Tuesday, October 11, 2011

Clay soil and Lily Bulbs - What to do?

A raised bed bordered by large landscape rock
A small proportion of clay in your soil is just fine, its presence helps to slow down moisture which reduces the need to constantly water trees, shrubs and other plants; plus it allows rooting stability for trees and shrubs against severe or sudden wind.  However, a yard of extensive "pottery-grade" sticky clay is not the best for a successful garden and measures need to be taken to increase the drainage by making a raised bed or adding drainage tiles so that roots do not suffocate from lack of oxygen.  Simply carving out a gallon-sized hole and filling it with sand or potting soil for planting lily bulbs only creates a "sump" - all of the water will drain immediately through the sand or potting soil into the hole and you will have simply made things worse. 
Clay soil is described in various ways as adobe, gumbo, or “heavy” and is composed of small mineral particles which are generally flat in shape and form a close bond with one another.  Water and air have difficulty passing through these tightly bonded particles.  In clay soil, oxygen is not supplied as readily to roots and your bulbs will suffocate and rot if given too much moisture.  This is especially true if your beds are newly planted and worm activity has not created channels for rainfall to soak readily in the underlying soil.  Mixing in builder's sand or Perlite, along with only a small amount of compost to lighten the soil evenly across the entire bed - above grade - helps prevent over saturation during rainy seasons but does take time. 
Berms are popular for creating adequate drainage in gardens and, if wide enough, will make large islands within the garden - creating an architectural design on their own.  Bear in mind that until the soil is well settled your mounds will continue to shrink for more than a year after creation if you only use soil with no under laying rock for drainage.  A dump truck load of natural garden soils spread one foot deep and compressed by foot traffic, garden tractor tires or whatever method you choose will still settle over time.  One season we had two loads of soil placed over top of our “close to the surface” septic tank to reduce odors after the house was newly constructed.  We garden at the bottom of a small valley with a high water table so our septic tank is shallow to allow for proper drop on our flat ground.  The next year another load was necessary because the soil level had dropped by one half because of heavy rainfall; the unmistakable scent was once more wafting into our nostrils.  After two more years and a higher proportion of clay soil added to the berm, the particles finally bonded to one another enough to keep the soil level constant and reduce odor. This same phenomenon happens to raised beds, they shrink over time on their own.  In addition, by pulling out weeds you are slowly reducing the level of the earthen embankment, so additional soil is needed from time to time.

If you are unsure as to whether you have clay soil, dig a hole in the garden one foot deep, fill it with water and wait.  If the water drains away within one hour, drainage is generally sufficient for lily bulbs (Lilium), however Eucomis (Pineapple Lilies) may still be marginal in that location.  If the water passes through the soil very sluggishly, consider a raised bed 8-10 inches above grade.

Sandy soil never “puddles,” since it has particles generally more rounded in shape which do not stick together when moistened.  Sandy soil (good for Eucomis) allows air and water to pass freely, but should be amended with additional organic materials such as compost or well-aged manure to add nutrients. Also, sandy soil needs to be fertilized more frequently than other soil types; organic mulches will disappear quickly because rainfall and irrigation easily wash away the granular components.  

Loam (a gardener’s dream) is a mixture of clay, sand, and organic materials which is fast-draining and naturally fluffy, allowing soil to stay slightly moist but not soggy between watering cycles.  Regular mulching with a variety of materials will encourage earthworm activity and improve marginal soil to become "loam" over time.  If your soil is still somewhat crumbly when wet and not overly sticky by the addition of water, then it is probably not necessary to mix in sand, Perlite or commercial potting soil.  This is especially true if rainfall has someplace else to go, such as when the bed is on a slope where moisture will constantly move away from the bulbs of its own accord – or in a raised bed or berm where the bulbs themselves are already planted above the natural soil level.

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