Soil and Bed Preparation for Growing Dahlias


Page 7

2. Dahlias prefer soil that drains well
but holds sufficient water to support vigorous growth.

Different soil types vary in the way they drain and hold water.

As the figure on the right indicates, if one inch of water is applied to sandy soil, the water will quickly proceed downward for at least 12 inches leaving very little water held in the top several inches.

The same amount of water applied to silt soil will drain downward more slowly reaching down to 8 or 9 inches. Some water will be retained by the silt soil in the top layers. The rest will recede to the bottom of the root zone draining sufficiently to allow roots to obtain air. Through capillary action* plants can obtain water even though their roots are not in the water.

One inch of water applied to heavy clay soil will drain slowly to a depth of 4 or 5 inches. The water is retained entirely within the root zone, closing air pockets, depriving roots of access to air, and keeping the roots continually surrounded by water.

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Adapted from "Improving Your Garden Soil", Ortho Books

What can be done to improve the drainage of heavy clay soil?

When humus is added to clay soil, the particles are pushed apart providing space for air and water. As spaces open up, water can continue to drain downward. It will take a good amount of humus to open up the clay soil sufficiently to ensure adequate drainage. If the humus is well composted you can apply large amounts without any problems. If the humus is sphagnum peat moss (not composted) applying large amounts can be damaging. However, two or three applications can be applied to the soil over the year allowing time between each application for the peat moss to decay in the soil. Once the peat moss has decomposed the danger of salt accumulations on the surface of the soil due to rapid drying is greatly reduced. Clay soils are typically alkaline having a high pH. Adding peat moss will provide the additional benefit of lowering the pH.

Dry Heavy Clay Soil

What can be done to improve the water retention of sandy soil?

When humus is added to sandy soil, it binds sand particles together retaining more of the available water. Large amounts of humus will be necessary. If the humus is well composted large amounts may be added without problems. If the humus is sphagnum peat moss, care must be taken not to add too much at one time. Sandy soil is likely to have a high pH. Adding peat moss would give the additional benefit of lowering the pH.

Sandy Soil

Since silt soil provides a balance between water drainage and retention, should humus be added?

If the soil is predominantly silt it will contain little humus. Humus should be added to improve the "tilth" of the soil.

"Tilth" refers to the soil's fitness for cultivation--the way in which particles of sand, silt, clay, and humus mix together when tilled to provide support, air, water and nutrients to plants.

Adding the recommended annual amount of composted humus will provide reasonably good "tilth" immediately. More may be added, but will not produce greater benefits.

Predominantly Silt Soil

*While gravity causes water to drain down, capillary action may cause water to move from wet to dry even though that may be uphill. Within limits, when surface water evaporates, water from deeper in the soil is sucked up to the surface because of pressure differences.

Continued on next page.


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