using horse manure, it is important to understand what changes can occur in one’s soil with proper usage. Horse manure can help raise soil pH due to its high concentrations of carbon, calcium, and magnesium. It can help to regenerate important nutrients in the soil, giving plants a better ability to grow.

However, improper use of manure can be catastrophic, as it can contaminate underground water sources. Concentrated manure solutions can also burn the fragile roots of young plants.

Acquire horse manure from reputable sources to ensure it’s void of parasites and diseases and use it sparingly in your home garden. Knowing its pros and cons will help you maximize its benefits.

Dr. Moritz Picot

Dr. Moritz Picot is a horticulture enthusiast and the founder of TheGrowingLeaf.com, where he serves as the lead content writer. He established the website in 2022 as a valuable resource for both gardening aficionados and beginners, compiling all the gardening tips he has accumulated over the past 25 years. Alex has a passion for nurturing plants, transforming backyards into inviting spaces, and sharing his knowledge with the world.

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