Page 239 - Environment: The Science Behind the Stories

leached from the E horizon include iron, aluminum, and sili- Soil texture Soil texture is determined by the size of parti-
cate clay. In some soils, minerals may be leached so rapidly cles (Figure 9.6). Clay consists of particles less than 0.002 mm in
that plants are deprived of nutrients. Minerals that leach from diameter; silt, of particles 0.002–0.05 mm; and sand, of parti-
soils may enter groundwater, and some can pose human health cles 0.05–2 mm. Sand grains, as any beachgoer knows, are large
risks when the water is extracted for drinking. enough to see individually and do not adhere to one another.
A crucial horizon for agriculture and ecosystems is the A Clay particles, in contrast, readily adhere to one another and
horizon, or topsoil. Topsoil consists mostly of inorganic min- give clay a sticky feeling when moist. Silt is intermediate, feel-
eral components, with organic matter and humus from above ing powdery when dry and smooth when wet. Soil with an even
mixed in. Topsoil is the portion of the soil that is most nutritive mixture of the three particle sizes is known as loam.
for plants, and it takes its loose texture, dark coloration, and Soils with large particles are porous and allow water to
strong water-holding capacity from its humus content. The O pass through quickly—so crops planted in sandy soils require
and A horizons are home to most of the organisms that give life frequent irrigation. Conversely, soils with very fine particles
to soil. Topsoil is vital for agriculture, but agriculture practiced have small pore spaces because particles pack closely together,
unsustainably over time will deplete organic matter, reducing making it difficult for water and air to pass through. Thus in
the soil’s fertility and ability to hold water. When a farmer prac- clay soils water infiltrates slowly and less oxygen is available
tices no-till farming, he or she essentially creates an O horizon to soil life. For these reasons, silty soils with medium-sized
of crop residue to cover the topsoil and then plants seeds of the pores, or loamy soils with a mix of pore sizes, are best for
new crop through this O horizon into the protected topsoil layer. plant growth and agriculture.
Soils differ in color, texture, Soil structure Soil structure is a measure of the “clumpi-
structure, and pH ness” of soil. An intermediate degree of clumpiness is gener-
ally best for plant growth. Repeated tilling can compact soil,
The six horizons shown in Figure 9.5 depict an idealized soil, reducing its ability to absorb water and inhibiting the penetra-
but soils display great variety. Scientists classify soils—and tion of plants’ roots.
farmers judge their quality for farming—based on properties
such as color, texture, structure, and pH. Soil pH Plants can die in soils that are too acidic or too
alkaline, so soils of intermediate pH values (p. 46) are best for
Soil color To a scientist or a farmer, a soil’s color can indi- most plants. Soil pH influences the availability of nutrients
cate its composition and its fertility. Black or dark brown soils for plants’ roots. During leaching, for instance, acids from
are usually rich in organic matter, whereas a pale color often organic matter may remove some nutrients from the sites of
indicates leaching or low organic content. exchange between plant roots and soil particles.

100 0
90 10

80 20
Clay 70 30 Silt
(less than 0.002 mm Clay (0.002 – 0.05 mm
Percent clay 50 Silty 50
diameter) 60 40 diameter)

Percent silt
40 Sandy clay 60
clay
Clay loam Silty
30 Sandy clay loam 70 Figure 9.6 The texture of soil
clay loam depends on its mix of particle
20 Loam 80 sizes. Using this diagram, scientists
Silt loam
10 Sandy loam 90 classify soil texture according to the
Loamy Silt proportions of sand, silt, and clay.
Sand
sand
0 100 After measuring the percentage of
100 90 80 70 60 50 40 30 20 10 0 each particle size in a soil sample, a
Percent sand scientist can trace the appropriate
white lines inward from each side
of the triangle to determine texture.
Loam is generally best for plant
growth, although some plants grow
Sand better in other textures.
(0.05 – 2 mm
diameter) What type of soil contains
20% clay, 60% silt, and
238 20% sand?

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