Why do some cricket pitches crack more than others? Is cracking affected by the type or amount of clay? And do construction methods have an impact? Colin Ashman unravels some of the mysteries
A cricket pitch must crack - because it is only when the pitch is at its driest that it produces the greatest level of performance. Usually, established pitches should only crack during the final stages of the drying process having stable edges and a relatively stable plating of the surface. This is normal, and is rarely of concern to players or umpires. Newly-laid wickets generally exhibit the greatest amount of cracking during the drying process, which can adversely affect play because of their size, fragility and extent. In such cases, some would say these pitches should be kept ‘wetter’ simply to achieve play without being dangerous but with a loss of performance. Cracking can therefore influence the decision when new pitches are ready to receive play.
Construction methods should ensure that the pitch profile is evenly consolidated and level, and that consolidation is at a high level : 1.2g/cm3 to 1.6g/cm3 (achieving a bulk density towards 1.8g/cm3 could be considered as highly desirable) regardless of which method is used. Using a belt-type topdresser and subsequent consolidation at every 3 mm depth is one effective method of achieving higher bulk densities.
Although consolidation is paramount during construction, porosity is seldom considered - and this may partly explain why cracking occurs. A well-consolidated pitch can be considered to have a reduced amount of porosity, simply because consolidation is designed to remove it. The loams used are generally slightly moist (if stored correctly), friable (minimal moisture used for lower strength) and are easist to consolidate. Moisture levels are low so the effects of consolidation are greater; there is less water to resist it.
Once consolidated, a new pitch will contain low levels of porosity but, importantly, its pore spaces are predominantly air-filled but small enough to be water-filled when wet. Small pores can hold water against gravity. Once newly laid pitches get wet they become saturated and have limited air-filled porosity. Drainage in this situation only occurs after the soil cannot hold any more water against gravity, which means the pores will remain largely saturated even at field capacity, which raises questions about the effectiveness of drainage carpets.
Here lies the problem: how can a waterlogged pitch dry sufficiently if the ‘pumps’- the turfgrass roots - cannot develop? Roots cannot grow in a soil with little or no aeration.
A saturated soil profile with little aeration only allows the turfgrass roots to dominate the part of the profile which dries sufficiently to enable their growth; the surface. Over time these roots can begin to develop further depending on remedial treatments, without which the underlying soil will remain saturated because all of the water is held against gravity and cannot dry.
The pitch that can be expected in this type of scenario is crusty, shallow rooted, soft at depth - despite the high bulk density/consolidation because the water content makes it soft - and highly prone to damage caused by rolling too heavy too soon, thus creating bow-waves and, of course, prone to cracking.
Cracking also depends on the type of clay minerals used and the percentage of soil components; sand, silt and clay. Some clay minerals are more prone to shrinking and swelling causing more cracking than others, and sand components especially (not as top dressing) can restrict it. Generally, cracking problems are due to moisture content, and the type of clay and clay loams can compound the problem. Higher clay contents create smaller pores after consolidation, requiring more root ‘encouragement’ and more drying.
Cracking on pitches with shallow rooting is caused by moisture loss as the pitch dries from the surface down (crucially well-rooted pitches dry from the bottom up). With such a high moisture content, the pitch is as such swollen, but starts to shrink when drying. Compaction through rolling can capitalise on this until the surface becomes dry enough to resist movement; it goes from being mouldable to becoming more brittle. Then shrinkage starts to pull apart the pitch at any weakness, forming cracks. The cracks then open up the lower profile to evaporation, which allows the deeper profile to dry and shrink thus deepening the crack.
At the same time, the profile between cracks and below the roots is still wet, and it is this that results in a cracked pitch with a platy surface that moves under force. Drying to depth can only be achieved through the turfgrass roots; that is their primary function.
So, can the amount of cracking be controlled? First, it is recommended that a soil sample determine the health and extent of the root system, to determine exactly what is happening within the profile. Solutions can be varied dependent on the amount of remedial work needed.
The priority is to increase root depth and mass, without the expense of losing too much consolidation or creating inconsistent surfaces. The initial consideration should be increased aeration, either by reducing water content or by increasing pore size (water drains from larger pores).
Increasing the pore size involves solid tine aeration with no heave. Using heave will result in actually reducing consolidation, and should therefore only be considered as a last resort for profiles that are overly consolidated. Solid tine aeration re-arranges soil pore space so lots of smaller pores are made even smaller to accommodate one large one (the tine hole). This tine hole is now suitable for root development, so a gradual process of solid tine aeration will begin to dry the pitch. Solid tine aeration should form part of a pitch maintenance plan as it ensures adequate root development between seasons, drying out the pitch ready for each match.
Reducing the amount of water held in the profile is very interesting. Water is held in soil pores through its surface tension - the same reason why it forms beads on window panes - so removing its surface tension will actually cause it to drain, even if the pores are small enough to retain it against gravity. Wetting agents and surfactants have this affect, so provided they are suitable for use in sports turf they could be used to increase aeration - and increase aeration without compromising consistency.
The ultimate solution may involve a combination of aeration and wetting agents to achieve greater or quicker success, however there is a downside. To prevent loss of playability or safety, solid tine aeration should be limited to the autumn and early pre-Christmas period, although it can still be used carefully afterwards.
Wetting agents also create problems, and should only be used considerately. A pitch relies on the surface tension of water molecules to ‘bind’ the soil together, so applying a wetting agent too close to the playing season will result in a decline in soil strength and could create a pitch that deteriorates rapidly and becomes very dusty. The use of wetting agents is in response to a specific problem caused by excess moisture in the rootzone; they should not be used as an alternative to aeration or used inconsiderately on pitches which do not exhibit major problems.
Prevention is better than cure, and this certainly should be the case with new constructions. What is important is to preserve the air-filled porosity (dryness) of the new pitch. Once the profile gets wet it stays wet. This is because there are not enough roots to dry the profile and after wetting are less able to develop sufficiently - a vicious circle of too wet for root growth, and too wet because there is not enough root growth!
This is where aftercare comes in. After laying a new pitch the surface should be covered against rainfall (especially heavy) during the germination phase and the early part of establishment phase so that the turfgrass roots can take advantage of the aerated soil conditions, before becoming wet. After germination and an acceptable level of establishment, the covers can be permanently removed ready for the following season. It is a difficult call to know when it is suitable to leave the pitch uncovered during rainfall, but any level of establishment that occurs prior to wetting is going to be advantageous the following season.
A final point of interest: it could be suggested that not only do turfgrass roots dry pitches but also by occupying pore spaces (bio-pores) they reduce the amount of water pitches can hold; established pitches stay drier than new ones, and are therefore less susceptible to cracking. Do roots preserve porosity from one season to the next?
Food for thought…from the author
This article is based on both observation and facts. In 2003, four new pitches were laid which don’t crack, and in 2006 three were laid that do. 2003 was a dry autumn, 2006 less so! In 2008 there has been another new construction and an attempt to cover it during rainfall in the early establishment phase to test the ‘idea’ set out in this article – and time will tell if this has proved successful. To date, wetting agents and solid tine aeration have been employed on the 2006 pitches to remedy them.
