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Re: [SANET-MG] Fwd: Re: Soil fertility....

Hi Michael,

WRT tropical soils

> There is little or no "soil", in the sense of nutrients not in
> current use.

This is an overgeneralization.  There are many decent soils in the
tropics, and some extremely poor soils in temperate regions.

> Strip away the cover of life and you get an inert material called
> laterite. This rapidly turns to brick if it isn't covered up again
> by its protective coating of life.

Soils prone to plinthite (laterite) formation are not very common, even
in those few tropical regions infamous for having them.  In those
areas, extent ranges from 5 to 15% according to Sanchez in "Properties
and management of soils in the tropics".

When I was at CIAT in the Cauca valley in Colombia I saw only deep,
black vertisol clay (physically idiosyncratic, but very productive).
Further East I saw mainly highly aggregated oxisols (low CEC, but
physically good).  In the mountains around Medellin I saw mainly dark,
rich, friable, volcanically-derived soils.  I'm sure these snap up
phosphorus like crazy, but the small farmers were growing real nice
maize, beans and all sorts of vegetables.

> Amazonian primitives practise slash and burn agriculture not because
> of some ancestral wisdom, but because it's the easiest way to get
> rid of that protective ground cover and expose a patch of land not
> being used for other purposes. It works so long as there is so much
> land available that small groups can move to a fresh area once the
> patch they're tending plays out and becomes barren...
> The long term prognosis is soil sterility.

Not with judicious use of lime, fertilizer (esp. P) and keeping the
soil covered by mulch or crop canopy.  The biggest problem, and the big
reason for shifting cultivation is weed pressure.


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