UNDERSTANDING TURF MANAGEMENTThe third in a series byR.W.Sheard, PhD., P.Ag.

SOIL STRUCTURE, DENSITY AND POROSITY

Ifthe mixture of sand, silt and clayparticles in the soil remained

separated as individual particles thesmaller particles of clay and siltwould migrate into the holes be-tween the larger sand fragments tocreate a dense material such asfound at depth in the subsoil of anormal soil.An analogy would be totake a bin of softballs, fill the binwith marbles, shake it till no moremarbles canbe added, then add SCDpellets until no more of them can beadded. This simulation would havelittle porosity and is analogous to adense, compacted soil.Such a soil would be very poor forroot growth as it would have littleair or water movement and muchresistance to root penetration. Goodturf management requires the use ofcultural practices which will help toreverse this situation.

SOIL STRUCTUREFortunately soil particles tend togroup together into larger, semi-per-manent arrangements known asaggregates (clods, peds, crumbs).The result of aggregation is calledsoil structure and its importance isthat it tends to make soils which arehigh in clay act like sand in terms ofair and water movement.There are a number of factors in-fluencing the ability of a soil to havea stable soil structure. The primaryfactor is soil organic matter whosebreakdown products create acementing effect, holding themineral particles together. Calcium,

Table 1: The relationship between compaction, apparent density andporosity.

Degree of Apparent Total Macro MicroCompaction Density Porosity Porosity Porosity

(gm/cm3) (%) (%) (%)

Low 1.31 50.5 21.5 29.0

Medium 1.49 43.7 15.8 27.9High 1.64 38.1 10.9 27.2

iron and the type ofclay alsohave aneffect.It isnecessary,however, to move theparticles close together before thecementing action can become effec-tive. This movement is broughtabout through root action, soil or-ganisms such as earthworms,freezing and thawing, and wettingand drying of the soil.Of prime importance is the resis-tance of the aggregates to disin-tegration under the destructive for-ces of wind, rain, vehicle and foottraffic;the latter two being the forcesofconcernon sport fields.Stabilityisvery closely related to the amountand type of organic matter present,with turf providing the most effec-tive means of promoting stable soilstructure. Nevertheless under con-tinued traffic, particularly when thesoil is excessively moist, the struc-ture can be destroyed and the soilwill compacted.Soilstructure is of great significancewhere the soil contains silt and clay.Soil structure, however, is not a fac-tor in sport fields constructed on

sand as sands will not form ag-gregates. This is why selection ofthe correct size distribution ofsands is critical to prevent migra-tion of fine sand particles into thespaces between the larger sandgrains, creating a dense root zone.

SOIL DENSITYThe degree of compaction of a soil ismeasured by determining the ap-parent (bulk) density of the soil.Thedetermination is a simple proce-dure, involving the insertion of asmall (5-emdiam. X2-emdeep) ringinto the soil. The ring is carefullyexcavated, trimmed level at the topand bottom and dried for48hours atlOOC. The density is the dry weightof the soil in each cubiccentimetre ofvolume;this volume includes bothsolid particles and the spaces be-tween the particles - the porosity.The more compacted a soil thegreater the density; that is, thegreater the weight of soil particlescompressed into each cubic cen-timetre and the smaller the airspaces (Table 1).

Soils may vary in density from 1.0 to1.75 gm/ cm3. Then ideal soildescribed. in the first article of thisseries would have a density of 1.32.It may be calculated to show thatsuch a value would have 50% porespace. Natural soils with densitiesbeyond 1.55 can be considered tohave compaction problems. Sportsfields constructed on sand, how-ever, can be expected to have adensity as high as 1.65 while stillretaining good air and water charac-teristics.

SOIL POROSITYThe first article in this series describethe relationship between mineraland organic material, air and water.The air and water in the soil existedin the pore space in the soil, thus anunderstanding of soil porosity is afurther step in understanding goodturf management.The size of the pores range from sub-microscopic between silt and clayparticles to the very large poresvisible to the eye which are formedby earthworms and decaying rootchannels - commonly called bio-pores. For this discussion they maybe divided into two broad groups -macro pores, the larger pores, andmicro pores, the smaller range ofpores.Analytical techniques exist whichpermit the assessment of the relativepercentage of each group of pores ina soil sample. The total porosity maybe calculated directly from the den-sity measurements. This singlevalue, however, says nothing about

the size of the pores which is impor-tant in water movement.Other techniques, which give an as-sessment of the relative size of thepores are based on the amount ofwater retained in the soil in a con-tainer, such as the ring describedabove, when it is placed in a specialapparatus to which suction can beapplied. When the soil is saturatedwith water 100% of the pores arefilled with water, thus the total porespace equals the volume of water ina measured volume of soil. If the soilis allowed to drain freely until all thewater that will flow out due to thepull of gravity has occurred themacro pores will be empty. Thevolume of water that is removed inthis process from a measuredvolume of soil is the macro porosity.Since the macro porosity plus themicro porosity must equal the totalporosity the micro porosity is ob-tained by subtraction.Macro porosity (non-capillaryporosity, aeration porosity) is im-portant in the rapid drainage ofexcess water from the soil after aheavy rain or excessive irrigation.Rapid removal of this water allowsthe air essential for root function toreturn to the soil..Micro porosity (capillary porosity)retains the water required for plantgrowth. It also becomes filled withair as the plant extracts the waterfrom these fine pores. The water inthese pores is not lost by the forcesof gravity. However, the smaller thepores, the greater the difficulty theplant experiences in extracting the

water.Compaction results in a reduction inthe number, size and continuity ofthe soil pores. Generally the macropores will be destroyed first (Tablel),reducingtherateofwaterinfiltra-tion and the drainage ability of thesoil, the aeration of the soil and theability of roots to penetrate the soil.The lack of oxygen further reducesthe grass roots ability to penetratethe soil. The more moist a soil themore easily it can be compacted be-cause the water acts as a lubricantallowing the particles to move intocloser arrangements.Fortunately, the grass ecosystem isrecognized as the optimum systemfor promoting soil structure, hencereducing the apparent density of thesoil and increasing the porosity ofthe soil, particularly the macroporosity. Hence using cultural prac-tices which favour vigorous turfgrowth, combined with adequatedrainage, is the best preventativesystem against compaction.