Tropics poor soil?

bigeasyjock(z8Ms)August 7, 2004

Sort of a botany question. I've always wondered why if most of the world's plants and animals are in the Tropics how come the soil is always said to be very poor? You would think with all the thick jungles around the compost would be miles deep. Is it due to a faster rate of decomposition?


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The soil is very poor because as soon as anything dead drops down to the earth it is immediately decomposed and taken up by the roots.

    Bookmark   August 7, 2004 at 4:26PM
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Yep thats the way I understand it ... but keep in mind fast nutrient cycling and low nutrient levels in the soil does not mean poor soil ... it simply means the bulk of minerals needed for plant life are stored in the plants not the soil ... some soils have high levels of certain minerals but plants are unable to absorb these minerals from the soil since they are "locked" into the soil or unavailable to the plant ... that could be a poor soil condition.

Good Day ...

    Bookmark   August 7, 2004 at 5:04PM
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Thats what I thought. I've always heard of the slash and burn method of farming leaving the soil in ruin due to erosion and poor soil starting quality. Like carbon is locked up in plants and the soil, so is the fertility of soils in areas of fast nutrient cycling.

    Bookmark   August 7, 2004 at 5:10PM
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garyfla_gw(10 Florida)

Another factor to this is rainfall. Check out some of the totals along the Orinoco or Amazon rivers. Many of these areas are devoid of soil. One area that interested me is where the Orinoco joins the Amazon.The difference between high and low levels is 70 feet!! Obviously any type of soil would be washed away.

    Bookmark   August 7, 2004 at 8:03PM
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froggy(z4/5 WI)

ok here is a few things to think about from a prairie botanist.

we all know how thick and lush rainforests can be...

now, imagine how thick and lush the soils of the great plains, in particular the tall grasses prairie lands of Iowa and east. an amazing cauldron of inhert and organics. Deeper and thicker than a steel plow could go, unless ofcourse they do it year after year...
the soils of the prairies are said to be like iceburgs, u only see 10% of what really hit the titanic. i like to say that the green stuff on the surface are solar panels and sex organs, the real meat of the prairie lies under. ive heard estimates that the biomass under soil surface = 85% vs above and i feel that is an accurate # of a mature prairie.
want an example? what is the biomass of the fruiting body of a fungi vs the vast underground structure or a butterfly vs a mole :)

here is something to chew on...

as plants ever evolve and find their way north/south into harsher and harsher areas of the world (given that diversity lessens as u move N/S), life has learned to 'dig in' to use more of the year's energy before the tilt freezes our part of the world.

its still a highly diverse ecosystem, its just that u cannot see it without a shovel.


    Bookmark   August 7, 2004 at 8:57PM
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garyfla_gw(10 Florida)

Much yet to learn isn't there?? The N.American great plains are almost unique when compared with other plains areas.First they are larger and almost contiguous unlike
the pampas and steppes areas. The African veldt is broken up much more and of course the climate is much different.
I've heard explanations that N.America was almost completely devasted by the last iceage so therefore much younger.Must certainly be true that the plains exist because of the iceages All the geological features certainly point that way.
Another thing is that the great plains has almost entirely been wiped out in the last 150 years by farming.
Doesn't even faintly resemble what was there before Lewis and Clark. Particularly the flora.

    Bookmark   August 9, 2004 at 2:28AM
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Can you give an explanation why the Plains had no trees? After all, trees grow in some of the harshest climates and most inhospitable places. josh

    Bookmark   August 9, 2004 at 5:20PM
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Gary, when the Mississippi flooded (before all the levees were built), didn't it deposit silt and therefore formed very good farming land? Wouldn't this happen to some extent with the Amazon/Orinoco Rivers? A lot washed away but as the waters receded leaving some behind? Do old maps show big changes in the width of the rivers...still wider each year? josh

    Bookmark   August 9, 2004 at 5:28PM
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froggy(z4/5 WI)

Can you give an explanation why the Plains had no trees?

simply, too much fire and not enough rainfall.

less rain = more grass and more fires. savanna and prairie ecosystems are fire dependent ecosystems. and ofcourse most forests are not :)

i live right on the edge of the prairies and the great piney north. we call it the tension zone. to the south are drier, warmer and grassy mainly cuz of rainfall and the cycle of burning grasses. to the north (and east) is heavier rains and less fire. i dont have an inverse chart of fire in the grassland vs rainfall, but im sure someone does. as u move from the rain shadow of the west twords the east, rain and prairie gets ever denser until u run into maybe ohio-ish, until the rain supports tree growth and thus less prairie.

something to think about, just think of the incredable water sink of the 10ft thick soils before the steel plow...


Here is a link that might be useful:

    Bookmark   August 9, 2004 at 9:26PM
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garyfla_gw(10 Florida)

I'm particularly interested in the last ice age and the impact on specie distribution within present day N.America.
Geologists say that the arctic circle extended to central Texas with pack ice reaching miles in depth as far south as
present day Kansas. Okay this would explain why the great plains is so large and flat and why the Mississippi is where it is,Why the great lakes are so large and the location. Why there is such a concentration of soil in this area. The massive specie extinction. The timeline of the duration is around 5000 years beginning 25000 years ago the
present day stable period at around 10000 years ago.
What I find hard to grasp is how to start an iceage and then reverse it in such a short period? After all , the most noticeable factor on specie distribution is the weather. There must be some laws of physics that are totally unknown.??

    Bookmark   August 10, 2004 at 6:17AM
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I remember hearing of theory that the ice ages are triggered by a change in the ocean currents. I don't remember details but it had to do with a chilling of the mid Atlantic currents. Once the current reached a certain low temp it triggered a global chilling that triggers a continued chilling of the M.A. current and so on becoming ever colder. I think it was called the "snowball effect" or some such.
Remember also the tilt of the Earth varies over time and this has an effect on local global temps.
Keep in mind also that I have no idea what I'm talking about. Rememeber a little knowledge ... can be a dangerous thing ;>

    Bookmark   August 10, 2004 at 7:19AM
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Froggy, Browsing for more info on how the prairies were formed, I found a great site about how the Ice Age affected Wisconsin. Surprised to learn there are even parts of the state which were protected from the glacier. You're probably familiar with this site...but I thought others might be interested. Now I promise I won't highjack any more on this thread. josh

Here is a link that might be useful: Wisconsin

    Bookmark   August 10, 2004 at 8:47PM
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TonyfromOz(z10 NSW Aust)

Back to the tropics . . .

At least 3 factors contribute to low soil fertility in rainforests of the Amazon, west Africa and much of southeast Asia.
1. ancient land surface, unaffected for millions of years by glaciation, vulcanism, or extreme upthrusting from plate collision
2. combination of high rainfall and warm temperatures, resulting in deep leaching of nutrients over very long period
3. most plant nutrients locked up in forest biomass, probably also due to sustained mesic conditions and hence long-lived plants. Slash and burn agriculture returns some but not all nutrients to soil - some lost in smoke.

I think this is the conventional wisdom among ecologists and soil scientists, though I may not be completely up to date. Seems plausible though.

    Bookmark   August 12, 2004 at 7:02AM
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garyfla_gw(10 Florida)

If you ever get the urge to actually see some of the tropics first hand and see some of these principles in action . Go to Costa Rica. In an area the size of west virginia There are 7 major climate zones with 100's of microclimates. Everything from montane rainforest to salt floodplains.From permanent frost to permanent tropics
Boggles the mind with specie diversity particularly with plants and insects. Only problem is there is so much to see
there's no way to see it all.Let alone understand it.
I've spent as long as 3 days looking at less than an acre!! Still feel I got only an overview of what's going on.
If you're interested in tropical botany a must see!!

    Bookmark   August 13, 2004 at 4:50AM
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