Andosols - Major Soil Groups of the World - page 316

becomes crumb structure. This is granulation . The soil then becomes
powdery, formed of composite grains of silt and often fine-sand size
(10 to 200 µ m, mean 40 µ m). These grains 'float' one over another
and offer hardly any resistance to erosion by concentrated runoff. In
this condition, Andosols are also susceptible to aeolian erosion. All
these prohibit the ploughing of slopes with a gradient that theoretically
permits mechanization. These slopes are exploited manually.
Lack of depth is often insurmountable. Soils that are too shallow are
used extensively for pasture.

In Central America and South America (the Andes), some volcanic
materials of pyroclastic nature have been consolidated during their
deposition in the form of tuffs over the past more than 10,000 years.
They form beds on the flanks of volcanoes, parallel to the slope. Their
stratification favours lateral circulation of water and ions.
On summits, in perhumid climate, these tuffs are partially weathered
to allophane and halloysite.
On the other hand, on piedmont slopes characterized by a long dry
season, tuffs are subject to induration by silica (opal) and sometimes by
calcite, products released at the same time as clay minerals (halloysite,
smectite) are formed. Fragic, duric and petrocalcic horizons are found.
They can be exploited for extracting construction materials. But the
overlying soils have often eroded and the indurated substratum was
exposed over large areas. These are Andean cangahua and tepetates in
Mexico and Central America (Zebrowski and Quantin 1991; Winckell
and Zebrowski 1992; Quantin and Zebrowski 1996; Quantin 1997).
Solutions have been found for rehabilitating these areas for agricultural
use: crushing of the naturally indurated materials, fertilization, organic
amendments (Podwojewski and Germain 2005), anti-erosive cultural
practices, adaptive crop rotations…

Compaction (Quantin 1997)

10.6.3 Agrochemical problems
The fertility of Andosols is largely conditioned by the natural fertility
or poverty of the volcanic parent materials that underlie them. We could
draw up some general remarks but for want of space to develop them.

Increase of pH and thus of the CEC through liming amendments is
difficult because of the strong adsorption power of the material. Also
there is risk of increase of deficiency in K and Mg.

Retention of cations and anions