Fundmentals Soil Basics (2) · Department of Environmental Science and Technology Use of Soil...

Department of Environmental Science and Technology

Basic Soils

June 1, 2010

Patricia Steinhilber, Ph.D.Department of Environmental Science and Technology

Ag Nutrient Management Program

University of Maryland College Park


What is Soil?

• the outer layer of the Earth’s crust (geologists)

• the ecstatic skin of the Earth (writer)

• the loose surface of the Earth in which plants grow• crucible of terrestrial life (Daniel Hillel, soil physicist)

• the pedosphere − the interface between the lithosphere, hydrosphere

and atmosphere (ecologists)


What are Soils?

• dynamic, reactive, three-phase ecosystems composed of solids, liquids and gases

MineralsAirWaterOrganic Matter

49%

25%

25%

1%

topsoil several days afterrainfall or irrigation


Soil Solids• mineral or inorganic solids

– often classified based on particle size

• organic solids a.k.a. “organic matter”


Major Mineral Classes in a Soil

mineral class

size of mineral particles feel of particles

sands 0.05 – 2 millimeters (mm) gritty

silts 0.002 – 0.05 millimeters (mm) like flour, cornstarch or talcum powder

clays less than 0.002 millimeters (mm) sticky when wet


Effect of Soil Texture on Soil Propertiessoil property textural group

coarse textured medium textured fine textured

water-holding capacity

low moderate High

susceptibility to erosion

low high moderate

leaching potential high moderate lownutrient retention capacity

low moderate high


The soil physical property that describes the proportion of sand, silt, and clay-sized particles in a soil is calleda) structureb) texturec) bulk densityd) porosity


The property that describes how particles are arranged into aggregates is called

a) textureb) porosityc) structure d) particle density


Two questions to ponder:

• How does organic matter affect soil texture?

• How does organic matter affect soil structure?


Organic Matter

Humus

Biomass

Residues &By-Products

75%

10%

15%


Biomass: What It Is• the living component of the soil• consists of a range of creatures

− as small as microscopic viruses & bacteria− as large as worms and other creatures that

are visible to the unaided eye− and everything between


Source: USDA


Biomass: What It Does• participates in nutrient cycling

– digest plant and animal materials (residues), using what they need and leaving behind what they don’t

• immobilization & mineralization


Biomass: What It Does (cont.)

• creation of biopores– larger organisms move through soil creating channels– channels promote water infiltration and create a

healthy balance between large and small pores• macropores (which allow rapid infiltration of rainfall and

replenishment of oxygen in the root zone) and • micropores (which store water for plant use)


Residues and By-products:What They Do

• dead stuff - crop residues, dead roots and bodies of soil creatures

• by-products - materials that plant roots and soil creatures release or exude into the soil


Residues and By-products:What They Do

• fuel and nutrients for soil organisms– energy and nutrient source for most of the soil

creatures• formation and maintenance of soil aggregates

(structure)– sticky and gummy by-products of residue

decomposition hold soil particles together in clumps or aggregates


• stable end product of residue decomposition• composes the majority of organic matter• resists further decomposition (1% per year) • it is not a good nutrient or energy source for

soil creatures

Humus: What It Is


Humus: What It Does

• very small in particle size & high surface area

• charged sites at many locations on the surface

• effective at holding water and nutrients


Soil Pores (Void Space)

• soil water– adequate (but not too much) quantity – adequate supply of nutrients– minimize runoff and leaching

• soil air– source of oxygen for roots and aerobic soil organisms– constantly enriched with carbon dioxide from roots

and soil organisms


Comparison of gases in the atmosphere and in soil air of well-structured topsoil several days after rainfall

gas in the atmosphere

in soil air

nitrogen 79% 79%oxygen 20.9% 20.6%

carbon dioxide 0.035% 0.300%


Soil Aeration• The exchange of O2 and CO2 between

the soil pores and the ambient atmosphere

Hillel


Balance between Water and Air

• macropores – drain quickly after rain or irrigation– conduits for gas exchange for soil atmosphere to

ambient atmosphere

• micropores– hold water against force of gravity– some water is plant-available; some is not


Texture and Porosity


The soil physical property that describes the proportion of void space in a soil is called

a) structureb) texturec) porosityd) bulk density


How much does soil weigh? What is its density?

• expressing density– English units – pounds/cubic foot (#/ft3)– metric units - grams/cubic centimeter (g/cm3)

• 2 kinds of density in soil– bulk density

• density of whole soil– particle density

• density of soil solids only


Typical Bulk Densities (g/cm3)

• organic soils (peats and mucks) – 0.1 - 0.6• volcanic soils – 0.6-0.8 • forest or native grassland – 0.8 – 1.1• cultivated silt loams – 0.9 – 1.5• cultivated sandy loams – 1.2 - 1.7 • plow pans – 1.7 – 2.0

• BD where roots can not penetrate – 1.6


The soil physical property that describes the density of the whole soil (solids and voids) is calleda) textureb) structure c) particle densityd) bulk density


The soil physical property that describes the density of the soil's solids only is called

a) bulk densityb) porosityc) particle densityd) structure


If a soil ped has a volume of 124 cm3 and a dry weight of 138 grams, what is its bulk density?

• BD = dry weight/volume = #/ft3 = grams/cm3

• BD = 138 grams/124 cm3 = 1.11 grams cm3


Formula for porosity

% PORE SPACE = 100 - % SOLID SPACE

OR

% PORE SPACE = 100 - (BD/PD X 100)


A very compacted plow pan has a bulk density of 1.74 g/cm3 and a particle density of 2.68 g/cm3. What is the percent porosity?

%PS = 100 – [(1.74/2.68) x 100]

%PS = 100 - 65 = 35%


Formula for soil moisture

X 100wet weight – oven dry weight

oven dry weight

%moisture = (weight of water/oven dry soil weight) x 100


Soil Solution

• adequate supply, well supplied with nutrients

• minimize adverse conditions for plant growth– acidity– salinity


Biological Classes of Water

www.landfood.ubc.ca


Biological Classes of Soil Water


Soils are Biochemical Reactors

• the various phases (soil air, soil water, soil minerals and organic matter) interact

• a wide array of chemical and biochemical processes occur


Solid-liquid Interface

• adsorption of water and nutrients occurs at surfaces

• small particles have more surface area than the same weight of large particles

• surfaces are often charged; most charge is negative

• negatively charged surfaces attract positively charged ions (cations)


Soils as Natural Phenomena


Why do soils differ across a landscape?

• parent material– rocks (and minerals)

• landscape position (topography)• biotic factors• climate

– temperature and rainfall• time


Let’s look at this soil mosaic again …


Soil Horizons

University of Wisconsin-Stevens Point


From this … … to this.


Use of Soil Surveys or WSS

• use & management– crop production issues

• suitability for cropping, vegetables, pasture, hay, orchards

– engineering issues• building site development, sanitary facilities

• tables of soil properties– drainage class– permeability– depth to high water table– productivity (yield)


County Harford Calvert

Soil Map Unit Soil Map Unit Name Symbol Slope

(%)Name Symbol Slope

(%)

Beltsville sil BeA 0-2 Westphalia fsl WaA 0-2

Beltsville sil BeB 2-5 Westphalia fsl WaB2 2-6

Beltsville sil BeC 5-10 Westphalia fsl WaC2* 6-12

Westphalia fsl WaD3* 12-20

Information Embedded in Soil Map Units

* 2 indicates moderately eroded; 3 indicates severely eroded


Questions?

Smithsonian – Dig it! The Secrets of Soil

Hillel, 2008front cover

Fundmentals Soil Basics (2) · Department of Environmental Science and Technology Use of Soil...

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