When rolling a cricket pitch we are aiming to achieve a number of outcomes:
1. To compact the soil – this makes the soil harder, meaning it deforms less when impacted by the ball. Without getting too technical, we want both the pitch and the ball to deform as elastically as possible (any deformation during impact is recovered – helping the ball bounce) but not to deform plastically (the deformation is permanent, absorbing energy from the impact and reducing pace and bounce). Hard pitches will offer good bounce, soft pitches will dent and the ball will then start to bounce variably (go up and down) depending on where it lands. The roller can make a pitch harder to a certain extent, but it is the combination of rolling and drying that causes pitches to get match hard – and this relationship between roller and soil moisture content is the basis of the research that Peter Shipton and I did at Cranfield University to develop the ECB Rolling Guidelines.
2. To make the pitch smooth – to remove any variations in level over the pitch that will cause variable bounce. Variations in level will come from a number of factors including shrinking and swelling of the soil, frost heave (in some winters), deformation by players (and groundstaff when its too wet), earthworms and the weather. The aim of rolling is to iron these variations out and make the pitch more consistent.
3. To modify the colour and condition of the grass plant. From work I have been doing with Lee Fortis, Head Groundsman at the Oval, we can see that grass coverage and colour, and in particular the consistency of grass colour, has on pitch pace and consistency of pace. We can manage the pace of a pitch by carefully controlling the amount of moisture in a pitch, the rolling of a pitch, the amount of grass removal and the cut height. A denser sward cover with a greater proportion of green leaf will offer more pace because of lower surface-ball friction, but excessive green grass can cause more seam movement, so content needs to be managed carefully. It can also lead to more variable pace. Managing the colour and density of the grass sward is the most critical challenge for the groundsman in determining the performance of pitches once the pitches are hard and smooth – particularly those used for 4-5 day games as the change over time is greater.
What do rollers do?
When used optimally, rollers will predominantly push the soil downwards forcing the particles of soil closer together making them more compact and therefore harder. The roller also pushes the soil forwards slightly which helps to smooth it out at the surface and make it even harder, but we do not want too much forward movement, or we could make a pitch uneven, and in the worst case – corrugated. The grass is critical in this process -providing a flexible reinforcement of the soil that helps to keep the soil moving downwards and not excessively forwards.
This can be seen in the videos I produced at Cranfield University back in 2009 and still available on YouTube https://www.youtube.com/watch?v=EtalCo06ae8
The effect of the roller is in part a function of roller design (in particular the mass of the roller, the diameter of the roller and the width of the roller), which all affect roller contact stress (force per unit area) and the direction of stress (larger diameter rollers have a greater component of vertical soil movement than smaller diameter rollers). Most commercially available cricket rollers are only effective to about 75 mm depth (depending on these roller factors and the moisture content).
Even more critical than roller design is the moisture content of the pitch – too wet and the pitch can be damaged by the roller causing horizontal movement (and very little vertical movement), but more commonly a lack of compaction because the water content in the soil cannot be compressed.
One of the key findings of the rolling trials at Cranfield University is that rolling is most effective at the optimum moisture content and that rolling method should be to carry out a few passes (say 4-6) and then let the soil dry before rolling again. A lot of time is wasted rolling wet soils that will not compact. A lot of time is wasted rolling dry soils that cannot be compacted by the weight of the roller. It is all about getting the rolling right for your pitch in relation to its moisture content and your roller. Early rolling when the soils are more moist allows improved smoothness and compaction, but pitches only really get harder as they begin to dry out and the soil shrinks. As rolling progresses through the pitch preparation process, the roller becomes less effective at smoothing and compacting, and more about preparing the grass plant. You can optimise rolling relatively easily – have a look at the rolling guidelines, experiment (safely – within small changes of moisture content and timing)) and draw on your experience as you prepare what works on your pitches.
The balance is to achieve no more rolling than is necessary to get good quality pitches. There is a point where you are wasting your time because the roller is having little or no effect, but under cook it and you will have slower pitches with variable bounce. With my own pitches, I want to be sure that I have done enough rolling before I worry about doing too much rolling – but as we are all volunteers with day jobs on our groundstaff, we do not have the time to do excessive rolling, and I know that even if I could spend all day on the roller is not improving our pitches because the increase in hardness with each pass is getting smaller and smaller – it is definitely the law of diminishing returns. The first-class grounds I work with all do their rolling slightly differently, but I can reassure you that none of them are spending all day on the roller.
Pre-season rolling Guidelines
1. When can I start pre-season rolling?
2. Is it worth it?
The questions need to be answered in the reverse order. Pre-season rolling in the right conditions is definitely worth it. Over winter, as soils get wet they swell and the surface can lift (usually unevenly), also the tolerances that can be achieved during end-of season renovations are not as tight as those needed for pitch preparation, and finally in very cold years, frost heave can be experienced too. Pre-season rolling should be done in four directions (across play, on each diagonal and then finishing in the direction of play – often referred to as the ‘Union Jack Pattern’). This allows optimum smoothness to be achieved and can remove small differences in compaction around the roller edge. Pre-season is often the only chance to roll across the square because later in the season once the season starts it can be difficult to roll in any other direction than in the direction of play because of the different preparation states of different pitches.
It is also harder later in the season to get the whole square to the optimum moisture content for rolling whilst – because it can be really hard to evenly wet a square with irrigation. As long as we have good surface levels – nature will get our squares evenly wet. In winter rainfall is higher, pitches are uncovered and evapotranspiration rates (the rate at which water is removed from the soil by the sun and the grass plant) are at their lowest. In summary the square needs to be compacted, it needs to be smoothed and conditions across the whole width of the square are at their optimum as long as the square is not too wet.
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