Chapter 8

Soil: Foundation for Land Ecosystems

SOIL AND PLANTS8.1

Soil vs. Dirt

• Soil is much more than just “dirt”

Top Soil FormationProductive topsoil involves dynamic interactions among the organisms, detritus, and mineral particles of the soil

Soil Characteristics - Texture

• Parent material– Mineral material of the soil, has its origin in the

geological history of the area– Parent material could be rock, or sediments

deposited by wind, water, or ice– Eventually parent material is broken down by

natural weathering

Weathering

• Gradual physical and chemical breakdown– Physical • Ex – ice wedging, erosion

– Chemical• Formation of H2CO3 of organisms, or other chemical

erosion

• As rock weathers, it breaks down into small stones (soil separates)

Soil Separates

• Sand– Particles from 2.0 – 0.02 mm in size

• Silt– Particles from 0.02 down to 0.002 mm

• Clay – Particles finer than 0.002 mm

Proportions

• Sand, silt, and clay particles constitute the mineral portion of soil

• Soil texture refers to the relative proportions of each type of the particle in a given soil

• Loam– 40% sand, 40% silt, 20% clay– Best type of agricultural soil

Major classes of soil are indicated on the triangle. For clay, read across horizontally; for silt, read diagonally downwards; for sand, read diagonally upwards to the left. The texture content of any soil should total 100% if the triangle is read properly.

Properties

• Larger Particles– Like sand– Have larger spaces separating them than smaller

particles• Visualize difference between packing softballs and golf

balls in the same size container

Properties

• Smaller Particles– Such as silt and clay– Have more surface area relative to their volume

than larger particles• (Visualize cutting a block in half again and again. Each

time you cut it, you create two new surfaces, but the total volume of the block remains the same)

Properties

• Nutrient ions and water molecules tend to cling to surfaces– Smaller particles will have a larger surface area,

therefore tend to hold on to these ions and molecules

Workability

• Soil texture affects workability– Ease with which a soil can be cultivated (typically

for agriculture)

Texture Water Infiltration

Water-Holding Capacity

Nutrient Holding capacity

Aeration Workability

Sand Good Poor Poor Good Good

Silt Medium Medium Medium Medium Medium

Clay Poor Good Good Poor Poor

Loam Medium Medium Medium Medium Medium

Soil Profiles

• Soil formation creates a vertical gradient of layers that are often quite distinct – Layers are known as horizons– Vertical slice through a horizon is called soil profile• 5 horizons

O-Horizon

• Consists of dead organic matter (detritus) deposited by plants, leaves, stems, fruits, seeds, etc.

• High in organic content• Primary source of energy for the soil

community

Humus

• Towards the bottom of the O-horizon, the decomposition process is well advanced and the original materials may be unrecognizable

• At this point the material is dark and is called HUMUS– Not to be confused with the chick-pea dip,

hummus.

A-Horizon

• Below O-Horizon• Mixture of mineral soil from below and humus

above• AKA Topsoil. • Fine roots from O-horizon can permeate this

layer • A-Horizon is usually dark because of the humus,

and may be shallow or thick depending on the ecosystem

E-Horizon

• E stands for eluviation– The process of leaching (dissolving away) many

minerals due to the downward movement of water

• This layer is often paler in color than the two layers above it

B Horizon

• Characterized by the deposition of minerals that have leached from the A and E horizons, – often high in FE, AL, CA, and other minerals.

• Referred to as the SUBSOIL• Often high in clay and is reddish or yellow in

color

C-Horizon

• Parent material originally occupying the site• Weather rock, glacial deposits, volcanic ash• Affected little by the biological and chemical

processes that go on in the overlying layers

Study Aide

• O• A• E• B• C

Soil Horizon Characteristic

O Humus (surface litter, decomposing plant matter

A Top soil (mixed humus and leached mineral soil)

E Zone of leaching (less humus; minerals resistant to leaching

B Subsoil (accumulation of leached minerals like iron and aluminum oxides)

C Weathered parent material (partly broken-down minerals)

Soil Classes

• Soil scientists have crated a taxonomy of soils– Order, suborder, group, subgroup, families

• Literally hundreds of soil classes• There are 12 major orders– We will investigate 4 orders

Mollisols• Fertile, dark soils found in temperate grassland

biomes• World’s best agricultural soil• Deep A-horizon and are rich in humus and

minerals• Precipitation is insufficient to leach the minerals

downward• Found in– Midwestern US, temperate Ukraine, Russia,

Mongolia, Argentina

Oxisols

• Tropical and subtropical rainforests• Layer of iron and aluminum oxides in the B-

horizon• Little or no O-horizon– Due to rapid decomposition of the plant matter– Most minerals are in living plant matter• Most oxisols are of limited fertility for agriculture

Alfisols

• Widespread• Moderately weathered forest soil• Not deep, but have well developed O, A, E,

and B horizons• Typically of the moist, temperate forest biome• Suitable for agriculture if they are

supplemented with organic matter or mineral fertilizers

Aridisols

• Widespread soils of dry-lands and deserts• Relatively unstructured in soil horizons • Thin, lightly colored• Irrigation used on these soils usually leads to

salinization– High evaporation rates draw salt to the surface

Soil and Plant Growth

• Plants need a root environment that supplies optimal amounts of mineral nutrients, water, and air

• The pH and salinity of the soil are also critically important

• Soil Fertility – Soils ability to support plant growth– Refers to the presence of proper amounts of

nutrients

Mineral Nutrient and Nutrient-Holding Capacity

• Phosphate, Potassium, Calcium and other ions are present in rocks and become available to roots through weathering– Processes is usually too slow for plant growth– Nutrients that support plant growth are supplies

mostly though breakdown of detritus

Leaching

• Nutrients may be washed from the soil as water moves though it

• Lessens soil fertility• Contributes to pollution• Soils capacity to hold nutrient ions until they

are absorbed by roots becomes very important

Fertilizer

• Unavoidable removal of nutrients from soil with each crop– Nutrients absorbed by plant are contained in

harvested material– Therefore agricultural systems require input of

fertilizers

Fertilizer

Organic• Includes plant or animals

wastes or both• Manure and compost

– Rotten organic material

• Includes nitrogen fixing plants– Alfalfa, soy (legumes), peas,

lentils

Inorganic • Chemical formulations of

required nutrients, with out any organic matter included

• Much more prone to leaching than organic fertilizers

Water/Water Holding Capacity

• Transpiration– Water is absorbed by the roots of plants, passed

up through plant, and exit as water vapor through microscopic pores in leaves (stomata)

Infiltration

• Water is resupplied to soil by rainfall or irrigation

• Soils ability to allow water to soak in is important

Water-Holding Capacity

• Poor water holding capacity implies that most of the infiltrating water percolates on down below the reach of the roots

Evaporation

• Evaporative water loss from the soil surface• Depletes soil’s water reservoir without serving

the needs of plants

Plant-Soil-Water relationships

Water lost from the plant by transpiration must be replaced from a reservoir of water held in the soil. In addition to the amount and frequency of precipitation, the size of this reservoir depends on the soil’s ability to allow water to infiltrate, to hold water, and to minimize direct evaporation

Aeration

• Roots need to “breathe”• Living organs need a constant supply of

oxygen for energy via metabolism (respiration)• Overwater fills air spaces preventing aeration• Compaction– Or packing of the soil due to large amounts of

traffic (foot or vehicular) reduces aeration

Relative Acidity (pH)

• Refers to the acidity or alkalinity of any solution

• Most plants do best with a pH near neutral

REVIEW

• To support good crop soil must– Have good supply of nutrients and a good nutrient

holding capacity– Allow infiltration and have a good water holding

capacity/resist evaporative water loss– Have a porous structure that permits good

aeration– Have a pH near neutral– Have a low salt content

Detritus

• Detritus accumulated on and in the soil supports a complex food web– Including bacteria, fungi, protozoans, mites,

insects, millipedes, spiders, centipedes, earthworms, snails, slugs, moles, and other burrowing animals

– Most numerous and important – BACTERIA

Humus

• As organisms feed, bulk of detritus is consumed through cellular respiration

• REVIEW• 6CO2 + C6H12O6 6CO2 + 6H20 + ENERGY• However, each organism leaves a part of the

detritus undigested– This is humus

Humus and the development of soil structure

On the left is a humus-poor sample of loam. It is relatively uniform, dense “clod”. On right is a sample of the same loam, but rich in humus. Note that is has a very loose structure, composed of numerous aggregates of various sizes

Soil Structure

• Refers to soil particle arrangement (whereas texture refers to size)– Ex a loose soil structure is ideal for infiltration,

aeration, and workability

Soil Interactions

• Between soil and biota• Roots of some plants and a fungi called

mycorrhizae– Mycorrhizae go deep into the detritus, absorbs

nutrients, and transfers them directly to the plant• No loss of nutrients due to leaching• Example of a mutualistic relationship!

Soil Interactions

• Nematodes– Small words that feed on living roots• Can be very destructive to some agricultural crops

– In flourishing soil ecosystems nematodes are controlled by other soil organisms• Like fungus

Figure 8-11

Soil Enrichment

• Green plants protect soil– Protects from erosion– Reduces evaporative water loss• Relationship between plants and soil can be broken

easily– Topsoil depends on additions of detritus and water

Mineralization

• Loss of humus and the consequent collapse of topsoil

SOIL DEGREDATION8.2

Degradation

• When key soil attributes required for plant growth or for other ecosystem services deterorate over time, the soil is considered degraded

Importance of humus to top soilTopsoil is the result of a balance between detritus additions and humus-forming processes, and their breakdown and loss. If additions of detritus are insufficient, the soil will gradually deteriorate

GLASOD

• Global Assessment of Soil Degradation map– Data came from questionnaires, very little

information has been validated by collecting data on actual soil conditions or information on crop productivity (but it is still cited)

– Starting to change• Ex – Burkina Faso

Erosion

• Soil and humus particles are picked up and carried away by water or wind.

• Occurs anytime soil is bared and exposed to the elements

Splash Erosion

• Water erosion starts here– Impact of falling raindrops breaks p the clumpy

structure of the topsoil– Dislodge particles wash into paces between

aggregates, clogging pores, and thereby decreasing infiltration and aeration

– Results in more water running off

Sheet Erosion

• Once pores are closed in, sheet erosion occurs• Water converges into rivulets and streams– Great velocity and can pick up more soil– Result is the erosion into gullies (gully erosion)

Figure 8-13

Desert Pavement

• Differential removal of particles– Lighter particles of hums and clay are first to be

removed– Rocks, stones, coarse sand remain behind• Remaining soil becomes more and more coarse, and

finally rocky

Figure 8-14

Drylands

• Cover 41% of Earth’s land area• Defined by precipitation, not temperature– 10 – 30 inches yearly

• Home to 2 billion people

UNCCD

• United Nations Convention to Combat Desertification– Issues on funding projects to reverse land

degradation– Local communities gather and share traditional

knowledge of effective dryland use

Causing and Correcting Erosion

• 3 major practices expose soil to erosion and lead to degradation– Overcultivation– Overgrazing– deforestation

Figure 8-16

Overcultivation

• Fist step in growing crops – plowing for weed control

• Exposes loose soil• Soil may remain bare for a while before

vegetative cover is planted

Negatives of Plowing

• Splash erosion– Seals surface so that aeration and infiltration are

decreased• Weight of tractors add to compaction of soil• Accelerates oxidation of humus and

evaporation

Crop Rotation

• Cash every third year– Hay and clover in between• Hay – adds organic matter• Clover – adds nitrogen

• Not very cost effective for farmers

No Till Agriculture

• Field is first sprayed with herbicide to kill weeds

• Crop seeds are planted• At harvest, process is repeated– Waste from previous crop becomes detritus– Soil is never left exposed

Fertilizer

• Inorganic versus organic fertilizers– Chemical fertilizer lacks organic matter to support

soil organisms to build soil structure

Contour PlowingCultivation up and down a slope encourages water to run down furrows and may lead to severe erosion. This problem is reduced by plowing along the contours at a right angle to the slope

ShelterbeltsBelts of trees around farm fields that break the wind and protect soil

NRCS

• Natural Resource Conservation Services– Established in response to the dust bowl– Provides info regarding soil or water conservation

practices– Performs inventory of erosion losses in the U.S.

Overgrazing

• 65% of dryland areas are rangelands• Native plants cleared out for grass for cattle to

graze on• Usually occurs because rangelands are public

lands not owned by the people who own the animals– “tragedy of the commons”

Deforestation

• Forest ecosystems are efficient at holding and recycling nutrients and for holding water

Sedimentation

• Sediments are carried into streams and rivers, clogging changes– Intensifies floods– Fills reservoirs– Kills fish– Eutrophication – Groundwater decreases

Irrigation

• Supplying water to croplands by artificial means– Increased crop production in arid regions– Water is usually diverted from rivers though canals

and flooded through furrows in fields• Flood irrigation

Center-Pivot Irrigation

• Water is pumped from a central well through a big sprinkler that slowly turns

Salinization

• Result of accumulation of salts in and on the soil where plant growth is suppressed

• Occurs because irrigation water contains at least 200-500 ppm dissolved salts

CONSERVING THE SOIL8.3

Soil Conservation

• Practiced at two levels– Individual land owner– Public policy

– Read and take notes on your own for section 8.3