Raised garden soil components

Asked April 15, 2017, 10:20 AM EDT

We are making raised garden beds. What is a good mix of components for the soil? We have some native soil from our clayey Arvada yard. Should we use any of it? Where can we purchase these soil components? Thank you ahead of time for your help.

Martha Schwartz
Arvada. CO

Jefferson County Colorado soil and fertility issues

1 Response

Hello Martha,
For our clayey soils, I recommend using purchased topsoil or planter's mix, 'double-digging' it into the native clay soil, so as to minimize the soil texture interface between the two. This will enable roots to extend below the topsoil more easily...should they grow so long. This is discussed in the CSU Garden Notes on Block Style Layout in Raised Bed Vegetable Gardens: http://www.ext.colostate.edu/mg/Gardennotes/713.pdf AND in the Missouri Extension publication on Raised Beds that includes diagrams: http://extension.missouri.edu/p/G6985

Soil mix

Good-quality existing topsoil may be used in raised beds. However, the addition of organic matter to any soil will improve its physical and chemical makeup, thus making it more productive. Peat moss, compost and decomposed manures are good sources of organic matter.

To take full advantage of the deep rooting potential with raised beds, work up the base soil by rototilling or hand-digging before bringing in additional soil. Many gardeners double dig beds.

Double digging involves removing the topsoil the depth of a spade, setting the soil aside, then loosening the subsoil another spade's depth (Figure 8). Finally, the topsoil is returned with added amendments, such as compost, manure or fertilizers. This labor-intensive soil preparation method provides an excellent rooting zone for plants. However, less-intensive methods also permit satisfactory plant growth.

Avoid hauling in new layers of soil without mixing them into existing soil. Distinct layers of soil create barriers through which water will not readily penetrate and roots will not easily grow.

Double digFigure 8

To double dig before establishing a raised bed, remove all the soil from the bed one spade's depth. Dig the next layer down, leaving the soil in place. Return the topsoil to the bed, and thoroughly mix the layers. Double digging permits deeper rooting by plants growing in the bed.

Note that there is not legal definition of composition of purchased topsoil, so you may still need to add organic matter (OM), such as compost. Ideally, there should be 5% OM. You can have your soil tested to monitor the % OM from season to season. Soil sampling containers/directions can be picked up at the Jeffco Extension office at the Fairgrounds in Golden. Collection kits are free; testing is done at the CSU soils lab in Ft. Collins ($35); you mail your soil sample to CSU,

As an alternative to purchased topsoil, you may create your own garden soil mix. A garden soil blend could consists of equal parts (1:1:1 by volume), of the following ingredients: http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2069&context=extension_curall

  1. Composted Organic Matter: animal manures, composted kitchen scraps, finely ground wood shavings, leaves, grass clippings, and/or peat moss
  2. Drainage Increasing Materials: vermiculite, or perlite
  3. Native soil or topsoil

You can purchase components or finished product at your favorite nursery or garden center.

Best regards,