Basic Soil Science Fundamentals of Nutrient Management Melissa L. Wilson Department of Environmental Science & Technology Ag Nutrient Management Program University of Maryland, College Park 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? Dynamic, reactive, three-phase ecosystems composed of solids, liquids and gases Five Components: Mineral particles Water Air Dead organic material Soil fauna and flora 48% 2% Image from: https://wildcatfarmers.wordpress.com/resources/soil-under-construction/the-dirt-on-soils/
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, 14 th ed.
Effect of Soil Texture on Soil Properties water-holding capacity nutrient retention capacity leaching potential susceptibility to erosion coarse textured medium textured fine textured low moderate high low moderate high high moderate low 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 claysized 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, 14 th 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 O 2 and CO 2 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, 14 th 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 g x 100 = 36%
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 75% Residues and By-products 15% Biomass 10%
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, 14 th 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, 4 th 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, 14 th ed.
Figure: Brady & Weil, 14 th ed.
Two questions to ponder: How does organic matter affect soil texture? How does organic matter affect soil structure? Photo credit: Brady & Weil, 14 th ed.
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 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 Figure from: University of Wisconsin-Stevens Point
How many soil horizons can you see in the picture? O AE Bt1 Bt2 A. 2 B. 3 C. 4 D. 5
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 (#/ft 3 ) metric units - grams/cubic centimeter (g/cm 3 ) 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 PD BD Figures from: passel.unl.edu
Soil Organic Soil 0.1-0.6 Volcanic 0.6 0.8 Forest & Native Grassland Typical Bulk Density (g/cm 3 ) 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/cm 3
Formulas for density: BD = oven dry weight/volume of pores and solids Units: #/ft 3 or grams/cm 3 Ex: 1.33 g /1 cm 3 = 1.33 g/cm 3 PD = oven dry weight/volume of solids Units: #/ft 3 or grams/cm 3 Ex: 2.66 g /1 cm 3 = 2.66 g/cm 3
If a soil ped has a volume of 124 cm 3 and a dry weight of 138 grams, what is its bulk density? A. 91 g/cm 3 B. 11.1 g/cm 3 C. 0.91 g/cm 3 D. 1.11 g/cm 3 138 g / 124 cm 3 = 1.11 g/cm 3
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/cm 3 and a particle density of 2.68 g/cm 3. 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
Video is also viewable at: https://youtu.be/gqoasuf-ngk
The End Questions?