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pnbrown

riddle me this

16 years ago

So I'm at my central fla place and it had rained pretty close to two inches by this morning since yesterday morn. I was digging in the garden and I noticed that only the top half to three-quarters of an inch of sand is soaked, with a sudden transition to quite dry sand. This is an area that was slightly bowl-shaped, so it isn't a case of the water running off. I checked around different areas, and it's the same deal everywhere except for spots that catch a lot of run-off.

So how does two inches of rain get captured in a half-inch of sand? I was under the impression that florida sands are extremely highly drained, but this evidence seems to contradict that. It seems more likely that the water sits right at the top on flat areas and either evaporates quickly when the sun comes out or is transpired by shallow-rooted plants if it remains overcast for long periods. I would also guess that this condition encourages trees and shrubs to be shallow and widely rooted, to use the water quickly before it evaporates since it does not penetrate.

I'm thinking that young trees need a lot of mulch around them and not grass, more than I may have thought.

Comments (6)

  • 16 years ago

    That 2 inches of water had to have gone somewhere. Maybe there is one spot in the bowl-shaped area that drains the water quicker than where you checked.

    At a previous home (not far from here) the soil/sand in the yard almost seemed to be water-repellent. The water actually seemed to bead up on top rather than soak into the soil. The soil there seemed to be sand mixed with a fine dust. After a heavy rain you could scratch the surface and the soil would be bone-dry underneath.

    At my current home the soil absorbs water much better. The top few inches is a dark mixture of sand and organic matter and deeper than that it is more of pure sand consistency. It seems to stay evenly moist.

    John

    John

  • 16 years ago

    Yeah I think John is right on. I've seen the top layer of sand quite dry after a good rain too. Talk about fast drainage! The rain seems to find "leaks" that suck up the water faster thereby missing that top layer.
    Organics is the key. Also the mulch will help the top layer stay damp instead of the super dry that seems to repel the water. IMO

  • 16 years ago

    I went back to that bare area just now, where I had scraped off giant weeds a few days ago, and dug in a few different places. There is still the distinct soaked top like a crust, consistent all over, but it does seem like deeper down some places are damper than others. The area is flat, but of course on a scale of feet there are places higher or lower by a few inches.

    Then I dug into the bahia sod in a few places, and one would be hard pressed to imagine that there was heavy rain recently. Due to the total diffusion of the root-mass, though, it seems that the moisture spreads more evenly in a heavy sod. And yes, of course other places like mulch around trees with high OM were much more wet.

  • 16 years ago

    It's what we call "waterproof dirt", once it dries out it is hard to wet it again. I have noticed something strange while hand watering in an area that has been cultivated, but allowed to dry out. The water will stand on top until a tunnel opens up and the water gushs down into a water "black hole". After that the hole closes up and all you see is a slight depression. Sometimes I wonder if I'm setting on top of a sinkhole.:>( I have used a little liquid soap in a hose end sprayer to wet it.
    john

  • 16 years ago

    johnjsr, I have the same black holes in my yard. Also the ground is spongy all over my yard. I always thought I had a serious mole problem...maybe it is the sink hole effect?

  • 16 years ago

    Yesterday I dug a trench a foot-and-a-half deep and two feet wide. I put about six inches of mushroom compost in the bottom, soaked it good, then about six inches of sand, another couple inches of compost, water, then filled it up the rest of the way with sand leaving it slightly concave on top. Sowed it with chinese cabbage and lettuce.

    My thought is that the thick layer of compost at the bottom will catch and store all the rainfall and/or irrigation water, while the foot or more of sand on top will prevent evaporation. Once the crop is large and rooted into that layer it should be pretty well drought-proof. The thin middle layer will act as a way station for the roots on their way down.

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