Soil is made of

What are the components of soil?

Typical garden soil is composed of 50% solid material by volume, 25% water, and 25% air. The solid material portion includes about 45% minerals and 5% organic matter.

Four main components

  • Minerals

    The mineral portion of the soil is derived from the bedrock from which it was formed. Some of the mineral particles, such as sand, still consist of rock. They are chemically and structurally the same as the parent rock, but have been ground by weather, water, glaciers, and other natural forces into small pieces. Other particles—including most of the microscopic clay particles—have been dissolved and precipitated again, perhaps many times, to reach a form that is quite different from the native rock.

    The sizes of individual particles of the mineral component are responsible for the texture of the soil. Soil textures are classified as sand (the coarsest texture), silt, and clay (the finest texture, with particles 1,000 times smaller than large sand particles). Loam is a mixture of sand, silt, and clay.

  • Organic matter

    Living and dead plant and animal matter in various stages of growth and decay constitute the organic part of the soil. Most native, or unamended, soils contain from less than 1% to 5% organic matter, whereas a well-amended garden soil may contain 30% or more. Adding organic matter is one of the best things you can do to your soil. In addition to being a reservoir of nutrients, organic matter improves the soil structure .

    Live organic matter includes earthworms, insects, microorganisms, and plant roots. Earthworms and plant roots perform a valuable service by creating tunnels for air and water to flow through the soil. An invisible world of soil bacteria, fungi, and algae is even more crucial. These microorganisms decompose organic matter and contribute to the chemical reactions that allow plants to absorb nutrients.

  • Water

    Soil water enables plants to absorb minerals by first dissolving them. Water is also needed for the physiological and chemical processes of plant growth.

    Water is so strongly attracted to small spaces, or pores, in the soil that it moves from large spaces to smaller ones, even if the movement is upward or sideways. That is why a soil with mostly small pores, such as clay, holds water so well. An ideal soil has a mix of large and small spaces, so that it holds both water and air.

  • Air

    Soil with a loose surface and large pores permits air to diffuse easily into it. Entry is limited if the soil is crusted over or compacted. Soil air is more humid than the air that humans breathe, and it has a higher carbon dioxide content. The oxygen it contains is vital to the root growth of plants. In fact, roots grow only where oxygen is present in the soil.

    Roots and decaying organic matter give off carbon dioxide, which diffuses to the surface and dissipates in the air as oxygen diffuses to the depths of the soil.

What are the major components of soil?

There are 5 types of mineral including Water. The percentage of the components vary from time to time.

  1. MINERAL- It is the largest component of the soil and makes about 45% of the soil. It consists of sand,silt and clay. It retains the soils ability to retain important nutrients.

  2. WATER- It is the second basic component of the soil. It may vary from 2% to 50% of the soil. It usually contains added salts and other chemicals.

  3. AIR/GASES- About half of the total mass of the soil has many cavities and holes. It makes 2 to 50% of the soil. The air found in the Earth or soil is very different from the air found in the atmosphere.

  4. ORGANIC AND BIOLOGICAL COMPONENTS- It is the next basic component found in levels of approximately 1% to 5%.It has a very high water holding capacity and thus it can increase the fertility of the soil.

  5. MICROORGANISMS- These are found in the soil in a very high number but covers only 1% of the soil. These are the primary decomposers of the raw materials present in the soil.they eat(consume) water,air and organic matter to recycle raw materials into humus. Other microbes like Nitrogen fixing bacteria helps the plants in intake of the nitrogen gas.


Question: What ingredients make up soil?

Answer: Soils contain solid particles of different sizes and different types. Some of the particles are minerals, and others are organic. There are spaces between the particles; the spaces may be filled with air or water.

Photo courtesy of the Natural Resources Conservation Service.

Minerals in soil and how they “weather”

The mineral particles in soil form as rocks and minerals break into smaller pieces, or change into other minerals due to “weathering.” Water, ice, plant roots, heating and cooling of rocks, and rocks bumping into one another can break the rocks into smaller pieces. This is physical weathering: only the size has changed. The smaller particles are the same kind of rock or mineral as the larger particles from which they came.

Chemical weathering, in contrast, changes rocks or minerals into other rocks or minerals. The particles also may be smaller, but they are no longer like the rock or mineral from which they came.

Think about it this way. If you have a cracker and break it in half, each piece is still a cracker, just smaller. If you break it again, each piece is still a cracker. You might even be able to put it back together. This represents physical weathering.

Now put the cracker in your mouth and start chewing. Your teeth break the cracker into smaller and smaller pieces, but the saliva in your mouth mixes with the cracker as you chew. This starts the digestion process, and afterward the pieces of the cracker are no longer a cracker. You could not take the pieces out of your mouth and put them back together to form the cracker. This represents chemical weathering in which the rock or mineral is changed into another kind of rock or mineral.

Many people have used vinegar and baking soda to make a volcano. This is an example of chemical weathering. Baking soda is a mineral and vinegar is a weak acid. When they are mixed together, a chemical change occurs, water and carbon dioxide are released, and the solid that remains is no longer baking soda.


Humus is composed of the organic particles in the soil. These particles are the remains of plants, animals, and microorganisms after they have been decomposed by other organisms. Like you, organisms in the soil “eat” plant and animal tissues. Like you, they get energy from what they eat, and they produce waste. The waste produced by these soil organisms is called humus.

Particle size

Soil scientists separate the solid particles based on size. Particles larger than 2 millimeters (less than 1/10th of an inch) in size are gravel. Particles between 2 and 0.05 millimeters are sands. Particles between 0.05 and 0.002 millimeter are silts. Particles smaller than 0.002 millimeter in size are clays. The mixture of sand, silt, and clay particles determines soil texture. These individual particles form clumps, often held together by humus. Soil scientists call these clumps “peds.” The shape and size of these peds determines the soil structure.

So, soils are composed of mineral and organic solids, water, and air. Plants, animals, and microorganisms living in the soil affect the soil properties we observe, especially the amount of humus and the soil structure.

–Answered by Eric Brevik, Dickinson State University

Have a question for Soils Matter? Post it as a comment below, or email us at [email protected]

In just a word, the story of soil goes something like this: “CLORPT!” It’s fun to say, and it helps explain how tough rock turns into the soft soil farmers need to grow food and feed the world.

Jim Harsh is a scientist at Washington State University and an expert on soil. He said a soil scientist, Hans Jenny, came up with five actors in the soil story: CLimate, Organisms, Relief, Parent material, and Time.

The texture, smell, and color of soil can often depend on the type of climate where the soil formed. Up in the cold, rainy mountains, soil will look and smell different than it does in the dry, hot desert. Red soils, black soils, white soils, yellow soils and even spotted soils can all be found around the planet.

Soil is bursting with life. When leaves, plants, grasses, small bugs, and animals fall into cracks of rocks, these organisms start breaking down and soil starts to form. It’s here where tiny microbes, such as bacteria and fungi, are busy working in the soil.

Usually when someone says the word “bacteria” it sounds like germs, but most microbes in soil are actually helpful.

Microbes have several jobs. First, they hang out on the surface of rock where they produce acid to break down rock. They eat nutrients in the soil and return them back to the earth. Second, they create sticky glue and thread-like strings to hold together the particles of sand, silt, and clay that make up soil. These are called aggregates. A good soil will have a lot of aggregates and create a solid place for plants to stick their roots. Microbes even eat earthworms’ waste. It’s a big, beautiful cycle.

The shape of land, or the relief, also impacts how soil forms. Soils on the sunny side of a hill are much warmer than those on the shady side of a hill. If the hillside is very steep, the soil can wash away as fast as it is formed, Harsh said.

Soil has to start somewhere and that’s why it needs parent material. Parent material can come from volcanic ash, sediments in rivers, or the rocks in your backyard. Even the Grand Canyon is parent material. Over time this rock breaks down through weathering and with the help of microbes.

Time is the other big factor. Some soils, especially the red ones, take hundreds of thousands of years to develop. Scientists have found fossilized soils and samples from African deserts that are more than 55 million years old. Now you really know what they mean when someone says “old as dirt.”

It isn’t officially part of CLORPT, but the last actor in the soil story is you. Farmers, scientists, and everyone who likes to eat dinner are stewards of soil. Just two feet of soil pretty much determines if the world has food. How we take care of the earth will determine what kinds of soils we have for the future.

Dr. Universe

Got a question? Ask Dr. Universe. Ask Dr. Universe is a science-education project from Washington State University and University Communications.

5 Different Soil Types – Know Your Soil Type

Last time, we discussed basic soil facts that you need to know before you started digging around in your garden. This time, we’re going to look at different soil types and how to determine the kind of soil that’ll be host to your plants.

There are 5 different soil types that gardeners and growers usually work with. All five is a combination of just three types of weathered rock particles that make up the soil: sand, silt, and clay. How these three particles are combined defines your soil’s type—how it feels to the touch, how it holds water, and how it’s managed, among other things.

1. Soil Type: Sandy

Sandy soil has the largest particles among the different soil types. It’s dry and gritty to the touch, and because the particles have huge spaces between them, it can’t hold on to water.

Water drains rapidly, straight through to places where the roots, particularly those of seedlings, cannot reach. Plants don’t have a chance of using the nutrients in sandy soil more efficiently as they’re swiftly carried away by the runoff.

The upside to sandy soil is that it’s light to work with and warms much more quickly in the spring.

Testing what type of soil you’re working with involves moistening the soil and rolling it into a ball to check the predominating soil particle. When you roll the slightly wet sandy soil in your palms, no ball should be formed and it crumbles through your fingers easily.

2. Soil Type: Silty

Silty soil has much smaller particles than sandy soil so it’s smooth to the touch. When moistened, it’s soapy slick. When you roll it between your fingers, dirt is left on your skin.

Silty soil retains water longer, but it can’t hold on to as much nutrients as you’d want it to though it’s fairly fertile. Due to its moisture-retentive quality, silty soil is cold and drains poorly.

Silty soil can also easily compact, so avoid trampling on it when working your garden. It can become poorly aerated, too.

3. Soil Type: Clay

Clay soil has the smallest particles among the three so it has good water storage qualities. It’s sticky to the touch when wet, but smooth when dry.

Due to the tiny size of its particles and its tendency to settle together, little air passes through its spaces. Because it’s also slower to drain, it has a tighter hold on plant nutrients. Clay soil is thus rich in plant food for better growth.

Clay soil is cold and in the spring, takes time to warm since the water within also has to warm up. The downside is that clay soil could be very heavy to work with when it gets dry. Especially during the summer months, it could turn hard and compact, making it difficult to turn. (When clay soil is worked while it’s too wet though, it’s prone to damage).

If moistened soil feels sticky, rolls up easily, and forms into a ball or sausage-like shape, then you’ve got yourself clay.

4. Soil Type: Peaty

Peaty soil is dark brown or black in color, soft, easily compressed due to its high water content, and rich in organic matter. Peat soil started forming over 9,000 years ago, with the rapid melting of glaciers. This rapid melt drowned plants quickly and died in the process. Their decay was so slow underwater that it led to the accumulation of organic area in a concentrated spot.

Although peat soil tends to be heavily saturated with water, once drained, it turns into a good growing medium. In the summer though, peat could be very dry and become a fire hazard. (I kid you not—peat is the precursor of coal.) The most desirable quality of peat soil, however, is in its ability to hold water in during the dry months and its capacity to protect the roots from damage during very wet months.

Peat contains acidic water, but growers use it to regulate soil chemistry or pH levels as well as an agent of disease control for the soil.

When wet peat soil is rolled, you won’t form a ball. It’s spongy to the touch and when squeezed, water could be forced out.

5. Soil Type: Saline Soil

The soil in extremely dry regions is usually brackish because of its high salt content. Known as saline soil, it can cause damage to and stall plant growth, impede germination, and cause difficulties in irrigation.

The salinity is due to the buildup of soluble salts in the rhizosphere—high salt contents prevent water uptake by plants, leading to drought stress.

It’s easy enough to test if you have saline soil. You’ll probably see a white layer coating the surface of the soil, your plants are growing poorly, and they’re suffering from leaf tip burn, especially on young leaves.

The Ideal Soil Type: Loam

The type of soil that gardens and gardeners love is loamy soil. It contains a balance of all three soil materials—silt, sand and clay—plus humus. It has a higher pH and calcium levels because of its previous organic matter content.

Loam is dark in color and is mealy—soft, dry and crumbly—in your hands. It has a tight hold on water and plant food but it drains well, and air moves freely between soil particles down to the roots.

The feel test for loam yields a smooth, partly gritty, partly sticky ball that crumbles easily.

Although loamy soil is the ideal material to work with, don’t despair if you don’t have it in your garden. That’s because soil will always favor one particles size over the two others. Then again, there are many ways to condition your soil—adding beneficial soil inoculants, covering your soil with compost, or simply spraying leaves and soil with compost tea.

One Last Soil Typing Trick

If you really want to know what soil type you’re working with but don’t want to get dirty, fill a smallish jar with soil sampling from your garden. Shake vigorously and let the soil settle overnight. Next day, you’ll see distinct soil layers. Sand stays at the bottom, clay at the top and silt in between. Their percentages will be your clue on your soil type.

Stay tuned for our next blog posts, where we’ll discuss how to manage sandy, silty and clay soils and the ideal plants to grow in each soil type. In the meantime, try the jar trick and tell us what type of soil you’re working with and how you’re managing that soil.

16 Feb Determining Your Soil Type For Your Best Garden Soil

Soil Types

Soil is made up of a variety of particulate material. The characteristics of your soil depend on these particles.

  • Clay: High in nutrients and a hero at moisture retention, clay particles are small, fairly uniform in size and tightly packed. Clay soil drains slowly and holds very little air. Clay soils compact easily and cause a number of issues; one being stunted root systems.
  • Sand: This soil type consists of large particles with a lot of space between them. It drains quickly and doesn’t retain nutrients as well as clay soil.
  • Silt: Comprised of medium-sized particles, this soil retains just the right amount of water. It’s better at retaining nutrients than sand, but not as good as clay.

Most of the time, you’ll have a combination of all three soil types. Depending on your region, you may find that your soil has more clay, while another region has more sand and so on. If your neighbor has mostly clay soil, you probably have the same.

The dominant particle type determines your soil properties. The perfect soil is a 40-40-20 percent mix of sand, silt and clay. This is what is known as loam. You get the best of the three soil types from loam: moisture retention with good drainage, good airflow and decent nutrient retention.

Simple Ways to Determine Your Soil Type

There are two simple tests you can do yourself to determine your soil type.

The squeeze test

  • Pick up a handful of damp – but not soaking wet! – soil.
  • Rub it between your fingers.
  • You have clay if the soil feels slippery and stays tightly balled.
  • You have sand if the soil feels gritty and crumbles.
  • You have loam if it is a little crumbly, but stays loosely balled.

The ribbon test

  • Pick up a handful of damp – but not soaking wet! – soil.
  • Roll it between your hands to form a ribbon.
  • If it won’t form a ribbon, you have sand.
  • If it forms a ribbon that breaks when you hold it up, you have around 25 – 50% clay soil.

If you don’t have loam in your garden, never fear. You can get there by adding organic amendments, or “fixes.”

Soil Improvements

Soil amendments are organic materials you add to your soil to improve it.

Improving Sandy Soil

The goal is to increase water and nutrient retention.

  • Add 3 – 4 inches of organic material like finished compost or well-rotted manure.
  • Add an additional 2 inches every year.
  • Apply a thin layer of wood chips, bark, hay, straw or leaves. Mulching helps soil retain water and provides nutrients as it decomposes.

Improving Clay Soil

The goal is to increase water retention and aeration.

  • Add 2 – 3 inches of organic material like finished compost, well-rotted manure or gypsum. Gypsum helps break up hard clay.
  • Add an additional inch every year.
  • Amend your soil in the fall.
  • Consider permanent raised beds for better drainage. This also eliminates foot traffic, which causes compaction or soil hardening.
  • Don’t spade or till any more than necessary.

Improving Silty Soil

The goal is to increase nutrient retention.

  • Add at least an inch of organic amendment every year.
  • Minimize compaction by limiting tilling and foot traffic.
  • Consider permanent raised beds.

Types of Organic Amendments

  • Compost: decomposed fruit and veggie scraps.
  • Compost manure: odorless farming by-product.
  • Builder’s sand: larger grains than beach sand; good for improving drainage.
  • Topsoil: commercial product, partially decomposed compost with rougher texture.
  • Potting soil: good for balanced water retention and drainage.

There are many other specialty mixes & “plant specific” organic soils available for the purpose of improving your soil. No matter what type of soil you start with, following these simple steps will help you get to the promised land of gardening: loam! Happy amending!

How soils form

Factors affecting soil formation

Soil forms continuously, but slowly, from the gradual breakdown of rocks through weathering. Weathering can be a physical, chemical or biological process:

  • physical weathering—breakdown of rocks from the result of a mechanical action. Temperature changes, abrasion (when rocks collide with each other) or frost can all cause rocks to break down.
  • chemical weathering—breakdown of rocks through a change in their chemical makeup. This can happen when the minerals within rocks react with water, air or other chemicals.
  • biological weathering—the breakdown of rocks by living things. Burrowing animals help water and air get into rock, and plant roots can grow into cracks in the rock, making it split.

The accumulation of material through the action of water, wind and gravity also contributes to soil formation. These processes can be very slow, taking many tens of thousands of years. Five main interacting factors affect the formation of soil:

  • parent material—minerals forming the basis of soil
  • living organisms—influencing soil formation
  • climate—affecting the rate of weathering and organic decomposition
  • topography—grade of slope affecting drainage, erosion and deposition
  • time—influencing soil properties.

Interactions between these factors produce an infinite variety of soils across the earth’s surface.

Parent materials

Soil minerals form the basis of soil. They are produced from rocks (parent material) through the processes of weathering and natural erosion. Water, wind, temperature change, gravity, chemical interaction, living organisms and pressure differences all help break down parent material.

The types of parent materials and the conditions under which they break down will influence the properties of the soil formed. For example, soils formed from granite are often sandy and infertile whereas basalt under moist conditions breaks down to form fertile, clay soils.


Soil formation is influenced by organisms (such as plants), micro-organisms (such as bacteria or fungi), burrowing insects, animals and humans.

As soil forms, plants begin to grow in it. The plants mature, die and new ones take their place. Their leaves and roots are added to the soil. Animals eat plants and their wastes and eventually their bodies are added to the soil.

This begins to change the soil. Bacteria, fungi, worms and other burrowers break down plant litter and animal wastes and remains, to eventually become organic matter. This may take the form of peat, humus or charcoal.


Temperature affects the rate of weathering and organic decomposition. With a colder and drier climate, these processes can be slow but, with heat and moisture, they are relatively rapid.

Rainfall dissolves some of the soil materials and holds others in suspension. The water carries or leaches these materials down through the soil. Over time this process can change the soil, making it less fertile.


The shape, length and grade of a slope affects drainage. The aspect of a slope determines the type of vegetation and indicates the amount of rainfall received. These factors change the way soils form.

Soil materials are progressively moved within the natural landscape by the action of water, gravity and wind (for example, heavy rains erode soils from the hills to lower areas, forming deep soils). The soils left on steep hills are usually shallower. Transported soils include:

  • alluvial (water transported)
  • colluvial (gravity transported)
  • aeolian (wind transported) soils.

Read more about soil erosion.


Soil properties may vary depending on how long the soil has been weathered.

Minerals from rocks are further weathered to form materials such as clays and oxides of iron and aluminium.

Queensland (and Australia) is a very old weathered landscape with many ancient soils.

What is soil made of ?

Soil is a mixture.

Get a little earth from a valley, from a plateau and from the side of a slope.

Put the earth from each field in a different glass. (If you haven’t got glasses, use empty bottles.)

Earth from first and second field

• In each glass or bottle put two fingers of earth.

Fill up with water.

• Stir the mixture well in each glass.

Put it down and do not touch it for Jive minutes.

• Stir the mixture well in each glass once more.

Put it down and do not touch it for an hour.

An hour later, what do we see?

The earth has dropped to the bottom of the glass and the water is clear.

Look at the earth: several layers have formed.

• At the bottom of the glass there is a layer of sand and some little pebbles.

• In the middle is a layer of silt.

• Above is a thin layer of clay.

If the water is not clear, that is because clay is still mixed with the water.

• On top of the water float pieces of leaves and roots. The leaves and roots rot and form humus.

Thus the soil is a mixture.


Sand is found everywhere.

Rain carries sand into hollows and streams. It is then white- or yellow and shining because it is clean.

In the soil, sand is grey and does not shine, because it is mixed with earth.

Let us take a closer look at some sand.

It is made up of little grains.

These grains are not all alike.

They are very hard. If you rub a piece of iron with them, they scratch the iron because sand is harder than iron.

Grains of sand

• Sand is permeable.

Take a can.

Make a hole in the bottom of it.

Fill the can with sand.

If you pour water on to the sand, it goes through.

If you pour water on to the sand, it goes through

We say sand is permeable because it lets water through.

• Sand is unstable.

Take some dry sand in your hand and let it run gently.

The grains slide over each other; you cannot make a ball of sand.

Sand runs like water

• Sand in the soil.

Almost all soils contain sand.

Soils that contain a lot of sand are called sandy veils.

Sandy soils

Like sand itself, sandy soils are:

• permeable.

When it rains on sandy soil, the water passes through easily.

You can walk on sandy soil after rain.

Sand does not stick to the feet like clay.

Sand does not make mud. It is

• easy to work.

After the first rains, sandy soils are easy to work; they do not stick to tools like clay.

Sandy soils are called light soils.

• unstable.

The grains of sand do not stick together.

In the rainy season, water easily carries them away.

In the dry season, the wind can lift them up and carry them a long way. In the northern savannas people speak of a sand wind.

Groundnuts, cassava, yams and coconut trees grow very well in sandy soils; the roots easily penetrate them.

But sandy soils hold water and mineral salts badly.


Adobe walls and pots are made of earth.

This earth is called clay.

Usually the clay is found in the third soil layer: the red layer.

Because of this, a hole has often to be dug to get earth for bricks.

Usually the clay is mixed, which is what gives it a red or sometimes a brown or black colour.

When the clay is not mixed, it is white. This is kaolin. It is pure clay. Kaolin is used to whitewash houses.

• Clay and water

Wet clay takes whatever shape it is given, such as bricks, pots and stoneware Jars.

Wet clay sticks to the fingers; it makes mud.

When it is dry, clay forms hard lumps.

If you crush a lump of clay, it becomes dust finer than flour.

When it is dry, brown or black clay loses its water and cracks.

Red clay also loses its water, but does not crack.

Red clay can be used to make bricks and stoneware jars.

If dry clay is made wet again, it becomes soft and sticky.

If clay is baked, it becomes very hard. The stoneware jars keep their shape.

Clay is impermeable.

Take a can.

Make a hole in the bottom of it.

Fill the can with clay.

If you pour water on to the clay, it does not go through.

We say clay is impermeable because it does not let water through.

Water does not go through

Clay in the soil

Almost all soils contain clay.

Soils that contain a lot of clay are called clay soils,

Clay soils

Like clay itself, clay soils are:

• impermeable.

When it rains on a clay soil the water does not go through the soil easily. The water takes a long time to disappear. You can see the mud. So clay soils are

• difficult to work.

Wet clay sticks to the hands. It also sticks to tools.

For this reason clay soils are very difficult to work after the rains.

Clay soils are called heavy soils.

During the dry season clay soils become very hard. Cracks form in them.

The lumps are difficult to break.

Water remains in the soil; Cracks form in the soil

Soils with much clay are difficult to work and are often too wet.

Soils with little clay are easily carried away by water and by the wind.


Certain soils are neither sandy nor clay soils. They are made of silt.

Silt is made up of grains much smaller than sand grains (see page 15).

Because of this, silt does not let water through as easily as sand does (see page 16).

Silt does not form dust as fine as clay dust; because of this it is not impermeable like clay (see page 19).

Wet silt does not stick like clay. However, silt can be made into lumps.

Soil that contains a lot of silt is called silty soil.

Silty soils

Like silt itself, silty soils are:

• not as light as sandy soils.

The silt grains are closer together than the grains of sand.

Thus water does not go through so easily; silty soils do not dry quickly.

They are harder to work than sandy soils.

Wind and rain do not carry them away so easily.

• Iess heavy than clay soils.

Silty soils stick less than clay soils.

They are less hard to work.

They do not crack when dry.


In the soil there are dead leaves and roots.

They rot and change into humus.

You cannot see humus as you see sand, clay and silt.

• Dead plants change into humus.

Leaves, branches and dead trees rot in the soil.

We say they decompose.

Even big trees rot in a few years on wet soil.

Many worms and insects live in a rotting tree. You can often see them.

But other living things cannot be seen. They are too small.

These are called microbes.

There are very, very many of them. In a lump of earth as big as a lump of sugar there are millions and millions of microbes.

They feed on leaves, on dead branches, on organic matter (see Booklet No. 2, page 23).

They also need air to breathe, and water.

If there is no air and water, the plant does not rot. The organic matter does not decompose.

Organic matter decomposed by the microbes in the soil is humus.

What humus does

• Humus makes soils richer.

From the soil the plant gets mineral salts (see Booklet No. 1, page 19).

The leaves turn the raw sap into elaborated sap (see Booklet No. 2, page 20).

The plant feeds on elaborated sap.

Wood and leaves are organic matter rich in mineral salts and carbon.

Organic matter rots and makes humus.

Humus returns to the soil the mineral salts used by the plant.

Humus returns the mineral salts to plant

Humus improves soils

Heaps of bricks, cement and sheet iron do not make a house.

To make a house they must be arranged, must be joined together.

Sand, clay, silt and pebbles without humus do not make a good soil.

They must be arranged, must be joined together to make a good soil.

The way in which sand, clay, silt and pebbles are joined together is called the soil structure.

It is the humus which joins together sand, clay, silt and pebbles.

Humus is necessary for soil structure.

Bad soil structure

This structure is bad because there is no humus.

The sand, clay and silt are not joined together.

Air and water circulate badly.

Roots penetrate badly, breathe badly and feed badly.

This is a bad soil structure.

Good soil structure

The structure is good, because the sand, clay- and silt are joined together by a mixture of clay and humus.

Air and water circulate well.

Roots penetrate well, breathe well and feed well.

Good soil structure

• Humus improves sandy soils.

Sandy soils with humus hold water better.

They are less easily carried away by wind and rain.

• Humus improves clay soils.

Clay soils with humus are less hard.

Air and water circulate better.

Soil without humus is not good soil.

Some practical advice

• Brush fires

In traditional farming; leaves and branches are burned.

The fire destroys the organic matter, so that no organic matter is left to make humus.

The brush fire destroys the organic matter and changes the soil structure.

Modern farmers do not make brush fires.

• Fallow

Cassava gives an example.

When cassava is harvested, the whole plant is lifted: the root for eating and the stems for replanting.

Almost nothing is left either on the soil or in the soil.

The cassava has taken humus from the soil, but the organic matter of the cassava is not returned to the soil. So the soil is less rich.

After growing, cassava farmers let the soil rest. They let it lie fallow.

During the fallow, the soil gets all the dead plants; it gets the organic matter from the dead plants, and the soil improves.

Soil must be given organic matter. The remains of the crop, grass, manure, provide organic matter.

• Some crops improve the soil.

When you harvest a bean plant, you take only the fruits.

The stems, roots and leaves are left to rot on the soil.

They decompose and give humus.

Beans are a crop that leaves organic matter in the soil.

The beans take humus from the soil but return organic matter to it.

• Manure

Modern farmers use animal manure.

Manure returns to the soil the organic matter taken out by the crop.

• Do not mix the soil and the subsoil.

The soil is rich in humus.

The subsoil has no humus.

In tilling, the farmer should never mix the soil and the subsoil, so as to keep his soil rich.

In planting a tree, do not mix the layers of soil.

Do not mix the layers of soil

Soil scientists separate soil using a stack of several sieves. The sieves have holes with slightly different sizes. The coarsest sieve is at the top of the stack, and the finest sieve is at the bottom. Most soils contain many kinds of material. All soils consist mainly of two kinds of material: particles of minerals and rocks, and organic matter. Organic matter is any matter that is or once was living.

Soil is likely to have several kinds of rock and mineral particles. A few kinds are very common. The three most common kinds are quartz particles, feldspar particles, and small pieces of rock. A soil sample is very likely to have a lot of at least one of these three kinds of particles. Quartz particles have irregular shapes. They look gray and glassy. Their surfaces are often stained brown or orange, because they are coated with rust. Feldspar particles are usually white or cream-colored. Their surfaces are often flat, at least partly, rather than irregular. There are many kinds of rock particles. You can tell them apart from the mineral particles because rocks are made of many different particles of minerals, all stuck tightly together.

The finest part of a soil sample is probably mostly very small flakes of clay. They are too small for you to see even with a hand lens. Sandy soils are loose and easy to dig. Soils with a lot of clay are harder to dig. Some plants like sandy soils and others like soils with more clay. Most soils have lots of organic matter. Some of the organic matter is in the form of living things, such as earthworms, insects, and microorganisms. Most soils are also rich in decaying plants. If the plant has decayed only slightly, you can usually recognize scraps of leaves, roots, and seeds. When the plant has decayed more, it turns into a soft, fine, dark material called humus. Humus is very important in soils. New plants can easily put their roots into humus. It is also good at holding water for later use by growing plants.

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