Soil Conditioning – Establishing a Successful Gardening Foundation

Andrew “Drew” Jeffers,
Horticulture and Natural Resource Agent,
Clemson University

HTG 0118

The soil is often referred to as the “foundation of life”. The foundation must be strong and healthy before great things can be developed. Most garden and landscape soils have poor soil texture and often lack organic matter, one of the most important additions any gardener can make.

The soil is a medium comprised of mineral elements, soil particles, water, and living organisms, all of which are important to the overall health of the soil. The three primary soil particles are sand, silt, and clay. The relative percentages of these components present make up the soil’s texture. Texture is important to overall soil and plant health as it relates to soil porosity space, the pore spaces where air and water reside. The ideal soil texture is balanced mix of sand, silt, and clay particles, also known as a loamy soil. In most cases the particles will not be balanced and the soil will need to be altered by adding amendments. To evaluate soil texture, use a simple jar test to determine the percentages of sand silt, and clay. For more information on the soil jar test, check out this video at the Clemson Extension Virtual Raingarden https://www.clemson.edu/extension/raingarden/virtual_rain_garden.html.

The ideal soil texture is a loamy soil that is a balanced mixture of sand, silt, and clay particles.

The ideal soil texture is a loamy soil that is a balanced mixture of sand, silt, and clay particles.
Andrew Jeffers © 2017 Clemson Extension.

Myths About Breaking Up Clay

Myth #1: Use sand alone to improve soil drainage.

Truth: However, clay particles are very fine and layered in sheets (similar to a stack of un-neatly stacked paper) making it difficult for water to pass through these particles readily. Sand that is worked into clay provides a surface for these clay particles to adhere to. The result is a concrete like mixture that is more difficult to work than clay itself.

Clay soils particles are very fine, and stacked together similar to un-neatly stacked sheets of paper.

Clay soils particles are very fine, and stacked together similar to un-neatly stacked sheets of paper.
Andrew Jeffers © 2017 Clemson Extension.

Myth #2: Another recommendation to break up clay and reduce soil compaction is to add gypsum.

Truth: The structural improvement effects of gypsum are often short-lived and need to be repeated from year to year. Tillage and aeration are the two surest ways to reduce compaction in the soil.

Sand that is worked into clay provides a surface for these clay particles to adhere to. The result is a concrete like mixture that is more difficult to work than clay itself.

Sand that is worked into clay provides a surface for these clay particles to adhere to. The result is a concrete like mixture that is more difficult to work than clay itself.
Andrew Jeffers © 2017 Clemson Extension.

Dealing with Sandy Soils

Sandy soils are much different than clay soils. Sandy soils tend to not hold on to plant nutrients very well. The goal of amending this soil type is into increase the nutrient holding capacity. Incorporating compost will help add additional binding sites for plant nutrients to be held. Caution must still be exercised, because sandy soils can still leach out great amounts of nutrients. Adding fertilizer in small, frequent applications can help reduce the amount of leaching nutrients.

In addition to helping loosen heavily compacted soils and improve overall soil health, adding organic matter is a good way to improve both clay and sandy soils. Aside from improving soil texture, adding organic matter can have other benefits as well. One of which is the addition of beneficial soil microbes, such as fungi and bacteria. These microbes help with plant nutrient uptake by making nutrients more available for root uptake.

Several sources of organic matter are available. Fresh sources tend to decompose rapidly, generally within 30 days, providing readily available sources of nutrients for microorganisms in the soil. The remaining compounds are more resistant to breakdown by these microorganisms. Composted materials have completed most of the rapid decomposition process in the compost pile. Therefore, the supplied organic matter tends to last longer in soil than fresh sources. The caveat is that composted materials do not provide microorganisms a long-term supply of usable compounds. However, properly composted materials (meaning the pile’s internal temperature has reached a consistent 140 °F) do have the benefit of being less likely to have weed seeds and pathogens.

Applications

Soil amendments are most commonly used in vegetable gardens and landscape beds. It is important to evaluate various types of soil amendments and determine which ones will provide the most benefit. This often leads to a combination of different amendments to supply organic matter, nutrients, and improve the soil texture. There are many combinations but a good general-purpose ratio is one part composted manure, to three parts garden compost to one part soil conditioner. This amendment mixture can then be tilled in prior to planting. The addition of organic matter is beneficial but too much can be detrimental to plant health. Start with adding 25% by volume or 2 inches of organic matter into the top 6 inches of the soil.

CAUTION: Incorporating more than 50% organic matter into the soil may negatively affect plant growth. This amendment mixture can also be used as a top dressing for tree, shrub, herbaceous ornamental, and vegetable plantings. The same mixture can also be used in turf areas such as for bare spot repair or incorporation for turf establishment. For raised garden beds, the mixture can be incorporated with native soils.

Sources:

  • Bell N., Sullivan D.M., Brewer L.J., and Hart J. 2003. Improving Garden Soils with Organic Matter. Oregon State Univ. Coop. Ext. Serv. http://extension.oregonstate.edu/polk/sites/default/files/MG_Handouts/ec_1561_improving_garden_soil_with_organic_matter.pdf
  • Chalker-Scott. L. The Myth of Gypsum Magic. Washington State University. Puyallup Research and Extension Center. https://s3.wp.wsu.edu/uploads/sites/403/2015/03/gypsum.pdf.
  • Morganello-Counts. K. Clemson University Cooperative Extension. Virtual Rain Garden. https://www.clemson.edu/extension/raingarden/virtual_rain_garden.html.