basic soil science - university of maryland, college park€¦ · air and water in soil •soil...
TRANSCRIPT
Fundamentals of Nutrient Management
Melissa L. WilsonDepartment of Environmental Science & Technology
Ag Nutrient Management Program
University of Maryland, College Park
Basic Soil Science
Photo credit: Edwin Remsberg
Today’s Objectives:
• Elements of soil formation
• Describe important soil physical & chemical properties
• Describe the ability to use and interpret soil survey information (printed and digital)
• List some site specific control practices for soil conservation
What is soil?
• Five Components:−Mineral particles−Water−Air−Dead organic
material−Soil fauna and
floraImage from: https://wildcatfarmers.wordpress.com/resources/soil-under-construction/the-dirt-on-soils/
• Dynamic, reactive, three-phase ecosystems composed of solids, liquids and gases
48%
2%
Mineral Particles•Minerals usually come from bedrock, either in place or from somewhere else (brought in by wind, water, or glaciers)
•Particle Sizes−Sand (feels gritty)−Silt (feels like corn starch)−Clay (is sticky when wet)
•Most soils are mixtures!
Where did the minerals in the majority of Maryland’s soil come from?
A. Bedrock
B. Glaciers
C. Wind
D. Aliens
Soil Texture•Soil texture – the amount of sand, silt, and clay in the soil−Sandy soils drain quickly and are easy to dig−Clay soils are sticky and hold a lot of water−Silt soils aren’t too sticky and don’t drain too
quickly but they blow away with the wind very easily
−Loam soils – a mixture of all three – are best for agriculture• Large particles to keep the soil loose • Small particles to retain water
What is the texture of a soil with 30% clay and 50% sand?
Photo: Brady & Weil, 14th ed.
Effect of Soil Texture on Soil Propertiescoarse textured medium
texturedfine textured
water-holding capacity
low moderate high
nutrient retention capacity
low moderate high
leaching potential
high moderate low
susceptibility to erosion
low high moderate
Photo credit: Paul Meyer, MSCD
Photo credit: Paul Meyer, MSCD
The soil physical property that describes the proportion of sand, silt, and clay-sized particles in a soil is called:
A. structure
B. texture
C. bulk density
D. porosity
Two questions to ponder:
•How does organic matter affect soil texture?
•How does organic matter affect soil structure?
Photo credit: Brady & Weil, 14th ed.
Air and Water in Soil
•Soil particles do not fit together like puzzle pieces
−Water and air fill up the gaps (called pore spaces)• If all the gaps are filled with water, then there is no
oxygen present• If all of the gaps are filled with air, the soil is dried out
−Percentage of pore space usually ranges from 25-50%
•Sometimes, soil can become compacted−Can be problematic for plant growth and water
movement
Water Movement in Soil
•How Water Moves Through Soils
Soil Aeration•The exchange of O2 and CO2 between the soil pores and the ambient atmosphere
Hillel
Balance Between Water and Air• Macropores (large pores)
− drain quickly after rain or irrigation− allow rapid infiltration of rainfall and
replenishment of oxygen in the root zone
• Mesopores (medium-sized pores)− “storage pores”− hold water in form most plants can use
• Micropores (very small pores)− water is held too tightly to be use to most plants− where soil microbes reside when soil is dry
Wetting front 24 hours after 2 inch rainfall
What makes the wetting front wavy/not uniform?
Photo: Brady & Weil, 14th ed.
Biological Classes of Water
Image from: www.landfood.ubc.ca
Biological Classes of Soil Water
Formula for soil moisture:
%moisture =
(weight of water/oven dry soil weight) x 100
or
wet weight – oven dry weight
oven dry weight
x 100
If the wet weight of a soil is 136 g and the oven-dried weight of that same soil is 100 g, then what is the % moisture?
A. 36%
B. 136%
C. 26.5%
D. 1%
136 g – 100 g
100 gx 100 = 36%
Humus75%
Biomass10%
Residues and By-products
15%
Organic Matter
• Importance in soil:‐ Stores nutrients‐ Holds water
• Gives soil a black or dark brown color
• Components are abiotic and biotic (both living and dead)
Humus: What is it?
• Stable end product of residue decomposition
• Makes up 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 Does
• High surface area• Very small in particle size• Charged sites at many locations on the
surface
• Effective at holding water and nutrients
Figure: Brady & Weil, 14th ed.
Biomass: What Is It?
• The living component of the soil
• Consists of a range of creatures − microscopic viruses & bacteria− worms and other creatures
that are visible to the unaided eye
− everything in-between
• In one tablespoon of soil:− Thousands of bacteria and 60
feet of fungal material
Figure: Withgott & LaPosata, 4th ed.
Biomass: What It Does
• Participate 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
Photo credit: R. Weil
Residues and By-Products: What are they?
• 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 by-products of residue decomposition hold
soil particles together in clumps or aggregates
Photo credit: Brady & Weil, 14th ed.
Figure: Brady & Weil, 14th ed.
Two questions to ponder:
•How does organic matter affect soil texture?
•How does organic matter affect soil structure?
Photo credit: Brady & Weil, 14th ed.
•How does organic matter affect soil texture?
The property that describes how particles are arranged into aggregates is called:
A. texture
B. porosity
C. structure
D. particle density
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
Why do soils differ across the landscape?
Why do soils differ across a landscape?
Soil forming factors:−Parent material
• rocks (and minerals)
−Landscape position (topography)
−Biotic factors−Climate
• temperature and rainfall
−Time
Soil Horizons
Figure from: University of Wisconsin-Stevens Point
•O – organic material
•A – mineral and organic components mixed
•E – eluviated horizon –loss of clays, Fe, Al
•B – illuvial accumulation of clays, Fe, Al, OM
•C – unconsolidated bedrock
•R – hard bedrock
How many soil horizons can you
see in the picture?
A. 2
B. 3
C. 4
D. 5
OAE
Bt1
Bt2
Soil Properties You Need to Know
•Already covered:−Soil texture−Soil structure−Soil moisture
•Upcoming:−Soil density•Bulk and particle
−Soil porosity
Soil density•What does it tell us?
−How much a soil weighs−Whether there is any compaction−How easily water can move
•Expressing soil weight as density−English units - pounds/cubic foot (#/ft3)−metric units - grams/cubic centimeter (g/cm3)
1 acre of soil to a depth of 6 inches weighs approximately 2 million pounds!
What is its density?
•Two kinds of density in soil−Bulk Density (BD)
•density of whole soil
−Particle Density (PD)•density of soil solids
only
Figures from: passel.unl.edu
BD
PD
Soil Typical Bulk Density (g/cm3)
Organic Soil 0.1 - 0.6
Volcanic 0.6 – 0.8
Forest &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 through which roots cannot penetrate: 1.6 g/cm3
•BD = oven dry weight/volume of pores and solids•Units: #/ft3 or grams/cm3
• Ex: 1.33 g /1 cm3 = 1.33 g/cm3
•PD = oven dry weight/volume of solids•Units: #/ft3 or grams/cm3
• Ex: 2.66 g /1 cm3 = 2.66 g/cm3
Formulas for density:
If a soil ped has a volume of 124 cm3 and a dry weight of 138 grams, what is its bulk density?
A. 91 g/cm3
B. 11.1 g/cm3
C. 0.91 g/cm3
D. 1.11 g/cm3
138 g / 124 cm3 =
1.11 g/cm3
Porosity
•The portion of the soil that is pore space−This is where the air, water, and many of the living things reside in the soil
−Impacts water flow and air exchange in the soil
Texture and Porosity
Formula for porosity (aka % Pore Space)
% 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 porosity?
A. 54%
B. 99%
C. 65%
D. 35%
%PS = 100 – [(1.74/2.68)x100] =
35%
Where can you find useful
information about soils?
The Soil Survey!
From this … … to this.
• Soil map units and range of slopes
• Profile and soil textures
• Drainage classes and hydric soils
• Physical and chemical properties
• Vegetative productivity
• Water management
• Land management
The EndQuestions?