Soil and Bed Preparation for Growing Dahlias

Page 8

4. Dahlias prefer soil that is well aerated.

Air in Soil

Well-aerated soils are essential for the health of dahlias. If soils are compacted or too wet, plants and the organisms that live below the ground are often deprived of the air they require. Usually aeration is not a problem as long as good soil tilth is maintained.

What can be done to improve the aeration of soil?

In predominantly "clay" and "silt" soils the enemies of aeration are insufficient humus, too much water (or poor drainage), and compaction.

If the soil contains little or no humus, the particles of clay and silt pack together tightly because of their size. The spaces between the particles are not large enough to hold adequate amounts of air.

If the soil is too wet, the water fills up the airspaces and drives out the air. Additionally, as the water drains away the empty spaces are filled by particles since the air had previously been driven out--this action is referred to as "compaction".

Compaction, of course, can also be caused by walking to close to the plants-- the heavy weight on the soil presses it down driving out the air.

Once soil has compacted, very little can be done to restore adequate aeration until the plants are removed. The work required to aerate the soil will damage the plants. When the plants are removed (presumably in the fall at the end of the season) it is time to apply large amounts of composted humus, till the soil thoroughly, and assure that drainage is adequate.

To properly aerate soil for dahlias, the humus should be tilled into a depth of at least 10 inches--preferably twelve. The soil should be thoroughly mixed so that the various ingredients are uniformly mixed throughout the entire depth. Do not attempt to till when the soil is wet--the result would be compacted clods. The soil should be damp to moist. If the soil is dry, it may till, but the dry particles will tend to aggregate by size and not produce a uniform mixture with sizable air pockets. When the soil is damp to moist, tilling produces a mealy mixture. The different particles cling together lightly leaving large air pockets.

If preparing a dahlia bed results in the bottom of the bed being dug into clay or compacted subsoil, the bed will not drain adequately. The clay or compacted subsoil will form a trough. In some clays, the water may stay there for long periods. There are several ways to overcome this type of problem depending on the terrain in which the beds are located.

For example, building beds by terracing the side of a hill, building high raised beds in locations where the water table is high, and various combinations in between. When beds are terraced on the side of a hill, a drainage ditch should be built at the intersection of the back of each step and the riser for the next step. This will prevent excess water and fertilizer or salts from spent fertilizers from running onto the next lower bed.

Walkways and beds should be clearly demarcated. This reduces the chances of compaction by the gardener and visitors. In the picture on the immediate right, used red carpet marks the walkways. This also serves to keep walkways from being muddy during and after rains and to provide a sense of neatness.


Sandy Soil typically has plenty of air and airspaces since the particles are large and irregular--they do not fit together well and do not compact. However, if sandy soil is located in an area where the water table is high, high water can drive out all of the air.

Raising the beds so that the bottom of the bed will be at least 2 or 3 inches above the high water table is about the only solution. Typically 12 inch high beds are necessary.

Sandy Soil

Continued on next page.

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