Unit 3: Agriculture and the environment
3.1 The soil
Soil is a habitat for plants and other organisms that live within it. It's made out of four
components:
- Mineral particles, a combination of rock fragments and other small inorganic items
- Organic content, 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
- Water, which is also held within the soil pores and is the water available for plants to
grow
The proportion of these components will vary depending on the type of soil, the way it’s
managed, the local climatic conditions, and the size of the mineral particles.
Where do the soil components come from?
The mineral soil particles take the most space within the soil. They 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 pieces, this is called weathering. The movement of
these fragments (and other soil components) is known as erosion. There are three types of
weathering:
Physical weathering is caused by frost, heat, water, ice, or wind. Eg: water expands the rock
as it freezes within its cracks, rocks heat up causing it to break, etc.
Chemical weathering can be caused by CO2 combining with H2O to form a weak acid
(carbonic acid). This can react with alkaline minerals in the rock, causing the rest to crumble.
Acid rain attacks the surface of the rock, causing it to break down.
Biological weathering is caused by processes such as the roots of plants growing into the
cracks in rocks, causing them to split open and small fragments fall off. The movement of
animals across them also acts as weathering.
Organic means delivered 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 have a role in the decomposition of their dead remains, releasing
nutrients that are available for plants to use. These processes and the movement of soil
animals (earthworms) create channels that allow air and water into the soil. Air enters by
diffusion, while water enters when there’s precipitation or irrigation.
Particle size
The size of the soil particles has an effect on the properties of the soil: how well it holds or
drains water, its capacity to hold mineral nutrients and the ease with which plants can grow
on it and be cultivated. There are three types of soil: sand, silt and clay.
Sand: The large size (2.0-0.02 mm) and irregular shape of the particles mean that they do
not pack together easily, so there are large pores between the grains that allow water, air
and soil organisms to move through the sand easily.
Silt: The particles are smaller than those on sand (0.02-0.002 mm). It feels silky or soapy to
the touch and when rubbed between the fingers the particles slip over each other easily,
unlike sand particles
Clay: Has the smallest size of particles (less than 0.002 mm). They pack together, reducing
the ability to drain through the soil. It feels sticky when wet and it’s easily moulded. When
dry, they stick together solidly forming a hard mass.
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, 3.2 Soils for plant growth
Most plants require a combination of factors to grow successfully:
- The availability of mineral nutrients
- Anchorage to hold the roots securely in one place
- A supply of water
- Oxygen around the roots to enable the root cells to respire
The availability of nutrients is the most important factor and the one that a farmer can have
most control over, by using fertilisers.
Mineral ions
Plants aren’t able to consume mineral nutrients in the same way animals do. Plants require a
supply of nitrogen, phosphorus, potassium, and other elements to construct proteins and
other chemicals. These mineral nutrients are obtained from the roots. Plants take the oxides
of nitrogen and phosphorus, and the potassium ions. The roots contain an electric charge
that enables the charged ions to bind with them and be taken up for use within the plant.
Soil pH
The uptake of nutrients by plant roots is also affected by the pH of the soil, a measure of the
acidity or alkalinity (1-14). This level depends on the type of parent rock and the pH of the
water that flows in the area. Every plant has its own preferred soil pH. The pH will also affect
the availability of the mineral nutrients within it because some ions are only available at
certain soil pH levels. This information gives the farmer two options to maximise their yield:
either they check the crop and look for deficiencies, and apply the relevant mineral nutrient
as it’s needed. Or they try to change the soil pH using specific fertilisers that have an
acidifying effect or make it more alkaline while using substances such as ground limestone.
Soil organic content
The organic content of soil is a combination of living organisms together with their dead
remains. The organisms that live within it form part of complex interactions within food webs
and the recycling of organic materials for future use. There are different types of
decomposers that return the nutrients of the dead matter back into a form the plants can use
(earthworms, fungi, bacteria, etc). High levels of organic matter have these positive effects:
- Increased water-holding capacity, so less irrigation is needed
- Increased air spaces, allows air to penetrate and use oxygen for root respiration
- Increased decomposers, so there’s a more abundant food source
- Prevents the loss of mineral nutrients
Sandy soils vs clay soils
The ideal soil is a mixture of sand, silt and clay, which is called loam soil. It has high quality
organic matter and a neutral pH. However this is very rare.
● Air content: Clay soils have small air spaces that can limit the amount of oxygen
reaching plant roots, and therefore limit the respiration and root growth. Sandy soils
have large air gaps allowing good transmission with high levels of oxygen and
therefore a healthy root system.
● Water content: Water tends to cling to the surface of the particles. A clay soil has far
more particles and therefore a larger surface area, compared with sandy soil. Clay
soils hold significantly more water.
Página 2 | 14
3.1 The soil
Soil is a habitat for plants and other organisms that live within it. It's made out of four
components:
- Mineral particles, a combination of rock fragments and other small inorganic items
- Organic content, 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
- Water, which is also held within the soil pores and is the water available for plants to
grow
The proportion of these components will vary depending on the type of soil, the way it’s
managed, the local climatic conditions, and the size of the mineral particles.
Where do the soil components come from?
The mineral soil particles take the most space within the soil. They 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 pieces, this is called weathering. The movement of
these fragments (and other soil components) is known as erosion. There are three types of
weathering:
Physical weathering is caused by frost, heat, water, ice, or wind. Eg: water expands the rock
as it freezes within its cracks, rocks heat up causing it to break, etc.
Chemical weathering can be caused by CO2 combining with H2O to form a weak acid
(carbonic acid). This can react with alkaline minerals in the rock, causing the rest to crumble.
Acid rain attacks the surface of the rock, causing it to break down.
Biological weathering is caused by processes such as the roots of plants growing into the
cracks in rocks, causing them to split open and small fragments fall off. The movement of
animals across them also acts as weathering.
Organic means delivered 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 have a role in the decomposition of their dead remains, releasing
nutrients that are available for plants to use. These processes and the movement of soil
animals (earthworms) create channels that allow air and water into the soil. Air enters by
diffusion, while water enters when there’s precipitation or irrigation.
Particle size
The size of the soil particles has an effect on the properties of the soil: how well it holds or
drains water, its capacity to hold mineral nutrients and the ease with which plants can grow
on it and be cultivated. There are three types of soil: sand, silt and clay.
Sand: The large size (2.0-0.02 mm) and irregular shape of the particles mean that they do
not pack together easily, so there are large pores between the grains that allow water, air
and soil organisms to move through the sand easily.
Silt: The particles are smaller than those on sand (0.02-0.002 mm). It feels silky or soapy to
the touch and when rubbed between the fingers the particles slip over each other easily,
unlike sand particles
Clay: Has the smallest size of particles (less than 0.002 mm). They pack together, reducing
the ability to drain through the soil. It feels sticky when wet and it’s easily moulded. When
dry, they stick together solidly forming a hard mass.
Página 1 | 14
, 3.2 Soils for plant growth
Most plants require a combination of factors to grow successfully:
- The availability of mineral nutrients
- Anchorage to hold the roots securely in one place
- A supply of water
- Oxygen around the roots to enable the root cells to respire
The availability of nutrients is the most important factor and the one that a farmer can have
most control over, by using fertilisers.
Mineral ions
Plants aren’t able to consume mineral nutrients in the same way animals do. Plants require a
supply of nitrogen, phosphorus, potassium, and other elements to construct proteins and
other chemicals. These mineral nutrients are obtained from the roots. Plants take the oxides
of nitrogen and phosphorus, and the potassium ions. The roots contain an electric charge
that enables the charged ions to bind with them and be taken up for use within the plant.
Soil pH
The uptake of nutrients by plant roots is also affected by the pH of the soil, a measure of the
acidity or alkalinity (1-14). This level depends on the type of parent rock and the pH of the
water that flows in the area. Every plant has its own preferred soil pH. The pH will also affect
the availability of the mineral nutrients within it because some ions are only available at
certain soil pH levels. This information gives the farmer two options to maximise their yield:
either they check the crop and look for deficiencies, and apply the relevant mineral nutrient
as it’s needed. Or they try to change the soil pH using specific fertilisers that have an
acidifying effect or make it more alkaline while using substances such as ground limestone.
Soil organic content
The organic content of soil is a combination of living organisms together with their dead
remains. The organisms that live within it form part of complex interactions within food webs
and the recycling of organic materials for future use. There are different types of
decomposers that return the nutrients of the dead matter back into a form the plants can use
(earthworms, fungi, bacteria, etc). High levels of organic matter have these positive effects:
- Increased water-holding capacity, so less irrigation is needed
- Increased air spaces, allows air to penetrate and use oxygen for root respiration
- Increased decomposers, so there’s a more abundant food source
- Prevents the loss of mineral nutrients
Sandy soils vs clay soils
The ideal soil is a mixture of sand, silt and clay, which is called loam soil. It has high quality
organic matter and a neutral pH. However this is very rare.
● Air content: Clay soils have small air spaces that can limit the amount of oxygen
reaching plant roots, and therefore limit the respiration and root growth. Sandy soils
have large air gaps allowing good transmission with high levels of oxygen and
therefore a healthy root system.
● Water content: Water tends to cling to the surface of the particles. A clay soil has far
more particles and therefore a larger surface area, compared with sandy soil. Clay
soils hold significantly more water.
Página 2 | 14
