Fall Soil Amendment

Although fall marks the end of the growing season, it is a great time to improve the soil for next season. Adding soil amendments in the fall allows time for soil reactions to take place that are necessary to realize benefits.

A soil test is the best method to determine the need for soil amendments (UMass Soil and Plant Nutrient Testing). It is very important to collect a representative sample of soil from the area to be tested. The soil’s physical appearance, texture, color, slope, drainage and past management practices should be similar throughout the test area. Separate samples should be collected from areas that differ in soil texture, soil color, slope, drainage and kinds of plants previously grown. Do not collect samples when soil is very wet.

Amendments for soil pH modification

Soil pH is a measure of soil acidity. This is a very important chemical property of the soil because it affects the availability of nutrients to plants and the activity of microorganisms in the soil. Micronutrients such as iron and manganese are more soluble and more available to plants in acidic soils. In acidic soils with pH below 6.0, the activity of beneficial soil bacteria is significantly reduced. Acidic soil conditions also reduce the effectiveness of some herbicides.

Most plants grow best when soil pH is between 6.0 and 7.0, except for some acid loving plants such as blueberries, azaleas, and rhododendrons that require a lower soil pH between 4.5 and 5.2. Due to the climate and geology of New England, our soils tend to be naturally acidic, with pH ranging between 4.5 and 5.5. Adding amendments in the fall to adjust pH allows time for the chemical reactions to take place that are necessary for pH change to occur.

Amendments for raising soil pH

Agricultural limestone is the most common material used for raising soil pH. Limestone consists of either calcium carbonate (calcitic limestone) or calcium and magnesium carbonate (dolomitic limestone). In soils low in magnesium, dolomitic limestone is preferred. Apply lime as recommended by soil test results. The soil testing lab determines the amount of lime using the target pH (based on the plants identified on the submission form) and the soil’s buffering capacity (soil’s tendency to resist pH change). When lime is added to the soil, chemical reactions neutralize the active acidity (hydrogen ions in the soil solution) and exchangeable acidity or buffering capacity (hydrogen and aluminum ions held on soil particle surfaces) to raise the pH.

Amendments for lowering soil pH

Lowering soil pH is required when growing acid loving plants. Lowering the pH may also be needed if the level of calcium and magnesium is too high in the soil. Elemental sulfur is the most common material recommended for lowering soil pH and should be applied as recommended by soil test results. The lab determines the amount of elemental sulfur needed to lower the soil’s pH based on the soil texture, the current pH, and the target pH. Elemental sulfur reacts slowly with soil and is best applied in the fall and incorporated into the soil to a depth of 6 inches. Apply elemental sulfur as recommended by soil test results. Sandy soil requires lower amounts of elemental sulfur than clay soils. For example, a sandy soil with pH 6.0 requires 4 pounds of elemental sulfur per 1000 sq. ft. to lower the pH to 5.5, while a clay soil requires 16 pounds per 1000 sq. ft. Large amounts of elemental sulfur are required to make even a small change in soil pH. Apply no more than 15 pounds of elemental sulfur per 1000 square feet at any one time.

Organic amendments to improve soil

Adding organic matter to the soil improves soil structure, increases nutrient holding capacity, improves drainage and water holding capacity, provides plant nutrients, and increases biological activity in the soil. Organic amendments can be added to the soil such as manure or compost. Manure or compost should be thoroughly incorporated into the soil when dry to prevent layering. This can be done by thoroughly rototilling the compost or manure after applying on the soil surface. Fall manure or compost application gives enough time for these to break down to provide plant nutrients and other soil benefits in the spring.

Application rates vary from 1/2 to 3 inches applied to the surface of the soil and incorporated into the top 6 inches of the soil. For fall application of organic amendments to vegetable gardens, apply in conjunction with the planting of a cover crop for best results. For established turf, topdress with approximately 1/8 to ¼ inch of finished compost, in conjunction with core aeration if possible.

When added to loose, sandy soil, organic amendments help bind the particles together and increase the soil’s ability to hold moisture and nutrients. In heavy clay soils, organic amendments bind to the small clay particles, forming larger particles that have larger air spaces between them, allowing for better drainage and air exchange.

While manures and composts are excellent soil amendments, they contain high levels of phosphorus. Regular soil testing is important to avoid soil phosphorus buildup from continuously applying manures and composts.

References

1. Allen, T. 2015. Adjusting soil pH. Soil and Plant Testing Laboratory, University of Massachusetts, Amherst, MA
2. Sidemen, B., Majewski, C., Haddad, N. and T. Buob. 2013. Guidelines for using animal manures and manure-based composts in the garden. UNH Cooperative Extension, Durham, NH
3. Browning, S. 2012. Soil Amendments for the garden. University of Nebraska Extension, Lincoln, NE
4. N. Bell, D.M. Sullivan, L.J. Brewer, and J. Hart. 2003. Improving garden soils with organic matter. Oregon State University Extension, Corvallis, OR.