Basic Soils

Is it alive?

Is it fragile?

Should we consider soil a fragile resource?

Soil loss through plowing

Soils

A medium for plant growth1,000 years to form 1” of topsoilDeplete 1” of topsoil in 100 years or less

Soils Composition

Soil is composed of three “fractions”An organic fractionA mineral fractionA water and gas fraction

Soils Composition

An organic fraction organic materials provide a reservoir of plant nutrients nutrients are continually recycled as organic materialsLiving organisms including plant parts, bacteria, fungi, worms, insects, etc.; dead/decomposing organisms

Soils Composition

The mineral fractionSand, silt and clay

Sand and silt – primary mineralsdecomposition of various rock materials referred to as “parent material”

Clays – secondary mineralsdissolved minerals in solutionprecipitate out to form clayscrystalline

Soils Composition

Gasses and waterEx. Oxygen (O), carbon dioxide (CO2), nitrogen (N), etc.; H2O

Fills in all of the pore spaces between the soil particlesLarger pore spaces tend to contain more gassesSmaller pore spaces tend to contain more water

The Mineral Fraction

So what’s a rock? Consolidated mass of mineralsThen what’s a mineral?Unique, repeatable combination of elements And what’s an element?Found on the periodic table, unique combination of protons, neutrons in a nucleus and electrons

Soils Composition

Rocks can be divided into three categoriesIgneous rockSedimentary rockMetamorphic rock

Soils Composition

Igneous rock – molten, liquid magma or lavaDivided into two sub-categoriesIntrusive igneous rockExtrusive igneous rock

Soils Composition

Intrusive igneous rock“magma” cools and hardens slowly within the crustforms distinct crystals from slow coolingoften referred to as plutonic rock ex. granite, quartz

Soils Composition

Extrusive igneous rock“lava” is extruded out of the Earth’s crustcools and hardens quickly outside the crust in contact with air or waterforms small to no crystals from rapid cooling ex. basalt, obsidian

Soils Composition

Sedimentary rockformed from layers of deposited sediments – sands, silts or claysdeposited by wind or waterex. sandstone

Soils Composition

Metamorphic rockrock changed from one form to another over timechanged deep in the earth by heat and pressureex. marble changed from limestone and slate changed from shale

Soils Composition

Sand and silt soils “primary” mineralsdirect product of erosion

Clays“secondary” mineralsresult of chemical weathering breaking down primary minerals minerals leached in solution deposited out as clays

Soils Composition

Soils are the uppermost layers of tectonic platesFormed from weathered and eroded rock or “parent materials”

Weathering

Chemical action of air and rainwater and the biological action of plants and animals Rock and minerals disintegrate, decompose and synthesize new compounds and clay minerals

Erosion

Wearing away of land surfaces by wind, water, ice and rock-on-rock

Soils

Combination of sand, silt and clay and organic material Sand 2.0 mm to 0.05 mm in diameterSilt 0.05 mm to 0.002 mm in diameterClay less than 0.002 mm in diameter

Soil Particle Sizes

Soil Particle Sizes

As the surface area for a given mass (or volume) of particles decrease in size, the total surface area increases geometricallyIn other words—in a given volume, there is more surface area on smaller sized particles than larger sized particles Sand particles are larger than clay particlesSo a given volume of clay particles will have more surface area than sand

Soil Particle Sizes

Imagine 2 identical pencilsOne broken in halfWhich has the greatest amount of surface area?

Bulk Density

Refers to weight per a given volumeWe talk about:"Light" soils having a higher sand content"Heavy" soils having a higher clay contentIn reality—a given volume of clay weighs more than the same volume of sand

Soil Texture

Percentages of sand, silt and clay per volume of soilLoam soils contain a blend of sand, silt and clayLoam soils make some of the better gardening soilsSoil texture determined by a "settling" test

Soil Settling Test100 grams oven dried, cleaned, screened soil into a blender with:5 ml 8% Calgon® (sodium hexametaphosphate) solution100 ml waterBlend 5 minutesPour and rinse blender contents completely into 1000 ml graduated cylinder

Soil Settling Test

Fill graduated cylinder to 1000 ml with waterCover graduated cylinder with palm of hand, invert several times getting soil into suspensionSet cylinder on flat surface Wait 40 sec., insert Bouyoucos hydrometer into solution

Soil Settling Test

Take hydrometer reading Repeat three times and average readingsAdjust for temperaturesTake another reading at 2 hoursAdjust for temperaturesPreform calculations

Soil Settling TestThe Bouyoucos hydrometer measures density of solutionAfter the 40 sec. reading what soil particles remain in solution—sand, silt or clay?Silt and clayWhat has settled out first?SandWhy?Sand is heaviest—it settles first

Soil Settling Test

After the 2 hour reading what soil particles remain in solution—sand, silt or clay? ClayWhat has settled out?Silt Why?Silt is heavier than clay—it settles next

Soil Settling Test

Assuming we started with 100 grams soilThe 40 sec. reading indicated 72 grams in solutionHow many grams have settled out?100g – 72g = 28g28g of what?Sand

Soil Settling Test

The 2 hour reading indicated 15 grams remaining in solutionHow much has settled out of solution?72g – 15g = 57g57g of what?Silt

Soil Settling Test

What is left in solution?15g clay100g soil – (28g sand + 57g silt) = 15g clayWhy is the clay still in solution?Clay is the lightest—it stays in solution

Soil Settling TestHow much sand, silt and clay is there? 28g sand = 28% sand +57g silt = 57% silt +15g clay = 15% clay =100g soil = 100% soilIt checks Refer to soil texture chart to determine soil texture What soil texture do we have?

Soil Texture Settling Test

Turn to the soil texture triangle and find the soil texture class.We have a “silt loam”

Ribbon Test

“Quick and dirty” test for soil textureWith a handful of moistened soil, attempt to squeeze out a ribbon of soil between your thumb and forefingerClay and silt soils feel smoothClay “ribbons” can be 2” longSandy soils feel grittySandy soils won’t form ribbonsSee handouts at end of PowerPoint

Soil Structure

Soil structure is determined by arrangement of soil particlesSoil aggregates are soil particles (sand, silt and clay) held together by iron oxides, calcium carbonate, clays and/or silica as well as humusTexture and structure combined help determine soil pore sizesTexture and structure greatly determine water movement through soils

Soil Porosity

Soil pore sizes determine how well water (and air) moves through soilInfiltration – downward movement water of into soilsPercolation – downward movement water of through soils

Soil Porosity

Sandy soils drain quickly – don’t hold waterSandy soils have mostly large pore spacesClay soils don’t drain well – hold waterClay soils have mostly small pore spaces

Soil Porosity

MicroporesSmall spaces between soil particlesHold water tightly – inhibits drainageRetain various nutrients in solution, making them available to plant rootsDo not aid in circulation of gasses in soil

Soil Porosity

Macropores Larger spaces between soil particlesHold water more loosely - essential to good drainageDo not retain nutrients in solution particularly well Aids in circulation of gasses in soil

Soil Structure

Soil HorizonsHorizon layers and composition are a product of their surroundings. . . .O-Horizon – organic matterA-Horizon – darkened by organic matterE-Horizon – materials leached away downwardB-Horizon – accumulates clays, salts and other fine materialsC-Horizon – loose or cohesive materials, rests on bedrockR-Horizon – bedrock solid rock

Humus

Residue formed after the decomposition of organic matterActs to "glue" soil particles togetherProvides a source of carbon to soilSoils with a higher carbon content referred to as an organic soil (between 12% and 20% organic carbon by weight)Can change soil structure over time

Changing Texture

Soil texture takes years to affect . . . Why?Sand, silt or clay must be added to change textureLarge amounts of sand can be added to change texture and increase drainage to clay soilsNot enough makes mortar!Expensive! Raises soil level – something must be removed

Changing Structure

Soil structure can be changed through tillageTillage can both loosen and compact soilsOrganic amendments can be added to soils to enhance the structureProvides humus through decompositionOrganic materials slow drainage in sandy soilsOrganic materials increase drainage in clays

Changing Structure

Peat moss helps water retentionNever add peat moss to heavy clay soilsPeat moss helps to reduce soil pHUse compost and soil sulfur in clay soilsAddition of salts through fertilizers or irrigation water can eventually inhibit drainage

“Soil Drainage Test”

Good drainage is a determining factor in successful landscapesSimple drainage test – dig a hole about 1’ deepFill the hole with water Determine time required for hole to drain1”/hour – adequate drainageLess than 1”/hour – problems - clayFaster than 1”/hour – problems - sand24 hours or more – serious problemsPuddles after irrigation or rain – indicators of problems

Soil Drainage Test

Soil Reaction

Soil reaction refers to the relative amount of free hydrogen ions ( H+ ) in the soil/water solutionIn other words how acidic or alkaline is the soil solutionSoil reaction is important in that it determines the availability if different nutrientsAlso the susceptibility of plants to various plant disease

pH – Percent Hydrogen

Soil reaction measured by pH on a scale of 1 through 14pH of 7 represents a neutral soilAcidic soils — pH 1 to less than 7Alkaline soils — pH greater than 7 to 14

pH Scale

pH Scale

pH Scale

Appendix: Home Soil Settling Test

You can try this one at home

Soil Texture Settling Test

Spread soil on a newspaper to dry; remove all rocks, trash, roots, etc.; crush lumps and clodsFinely pulverize the soilFill a tall, slender jar (like a quart jar) ¼ full of soilAdd water until the jar is ¾ full.Add a teaspoon of powdered, non-foaming dishwasher detergent

Soil Texture Settling Test

Put on a tight fitting lid and shake hard for 10 to 15 minutes. This shaking breaks apart the soil aggregates and separates the soil into individual mineral particlesSet the jar where it will not be disturbed for 2 to 3 daysSoil particles will settle out according to size; after 1 minute, mark on the jar the depth of the sand

Soil Texture Settling Test

After 2 hours, mark on the jar the depth of the siltWhen the water clears mark on the jar the clay level. This typically takes 1 to 3 days, but with some soils it may take weeks

Measure the thickness of the sand, silt, and clay layers.a. Thickness of sand deposit _____.b. Thickness of silt deposit _____.c. Thickness of clay deposit _____.d. Thickness of total deposit _____.

Soil Texture Settling Test

12. Calculate the percentage of sand, silt, and clay [clay thickness]______________ = _____ percent clay[total thickness][silt thickness]______________ = _____ percent silt[total thickness][sand thickness]______________ = _____ percent sand[total thickness]

Ribbon Test