by the surface area actually in contact withthe soil. The area of contact should be aslarge as possible; thus the use of turf tiresor flotation tires on turf maintenanceequipment.

James Beard provides some interestingnumbers in his book 'Turfgrass, Science& Culture' on the effect of the athletes'footwear on the pressure applied to the soilsurface. A comparison is made of a 220-lbperson wearing football shoes versuswearing regular street shoes. A street shoecontains approximately 32 sq. inches ofeffective surface area, thus in walking the220-lb person would exert a pressure of6.25 lb/sq. inch. In contrast, a footballshoe has seven, 0.9-cm diameter cleats,providing an effective surface area of 1.3sq. inches in contact with the soil surface.Thus the static pressure exerted by the220-lb person increases to 150 lb/sq. inchunder his running foot.

Compaction due to the athlete or to turfmaintenance equipment is generally con-fined to the upper three inches. In manycases it may be a zone of one inch or less.

The remedy for compaction of existingturf is turf cultivation, often called aerifi-cation. The principle types of cultivationfor the relief of compaction are coring,slicing, and spiking.

The three cultivation machines may bedivided into two distinctly different sys-tems based on their physical operation.Coring is the practice by which hollowtines or spoons are used to extract cores ofsoil to a depth of two to three inches. Onthe other hand, slicing is a procedure inwhich a solid knife, mounted on a rotatingdrum, is sliced into the top two to threeinches of soil. Spiking is a similar proce-dure in which a solid spike or prong isforced into the soil up to six inches andwithdrawn. The spike may be vibrated toa degree to cause some shattering effect to

UNDERSTANDING TURF MANAGEMENTThe Thirteenth in a series by

R.W. Sheard, P.Ag.

SOIL COMPACTION

A major problem facing every man-ager of intensi vely used sports fields

is compaction. While surface wear fromintensive use will visibly remove aboveground vegetation, simultaneously a non-visible problem may be occurring belowthe surface in the form of compaction.

The combined wear and compaction cre-ates a double restriction on root develop-ment of the grass plant. Removal of topgrowth reduces photosynthesis, thus thefood required for root growth. Compac-tion reduces the porosity of the soil, re-stricting normal exchange of oxygen andcarbon dioxide and other potentially toxicgases. Under very compacted conditionsroot elongation may even be impeded. Thetotal effect on root growth is thus muchgreater than would be expected throughremoval of top growth only.

Compaction occurs when the soil parti-cles moved into closer fitting arrange-ments. Water is the lubricant whichfacilitates the movement. Thus play underwet conditions when the soil is near satu-ration is more likely to lead to compactionthan when the soil is dry. Drainage toremove excess water rapidly from the rootzone is the one step which may be takento assist in long term correction of a com-paction problem.

The clay content of the soil is anothervery important factor. While sand and siltparticles may be spherical or angular inshape, clay particles tend to be flat; hencethey are often referred to as clay plates.These plates easily slide over each otherand tend to become layered under pressureat high moisture contents. As the layeredarrangement becomes more prevalent, po-rosity decreases. In addition the muchsmaller clay particles tend to slide into thepores between the larger sand and silt par-ticles, intensifying the reduction in poros-ity.

If you have ever observed a road beingconstructed you would have seen thewater truck spraying on water, followedby the various types of rollers. The con-tractor is attempting to maximize compac-tion by providing heavy traffic at a highmoisture content.

But a compacted roadbed is obviouslynot the media for growing grass.

Thus compaction results from traffic,whether it comes from the players foot orfrom the machines used in construction ormaintenance. Most sports fields are seri-ously compacted at the end of the con-struction process; and to a significantdepth. The size of the equipment used inmoving the root zone into place, combinedwith the repetitive trips by the motorgrader establishing the finished grade, in-variably result in a compacted system be-fore the turf is ever seeded or sodded.

Often the contractor is under time re-straints. Thus some, or all of the work maybe conducted with large machines underexcessively wet conditions. The primarybusiness of most earth moving contactorsis in road construction; they just followingtheir normal practices when they get arelatively small contract for a playingfield.It is important that the sports turf man-

ager realizes that compaction may existbefore the field is sodded and he shouldendeavour to correct the problem first.Deep tillage with heavy duty aerifiers oreven the use of a farm-type chisel plough,operated in several directions will assist inreducing the compaction. For maximumeffect the trick is to do the remedial aeri-fication or tillage when the soil is some-what less than field capacity; that is - dry.

Compaction is the cumulative effect offrequency and intensity of pressure ap-plied to the soil surface. Pressure is theweight of the pedestrian or vehicle divide

the surrounding soil.The two systems are distinctly different

in operation because they involve two dif-ferent principles. Coring involves the re-moval of an intact core of soil which isdeposited on the surface or removed fromsite. It results in a minimum disturbanceof the surrounding soil (Fig. 1). Compac-tion is alleviated by breaking up the re-moved cores and matting them back intothe holes, or removing the cores from siteand backfilling the holes with a suitabletopdressing material. The procedure isbest performed when the soil moisture isat field capacity to reduce the force re-quired to penetrate the compacted soil andto assist in removing intact cores. Follow-ing two or three hours of drying of thecores on a sunny day, they may be easilybroken and matted or vertical mowed backinto the surface. On a clay soil, allowingthe cores to completely dry may result innearly rock-like objects which cannot bebroken until the next rain or irrigation.

Slicing and spiking involve forcing aknife or prong into the soil and opening a

slit or hole. The soil existing in the volumeoccupied by the slit or hole must be forcedsideways or upward, thus increasing thedensity of the soil in the immediate vicin-ity of the slice or hole (Fig. 1). Relief ofcompaction occurs when there is a vibra-tion effect associated with the penetrationof the knife or prong which shatters thecompacted zone. To maximize the vibra-tion effect, the cultivation should be doneunder relatively dry conditions, which intum requires more force to permit pene-tration of the knife or prong.

Slicing and spiking are generally lessdisruptive of the turf surface and are lesslabour intensive than coring. They areused, therefore, as a routine operation forentire fields whereas coring is restricted tohigh traffic areas where overseeding is tobe done or the root zone is to be modifiedby the addition of a topdressing material.

Coring is a practice which should beintegrated with overseeding as loose soilis provided to aid in covering of the seedand improvement of germination. At thesame time it must be realized that weed

Figure 1: An illustration of coring versus spiking. The lenght of arrows represent therelative amount of compression of displaced soil into the surrounding soil. The wavylines indicate the zone of shattering.

COREAERIFIER

SPIKEAERIFIER

seeds are also encouraged to germinate, afactor which applies to all types of turfcultivation which disturb the dense can-opy of turf. Scheduling turf cultivation toseasons when annual weed germination isnot a significant factor, such as early fall,can reduce the problem.

The total removal of a compacted zoneby coring will take a number of years. A0.75 inch diam. core, on two-inch centres,will remove 11 sq. inches of surface areaper square foot per operation. Assumingthat the machine may be driven in eachoperation so that no recoring of the samehole occurs, it would take a minimum of13 operations to remove the compactedlayer.It is generally recommended that culti-

vation not be performed when the turf isunder heat or moisture stress. The otherfactors, such as irrigation,nitrogen fertili-zation and correct mowing height, whichcontribute to vigorous turf, should be inoperation before cultivation occurs. Nev-ertheless, the need for relatively dry con-ditions to optimize the shattering effect ofspiking should not be overlooked.

One of the principle reasons for the con-struction of all sand rooting zones for turfis to avoid the problems of compaction.Sands, selected according to the USGAspecifications, do not compact to any sig-nificant degree beyond that which existsat the completion of construction. Whilethe resulting porosity of all sand rootingzone may be ten percentage points lessthan that of a normal soil, one is assuredthat it will remain constant, as will therelationship between micro and macro po-rosity, even under a high level of use. Thesame can not be said for fields built withclay soils where the macro porosity willbe reduced by compaction.