CHAPTER 3
AGRICULTURE AND THE ENVIRONMENT
Learning outcomes
• By the end of this chapter, you will be able to:
• describe what soil is made of
• describe the components of soil that make it good for plant
growth
• explain how sandy and clay soils are different
• discuss the different types of agriculture
• discuss how agricultural yields can be improved
• discuss the impact agriculture has on people and the
environment
• explain the causes and effects of soil erosion and how it may be
reduced
• discuss different ways in which agriculture can work
sustainably.
Food for thought
Whether measured by the area of land used or the numbers of people
involved, agriculture, the provision of food, is the largest industry on the
planet. It is not surprising, therefore, that it can potentially have a major
environmental impact. Knowing about all the potential effects and how they
can be managed is of great importance to the health of the planet. The
challenges are large and the solutions are complex: the human population is
growing rapidly and needs to be fed, and existing farmland is often overused,
depleted of mineral nutrients and suffering from soil erosion.
What are possible answers?
Cutting down more forest to
turn into farmland will mean
a reduction on natural habitat
for other animals and plants.
Increased use of artificial
fertilisers and pesticides will
increase the amount of food
but can also damage the
ecosystem.
More irrigation will help crops grow better but can also mean less drinking
water for the increasing human population. Some parts of the world are
already short of water.
Should we be banned from eating 'luxury
foods' so that the resources can be used
to produce larger quantities of staple
foods that will feed a larger population?
Would such an idea cause riots and wars?
What about freedom of choice?
There are no easy answers, but some creative solutions to the problems are
presented in this chapter. This chapter will help you understand the
fundamentals of the precious resource soil, how it can be used and misused.
This understanding will help you identify strategies that are needed for feeding
the world population now and in the future.
PART 1: THE SOIL
Soil is fundamental to the growth of plants, and ultimately the success of all
other living creatures. It can be argued that a person's standard of living is
determined by the quality of the soil where he or she lives, and the types of
plants and animals that are farmed there as a result.
Soil is a natural resource and needs careful management to keep it in good
condition.
What is soil?
Soil is a habitat for plants and other organisms that grow within it. It is made
up of a number of components. There are four main groups:
• mineral particles, which are a combination of rock fragments and other
smaller inorganic (non-living) items
• organic content, which is a mixture of living plants and animals and their
dead remains
• air, which is held within the spaces (pores) between the particles and
organic content of the soil
• water, which is also held within the soil pores and is the water that is
available for plants to grow.
The proportion of each of these four
components will vary depending on the
type of soil and the way it has been
managed, the local climatic conditions and
the size of the mineral particles.
Figure 1 shows the composition of a
typical cultivated soil. In most soils the
volume of solid material (organic content
and mineral particles) will be in excess of
50–60%. The pore spaces will contain a
mixture of water and air, and the
Figure 1 The proportions of components in a typical
cultivated soil. Note that the proportions of air and
proportions of these will vary constantly
water will vary depending on plant water use and depending on rainfall and the uptake of
rainfall.
water by the plants' roots. The amount of
air in the soil will also be reduced by soil compaction and increased with
cultivation.
Where do the soil components come from?
The mineral particles occupy the largest volume within soil. The particles are
formed from the weathering and erosion of the parent rock (the rock
underlying the soil). Over time these rocks are broken down into smaller and
smaller particles. This process is known as weathering. The movement of these
fragments (and other soil components) is known as erosion. The weathering of
rock can take a number of forms.
Physical weathering is often
caused by frost, heat, water
and ice or wind. The rock can
be broken down in a number
of ways, such as water
expanding as it freezes
within cracks in a rock, rocks
heating up, causing them to
expand and crack, or wind
blowing fragments of rock
that collide with other rocks,
Figure 2 Landforms created by the process of physical weathering
wearing them away. Glaciers
also cause weathering as
they move through a valley: the large volume of ice causes friction against the
rocks, wearing them away.
Chemical weathering can be caused by carbon dioxide (in the air) combining
with water to form a weak acid, carbonic acid. This acid can react with alkaline
minerals in the rock, causing the rest of the rock to crumble. The increase in air
pollution has also increased the amount of other acids within precipitation,
often described as acid rain. These acids also attack the surface of the rock,
causing it to break down.
Biological weathering is caused by processes such as the growth of plant roots
into the cracks in rocks, causing the rock to split open and small fragments to
break off. Rocks can also be broken by the movement of animals across them.
Organisms in the soil can also produce carbon dioxide, which, when combined
with soil water, will form carbonic acid, adding to the chemical weathering
effect.
The word organic means derived
from living things. The organic
component of the soil is a mixture
of living plants, animals and
microorganisms along with their
dead remains. Many of the
organisms present in soil have a
role in the decomposition of
these remains, releasing nutrients Figure 3 Earthworms are very important for soils.
that are then available for use by
plants. These processes and the movement of soil animals (such as
earthworms) make channels within the soil that allow air and water to be held
in the soil pores (spaces). Air enters the soil by diffusion; water enters the soil
when there is precipitation or when the soil is irrigated.
KEY TERMS Particle size
Weathering: the processes that The size of the soil particles has a major
cause rock to be broken down into
smaller particles effect on the properties of the soil: how well
it holds or drains water, its capacity to hold
Erosion: the movement of rock and
soil fragments to different locations mineral nutrients and the ease with which
plants can grow in it and be cultivated.
Organic: derived from living
organisms
Soil mineral particles can be classified according to their size into three groups:
• sand
• silt
• clay
Sand is the largest mineral component of soil. The International Society of Soil
Science (ISSS) describes sand as having particles between 2.0 mm and 0.02 mm
in size. It is easy to see the individual grains with the naked eye, and sand feels
gritty to the touch. The large size and irregular shape of the particles mean
that they do not pack together easily, so there are large pores between the
grains of sand; this allows water, air and soil organisms to move through sand
easily.
Silt particles are smaller than those of sand, being between 0.02 mm and
0.002 mm in size. Silt feels silky or soapy to the touch and when rubbed
between the fingers the particles slip over each other easily, unlike sand
particles.
Clay particles are the smallest mineral component, being smaller than 0.002
mm in diameter. Their small size means that they pack closely together, which
reduces the ability of water to drain through the soil. When damp, clay
particles feel very sticky and are easily moulded; when dry they stick together
solidly. This is why clay is used to make pottery: imagine trying to make pots
out of sand! Clay particles have surface electrical charges that affect the way
they behave and the way they link with and release nutrients. Table 1
compares the characteristics of sand, silt and clay.
Particle type Size of particle Texture Characteristics
Sand 2.0–0.02 mm Feels gritty Large pore sizes
Drains well
Contains large air spaces
Silt 0.02– 02.002 mm Feels silky or Less friction than sand
soapy Particles slippery
Clay <0.002 mm Sticky when Particles held together
wet tightly
Poor air spaces or drainage
Forms a hard mass when
dried
