Best soil for a raised bed?

Sun, 13 Jul 2014 09:33:50 PDT
>Mike, I always think that the inclusion of very small-sized gravel helps 
>make a good soil mixture for bulbs.

I don't know anything at all about winter-growing bulbs, but in arid and 
semi-arid environments, plants grow in sandy or coarse soils in order to 
utilize every bit of precipitation. Nothing to do with "drainage" (a word no 
visitor to the garden here is allowed to use, unless they also happen to be 
a plumber).
Clay soils hold more water, but less is available to the plant because the 
water binds to the clay particles like glue. In this part of the world, the 
evaporation rate in clay exceeds the percolation rate, so it takes a great 
deal more water to get to plant roots growing in clay than in coarse soils.
Called the "inverse texture effect"; "inverse" because it seems 
counterintuitive compared to information (and drainage talk) from climates 
with regular rainfall. Even if clay is saturated, there is still less water 
going to roots than in coarse soils. My raised beds are piles of sand and 
gravel, nothing else. The plants growing there require less irrigation (in 
fact, none) than the plants growing in the heavy clay here.
Don’t know if this translates well to mediterranean climates (I suspect it 
does), but might be worth thinking about if you're not going to mimic the 
native soil of each bulb.

Bob Nold
Denver, Colorado, USA

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