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High Yield Gardening: High Yield Gardening in Raised Beds, 1 by tapla

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tapla wrote:
Hi guys! Linda turned me on to your conversation & I'll join it if it's ok? I didn't read the whole thread, but I went upthread quite a few posts & I'll make a few comments that will probably generate additional exchanges - all happy ones, I hope. ;o)

On the 18th @ 1:22 Corey quoted me as saying " in raised beds (and, I guess on the ground, too), the earth acts like a giant wick, pulling moisture downward.

then offered his own comment "On the other hand, when the underlying clay or heavy soil is compacted too much, and the rain is too heavy, impervious soil under the bed acts like a big plastic pot with no drainage hole. You have to "drill holes in the side of the container" - - - i.e. raise the soil above grade and let it drain out through cracks or holes in the RB walls."

I'd like to point out that this is only true to a minor degree, and I'll use an example to illustrate: Lets say you have a fully saturated sponge representing a raised bed and it's resting on a counter top. If we add ANY water at all to the sponge, what happens? It flows laterally across the counter top, right? The more water you pour on the sponges, the more water comes out and moves laterally - yes? That's because the weight of the water column - the water pushing down on the water already IN the sponge is enough to overcome the capillary attraction of the water in the sponge, so the sponge 'seeks a level' that is pretty constant. OK - that would seem like that's all there is too it, but the counter top is not a good comparison to a real raised bed situation. If the soil below is permeable, water continues to percolate downward & right out of the bed, but let's say it is clay, like Corey alluded to. How can we better illustrate what actually happens in clay? Well, lets set that sponge on a newspaper or a paper towel, which does offer a favorable comparison to clay soils because you have the impermeable counter top right underneath. The newspaper or paper towel will continue to 'pull' moisture from the sponge until they are saturated or until the sponge is just damp. The same thing happens in a raised bed over clay.

The FIRST part of a clay soil to dry is the upper surface. The water in the raised bed either FLOWS out of the bed over the surface of the clay, or it is absorbed by the surface of the clay as the surface dries. In either case, water usually exits a raised bed over clay quickly because of a combination of lateral flow AND evaporation from the surrounding surface. If you unfold 1 sheet of newsprint to represent the soil surrounding a raised bed, and set a saturated sponge in the middle, the newspaper soon pulls all the excess water from the sponge. Because the evaporative surface of the newsprint is far, far larger than the sponge, evaporation occurs very quickly.

IF, though, you double dig and AMEND beneath your clay beds, you create the bathtub effect which in large part negates the lateral flow AND evaporation, causing problems related to the the soil beneath the bed remaining wet for extended periods because you now have to depend on extremely slow percolation through the clay for the water to move out of the soil.


Linda's analogy of pearls & mud is a good way to understand that you cannot start with a heavy container soil and amend it to reduce the ht of the PWT, improve drainage (flow through rates or improve aeration. It is only AFTER you have added much greater than a 50% fraction of the larger particles that these areas begin to improve. At that point, because you have more than 50% larger particles, you/re actually amending the LARGE particles with a lesser fraction of peat/compost/ or other fine amendments. That's a very important concept - good job, Linda!

As you saw, though, raised beds and containers are very different insofar as how water behaves. Raised beds are almost the same as growing in the ground, where container culture is much closer to hydroponics than conventional 'in-ground' gardening.

Great thread & a good discussion Linda and the rest of the participants. Lots of good info being exchanged!!

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