Soil fertility and especially soil biological fertility is promoted within organic farming for reasons of
nutrient cycling, structure improvement or biodiversity.
— von Fragstein, 2006

Organic farming strives for a balance between a reasonable good yield, a high produce quality and a limited environmental impact. Zanen, M et al. 2008 PDF (1)

Photo by belchonock/iStock / Getty Images
Photo by belchonock/iStock / Getty Images

What is a soil amendment?

Soil amendments are anything mixed into topsoil to promote healthy plant and root growth. They can improve soil tilth, change the pH of soil or supply nutrients. Traunfeld, J., Nibali, E 2015 PDF (2)

Organic amendments also improve physical properties (texture/tilth/structure), provide pore space and water holding capacity, improve drainage and aeration, moderate soil temperature, increase soil microbial community, adjust pH and add nutrients. Whiting, D et al. CMG213 2015 PDF (3) They include plant residues, composts (humus or decomposed plant material), animals manures from various origins and stages of decomposition and natural inorganic nutrients like rock dust. Zanen, M et al. 2008 PDF (1) 

Why Water-Filled Pore Space (WFP) is so important?

A soil with good tilth has large pore spaces for adequate air infiltration and water movement. Soil texture influences the size of the pore spaces. It refers to the size of the particles that make up the soil. Sand, silt, and clay refer to relative sizes of the individual soil particles. Whiting, D et al. CMG213 2015 PDF (3)

Amendments will improve the physical property or texture of the soil. The decision on what type of amendment will depend on the relationship between the amount of clay and sand found in your soil, as well as organic matter. 

Ideally, your soil should have a combination of large and small pore spaces. Organic matter encourages the formation of aggregate, or crumbs, or soil. Organic matter also absorbs water and retains it until it is needed by plant roots. An ideal soil has 50% pore space (with the remainder consisting of minerals and organic matter). LaLiberte, K. 2016 (4)

Before you amend your soil, check the following:

Mineral Component: This comprises 12 soil texture classes. These are sand, loamy sand, sandy loam, silt, silt loam, loam, sandy clay loam, silty clay loam, clay loam, sandy clay, silty clay and clay. Ideally, a desirable soil has a representative amount of sand, silt, clay and organic matter. A good soil texture for landscape beds is a silt loam (40% sand, 40% silt and 20% clay). Richardson, W. et al. 2007 PDF (5)

Organic Matter (OM):  When bedding plants or vegetables, you will need to add organic matter, such as compost, to the bed each time you prepare it for planting. Organic matter retains moisture, improves drainage, provides nutrients and attracts beneficial organisms like earthworms. Other sources of organic matter include aged or composted manure, leaf mold (partially decayed leaves), peat moss, composted finely ground pine bark and soil conditioner. Richardson, W. et al. 2007 PDF (5)

Coverage: 3 cubic feet of organic matter will cover 36 sq. ft. to a depth of 1 inch. Traunfeld, J., Nibali, E 2015 PDF (2)

Useful conversions: 7.5 gallons = 1 cu. ft., 1 cu. ft. = 1.25 bushels, 27 cu. ft. = 1 cu. yd.

As the organic content increases, earthworms and soil microorganisms become more active; this over time improves soil tilth.  The ideal soil for most gardens has 4-5% organic matter, and at this level, additional fertilizer will not be needed. Figure 1 explains the amount of N to add based on the level of OM found in the soil. Whiting, D et al. CMG711 2016 PDF (6)  

Figure 1:

  Whiting, D.  et al. CMG GardenNotes #213. 2015. Managing Soil Tilth, Texture, Structure and Pore Space.   Colorado State Extension Program. 

Whiting, D.  et al. CMG GardenNotes #213. 2015. Managing Soil Tilth, Texture, Structure and Pore Space. Colorado State Extension Program. 

Before you start to plant, you should conduct an analysis of the planting area. Test your soil using the R Squared Diagnostics pH and Soil Texture test kit and Nitrate Nitrogen test kit. This will indicate pH, soil texture class, amount of organic matter and the amount of essential nutrient (N) already present.  These first steps will help to correct soil pH, soil texture, nutrient levels and improve soil drainage. 

STEPS TO AMEND SOIL FOLLOWING THE SOIL ph and TEXTURE TESTs:

As determined by particle size, sandy and silty soils lack sufficient structure.  Soils with more clay content, such as the various loams, aggregate into larger chunks called peds.  Highly aggregated soils are optimal for root growth and aeration, but can be easily destroyed by any activity that results in soil compaction. One of the least invasive and most cost-effective ways to improve soil structure is by adding organic mulches. Chalker-Scott, L. 2008 PDF (7)    

Clay soils: Apply sand and some form of organic matter. Also follow up with routine applications of organic matter to foster the activity of soil microorganisms and earthworms.  As soil microorganisms decompose the organic matter, the tiny soil particles bind together into larger clumps, called “aggregates”, increasing large pore space. Work 2 to 3 inches of organic matter into the surface of the soil, preferably during the fall season. Then add at least 1 inch more each year after that. Adding gypsum (calcium sulfate) at the rate of 50-100 lbs./1000 ft2 may also improve internal drainage of clay-based soils. Use permanent raised beds to improve drainage and keep foot traffic out of the growing area. Minimize tilling and spading. LaLiberte, K. 2016 (4)

Sandy soils: If amending a sand or loamy soil, add organic matter only. Every 6 inches of sandy or loamy soil need 1 to 2 inches of organic matter. Work in 3 to 4 inches of organic matter such as well-rotted manure or finished compost. LaLiberte, K. 2016 (4)

Peat moss, particularly Sphagnum peat is a good soil amendment, especially for sandy soils making them more absorbent and capable of retaining more water after sphagnum peat application. Sphagnum peat is generally acidic (i.e., low pH) and may help gardeners grow plants that require a more acidic soil. Whiting, D. et al. CMG241 2015 PDF (8)

Silty Soil: Silty soils contain small irregularly shaped particles of weathered rock, which means they are usually quite dense and have relatively small pore spaces and poor drainage. They tend to be more fertile than either sandy or clayey soils. To improve silty soil, add at least 1 inch of organic matter each year. Concentrate on the top few inches of soil to avoid surface crusting. LaLiberte, K. 2016 (4)

Acidic Soil: If the pH of your soil is less than 6.5, it may be too acidic for most garden plants (although some, such as blueberries and azaleas require acidic soil). Soils in the eastern half of the U.S. are usually on the acidic side. LaLiberte, K. 2016 (4)

The most common way to raise the pH of your soil (make it less acidic) is to add powdered limestone. Dolomitic limestone will also add manganese to the soil. Apply it in the fall because it takes several months to alter the pH.

To raise the pH of your soil by about one point: LaLiberte, K. 2016 (4) :

  • In sandy soil: add 3 to 4 pounds of ground limestone per 100 square feet.
  • In loam (good garden soil): add 7 to 8 pounds per 100 square feet.
  • In heavy clay: add 8 to 10 pounds per 100 square feet.

Alkaline Soil: If your soil is higher than 6.8, you will need to acidify your soil. Soil is usually acidified by adding either iron sulfate or ground sulfur. You can also incorporate naturally acidic organic materials such as conifer needles, sawdust, peat moss and oak leaves. LaLiberte, K. 2016 (4)

To lower soil pH by about one point:

  • In sandy soil: add 1 pound ground sulphur per 100 square feet.
  • In loam (good garden soil): add 1.5 to 2 pounds per 100 square feet.
  • In heavy clay: add 2 pounds per 100 square feet.

References:

  1. Zanen, M., Bokhorst, J. G. & Koopmans, C.J.  June 16-20, 2008. Soil Fertility and Biodiversity effects from Organic Amendments in Organic Farming.  16th IFOAM Organic World Congress, Modena, Italy

  2. Traunfeld, J., Nibali, E., 2015. Soil Amendments and Fertilizers, Fertilizing Guidelines Included by Plant Group. Home and Garden Information Center University of Maryland Extension

  3. Whiting, D. et al. CMG GardenNotes #213. 2016. Managing Soil Tilth, Texture, Structure and Pore Space. Colorado State Extension Program, Colorado Master Gardener Program

  4. LaLiberte, K. Vegetable Gardening Building Healthy Soil Blog. 2016.  Gardener' s Supply Company

  5. Richardson, W. B. et al. 2007. Landscape Bed Preparation For Ornamental Plants, Louisiana State University Agricultural Center. 2670, 04/07 Rep.

  6. Whiting, D. et al. CMG GardenNotes #711. 2015 Vegetable Gardens: Soil Management and Fertilization. Colorado State Extension Program, Colorado Master Gardener Program

  7. Chalker-Scott, L. The Myth of Soil Amendments Part III. 2008. Puyallup Research and Extension Center, Washington State University

  8. Whiting, D. et al. 2015. CMG GardenNotes #241: Soil Amendments, Colorado State Extension Program, Colorado Master Gardener Program