soil temperature variation with depth in a bulb garden

Gastil Gastil-Buhl
Tue, 19 Nov 2013 18:35:30 PST
I would like to determine whether bulb chilling requirements may be met by planting deeper when in a climate otherwise too warm. Do any of you have experience setting up an experiment to measure soil temperature in a bulb garden? I have heard from a lily grower in a cold zone in Canada with some good advice. My experiment is the opposite: not how warm it is deeper but how much cooler the soil may be deeper. Soil averages the day's highs and nights lows of air temperature, damping that oscillation with depth. I would like to know how much the soil temperature varies in my bulb garden during the winter growing season. 

I have 12 tiny temperature loggers, each one about the size of a stack of three quarters (the US 25 cent coin). I have buried them in my garden for a quick 3-day test to see how well this works and then plan to put them out for 8 months.  There are a few questions I wish to answer. 

How much is the temperature daily fluctuation muted with depth in the sand bed and in soil beds? 
How much is that affected by a half-inch layer of white pumice as mulch?
Can planting a bulb more deeply simulate a colder climate? 
What is the effect of sunlight-warming on being planted near the side of a raised bulb box? 

And there are some practical method questions I hope to solve with this small initial experiment, most importantly whether these thermistors require more protection from moisture than their little steel cases. I plan to put some in those tiny ziploc baggies Dell so kindly provides with each bulb allotment.

The initial test started this morning in three spots in the planting beds, shown here:…

Saturday I will dig them up, download their data, and check to be sure I understood how to program them. Then I plan to re-plant them for 8 months at a 3 hour sampling period. So if you see flaws I may correct in my experiment, please speak up. I am open to suggestions. 

Given only 12 thermistors, would you suggest it is more important to measure more depths or more spatial variability?

This bulb garden is primarily full-sun, with some areas lightly shaded part of the day, but no significant shade.

- Gastil
Santa Barbara, California
too cold for Veltheimia unprotected but too warm for most Tulipa

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