more obvious than along the wider stretches of the
Amazon River. At midday, skies immediately above the
great river tend to be clear and blue, but should you
look over the distant forest on either of the riverbanks
you will likely see big, puffy white clouds, formed by
the condensing moisture transpired by the forest; you
are literally watching the forest breathe (plate 6- 4).
Indeed, studies show that approximately 50% of the
precipitation falling on the Amazon Basin is directly
recycled via transpiration from the vegetation.
Roots acquire essential minerals by transpirational
uptake of water from soil. But transpiration can be a
mixed blessing. Plants may lose too much water when
subjected to constant high temperature, which causes
desiccation. Many tropical plants retard evaporative
water loss both by closing their stomata (openings on
the leaves for gas exchange) and by producing waxy
leaves that inhibit excessive evaporation of water.Leaching
High rainfall typical of the tropics in rainy season
washes essential minerals and other chemicals from
leaves, a process called leaching. Leaching is especially
severe in areas subject to frequent heavy downpours.
The protective waxy coating typical of tropical leaves
contains lipid- soluble (but water- insoluble) secondary
compounds, such as terpenoids, that act to retard water
loss and discourage both herbivores and fungi. Drip
tips (the sharply pointed tips of many tropical leaves)
likely reduce leaching by speeding water runoff. Such
adaptations enable a typical tropical leaf to retain both
its essential nutrients and adequate moisture.
Rainfall leaches minerals from soil, washing them
downward into the deeper soil layers. The degree of
leaching in any area is in part related to soil particle
size. The largest soil particle size is rocky gravel, but
gravel is not a prominent part of tropical soils except
in regions with recently formed volcanic soils. More
typically, soil is a mixture of sand, silt, and clay. Sand
particle size ranges from 2.0 (0.07 in) to 0.5 mm. Silt
particles are dust- like, ranging from 0.5 to 0.002 mm.
Clay particles are microscopically small: coarse clay
ranges from 0.002 to 0.2 μm (micrometer) and fine clay
is less than 0.2 μm. Tropical soils are typically rich in
clay, the structure of which strongly affects leaching as
well as other characteristics, such as making tropical
soils slippery when wet.Clay particles, like humus, have negative electrostatic
charges that attract minerals with positively charged
ions, or cations, such as sodium, magnesium, calcium,
and potassium. Clay also attracts alkaloid compounds,
such as those produced in most tropical leaves, because
these compounds also carry positive charges. Because
rainfall adds positively charged hydrogen atoms to the
soil, these abundant H+ ions exchange with those of
elements such as calcium or potassium, which, when
free, then leach to a deeper part of the soil or may
leach out of the soil entirely by washing into streams
and rivers. This process has the potential to reduce soil
fertility. Rainfall also influences soil acidity, because the
accumulation of hydrogen ions, either on humus or clay,
lowers the pH (the definition of pH is hydrogen ion
concentration), thus increasing the acidity of the soil. In
the tropics, the combination of high temperatures and
heavy rainfall may cause much leaching and result in
strongly acidic soils. The pH of tropical soils is usually
but not always acidic, typically ranging between 4.0 and
6.0 (but it can be less than 4.0 or as high as 6.7).
Amazon soils are typically mineral- poor, high in
clay, acidic, and low in available phosphorus, and the
nutrient- poor nature of the soil is a limiting factor to
plant productivity. A study by a team of researchers
headed by J. J. Nicholaides concluded that nearly 75% of
the soils in the Amazon Basin are acidic and generally
infertile. Age also affects soil mineral content. Older
soils are more leached of minerals than younger soils.
This reality is all the more interesting because although
much of the soil is considered nutrient poor and acidic,
forest plants have nonetheless adapted to thrive on
most of these soils. How do they manage that?Plate 6- 4. A wide point in the Amazon River at midday. Notice
that clouds have formed over the forest but not over the
river. The clouds reflect the transpiration process vital to the
physiology of the forest trees. Photo by John Kricher.chapter 6 essential dirt: soils and cycling 83