The big picture

This short video gives some indication of the importance of soil.

People have different ideas about soil. A child sees an endless source of fun digging in search of worms and the just getting dirty. An adult sees it tracked across the floor and knows it has to be cleaned up. A teenager wants to step over it or around it so they don’t ruin their "look". Figure 1 suggests something else about soil. What could this be? Make up your own mind about what soil means to you before you continue with this topic.

Figure 1. From soil comes everything.

We all know what soil looks like. If you have forgotten go outside and find some, feel it, look at it closely and see what is in it, around it or growing on it. Like everything else in ESS soil is in fact a system and an ecosystem. To be a system is must have inputs, flows, stores, and outputs. To be an ecosystem soil must have a living component.

The inorganic part of soil is what we see – the particles that come from rocks or other parent material. Other inorganic components of soil include air and water. None of them are living; the nutrients that must be in there to nourish plants are also inorganic. So where does the living part come in? Lots of large organisms live in the soil e.g. worms, moles spiders and many more. Not only do they make it an ecosystem but they process the soil, change it and sort it into layers.

Soil is dynamic, changing all the time. As rocks weather soil increases however, as wind blows it may be blown away. Water washes out the nutrients but decaying organic matter puts them back again. Plants take nutrients and then die and decompose returning the nutrients back the soil. Plants absorb carbon dioxide from the atmosphere and emit oxygen and then animals reverse the process. The list is endless.

Figure 1 – from soil comes everything, that's a strange idea until you stop and think about it carefully. Plants need soil, and plants are at the base of most of the planetary food chains and webs. No plants mean limited food supply for humans. We are developing ways to grow food without soil e.g. hydroponics but at the moment they are limited.

The qualities of a soil will determine the primary productivity of an area, and therefore its food production potential. The qualities are determined by the soils texture and structure. A good agricultural soil will:

• Act as an anchor for the plants so the soil needs to have good crumb structure for root development.
• Supply the right amount of water so it needs the right texture to hold just the right amount of water but not too much.
• Supply the right amount of nutrients which means it needs to retain the organic matter.
• Not be too freely drained otherwise leaching becomes a problem.

The best soil for agriculture is a loam with a more or less even mix of sand, silt and clay.

Figure 2. A good soil provides everything.

What is soil?

We are all familiar with soil, at least at a superficial level. We know it exists and that it is probably quite important. Do we know what it is? How it is formed? How it is a system? The answer to most of these questions is probably no.

Soil is a mixture four basic parts – minerals, organic matter, air and water and it serves four primary functions:

1. It is a medium for plant growth. Soil supplies the plants with nutrients and water and a place for them to anchor (roots).
2. It is a major water storage and purification system.
3. It provides a habitat for over one quarter of all living species from bacteria, to worms, moles and many more. These organisms then modify the soil.
4. It modifies the atmosphere through respiration of the soil organisms and plant roots.

Soil (pedosphere) interacts with the lithosphere (rocks), hydrosphere (water), atmosphere (air) and biosphere (living). Figure 1 shows a few of the possible interactions, there are plenty more. Try re-drawing the diagram and add more interactions - this will help you revise some of the information you have already covered.

Figure 1. Interactions between spheres.

Factors affecting soil characteristics

The characteristics of the soil in a particular area will be dependent on:

• Climate: There are many aspects of climate that will affect the soil but the most important one is the precipitation / evaporation balance which determines the dominant direction of water movement.
• Organisms: Soil organisms breakdown the dead organic matter and mix it in to the upper layers of the soil.
• Relief: The elevation of the land, the aspect of the slope (the direction it faces) and the angle of the slope will all impact the soil.
• Parent material: This is the original material that the soil develops from. It will either be the bedrock (solid rock) or a drift deposit (lake or glacial) that has been laid down on top of the bedrock.
• Time: The development of soil is a long slow process hence soil is considered non-renewable natural capital. Therefore the amount of time the soil has had to develop will affect its characteristics.

Theory of Knowledge

Reason is probably the dominant way of knowing that these five factors affect the characteristics of soil. How can we be certain that these are the only five factors that matter?

Figure 2. Soil starts as bedrock.

Soil as an ecosystem

Check out the definition of an ecosystem. An ecosystem must have biotic and abiotic components that interact. The abiotic components are covered in the subtopic, Soil as a system. Here we will consider the biotic component.

In a handful of soil there are billions of organisms including:

• Micro-organisms such as bacteria, algae and fungi.
• Macro-organisms include earthworms, insects, mites, millipedes and mammals e.g. moles.

Leaves and organic debris accumulates on the surface of the soil where it will be broken down by decomposers in the top 10cm of the soil. The process starts with bacteria and fungi and ends with small insects mites, springtails and earthworms that consume the dead organic matter (DOM). Decomposition produces plant nutrients and microbial remains that bind the soil to give a crumb structure.

Some soils contain Rhizobia bacteria that live symbiotically with legumes. The bacteria take carbohydrates from the plant and provide nitrogen for the plant. Fungi have a symbiotic relationship with plants as they extend the plant root system by up to 400 times.

Earthworms perform the final stage of decomposition. They breakdown the DOM and convert it to humus then as they move to the lower horizons they mix it into the soil. Earthworms are a vital part of the soil ecosystem as they change the soil chemically and physically. They distribute microbes throughout the soil in their castings; they increase the availability of phosphorous, they aerate the soil and improve drainage and they mix the soil.

Mites are abundant in most soils. Each species browses on a particular type of fungi so the populations are kept in balance. Other organisms such as slugs and snails burrow through the soil and improve its fertility and mixing.

Figure 3 shows a typical food web in a healthy soil. It shows some of the species that make up the soil ecosystem.

Figure 3. Soil ecosystem food web.

International-mindedness

Figure 3 shows one example of a soil ecosystem food web. Consider how these may vary in different parts of the world.

Soil as a system

Soil is not static; it is continually changing and developing through physical, chemical and biological processes such as weathering, erosion and translocation (internal reorganisation of matter and energy). As soil has processes it is probably a system.

Systems have inputs, outputs and stores as well as processes. It is often hard to separate the parts of the system because; as with all systems everything is interlinked.

Figure 1. Soil.

This section deals with natural inputs, stores and outputs for soils. Processes are dealt with in the next section. It should be noted that humans have a significant impact on soils but that is dealt with in section 5.3.3.

Theory of Knowledge

How do we know that the systems model is the best model to apply to soil?

Inputs

As discussed earlier soil is made up of four basic parts minerals, organic matter, air and water. The minerals and organic matter are the major inputs to the soil system.

• Minerals: The minerals of a soil come from the weathering of the parent material. Weathering is the breakdown of the parent material by physical, chemical and biological processes. You do not need to know the details of weathering but if you are interested go to Physical Geography.
• Organic matter: Organic matter comes from the living organisms that are on and in the soil. Soil and vegetation develop together. During succession the early plants colonize the area before the soil has really developed. When the early plants die they add organic material to the soil. The soil is considered an ecosystem and as such it contains living organisms. These organisms are themselves inputs, they add organic material to the soil and they carry out some of the soil forming (pedogenesis) processes.

Figure 2. Organic matter accumulation on the surface of the soil.

• Gases: Certain plants also add some inputs to the soil. Certain plants fix atmospheric nitrogen and change it into nitrates and ammonia compounds in the soil. The nitrogen fixation is a process that forms an input into the soil system. The living component of the soil (various organisms) also respire removing oxygen and adding carbon dioxide to the soil.
• Water: The way in which water enters the soil is dependent one whether the soil is on a slope and where it is on that slope. At the top of a slope the soil receives most water through direct precipitation. Lower down the slope the soil will receive water via precipitation but also as throughflow from the soils higher up the slope.

Stores

The stores for the soil system are really quite simple as it is the four basic elements of a soil plus organisms and nutrient.

• Organic matter: Is actually the dead organic matter (DOM) that comes from the biotic elements of ecosystems. Anything that once lived in an ecosystem becomes the organic matter store in soil. So the DOM is a store of nutrients that can be taken up by the plants to be used for growth.
• Organisms: The soil is an ecosystem with millions of organisms. These organisms add nutrients to the soil when they deposit waste. They remove oxygen and add carbon dioxide and they breakdown DOM.
• Minerals: The parent material is made up of minerals some of which form the body of the soil whilst others are used by the plants.
• Air: The amount of air (gases) in the soil is extremely variable. The store of air is dependent partly on weather conditions e.g. precipitation, temperature and wind, and the number of organisms and plant roots in the soil. Also, different soil textures will contain different proportions of air spaces e.g. clay vs. sand.
• Water: This is also very dependent on the weather and the soil texture.
• Nutrients: These may be stored in the organic matter but may also be free in the soil.

Figure 3. Soil organisms are a soil store.

Outputs

Soils lose minerals, organic matter, water and gases thorough the action of wind, water and plants and animals. Wind can physically remove soil by blowing loose soil away. Water leaches minerals from the soil. Water also washes the finer clay particles out of the soil.

Plants take nutrients and carbon dioxide from the soil for growth and to photosynthesize. If these plants stay in the area they become inputs as they die, decompose and become soil nutrients. If the plants are harvested or removed in some other way then those nutrients are lost.

Many species of animals eat clayey soils for medicinal purposes. Herbivores often ingest plant alkaloids and other toxins and the clay renders these harmless. These elements of the soil thus become outputs.

So soil is clearly a system. The end product of all these processes is the soil profile. The inputs are acted upon by the processes and the material is sorted into layers referred to as the soil profile.

Processes, transfers and transformations

Systems are dynamic so the inputs don’t just go in and stay there as stores, processes act upon them and move materials around. Soil has a number of processes some of which has already been mentioned, gas exchanges through respiration, photosynthesis and nitrogen fixation, weathering to breakdown the parent material and evaporation to name a few. Go back over this section and make a note of all the processes that have been mentioned so far.

Leaching and evaporation

Water is a major input and output of the soil system. As discussed in "What is soil?" the P/E balance determines the dominant direction of water movement in a soil. Any water that enters the soil will initially move down through the soil layers. As it moves through the soil it will dissolve any of the soluble minerals (transformation).

In dry climates (deserts) evaporation rates are high so the dominant direction of water movement is upwards. High temperature draws soil water (plus the mineral salts) up to the surface. The water evaporates and the salts are left behind at or near the surface - salinization.

If there is heavy rainfall and low temperatures the dominant direction of water movement is downwards. As the water moves downward it dissolves any soluble minerals and washes them out of the soil – leaching.

Decomposition

The fungi, algae and bacteria are some of the decomposers found in soil. They breakdown the DOM in the soil and release the plant nutrients. This transformation of organic matter to nutrients gradually increases the soil fertility.

Weathering

The breakdown of parent material through the process of weathering adds minerals to the soil. The rate at which this takes place is largely dependent on climate and the type of rock. Chemical weathering dissolves the minerals in rocks and leaves behind very little raw material for soil formation. Physical weathering just breaks the rocks up in to smaller pieces and thus gives plenty of raw material for soil formation. The type of weathering that takes place depends on the type of rock present in the area e.g. limestone weather chemically whereas granite weathers physically.

Figure 4. Plants cause chemical and physical weathering.

Theory of Knowledge

In this model we are ignoring the human impact on soils. Is a model accurate if we ignore such a major factor?

The soil system processes move the inputs around and sorts them. The result is the well defined layers of the soil profile.

The soil profile

The soil profile (Figure 5) is a vertical section of the soil that goes from the surface down to the parent material. It may be exposed naturally at the top of a cliff, on a riverbank or where there has been a landslide. Alternatively you can dig a soil pit (though if you intend to do so – get permission first) and expose the layers of the soil.

Figure 5. Typical soil profile.

There are four main horizons – O, A, B and C. These can be further divided into many more sub-horizons but for the purposes of this course – four is enough. Soils will not always have all the horizons.

The Organic horizon (referred to as the O horizon) is on the top of the soil and includes all the DOM that accumulates on top of the soil. Initially the remains can be identified but as decomposition progresses the DOM becomes an unrecognizable jelly like substance (humus) that mixes into the soil over time.

The A horizon is the top soil or mineral layer. This layer is usually dark in colour due to the high proportion of organic matter. The high organic content means it is a zone of highest biological activity. This layer has often lost its clay as it has been eluviated or washed out. This sometimes is often absent.

The B horizon is the sub-soil and tends to be the zone of illuviation or accumulation. Minerals and particles are washed into this horizon from the ones above. Plant roots are likely to be found in this layer but very little humus is found here.

The C horizon is the decomposed parent material. In most cases this layer is not really affected by soil processes but it has weathered. This layer may contain large lumps of parent material.

You do not need to know the details of the soil profile but if you are interested check out this Website Plant and Soil Sciences.

International-mindedness

Soil profile vary considerably around the world. Consider the differences between a soil profile close to where you live and somewhere else in the world.

Soil structure and texture

Soil texture and structure have a very important impact on the soils mineral and nutrient content, drainage, water-holding capacity, air spaces, biota and organic matter retention.

International-mindedness

Soil affects primary productivity and thus agricultural potential. Consider how different farmers manage their soils effectively.

Soil texture

Soil texture is the look and feel of the soil and is linked to the relative proportions of sand, silt and clay particles. There are other sizes of particles but these three are the most important.

Soil texture is largely the result of the parent material and the type of weather. Physical weathering yields coarser textured soils while chemical weathering tends to give finer textures. However, no matter what type of weathering takes place – the longer it continues the smaller the particles.

The texture class of soil is shown on the soil triangle and this video shows you how to use the soil triangle.

Figure 1 shows a soil triangle. The numbers on the axes are usually tilted to show which lines they apply to. The coloured arrows show you which lines to use. Orange for clay, blue for silt and red for sand.

Figure 1. Soil texture triangle.

The arrows on Figure 1 correspond to the figures in Table 2 for the loam. Loam is the name given to a soil with a relatively even mix of sand, silt and clay. See if you can name soil A in Table 2.

Soil properties

The proportions of sand, silt and clay impart certain properties of a soil.

Sandy soils feel gritty, as the particles are quite big. The large particles create large pores spaces between them. This means that soils high in sand are:

• Well drained so rarely get water logged.
• Subject to drought in times of low rainfall.
• Warm up quickly in summer due to high air content.

Clay particles are the smallest and give soil a sticky feel. Small particles give small pore spaces and clayey soils are:

• Poorly drained and prone to water logging.
• Take a long time to dry out after rainfall.
• Warm up slowly in summer due to high water content.

Silt particles are too small for the human eye to see and soils high in silt have a smooth feel. The smaller particles give smaller pore spaces so their properties are between sand and clay.

Figure 2. Agricultural soil is usually a loam.

Soil texture influences nutrient retention as finer texture soils tend to store soil nutrients. Coarse soils tend to be well drained and so are more prone to leaching and so lose the nutrients.

It is rare for soil to be made of only one size of particle. A relatively even mix of sand, silt and clay is called a loam and it is ideal for agriculture. In the case of soil, loams are greater than the sum of the individual parts. The loam soil system has the best properties of sand, silt and clay and very few of the problems.

Loams have 20 – 50% sand, 10 – 25% clay and 30 – 50% silt. But there are other types of loam. Clay loams, sandy loams and silt loams and everything in between.

Examiner Tip

You will have to know how to use the soil triangle:

• You must be able to give the name of the soil from given proportions or mark the point of those proportions.
• You may be shown a point on the soil triangle and be asked to state the proportions of that soil.

Soil structure

Soil structure describes the way the sand silt and clay particles stick together. Soil structure is affected by the presence of organic matter and soil organisms. Materials excreted by soil organisms bind the soil together to form aggregates (lumps). Good agricultural soils allow for plant root growth, nutrient retention and free air and water movement. They are described as friable (crumbly) and break up easily. Poor soil structure has coarse lumps that do not break apart.

Good quality soils are friable (crumbly) and have fine aggregates so the soil breaks up easily if you squeeze it. Poor soil structure has coarse, very firm clods or no structure at all.

Figure 3. Good soil structure is friable.

Theory of Knowledge

The terminology to describe soil structure is very descriptive, does that invalidate it as a science?