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.
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.
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:
- 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.
- Finally, flooding your soil a few times per year can help to wash salts out of your soil. This is especially important in tunnels and other covered environments that do not have access to rainwater.
Best practices for adding compost and manure
- Avoid excess applications of animal-based composts (composted manure), which tend to be high in phosphorus and calcium.
- Test your soil regularly to make sure you are not over-applying certain nutrients.
- Deep compost mulch systems can be effective but should be used with primarily plant-based composts to avoid the problems described above.
- Do not rely on compost as your primary or only soil building strategy. Use cover crops to build organic matter in your soil, and account for the nutrients provided by both cover crops and compost in your decisions about fertilizer use.
- Make sure that any compost you apply to your farm or garden has fully completed the composting process.
- The compost should resemble the texture of the soil, with no chunks of un-decomposed organic matter.
- It should also be more or less the ambient temperature. Compost that is over 100 degrees F is likely not finished.
Reviewed in 2021