Soil
Soil has an important part to play in the production of horticulture crops. The soil type helps to determine what can be grown on a particular site.
Plants need soil to supply them with water and nutrients. Roots need a supply of air and soil to hold them firmly in the ground.
Understanding soil will help you grow more productive crops.
How soil is made up (soil components)
Inorganic matterOrganic matter
Air
Water
Soil organisms
Inorganic matter
This is part of the soil that has never been living.
This is part of the soil that has never been living.
Inorganic matter comes from rocks that have been weathered and broken down into smaller particles.
Types of inorganic matter in the soil are classified according to the size of the particles. These include sand, silt, clay and inorganic materials.
An ideal soil for growing crops will contain about 45% inorganic matter.
Organic matter
Organic matter includes: compost, dead plants, dead animals and animal manure.
Decomposers such as bacteria and fungi help to break it down (decompose it). Organic matter that is completely decomposed forms a dark, brown coloured material called humus.
When organic matter and humus are added to inorganic matter, soil is formed.
Organic matter makes up about 5% of an ideal soil.
Poor, unproductive soils often lack organic matter.
Air
Pores containing air or water spaces
Plant roots need air to respire 
and grow properly.
Air is also needed for soil organisms to survive.
Air is found in the spaces between the soil particles. These spaces are called pores.
Water
Water also sits in the pores between soil particles. It is found clinging to the outside of soil particles and in the smaller pore spaces.
Water is used by plants for photosynthesis and is lost through transpiration.
Soil water also contains dissolved minerals that can be used by plants.
If there is too much water in the soil there is no room for air. If there’s no air the plants die because the plant roots can’t respire.
On the other hand if there’s too little water in the soil the plants wilt and can die. This is because plants continually lose water to the air in the process of transpiration.
Plants grow best if there’s a good balance of water and air in the soil.
Soil organisms
The soil has many organisms living in or on it. The larger organisms play a part in soil formation because they stir up the topsoil and open it up to the air and rain. Then when they die their bodies form humus in the soil.
The larger organisms include worms, insects, insect larvae, slugs and other molluscs and some fungi.
Micro-organisms such as bacteria and fungi cause the decay of organic matter. The actions of the micro-organisms release nutrients into the soil. The plant roots can then take up these nutrients.
Some fungi and bacteria cause plant root diseases.
Complete these activities.
Fill in the gaps in the diagram below to show each soil component and its percentage value. One has been done for you.
Drag and drop each soil component in the right-hand column to its correct description.
How soil is formed
The two main processes constantly at work forming soil are weathering, and the addition of organic matter.
Weathering
The weathering process is the first stage in the formation of soils.
The earth is made of rock. When the earth’s surface is exposed to the action of the weather it’s gradually broken down into smaller and smaller particles.
The rock that a soil is formed from is called the parent rock.
Weathering gradually over a long time breaks down the parent rock into smaller pieces called parent material. It’s rather rocky, but some plants can grow in it, for example, gorse.
Weathering breaks down the parent material into smaller and finer particles called subsoil. It’s more suitable for plants, but isn’t soil yet.
The main weathering agents are:
- air
- water
- changes in temperature.
Air and water act on the minerals in rocks and this makes it easier for wind to wear them away.
Water carries away the broken pieces and grinds them smaller.
Water can also attack the chemical structure of rocks, breaking them down into smaller particles and making new chemical compounds.
The process of freezing and thawing of water in cracks in rocks can break up rocks, especially if it’s frosty after a hot day.
Press the Start button below to watch this process in action.
Erosion is where weathered particles are carried away. Erosion affects soil depth. How steep the land is affects the amount of erosion that occurs. On steep land more particles are washed or blown away. Often the soil from hills ends up on flat land so the soil depth there is often deeper.
Organic addition
The addition of organic matter to inorganic particles is the second stage in soil formation. As weathering carries on and the rock particles become finer, more plants establish themselves. Animals appear because they now have something to eat. As plants and animals die, their bodies decay and form humus. This makes the parent material darker. Once this dark layer can be seen, even if it’s quite thin, we have soil.
Inorganic material + organic material = soil
Soil formation is a natural process. It takes place slowly, over a long period of time.
Key points
Soil contains:
- inorganic material including sand, silt and clay particles
- air spaces
- water
- organic material including dead plants and animal material decayed by micro-organisms to form humus
- small animals and insects
- spaces between soil particles called pores.
Weathered inorganic material + organic material = soil.
What's next?
Go to: 2 Soil profiles.
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