Published on October 14, 2013
Hill Country Soils and Erosion Control September 12, 2012 Amanda Bragg Resource Soil Scientist San Angelo, Texas The USDA is an equal opportunity provider.
Presentation Overview – How soil forms – Physical soil properties – Soil Biology – Chemical soil properties – Soil vegetation relationships – Web Soil Survey – Soil erosion Source: http://soil.gsfc.nasa.gov
The Five Soil Forming Factors • Parent Material • Climate • Biotic Activity (Living Organisms) • Topography • Time
Parent Material • The material that the soil is weathering or forming from • Includes: – – – – Bedrock Alluvium (water-laid sediments) Colluvium (moved and deposited by gravity) Eolian (wind-blown sediments) • As the parent material is weathered, transported, deposited, and precipitated it is transformed into a soil.
Different Types of Parent Material • Residuum – developed from underlying parent rocks…Found on mesas, plateaus, and plains (~3% of all soils in U.S.) • Aeolian or Eolian – moved and deposited by wind – Because it forms stable aggregates clay is rarely moved by wind. • Alluvium – moved and deposited by flowing water • Lacustrine – sedimentary deposits in lakes • Marine deposits – beds of ancient seas revealed as the land has uplifted • Glacial deposits • Colluvium – moved and deposited by gravity
Parent Material Affects • • • • • • Sand, silt, and clay Chemical content Bulk density Structure Kinds and amounts of rock fragments Soil fertility
Weathering • Physical and chemical decomposition of parent material caused by climatic and biotic activity • Physical Weathering: – Abrasion • Chemical Weathering: – Dissolution of rocks by acid • Carbon dioxide dissolved in rain • Plant produced acids (like pine needles)
Climate • Dominant factor in soil formation • Rainfall / Moisture – Speeds chemical weathering – Runoff speeds physical weathering – Promotes biotic activity – Erosion and deposition • Wind – Abrasion – Erosion and deposition
Texas Precipitation --- 9 to 67”
Climate • Temperature – Heating and cooling cycles cause rocks to expand and contract – Water freezing in cracks expands and breaks up parent material – Warmer conditions promote biotic activity and speed chemical weathering – Decomposition of organic matter increases with temperature
Climate Influences • In areas of low rainfall – shallow accumulation of lime as caliche • In humid areas – formation of acidic soils • Erosion of soils on steep hillsides • Deposition of eroded material • In warm and humid areas – very intensive chemical weathering, leaching, and erosion
Biotic Activity • All activities of living organisms that result in parent material weathering • Includes: – Plant roots – Burrowing animals and insects – Some plants produce rock dissolving acids – Organic matter produced improve conditions for other, higher, organisms – Microorganisms
Wiki- organisms • Animals and micro-organisms mix soils as they form burrows and pores, allowing moisture and gases to move about. • Plant roots open channels in soils • Plants with deep taproots can penetrate many meters through different soil layers to bring up nutrients from deeper in the profile. Plants with fibrous roots that spread out near the soil surface have roots that are easily decomposed, adding orgamic matter. • Microorganisms – effect chemical exchanges between the roots and soil and act as a reservoir of nutrients. Humans – remove vegetative cover -> erosion
Wiki - Vegetation • Prevent erosion • Shade soils – keeping them cooler – slowing the evaporation of soil moisture • Transpiration can cause soil to lose moisture • Form new chemicals that can break down minerals and improve soil structure
Topography • Layout of the landscape • Impact of elevation and slope on water movement and erosion – Soil runoff – Infiltration • Aspect • Temperature – Cooler with higher elevation • Vegetation Source: Soil Survey of McCulloch County, Texas, 1974
Time • Cumulative effects of all other factors over hundreds to thousands of years • Takes about 500-1000 years to form 1” of soil • With age typically: – Soil depth increases – Soil resembles parent material less – Soil becomes more developed • Exceptions – Areas with constant sediment deposition – Areas where erosion is greater than soil formation
Soil Horizons • Distinctive layers within a soil that is parallel to the soil surface and used to describe and classify soils • Surface horizons: – O – Layer of organic matter – A – Mineral layer at soil surface
Soil Horizons • Typical subsurface horizons: – E – zone of eluviation or leaching of clays and minerals – B – more developed than the A horizon; sometimes considered a zone of accumulation of clays and minerals like calcium carbonates – C – unweathered to slightly weathered parent material; little to no soil development – R – Consolidated bedrock
Soil Development O A R O A O A O A B A B E B R C R R C Time
Drainage and Topography
Physical Soil Properties • • • • Texture Structure Density Porosity • Consistency • Color • Temperature
Major Soil Components Typical Soil Composition Mineral Organic Water Air Wet Dry Year Year Pore Space* 50% Pore Space 25% water* 25% air* Solids 50% Solid Material 45% mineral material 1 to 5% organic matter
Soil Texture • How a soil feels • The percent by weight of sand, silt, and clay in the soil • Most important physical soil property • Affects: • Soil structure • Water-holding capacity • Nutrient-holding capacity • Essentially impossible to change unless you remove it or add large amounts to it
Soil Texture – Soil Particle Sizes Separate Name Size in mm Number of particles per gram Surface Area (g/cm2) Shape Sand 0.05 to 2.0 90 – 722k 11-227 Round Silt 0.002 to 0.05 5.8 million 454 Round Clay Less than 0.002 90.3 trillion 8 million Flat From: Henry Foth – Fundamentals of Soil Science, 7th Edition From: Discovery Education Resources
Soil Particle Sizes and Pore Space
Affect of particle size on soil properties • Sands – Largest in size – Chemically – not very active – Supplies bulk – Porosity • Do not pack closely together (MACROPORES) • Easy root penetration • Allows water flow and exchange of gases
Affect of particle size on soil properties • Silts – In-between size – Express properties of both sands and clays – Chemically – more active than sands, less active than clays – Porosity • Smaller pores than sands • Can retain water against gravity • Water more available to plants than with clays
Affect of particle size on soil properties • Clays – Smallest size – (Microscopic) – Chemically – most active – Negative ionic charge attracts positive ions – Surface holds required minerals (cations) required by plants – Porosity • • • • Smallest pores (Micropores) Can retain water against gravity (Capillary action) Holds the most water cumulatively Not all water is available to plants.
Chemical structure for Smectite
Soil Structure Highest density? Fastest infiltration? Best for roots / plant growth?
Porosity Volume of soil not occupied by mineral and organic matter. Macropores – typically hold air Micropores – typically hold water Allows for the movement and storage of: Air Water Dissolved nutrients Tillage - increases % of macropores temporarily but results in destruction of soil structure.
SOIL COLOR Write color as: Hue Value/Chroma (10YR 6/3) HUE CHROMA VALUE
Dark Colors - High Organic Matter
Light Colors - Low organic matter content
Mottles/Redoximorphic Features Mottles or redoximorphic features are caused by oxidation or reduction of iron in the soil. • Red, orange and yellow colors are iron accumulations (oxidized iron) • Grays are iron depletions (reduced iron)
Yellows, Browns, Reds – Well Drained – oxidized iron water-shedding areas
Red Colors Highly weathered –oxidized iron
Gray – wet water accumulating areas
Soil Organic Matter (Humus)
Soil Biology • • • • Plant Roots Bacteria Fungi Actinomycetes • • • • Nematodes Protozoa Arthropods Earthworms
Soil BiologyFacts • A single shovelful of soil can contain more species of organisms than live aboveground in the entire Amazon rain forest • One cup of soil may hold as many bacteria as there are people on Earth • The weight of all the bacteria in 1 acre of soil can equal the weight of 1 or 2 cows • Mature trees can have as many as 5 million active root tips. • A teaspoon of forest soil may hold more than 10 miles of fungi. • Almost all freshwater travels over soil or through soil before entering rivers, lakes, and aquifers. • Plants can remove 400 to 2,000 lbs of water for every 2 lbs of plant material produced. • About 85% of the CO2 in the air comes from the actions of soil microbes feeding on organic matter. • Microbes have been found as deep as 10 miles.
Soil pH • Acidity or Alkalinity • Classes: – Acid – pH < 7 – Neutral – pH = 7 – Alkaline – pH > • Potential acidity is contained on the surfaces of the clay and organic particles • Active acidity is the liquid portion of the soil • High rainfaill climates produce more acid soils
Soil Reaction • Determines solubility and availability of certain elements for plant growth • Nutrient levels are optimum 6.0 to 7.5 for most plants. • Below 6.0: reduced availability for phosphorus, potassium, calcium, magnesium, and sulfur • Above 7.5: reduced availability for copper, zinc, iron, and manganese
Soil Fertility • Ability of the soil to supply nutrients in proper amounts and proportions as needed for establishment, development, and reproduction of the plant species • Mainly based on the cation-exchange capacity – Clay fraction – Organic matter
Salinity • A localized problem on Gulf Coast • Almost all soils have some salinity • Causes soil to become hard • Damages roots and stunts plants • Reduces the water that is available to plants (moisture can be in the soil, but the sodium “ties” it up to where it is not available to plants • Damages steel
Soil Compaction • High traffic areas • When soils are manipulated when wet • Plowing or incorporating organic matter decreases compaction • Minimize hazard of compaction by having a good thick layer of mulch on surface
MLRAs of Texas
MLRA’s Texas Hill Country
Low Stony Hill
Adobe Seep Muhly
What happens when it rains? • Infiltration – Texture – Structure – Organic matter – Depth to root limitation – Soil Moisture – Temperature • Runoff
Soil Erosion • Erosion - Wearing away of a substance • Two types of soil erosion – Geologic – slow method by which natures forces try to bring all surfaces into one plane – Accelerated – erosion that occurs because of the effects of man and occurs at a much faster rate • Example – Undisturbed land – 0.02 tons of soil lost per acre per year – Cultivated with mismanagement and dramatic climate conditions – 4 to 50 tons lost per acre per year
Soil Erosion • Reduced productivity • Inability to hold excess rainfall – Overland flow losses and ends up in drainage-ways and rivers – Can cause damage from flooding – Does not recharge aquifers • Formation of gullies • Silted waterways • Turbid water
Detachment – The beginning of soil movement Raindrops break the bond between soil particles and splash them a short distance. The detached particles are vulnerable to water moving over the soil surface
Sheet Erosion When rain falls faster than the soil can absorb it, water will collect and flow over the ground surface. The surface water will begin to carry soil particles that were detached by raindrops.
Rill Erosion Surface flow will soon establish paths. If the soil is unprotected, some of these paths become small eroding channels. Moving further downslope, flow in rills becomes more erosive, causing them to enlarge and join with others.
Concentrated-Flow Erosion (Ephemeral Gullies) The topography of the landscape in many places will allow flow to collect in a few major water courses before it leaves the field. Rills are erased by tillage, but channels eroded by concentrated flow tend to re-form in the same location each year.
Gully Erosion Rapidly moving water may cause severe “head cutting” and erosion of the gully’s sidewalls. Headcutting will continue as the gully advances up slope. Unless treated, it will not stop until it reaches the top of the slope.
Poor land treatment Sedimentation Nutrients attached to sediment Nutrients in solution or in air Effective Land Treatment Reduced sedimentation and nutrient loading
Approximately 1 billion tons of topsoil are lost annually in the U.S. due to wind erosion. 5 million acres of land in the Great Plains Region of the US are damaged each year. NRCS Darren Richardson The loss of fertile topsoil is severe enough in many places to permanently reduce inherent soil productivity.
Abrasion from wind blown soil particles can damage or destroy crops. The quality of many crops, particularly vegetables, may be reduced because of soil deposition or damage. NRCS Darren Richardson Peanuts
Aerosols Some soil from damaged land enters the atmosphere obscuring visibility, polluting the air, causing vehicle accidents, fouling machinery and impairing animal and human health. Approximately 20 infectious diseases, including tuberculosis and anthrax have been associated with dust particles.
Deposition of wind blown soil particles can clog canals, lakes and water courses. Other sediment deposited by wind is susceptible to movement by rainfall into water bodies. This sediment and the associated substances attached to it can significantly deteriorate water quality.
Wind erosion may occur wherever the soil is exposed, loose and dry. In Texas it is most severe in the Southern High Plains, Southern Rolling Plains, Rio Grande Plains and Gulf Coastal Regions.
How Wind Moves the Soil Under field conditions, soil begins to move when the wind velocity reaches about 13 miles per hour at 1 foot above the ground surface. People tend to notice dust high in the air. Soil particles carried high in the air are the smaller particles that include the most valuable portions of topsoil – clay and organic matter. However, most soil movement occurs within a foot of the ground. Only a small percentage is carried high in the air. ARS-WERU
Wind Erosion Processes Saltation Surface Creep Suspension
SALTATION • Fine and medium sand and sand sized particles move in this manner. • The wind lifts them a short distance into the air and they “bounce” to the soil surface. Most saltation occurs within 12” of the soil surface. • The “bouncing” impact detaches soil aggregates that can be carried by wind. • Saltation destroys protective surface crusts, making the soil more erodible. • Saltation accounts for 50 – 80% of total soil movement.
Suspension • Very fine soil particles are lifted by the impact of saltation. • They are carried high into the air and suspended for long distances. • Suspension generally accounts for only a small part of the total soil moved by wind. • Small suspended particles are the most fertile parts of eroded soils. • Movement of fine particles off-site tend to make sandy soils sandier.
Surface Creep • The movement of larger, sand sized soil particles along the surface. • Too large to be lifted by the wind, these particles move along the surface in a rolling motion. • Surface creep can account for as much as 25% of the soil moved by wind.
Not all sand sized particles are sand. Certain clay and clay loam soils will form stable sand sized aggregates. Calcareous soils particularly exhibit this type of aggregate stability.
Preventing Erosion Stubble Mulch No-Till Contour Farming Strip Cropping Terracing Prescribed Grazing Secondary Benefits: - Moisture Conservation Increased Crop Yield Prevents Loss of Fertilizer Saves Fuel Improved Structure & Rooting Depth - Sustainability P. 67 Brady
Preventing erosion helps in that… • Moisture Conservation leads to Increased Yields (especially in drier years) and makes money. • Improved Soil Structure leads to Moisture Conservation (see above). • Preventing Fertilizer and PesticideLoss leads to Increased Yields and saves money. • Takes less horse power to use a spray rig than tillage (saves money). • Reduced pollution from fertilizer and pesticides
References • The Nature and Properties of Soils, by Brady • Essentials of Physical Geography, by Gabler, Sager, Brazier and Wise • Field Book for Describing and Sampling Soils, by Schoeneberger, Wysocki, Benham and Broderson • http://munsell.com/ • http://soilcrop.tamu.edu/ • http://soils.usda.gov/ • http://www.tx.nrcs.usda.gov/
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