UNDERSTANDING TURF MANAGEMENT The second in a series by R.W. Sheard, PhD., P.Ag. SAND, SILT and CLAY A ny sample of soil is made up of a continuous array of particles which may range from the very smallest clay particles to large gravel. In order to describe soils scientist have established a clas- sification system for ranges of particle sizes; size ranges which reflect their role in many of the soil properties we are familiar with. This classification system divides the particles into four classes - gravel and stone, sand, silt, and clay. Table 1 lists the Canadian classification system for the size range of particles which may be found in a soil sample. CLAY The scientists have assigned the name clay to the finest particles and not without reason. Clay size par- ticles are the source of most of the chemicalproperties of soil.They are responsible for the retention of many of the plant nutrients in the soil, such as calcium, magnesium, potassium, trace elements and some of the phosphorus. Clays react with the breakdown products of organic matter to stabilize the humus in the soil.A soilwithout clayparticles can be a very infertile soil. Clays, because of their very small size and very large surface area, are able to retain greater amounts of water than sandy soils. On the other hand, as will be discussed in a latter article, clays hold the water more closely and do not release the water as readily to grass roots as sands. Clay particles have a vastly greater tendency to stick together than sand, thus it is common farmer knowledge that soilshigh in clayare difficult to till.When a small sample of a clay soil is wetted and rubbed between the fingers it will feel very Table 1: The classification system for soil particles; their sizes. surface area and visibility. Soli Separate Size Range Visibility Surface area (rnrn) Clay Silt Very fine sand Fine sand Medium sand Course sand Very coarse sand Gravel Boulders less than .002 .002 to .05 .05 to .10 .10 to .25 .25 to .50 .50 to 1.0 1.0 to 2.0 2.0 to 100 larger than 100 23.000 2.100 electron microscope light microscope light microscope human eye human eye human eye human eye human eye human eye 210 21 sticky and is easily formed into a string. SILT The particles classified as silt are in- termediate in size and chemicaland physical properties between clay and sand. The silt particles have limited ability to retain plant nutrients, or to release them to the soil solution for plant uptake. Silt tends to have a spherical shape, giving a high silt soil a soapy or slippery feeling when rubbed be- tween the fingers when wet and is more difficult to form into a string than clays. Because of the spherical shape, silt also retains a large amount of water, but it releases the water readily to plants. While silt soils are generally considered very fertile for the growth of plants, largely due to their water characteristics and ease of cultivation, engineers dread working with them due to their rela- tively easy release of water and lack of ability for the particles to stick together. SAND Sand particles are essentially small rock fragments, and as such, have little or no ability to supply grass with nutrients or to retain them against leaching.Asrockfragments, sandy soils feel gritty between the fingers. The sand grains have little ability to stick together; thus sandy soils can not be rolled into a string when wetted. It is well known that sandy soils are droughty soils because they retain little water when wetted. Neverthe- less what water is retained is
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UNDERSTANDING TURF MANAGEMENTThe second in a series byR.W. Sheard, PhD., P.Ag.
SAND, SILT and CLAYAny sample of soil ismade up of
a continuous array of particleswhich may range from the verysmallest clay particles to largegravel. In order to describe soilsscientist have established a clas-sification system for ranges ofparticle sizes; size ranges whichreflect their role in many of the soilproperties we are familiarwith. Thisclassification system divides theparticles into four classes - graveland stone, sand, silt, and clay.Table1 lists the Canadian classificationsystem for the size range ofparticleswhich may be found in a soilsample.
CLAYThe scientists have assigned thename clay to the finest particles andnot without reason. Clay size par-ticles are the source of most of thechemicalproperties of soil.They are
responsible for the retention ofmany of the plant nutrients in thesoil, such as calcium, magnesium,potassium, trace elements and someof the phosphorus. Clays react withthe breakdown products of organicmatter to stabilize the humus in thesoil.Asoilwithout clayparticles canbe a very infertile soil.Clays, because of their very smallsize and very large surface area, areable to retain greater amounts ofwater than sandy soils.On the otherhand, as will be discussed in a latterarticle, clays hold the water morecloselyand do not release the wateras readily to grass roots as sands.Clay particles have a vastly greatertendency to stick together thansand, thus it is common farmerknowledge that soilshigh in clayaredifficult to till.When a small sampleof a clay soil is wetted and rubbedbetween the fingers it will feel very
Table 1: The classification system for soil particles; their sizes. surfacearea and visibility.
Soli Separate Size Range VisibilitySurface area
(rnrn)
ClaySiltVery fine sandFine sandMedium sandCourse sandVery coarse sandGravelBoulders
less than .002.002 to .05.05 to .10.10 to .25.25 to .50.50 to 1.01.0 to 2.02.0 to 100
larger than 100
23.0002.100
electron microscopelight microscopelight microscope
human eyehuman eyehuman eyehuman eyehuman eyehuman eye
210
21
sticky and is easily formed into astring.
SILTThe particles classifiedas silt are in-termediate in sizeand chemicalandphysical properties between clayand sand. The silt particles havelimited ability to retain plantnutrients, or to release them to thesoil solution for plant uptake. Silttends to have a spherical shape,giving a high silt soil a soapy orslippery feeling when rubbed be-tween the fingers when wet and ismore difficult to form into a stringthan clays.Because of the spherical shape, siltalso retains a large amount of water,but it releases the water readily toplants. While silt soils are generallyconsidered very fertile for thegrowth of plants, largely due totheir water characteristics and easeof cultivation, engineers dreadworking with them due to their rela-tively easy release of water and lackof ability for the particles to sticktogether.
SANDSand particles are essentially smallrock fragments, and as such, havelittle or no ability to supply grasswith nutrients or to retain themagainst leaching.Asrock fragments,sandy soils feel gritty between thefingers. The sand grains have littleability to stick together; thus sandysoils can not be rolled into a stringwhen wetted.It is well known that sandy soils aredroughty soils because they retainlittle water when wetted. Neverthe-less what water is retained is
Fig. 1: The textural triangle used for assigning textural class names. (Astraight line drawn along the grid for the percentage clay and for thepercentage silt will intersect within a class name area. Try it using thenumbers in Table 2.)
..% sand
Table 2: Some typical particle size analysis and the correspondingclass name.
Panicle size analysisSand Slit Clay Textural class name
- % by weight -
92.5 5.5 2.0 Sand61.0 28.0 11.0 Sandy loam40.0 41.0 19.0 Loam20.0 61.0 19.0 Silt loam3.5 89.0 7.5 Silt28.5 42.0 31.0 Clay Loam10.0 31.0 59.0 Clay
released to plants easily. When rainor irrigation occurs the water readi-ly penetrates the soil surface, theexcess moves through rapidly andthe soil remains well aerated. Theseproperties make sands a desirablemedium for growing sports turfwhere there is no limitation in ap-pling water and nutrition, asneeded, throughout the season.The analysis for the distribution ofthe various particles in a soil sample(particle size analysis) for the sandportion is done using a set of screenswhich have mesh sizes that retainthe various ranges of sand particles.Estimating the silt and clay is morecomplicated as it requires measur-ing the rate of settling of theparticles in water. The analysis isbased on a law of physics whichsays that small diameter particlessettle more slowly in water thanlarger diameter particles. By dis-persing a sample of soil in a columnof water so that all particles will set-tle independently of each other andtaking samples at a set depth andtime interval, the technician can cal-culate how much silt and clay is inthe sample.Because there can be an infinitearray of percentages of sand, silt andclay in soils, scientist have devised aprocedure for classifying the poten-tial combinations into 12 groupswhich reflect broad soil properties.These groups are called texturalclass names and are obtained byap-plying the particle size analysis to atextural triangle (Figure 1). Thus asoil which contains 40% sand, 40%silt and 20% sand would be called aloam soil. Table 2 lists several tex-tural class names and a typicalparticle size distribution for each.Some confusion occurs between theuse of these terms by many turfmanagers and the soil scientist. Thesoil scientist considers the nameloam or clay to refer to a range ofparticle sizes in a soil. The turfmanager often refers to a loam as asoil which is easy to till or is notcompacted whereas a clay is dif-
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Sand, Silt and Claycontinued from page 5
ficult to work, is compacted or maybe a subsoil.In summary many chemical andphysical properties of a soil can beassessed in rough terms from aknowledge of a particle size analysisor the textural class name. For ex-ample in comparing a clay to asandy loam it can be expected that1) the clay will be relatively more
fertile,2) the clay will have greater
nutrient holding capacity forpotassium, calcium andmagnesium,
3) the clay will have more organicmatter,
4) the clay will hold more plantavailable water and be lessdroughty,
5) the clay will have smaller poresresulting in slower air andwater movement within thesoil, and
6) the clay will have greaterstickiness, ability to be retain ashape when moulded and beharder when dry.
Golf & Spans TURFMagazineIf you are not getting Golf &SportsTURF Magazine, you aremissing out on a well writtenand informative magazine ,-$50.00 U.S. funds.Order your subscription bysending in your name, address,city, province, postal code andpayment to:Golf & SportsTURFP.O Box 8420,Van Nuys, Ca91409USA
The Sports Turf Associationextends a warm welcometo New Members ...
...who have joined since August of 1990.
Dennis AmottPeter BookerTom Coffey ..Dave CowanJoe Holmes.Darrell PomfretGerry CompanionDan Ellis ...Grant McKeichRon ScheepersJane Rivers . .Mike Gorman . .Herman KrecherRick ReevesJeff Maslanka .Fred WilliamsIan Blundy ..John HopkinsSteve PollardHeather KennyMarie Thorne .Larry White . .Richard DwyerMark BishopPaul Turner . .Karen Nesbitt .Bruce SheppardTrent Baccardax
Hamilton Parks and RecreationHamilton Parks and RecreationHamilton Parks and RecreationHamilton Parks and RecreationHamilton Parks and RecreationHamilton Parks and RecreationYork Region Board of EducationYork Region Board of EducationYork Region Board of EducationYork Region Board of EducationOakville Parks and RecreationTown of Vaughan Parks DepartmentTown of Vaughan Parks DepartmentTown of Vaughan Parks DepartmentKitchener Parks DepartmentCity of Kingston ParksSeneca CollegeLawrence Park Bowling GreenGreen Thumb Horticultural ServicesGreen Cross Ltd.Green Cross Ltd.Pickseed Canada Inc.Pickseed Canada Inc.Bishop Seeds Ltd.Bannerman Turf EquipmentNesbitt AggregatesUnited Agricultural ProductsRain Bird Canada Ltd.
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