While adding compost to your soil can increase soil organic matter and improve soil health and fertility, too much compost can cause problems for the health of your plants and the environment.
In recent years, practices like deep compost mulch, adding extra compost to high tunnels, and growing in raised beds with pure compost have created problems for growers. Learn how to assess the condition of your soil and ways to remedy excessive compost applications.
As an avid gardener, I’m always looking for ways to boost my soil health and fertility organically. Horse manure is often touted as an exceptional soil amendment that can significantly increase yields and plant growth. However I’ve often wondered – can you put too much horse manure in your garden?
In this article, I’ll examine whether horse manure can be over-applied, the potential risks of using excessive amounts, and best practices for utilizing horse manure safely and effectively.
How Much Horse Manure is Too Much?
Determining the ideal application rate for horse manure takes careful consideration. Many sources recommend spreading 1-2 inches of manure and tilling it into the top 6-8 inches of soil before planting. However even these seemingly modest amounts could become problematic if applied repeatedly year after year.
With heavy, repeated applications, nutrients like phosphorus, potassium, magnesium, and sodium can accumulate to excessive levels over time. This can inhibit uptake of other nutrients and induce deficiencies. High salinity from over-applied manure can also damage plant roots, leaves, and fruits while creating drought stress conditions.
So how do you know if you’ve put down too much horse manure? Watch for these signs of over-application:
- Visible manure chunks in soil
- Strong manure odor
- Excessive salinity and high pH
- Nutrient deficiencies despite high fertility
- Lush, weak growth and increased pest/disease problems
- Nutrient runoff and contamination
Routine soil testing every 1-2 years provides valuable feedback on the effects of your manure applications. This helps ensure you’re maintaining balanced nutrient levels for optimal plant health.
Potential Risks of Using Too Much Manure
Applying excessive horse manure carries several risks:
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Nutrient imbalances: Too much manure throws off nutrient ratios, leading to deficiencies. High phosphorus, potassium, magnesium and sodium inhibit uptake of nitrogen and calcium.
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Excessive salinity: Salts in manure accumulate over time, making it difficult for plants to absorb water and causing drought stress.
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Contaminants: Heavy applications increase the risk of contaminants like weed seeds, pathogens, antibiotics, and heavy metals.
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Pollution: Nutrient runoff from over-applied manure can contaminate water sources and lead to algae blooms.
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Weak growth: Too much nitrogen from manure leads to excessive, weak growth prone to pests and diseases.
Best Practices for Using Horse Manure
Follow these best practices to harness the benefits of horse manure while avoiding over-application:
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Test your soil regularly to monitor nutrient levels and pH. Adjust manure applications accordingly.
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Use aged or composted manure to eliminate risks from pathogens, weed seeds, and raw manure.
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Limit heavy applications to 1-2 inches per growing season. Avoid applying 3+ inches at once.
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Rotate manure applications with other amendments like compost, green manures, and mineral fertilizers.
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Thoroughly incorporate manure into the top 6-8 inches of soil to prevent nutrient runoff.
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Account for nutrients supplied by manure and reduce additional fertilizer inputs.
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Monitor salt and pH levels yearly. Adjust practices to prevent excessive alkalinity.
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Prevent crop contamination by keeping freshly applied manure away from plants. Follow safe harvesting, handling, and washing practices.
The organic matter and nutrients in horse manure can profoundly enhance soil health when used properly. But it’s a powerful amendment that demands judicious application. Used in moderation and integrated with other organic and mineral inputs, horse manure boosts the garden ecosystem. Over-applied, it can be equally destructive. Careful monitoring and soil testing helps strike the right balance.
Horse manure can provide incredible benefits, but determining optimal application rates is essential to avoiding over-application. Regular soil testing, proper application guidelines, and integrating manure with other amendments are key to maximizing its advantages while minimizing associated risks. With careful use and monitoring, horse manure can transform your garden’s fertility and lead to healthier, more productive plants.
Adjust the pH, alkalinity, and salt concentrations in your soil
The ideal pH range for growing fruits and vegetables is 6.0 – 7.0 (with the exception of a few specific plants such as blueberries, which thrive in acidic conditions). Soils that are too acidic (below 6) can be amended with agricultural lime. A basic soil test will provide instructions for the amount of lime needed to adjust your soil.
More commonly, soils that have received excess compost tend to be too basic (pH above 7). It is more difficult to acidify soil than to make it more basic, but there are a few things that growers can do:
- Leave the plastic off your high tunnel for the winter in years when you are replacing it. This allows rain water or snow, which is very low in alkalinity, to infiltrate the soil.
- Stop adding compost to prevent the continual addition of cations (positively charged nutrients) to your soil.
- Soils with excessive compost tend to have elevated phosphorus concentrations as well.
- Stick to nitrogen-only fertilizer sources (ammonium nitrate or ammonium sulfate, urea, blood meal) until your calcium, potassium, and phosphorus levels have come back down.
- Consult your local Extension educator on the best fertilizer sources to add based on your soil test.
- Test your soil every year until conditions stabilize.
- Adding sulfur to your soil can bring down the pH. See the nutrient management guide for commercial growers for a list of sulfur products. Sulfur additions will likely need to be repeated to maintain a lower pH, and it can take from months to years to see results. The following amounts of sulfur can be used to lower the pH by one unit (e.g. from 8 to 7):
- In sand, loamy sand, and sandy loam soils, add 0.8 lb / 100 sq. feet, 8 lb / 1000 sq. feet, or 1 lb per cubic yard.
- In loam or silt loam soils, add 2.4 lb / 100 sq. feet, 24 lb / 24 sq. feet, or 3 lb per cubic yard.
- Lowering the pH of clay soils is difficult due to their high buffering capacity. Because of potential salt build-up with acidifying amendments and poor internal drainage, lowering the pH of clay soils is not recommended.
- For more information on acidifying your soil, see the Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens, and Landscape Plants guide. We do not yet have good recommendations about the amount of sulfur needed to bring down the pH of a tunnel, as it can depend on soil texture, moisture and calcium levels.
- Acidifying your irrigation water can also help to bring down the pH. This is especially important in high tunnels, where a lack of rainfall results in the build-up of salts in the soil. Acidic water can help to neutralize these salts.
- In order to do this effectively, you need to know both the pH and alkalinity of your irrigation water. The University of Minnesota Soil Testing Laboratory offers water tests for $10.
- Your irrigation water should have a pH of between 5 and 7, and alkalinity between 37.5 and 130 ppm (but a maximum of 65 ppm for seedlings).
- Once you know the pH and alkalinity of your water, use the AlkCalc tool to determine the amount of acid you’ll need to add to your water.
- If you are growing in a small space such as a raised bed, consider simply removing some of the compost and spreading it over a larger area.
- While flooding high tunnels is commonly cited as a way to mitigate salts, this practice can result in nitrate leaching, which is an environmental concern. Practice other methods to mitigate salts before resorting to flooding.
Assess the nutrients in your soil
Soils with excessive compost applications, particularly manure, tend to develop high concentrations of nutrients such as ammonium, calcium, magnesium, potassium and sodium. These soils can also develop high concentrations of bicarbonates, carbonates and hydroxyls.
Too much of any nutrient can inhibit the uptake of other nutrients, resulting in deficiencies.
- High ammonium can inhibit the uptake of calcium, magnesium and potassium.
- High concentrations of base cations like calcium, magnesium, potassium and sodium are associated with increased soil alkalinity.
- Highly alkaline soils tend to have a high pH (a measure of acidity), and many nutrients become less available in high pH soils. As a result, your plants may exhibit nutrient deficiency symptoms, despite an excess of nutrients in the soil.
Another issue of soils that receive excessive compost is the potential for increased soluble salts to levels that would cause salt toxicity.
- In high tunnels, soluble salts can accumulate to excessive levels because leaching is minimal.
- Composted manure is generally higher in salts than composted vegetative matter.
- Raw manure can be very high in salts and ammonium and is not recommended for use in high tunnels.
The first step to remediate an excessive application of compost is to test the soil. Your soil test should include the basic series (pH, organic matter, phosphorus and potassium), as well as soluble salts, calcium, magnesium and sodium, as well as ammonium.
Read more about Interpreting soil tests for fruit and vegetable crops.
Dos and Don’ts for Horse Manure in the Garden and the Compost Pile
FAQ
What vegetables don’t like horse manure?
Vegetables like tomatoes, peppers, and cucumbers do not like horse manure. Fresh manure’s high nitrogen can burn plant stems and roots, especially in tender plants such as lettuce and radishes. Root vegetables like potatoes and carrots are particularly sensitive.
When should you not use horse manure?
Food safety guidelines are generally about using ‘fresh’ (not well composted) manure, and IIRC are generally minimum recommended in the range of 4 to 6 months before harvesting. Horse manure that’s been outside for two years or more should be perfectly fine.
How much manure is too much for a garden?
Specialists at the University of Georgia recommend a rate of 150 pounds of cattle manure or up to 200 pounds of horse manure or 50 pounds of poultry manure per 1000 square feet of garden soil.
Is excessive use of manure harmful?
… of the application of manure are runoff of manure or manure components into surface water and leaching of nitrate (NO3) and P into the ground water