Geography Reference
In-Depth Information
Green plants cannot grow properly without 16 essential ele-
ments available in the correct proportions. The availability of these
essential nutrients for plants is influenced by soil pH. On the basis
of their concentration in plants the 16 elements are divided into
macronutrients (carbon, oxygen, hydrogen, nitrogen, phosphorus,
sulphur, calcium, magnesium, potassium and chloride) and micro-
nutrients (iron, manganese, zinc, copper, boron and molybdenum).
As a comparison, the typical amount of potassium in a soil is 1.5
per cent of the total mass whereas molybdenum only accounts for a
hundred thousandth of a percentage point. A pH range of six to
seven is generally best for plant growth as most plant nutrients are
readily available in this range. High soil pH results in phosphorus
and boron becoming insoluble and unavailable to plants. Most
nutrients are more soluble in low pH (acidic) soils, which can result
in high or toxic concentrations of them. On the other hand phos-
phorus and molybdenum become insoluble at low pH and are
unavailable to plants.
Humans and soil
Human activity can alter the soil. It is thought that the total global
area of soil degraded by humans (over 20 million square kilometres)
exceeds the amount currently being used for farmland. This degra-
dation is due to deforestation, overgrazing and poor agricultural
management. Degradation includes soil erosion, soil acidification,
soil pollution, the reduction of organic matter content and
salinisation
.
Over the last half century it is thought that between one-third
and a half of the world's arable land has been lost by erosion. While
erosion is a natural process, humans have dramatically accelerated
this process. Farmland is often left bare or has a low vegetation
cover for a considerable period of the year and old obstacles to
erosion like large woody plants have often been removed. This
makes it easier for the wind or wash processes to transport surface
sediment. Tractor wheels also compact the soil making excellent
fast routes for water flow over the surface often accelerating
erosion. Accelerated soil erosion results in a reduction of soil depth
because soil is lost at a faster rate than it is being formed, often with
the more organic rich upper layers being lost first. The erosion can
