Being a big garden fan, I’m always looking for natural ways to improve the health and fertility of my soil. A lot of people say that horse manure is a great way to improve the soil and make crops grow better. I have often thought, though, if it’s possible to put too much horse manure in your garden.
This article talks about whether horse manure can be used too much, the possible risks of doing so, and the best ways to use horse manure in a safe and effective way.
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 used too much horse manure? Look out for these signs:
- 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 the ratios of nutrients, which can lead to deficiencies. High levels of phosphorus, potassium, magnesium, and sodium make it harder for nitrogen and calcium to be taken in.
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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). To make soil more basic is easier than to make it more acidic, but growers can still do the following:
- When you replace your high tunnel every year, leave the plastic off of it for the winter. This lets rain or snow, which have a low alkalinity, seep into the ground.
- If you don’t want to keep adding cations (nutrients with a positive charge) to your soil, stop adding compost. Soils that have a lot of compost tend to have a lot of phosphorus as well. Until your calcium, potassium, and phosphorus levels are back to normal, use only nitrogen-based fertilizers like ammonium nitrate or ammonium sulfate, urea, and blood meal. Based on your soil test, talk to an Extension educator in your area about the best fertilizer sources to add. Test your soil every year until conditions stabilize.
- Adding sulfur to your soil can bring down the pH. A list of sulfur products can be found in the nutrient management guide for commercial growers. Adding sulfur will probably have to be done more than once to keep the pH low, and it could take months or even years to see results. To drop the pH by one unit, use the following amounts of sulfur: g. from 8 to 7: Add 0 to sand, loamy sand, and sandy loam soils. 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. Because clay soils have a high buffering capacity, it is hard to lower their pH. It is not a good idea to lower the pH of clay soils because acidifying amendments can cause salt to build up and the soil doesn’t drain well. The Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens, and Landscape Plants guide has more information on how to make your soil more acidic. However, we don’t have good advice on how much sulfur to use to lower the pH of a tunnel yet because it depends on the type of soil, how wet it is, and how much calcium is in it.
- Low pH can also be reached by making your irrigation water more acidic. This is especially important in high tunnels, where salts build up in the soil when it doesn’t rain. Acidic water can help to neutralize these salts. To do this right, you need to know both the pH and alkalinity of the water you feed plants. For $10, the University of Minnesota Soil Testing Laboratory will test your water. Alkalinity should be between 37 and 45% and pH should be between 5 and 7. 5 to 130 ppm (but no more than 65 ppm for seedlings). You can use the AlkCalc tool to find out how much acid you need to add to your water once you know its pH and alkalinity.
- If you are growing in a raised bed or another small space, you might want to take out some of the compost and spread it out over a bigger area.
- People often say that flooding high tunnels is a good way to get rid of salts, but this can cause nitrate leaching, which is bad for the environment. 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.
- Too much ammonium can stop calcium, magnesium, and potassium from being taken in.
- More alkaline soil is found where there are lots of base cations, such as calcium, magnesium, potassium, and sodium.
- Soils that are very alkaline tend to have a high pH, which is a measure of acidity. In high pH soils, many nutrients are less available. So, even though there are plenty of nutrients in the soil, your plants may show signs of nutrient deficiency.
Another issue of soils that receive excessive compost is the potential for increased soluble salts to levels that would cause salt toxicity.
- When leaching is low in high tunnels, soluble salts can build up to too high of levels.
- Most of the time, composted manure has more salts than composted plant matter.
- It is not a good idea to use raw manure in high tunnels because it can have a lot of salts and ammonium in it.
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?
There are two main ways that manure is used: it runs off into surface water or its parts, and nitrate (NO3) and phosphorus (P) are leached into ground water.