3. FACTORS DETERMINING PITCH PERFORMANCE
There are four main elements to a good pitch:
(i) the nature of the bulli;
(ii) the way in which the pitch is prepared;
(iii) the way the water content of the bulli is controlled; and
(iii) the type of turf used.
(i) The bulli
(a) bulli composition
The nature of the bulli determines the maximum pace and bounce that can be achieved. The South African and Australian pattern is to use bulli with strong swell/shrink characteristics i.e. bulli which swells when it is wetted and shrinks when it dries, and which is able to provide a pacey bouncy pitch. This characteristic is brought about by a high content of what is known as 2:1 swelling clays. These clay soils have a high binding strength (which is closely correlated with bounce and pace - the greater the binding strength the greater the pace and the higher the bounce). Organic matter in the bulli will have a dampening effect of both pace and bounce.
Alternatively, a high sand content in the bulli will help to provide grip to the ball when it comes into contact with the pitch. This will slow the pace of the ball but will allow for greater turn.
Published reports suggest that ideal bulli should comprise the following fractions:
Clay: 50 - 60% (Note that English and New Zealand pitches
have much lower levels than this - down to 30%)
Coarse sand: < 10%
Calcium carbonate: < 5% (this is often found as whitish nodules in our
heavy clay soils, so avoid such soils). No visible
reaction should take place when hydrochloric acid
(10% dilution) is added to dry soil
Sodium: < 5% (sodium levels are, unfortunately, rather high in
the coastal pitches but there is nothing much that can
be done about it except to make sure the problem is not
accentuated by using bulli with a high sodium content,
or regularly watering during the heat of the day when
evaporation rates are high)
Linear shrinkage: 0.08 to 0.15 (the higher this is the more the pitch will
crack)
Organic matter: < 5% (organic matter in the soil reduces binding strength
and so reduces pace and bounce)
If the clay content of the bulli is too high, cracking becomes a major problem. Cracking is not only dependent on the clay content but also to a large extent on the type of clay mineral dominant in the bulli. It seems also that cracks can become permanent in that they may not close up even on watering if a thick stone drainage layer exists under the bulli. This complicates pitch preparation and it is probably impossible to consistently produce a good pitch with such bulli. Note also that clay percentages can be lower than the above for club and school pitches which do not need to last for four to five days of continuous play.
The bulli should contain no gypsum or any other salts, since this promotes crumbling. Finely divided limestone in the bulli will also cause crumbling, as will a high salinity.
(b) bulli depth
Recommendations for the depth of bulli required are that it should be sufficiently deep to create a hard bound surface but not so deep that drying becomes difficult. Recommended depths range from as little as 70 mm to 150 and even 300 mm. The longer the pitch has to last, the deeper must the bulli be and for Provincial pitches we suggest that it should not be less than about 150 mm. One means of reducing the amount of bulli used during construction is to increasingly dilute the bulli with sand with increasing depth down the profile, but the upper 100 mm or so should always comprise pure bulli.
(ii) Pitch preparation - what to aim at
(a) The condition of the turf
We suggest that our aim should be a pitch well covered with grass and with some green grass on the surface when play starts (although not all would agree with the need for surface green grass). The evenness of the grass turf is particularly important since patchy grass will cause patchy drying and areas which are not occupied by grass will crack excessively at the surface but remain moist at depth (because there are no roots to pump out the water).
Of interest here is the reason for the apparent gain in pace of the ball when it strikes a greentop and the very definite deviation which can be achieved. The former results because the ball looses very little pace when it comes into contact with the 'lubricated' surface produced by the green leaf material and, because it tends to keep lower than on a non- greentop, it reaches the batsman sooner than it would have had it bounced higher (note that it is physically impossible for the ball to actually gain in pace on pitching). The ball seams more readily because the difference in the friction between the seam and the playing surface and the non-seam and the playing surface is higher on a greentop than a non-greentop. We assume here that the seam still bites into the pitch whereas the non-seam portion of the ball will slide through with little resistance from the succulent grassy surface. The ball will therefore seam in the direction of the seam.
(b) Moisture content of bulli
The ideal water content for maximum compaction is known as the optimum water content (Hillel, 1980) (Figure 1). Determining the optimum water content is time consuming, but once known for a specific bulli, it can be used to assist in achieving a range of compaction options.
The pitch should preferably have an even moisture content down to at least 75 mm and deeper since the nature of the bulli throughout this depth will affect the pace of the pitch. This can be achieved only at the start of play and then only provided there is a sufficiently well developed root system and enough active grass leaf to dry out the subsurface layers. During preparation it may be necessary to reduce moisture loss from the surface by covering the pitch with hessian or shade cloth (or by spreading grass cuttings over areas which are not well grassed - more of this later). Alternatively, the pitch can be syringed (wetted with a fine spray) at intervals during the day or in the evening before the covers go on to moisten the surface. Syringing in the evening is probably preferable as it allows time for the water to soak into the surface layers of the pitch. Please note that slow and deep drying is preferable to rapid and surface-only drying.
Figure 1. Typical water content - density curve indicating the maximum density at the optimum water content for a particular compaction effort.
The pace of the pitch will increase as it dries from some very wet state to some intermediate moisture content, when it will then slow down as it dries further (see Figure 2).
Figure 2. Relationship between water content and bounce on a clay sample over a twenty day period under laboratory controlled conditions. The moisture content at which the change takes place varies with the nature of the bulli. For four and five day games, the pitch at the start of play should normally be at a moisture content greater than that which will give maximum pace so that it initially speeds up with further drying. For a one day limited-overs game, the pitch should be near that moisture content which will give maximum pace.
(c) Compaction
The greater the compaction, the higher the bounce and the faster the pitch. To maximise these two, compaction down to a depth of 100 mm should be at least 80% of the maximum potential of the bulli to compact (measured as the bulk density of the bulli i.e. the weight of bulli per unit volume). Such compaction is best achieved by rolling with a roller of appropriate weight when the bulli is at an appropriate moisture content. The marrying of these two is difficult for any groundsman. It is possible to establish this relationship through research (it will vary from bulli to bulli) but the problem is in applying this information in practice. Some results of the different densities achieved when different energy levels are applied are shown in Figure 3. It is clear that when a light roller is used, the optimum water content is higher than would be the case when using a heavier roller, although one would then not achieve the same density.
Figure 3. Hypothetical example of different energy levels (E1>E2>E3>E4) producing different densities at different water contents. (Optimum water content differs at different energies).
Clearly, however, one has to start with uniform deep wetting of the pitch. Since the infiltration rate of bulli is extremely slow, it is preferable to apply water slowly and over a long period, and if possible at night since rapid evaporation during the day can lead to salt accumulation at the surface. Even where a pitch is badly cracked, the water which penetrates through the cracks still has to soak into the solid blocks of bulli between the cracks, and this takes time. Watering must be uniform or the resulting pitch will play inconsistently.
Compaction is then achieved by rolling regularly as the pitch dries. A frequent recommendation is to start with a light roller and to then increase the weight of the roller as rolling proceeds. The reason given for this is that it will create a surface seal. We are not convinced that this is necessarily a good idea. It is possible, though, that this seal will slow the rate of drying at the surface and, provided there is sufficient grass to ensure that water continues to be extracted from sub-surface layers, it may cause a more even drying of the soil profile. We must add, though, that our investigations of coastal pitches lead us to conclude that there was no need for a light roller, but we are prepared to be convinced that it has a role to play. Quite clearly, a heavy roller is needed to compact at depth and it should therefore be used before the deeper layers dry out too much. It must be appreciated that it is the weight of each single roller in a multi-drum roller that is important, and not the combined weight of all the drums.
A good pre-season heavy rolling should help the groundsman to achieve the required compaction more easily than if the pitch is not rolled with a heavy roller at this time.
Rolling on a dry surface, or the use of a vibrating roller, are not generally recommended. Both promote horizontal cleavage in the bulli and may severely damage the root system of the turf grasses.
(d) Thatch
Thatch results from the build-up of organic matter at the surface. (Interestingly, dung used to be sprayed onto pitches in England in the early days to deliberately produce lifeless pitches - no doubt spurred on by the great WG). Thatch build-up should be avoided at all costs. Whether it lies on the surface or within the profile, it acts to cushion the impact of the ball and will slow the pace of the pitch (although it will increase the bounce of moderate to slow paced pitches). When buried in the profile (where it forms an organic matter sandwich) it gives rise to independent layers of bulli. This has a severely negative impact on the pace and bounce of the pitch and is considered to be the main cause of low and uneven bounce on otherwise well prepared pitches. Thatch accumulation is therefore to be avoided at all costs. Dead material should be removed at regular intervals through the season (preferably by verticutting, but if a verticutter is not available, then scarifying and heavy brushing) and particularly before any top-dressing material is applied. All the dead material which is loosened must be removed and care needs to be taken not to sweep this material into cracks. This would prevent the bulli from binding when wetted.
A well known method of stabilizing a degraded pitch is to put some fresh grass clippings on the area to be stabilized and to roll the clippings into the surface of the clay while it is still fairly wet. There are questions as to when and how much of the clippings to put on and what the long term effect of this practice is on the pitch? Figures 4 & 5 provide a summary of some work done in this regard.
Figure 4 shows the effect of adding clippings to the bounce which is achieved. Since moisture content declined steadily with the number of days after compaction that readings were taken, this graph also shows the effect of declining moisture content on bounce.
Figure 4. Qualitative contribution of rolled-in clippings to bounce over a period of twenty days under Laboratory controlled conditions.
Figure 5 shows the negative effect of adding organic matter to bulli on the modules of rupture (crust strengh) of the bulli. The lower this is, the sooner will the pitch begin to crumble. Adding clippings will therefore promote the early break-up of a pitch.
Figure 5. The influence of dry organic matter on the modules of rupture (MR) - a measure of soil strength. (MR = 764.54 - 162 x %OM (r = -0.93)) The danger of layering caused by dead material is the main reason why we do not recommend the use of grass cuttings on pitches since once rolled into the pitch, this material is often difficult to remove completely.
Note that any organic material, including dead and living roots in the profile, will reduce soil strength and therefore the pace and bounce of the pitch, although living roots in particular should
help hold the pitch together and prevent the cracks from opening up too much as it dries. However, there is not a lot we can do about this. We can, however, prevent thatch build-up at the surface and, by correct mowing practices, reduce root die-back.
(e) The use of variable top-dressing
Topdressing with material which has very different swell/shrink characteristics from the existing material will also cause layering since the different materials will not bind together well. Such layering is then often exaggerated when grass roots follow the horizontal cracks between the layers of different material. It is important, therefore, to use top-dressing which has similar swell/shrink characteristics to the pitch bulli.
(f) Mowing
Not more than 30 to 40% of the leaf material should ever be removed at a single mowing. More intense mowing than this would severely retard the development of the root system and cause some of the roots to die - further adding organic matter to the pitch. Therefore, it is often recommended that the cutting height be gradually reduced during preparation. This also means that a greater amount of leaf will remain on the pitch during the preparation period, allowing the grass to more effectively dry out the sub-surface soil.
(g) Layering
All pitches should be free of horizontal layering since this deadens the pitch (and so reduces bounce and pace), promotes shallow rooting (since roots will penetrate to the layer and then grow horizontally instead of proceeding into the next layer) and results in uneven moisture distribution through the pitch profile. Layering is caused by the incorporation of organic matter into the profile, as when topdressing is applied on a pitch which has a quantity of plant material on the surface (to form an organic matter sandwich); when different types of bulli are laid on top of each other; and when a pitch is rolled when its surface is dry but the bulli below the surface is moist. Layering can also be caused by using a vibrating roller, particularly when the pitch is relatively dry.
(h) Irrigation
- Irrigation is best done by using sprinklers which give a fine mist spray. Large droplets are likely to cause 'potholes' on the pitch.
- Avoid pools of water lying on the pitch surface. Water penetrates bulli very slowly.
- After irrigation, grass cuttings often collect at certain points on the surface. These cuttings must be brushed off the pitch.
- A hessian or bidum cover on the pitch in hot, dry, windy weather ensures good moisture retention by the pitch.
(i) Aeration
Grasses, being dependent upon an adequate supply of oxygen, respond favourably to aeration. Aeration is achieved by a soil-slitting machine, or by forking the surface at close spacing. A day after aerating, roll the pitch in a moist state with a light roller. Aeration practices assist in the penetration of irrigation water and of applied fertiliser.
(iii) The type of turf to use
In temperate countries, ryegrass is generally the chosen grass to use on pitches. In the tropical and sub-tropical regions, however, varieties of Bermuda (Cynodon, couch, kweek) are used almost exclusively for cricket pitches. Bayview Bermuda and Skaapplaas are now widely used, having replaced the Magennis which was used in the earlier days.
The characteristics of the grass which are important are as follows:
(a) the grass must obviously grow well in a heavy clay soil and under the local climatic conditions
(b) it should be able to tolerate wear well (so it should have a good underground root system)
(c) it should preferably also have a strong rhizome (underground stem) system since these rhizomes can play an extremely important role in binding the pitch together and in preventing cracks from opening out too wide (provided the pitch is evenly grassed).
(iv) Cracking
As a pitch dries, the bulli shrinks and so cracks develop. These should disappear when the pitch is again wetted.
The impact of cracking is to reduce bounce and pace, and increase variability. It should also increase the amount of spin that a pitch will take, but turn in badly cracked pitches is likely to be inconsistent.
The cracking pattern will influence the behaviour of a pitch. It would clearly be undesirable for cracking to become so severe that the blocks of bulli between the cracks become unstable. This is likely to occur if the pitch is layered close to the surface. We believe, however, that a pattern of many fine cracks is preferable to only a few large cracks.
As mentioned earlier, a strongly rhizomatous grass is preferable to a surface grower as the rhizomes can be extremely effective in holding the pitch together.
(v) Powdering and crumbling
When a pitch powders or crumbles as it dries, the surface roughens. This allows the ball to grip when it pitches and so it will turn. The lower the clay content of the bulli, the greater will be its tendency to powder and crumble as it dries, and so the greater the tendency for the pitch to take spin.
(vi) Do pitches deteriorate with age
Three things are likely to happen to a pitch as it ages:
(a) it will develop layers, either because layers of dead leaf material are incorporated into the pitch or because different types of bulli are used as topdressing,
(b) organic matter will accumulate from dead roots, making the pitch spongy, and
(c) salts, derived from either fertiliser or from irrigation water, may accumulate at the surface.
Careful management will control the build-up of thatch and the accumulation of salts at the surface, but there is nothing much that can be done about the build-up of root organic matter.
