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.

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