Formation, Fertility and Loss

Soil

Soil Formation

There are several factors that interact to bring about soil formation.These include:

1. Weathering2. Humification3. Chelation4. Organic sorting5. Translocation, 6. Aggregation7. Time

8. Topography9. Vegetation10. Climate11. Parent material

1. Soil Formation: weathering

Weathering: The break down of rock, which does not involve transport. Weathering creates the mineral particles that will form the soil. There are several different types:

1. Chemical weathering2. Physical weathering3. Biological weathering

2. Soil Formation, humification

Humification: the formation of humus from dead organic matter by decomposers and saprobionts. Humus contributes to the soil in several ways:

1. It sticks together the mineral particles together reducing erosion.

2. It holds water.3. As part of the cation exchange complex (along with

clay) it improves the soils ability to hold water

3. Soil Formation, organic sorting

Organic sorting: Living organisms such as earthworms literally eat the soil particles and leaf litter. As a consequence this turns over the soil in effect ploughing it.

4. Soil Formation, chelation

Chelation: Living organisms create organic compounds which release acids.Water passing through the leaf litter on the top of the soil carries these acids through the soil which breaks down and mobilise mineral particles and organic matter.

Some types of vegetation eg. Pine needles release more organic acids than others and thus create more chelation. Soils under coniferous trees such as pine tend to be acid as a consequence.

leaf litter leaf litter

5. Soil Formation, translocation

Translocation: the movement of material in solution or suspension from one soil horizon to another.

Water table

Illuviation The process by which the

mineral particles and nutrients are washed into

the lower layers of the soil.

5. Soil Formation, eluviation and illuviation

Soil surface

EluviationThe process by which the

mineral particles and nutrients are leached out of the upper layers of the soil.

5. Soil Formation, aggregation

Aggregation: When soil particles cluster and stick together to form a single unit such as a crumb ped.

6. Soil Formation, time

Time: Soils develop over long periods of time. In sand dune as succession proceeds the organic content of the soil increases and, as it does so, the water retention increases and there are changes in pH, nutrient content and the diversity of living organisms.The rate of soil loss in many parts of the world greatly exceeds the rate of formation

Soil Formation

Soil

Parent MaterialOrganisms

Climate Topography

Weathering

Weathering

Precipitation, temperature, etc

Leaching, hydrolysis,Rate of formation etc.

Microclimatic effects of vegetation cover

Depth of soil, drainageand runoff.

Resistance to weathering

Chemical compositionand particle sizeAmount and type

of humus, soil, mixing and aeration

Availablemineralnutrientsand water

Humidity,temperature,

light

Soil FertilitySoil Fertility

A function of texture, structure, depth, aeration, pH, moisture and nutrient content

Soil FertilityComponent High Fertility Low Fertility

Structure

Depth

Texture

A soil with a mixture of about 1/3 clay, 1/3 silt and 1/3 sand, (Loam), will give the right balance of drainage, nutrient retention, aeration etc.

Clay soils will be be harder for plant roots to penetrate and be more poorly aerated. Sandy soils although well aerated lack the ability to retain nutrients and water

A crumb ped gives the best arrangement of particles and DOM to allow water/nutrient retention, aeration etc.

Blocky and platy peds are harder for plant roots to penetrate

A deep soil will inevitably have a greater capacity for anchoring plants and supplying them with the water and nutrients they require.

A soil may be shallow for a variety of reasons e.g. angle of slope, age erosion, etc. Shallow soils are less capable of anchoring plants and tend to have fewer nutrients, water

Soil FertilityComponent High Fertility Low Fertility

pH

Moisture

Aeration

A fertile contains air. Which provides oxygen for aerobic respiration by plant roots, decomposers, saprobionts and nitrifying bacteria

Waterlogged soils tend to lack oxygen. As a consequence there is less decomposition of DOM by decomposers and saprobionts. Nitrates undergo denitrification to nitrogen gas.

A soil with a pH 6-7, (neutral) nutrients are accessible to plant roots

In Soils with extremes of pH some nutrients become less available. Acid soil leaching of minerals and nutrients occurs.

Plants require water for growth.

Too little water will limit plant growth partly by limiting nutrient avalability. Too much water will reduce aeration and may cause leaching.

Soil FertilityComponent High Fertility Low Fertility

Nutrients

Plants require nutrients such as nitrates , phosphates and potassium (N, P and K). Nutrient retention will be affected by texture, ped, aeration, water content, pH etc, Humans may modify these factors to improve retention. Humans may add nutrients by using inorganic/organic fertilizers or by rotating with legumes such as clover

Without nutrients plant growth will be stunted. There are several possible reasons low nutrient levels. Poor texture e.g. sandy soils, Poor structure, translocation of minerals/nutrients, waterlogged soil, extremes of pH, harvesting of crops which removes nutrients from a field.

Soil Fertility, pHpH: The pH of soil effects the availability of plant nutrients and thus soil fertility. A pH of 6.5-7.0 is optimum for the majority of

nutrients. Farmers adjust the pH of the soil by adding lime to raise the pH of acid soils and thereby increasing the availability of

nutrients such as phosphorus and potassium.

6.5

Soil LossSoil Loss

Soil Loss

Why are soils important?

What are the many causes of soil loss?

Soil Loss, erosion

Erosion: those processes which wear away the surface of the land

Soil LossSoil erosion can be divided into

three types:

ii. Rill: The removal of surface material by running water down a gradient with the formation of small channels, rills.

iii. Gully: The formation of large deep channels from smaller ones (rills) by the rapid movement of water down a gradient. Produces severe soil erosion

i. Sheet erosion: the removal of surface material by water flowing over the surface usually on low gradient slopes

Soil Loss

1.Rain splash2.Surface runoff3.Wind blow4. Deforestation5. Overgrazing6. Reduced dead organic matter7. Soil exhaustion8. Ploughing of vulnerable soils

There are several possible causes of soil erosion

Slope

Movement of soildown slope

Soil accumulating at thebottom of the slope

Rain

Rain Splash: On a slope the impact of raindrops

causes soil particles to be carried down the slope.

Soil Loss

rain splash

Movement of soildown slope

Soil accumulating at thebottom of the slope

Rain

Surface runoff: The water flowing over the land surface will carry soil particles with it which will then accumulate at the bottom of the slope.

Rain

Soil Loss

surface run off Slope

Surface runoff

Soil Loss, windblow

Windblow: Soils exposed to the wind may be picked up and carried long distances. The greater the windspeed the larger the particle that can be moved.

Soil Loss, exhaustion

Exhaustion: Harvesting of crops removes nutrients and organic matter from the soil. As a consequence the soil may lose its structure and this will increase the rate of erosion. The loss of nutrients will reduce the crop yield.

Soil Loss, deforestation

Deforestation: The removal of the tree cover leads to less interception and more surface run off. The consequent greater speed and amount of water flowing over the surface increases the rate of erosion.

Soil Loss, overgrazing

Overgrazing: The consequences of overgrazing are impaction of the ground and removal of covering vegetation. The results are increased surface runoff and increased soil erosion.

Soil Loss, reduced DOM

Reduced DOM: Dead organic matter binds the soil particles together, contributes to the soil ped and helps retain soil nutrients. Harvesting reduces the amount of DOM entering the soil. Fertilisers speed up the decomposition of the DOM. The consequence is the soil gradually losses its structure and erosion will increase.

Soil Loss, ploughing of vulnerable soils

Ploughing: ploughing removes plant cover which is largely responsible for holding the soil in place. Shallow soils on slopes, particularly those with a sandy texture are likely to erode easily after ploughing. Contour ploughing reduces the movement of the soil down down the slope.

Soil Loss, sediment traps

To measure the amount of soil erosion sediment traps can be equally placed down a slope and the amount of soil carried down the slope by surface run off can be measured. By weighing the sediment trapped at regular intervals

Soil and CivilizationThe Fertile Crescent 1

In the Western world agriculture began in an area of land called the fertile crescent at the western end of the Mediterranean about 7000 to 6000 years ago. This led to the development of settled civilizations such as the the Sumerians who developed the first writing cuneiform. Poor agricultural techniques led to deforestation, salination and erosion. Without food to support their populations these civilizations collapsed.

3500 BC: Wheat and barley are nearly equal in occurrence in southern Iraq.

2500 BC: The less-salt-tolerant wheat accounts for 1/6 of the crop.

2300 BC: Wheat is no longer grown in the Sumarian city-state of Agode

2200 BC: Wheat accounts for 2% of the crop in Girsu.

2100 BC: The city of Ur abandons wheat production. Wheat now just 2% of the crop in the Sumerian region.

2000 BC: Isin and Larsa no longer grow wheat.

1700 BC: Wheat abandoned completely in the south.

Soil and CivilizationThe Fertile Crescent 2

Crop yields were recorded in cuneiform tablets in the region of Girsu:

2400 BC: yields 253.7 m3/ km2

2100 BC: yields 146. m3/ km2

1700 BC: yields 89.7 m3/ km2

Deforestation and over-grazing in the eastern Turkey led to widespread erosion and is thought that this is the source of the silt load which destroyed and the irrigation system on which the population depended. This is believed to be the root cause of the decline of the Mesopotamian civilization. Between 700 BC and the 20th century AD the coastline at the mouth of the Tigris and Euphrates Rivers changes due to siltation. The Persian Gulf is shortened by 150 miles by the silt load of these rivers.

A Cuneiform Tablet

Rome and North Africa

In Roman times as many as 15,000,000 bushels of grain were transported annually from North Africa and Egypt to Rome, where now their is only desert. North Africa had a large, prosperous agriculture during 200 BC-600 AD, as much evidence shows. Erosion largely caused by deforestation is thought to be the reason for the decline. The land was probably already more-or-less ruined by erosion when Arabs conquered the area near the end of the 7th century AD. By the 10th century the situation was said to have become even worse. No more than 10% of the original forests that once stretched from Morocco to Afghanistan as late as 2000 BC still exist .

Soil and Civilization

A Roman African

Easter IslandEaster Island, off the coast of Chile has, fertile volcanic soils. Archaeological finds and ancient pollen show that when Polynesians arrived on Easter Island, it was covered with a lush subtropical forest of palm trees and giant sunflowers, inhabited by land birds and breeding sea birds. Easter Island sustained a population of 58 people/ km2. But the population stripped the forests bare and killed the native animals. By 1500, all that was left was grassland. "People turned to the largest protein source around-cannibalism. Easter Island society collapsed in an epidemic of warfare. When the first Europeans reached the island in 1722, two-thirds of its population had died.1550: Population of Easter Island 7000.1722: Population of Easter Island 23331850: Population after deforestation and collapse of natural and agricultural systems: 100

Soil and Civilization

Soil and Civilization

The Dust Bowl

Poor agricultural practices and years of sustained drought caused the Dust Bowl. Plains grasslands had been deeply ploughed and planted to wheat. During the years when there was adequate rainfall, the land produced bountiful crops. But as the droughts of the early 1930s deepened, the farmers kept ploughing and planting and nothing would grow. The ground cover that held the soil in place was gone. The Plains winds whipped across the fields raising billowing clouds of dust to the skies. The skies could darken for days, and even the most well sealed homes could have a thick layer of dust on furniture. In some places the dust would drift like snow, covering farmsteads

The Grapes of Wrath:John Steinbeck wrote in his 1939 novel "And then the

dispossessed were drawn west- from Kansas, Oklahoma, Texas, New Mexico; from Nevada and Arkansas,

families, tribes, dusted out, tractored out. Car-loads, caravans, homeless and hungry; twenty thousand and

fifty thousand and a hundred thousand and two hundred thousand. They streamed over the mountains, hungry and restless - restless as ants, scurrying to find work to do - to lift, to push, to pull, to pick, to cut - anything, any burden to bear, for food. The kids are hungry. We got no place to live. Like ants scurrying for work, for food, and most of

all for land."

Soil and Civilization

Soil and Civilization

Scientists say impoverishment of the soil is a major threat to the Earth's ability to feed itself. They have found that nearly 40% of the world's agricultural land is seriously degraded. The damage has already had "a significant impact" on the productivity of about 16% of the planet's farmland. The economic and social effects are currently having a greater impact in developing countries than in rich ones. However loss of soil is still significant in developed countries. In California it is estimated that it takes 2000 years for one inch of topsoil to be created. Currently one inch is being lost every 25 years. It is being lost 80 times faster than it is made .

Ozymandias

I met a traveler from an antique landWho said: Two vast and trunkless legs of stone

Stand in the desert. Near them, on the sand,Half sunk, a shattered visage lies, whose frown,And wrinkled lip, and sneer of cold command,Tell that its sculptor well those passions read,

Which yet survive, stamped on these lifeless things,The hand that mocked them, and the heart that fed,

And on the pedestal these words appear:"My name is Ozymandias, King of Kings:

Look upon my works, ye Mighty, and despair!"Nothing beside remains. Round the decayOf that colossal wreck, boundless and bareThe lone and level sands stretch far away.

-Percy Bysshe Shelley1792-1822