By Julia Rudolph RootZone Laboratories International

It is important to know what to look for when selecting soils to be used in a pitch, either as topdressing or for a complete reconstruction…

This article will concentrate on the physical properties of wicket soil, however chemical properties such as pH and sodium content should also be considered.

What are the Physical Properties of a Good Cricket Wicket Soil?

This apparatus is used to measure the crushing strength, or cohesion, of a soil

Clay Content
To provide a sufficiently hard pitch for good ball bounce under Australian conditions, and to show acceptable change over the course of a match, the clay content needs to be between 50% and 70%. A higher clay content is likely to cause cracks that are too wide. Sand grains tend to initiate cracking but there should not be too much coarse sand as this can cause ball damage. If there is excess silt cohesion will be reduced, powdering will occur and the amount of cracking can be reduced.

Cracking characteristics
The clay used must be a cracking clay, and as a rule the Smectite (Montmorillonite) and Illite clays make the best wickets. Smectite clays are preferred as they provide the greatest plasticity, cohesion and hardness. A soil high in Kaolinite should be avoided as strong hydrogen bonding means that kaolinite clay remains strongly bound together when wet, with the result that the soil does not swell, shrink or crack. Cracks are essential for good grass growth, as they allow both air and water to penetrate the block which allows the grass roots to grow. If the clay has very little cracking, or the cracks are too fine, the top of the wicket quickly seals up and it is very difficult to wet the whole clay profile.

If the clay cracks excessively, the cracks become too wide and this can become dangerous to the batsman, as a ball from a fast bowler can hit the edge of a crack and deviate wildly. The cracking pattern is important. If cracks are too far apart grass growth will be poor because there will be insufficient air and water reaching the roots. If cracks are too close together the surface may become too crumbly. Ideally crack width should never be more than 10mm. The cracking pattern and size of the blocks formed mainly depend on the soil type and its salt content. The depth of the soil layer and rate of drying, as well as rolling also has some impact.

Clay shrinkage
The soil should swell and shrink with changes in water content. Clay mineral types differ in their potential for shrinkage as they dry. Shrinkage is needed in order to get cracking in pitches. However, excessive shrinkage and a very large distance between cracks will result in cracks forming which are too wide under dry conditions.

Crushing strength
A good cricket wicket soil should have an adequate cohesion (or crushing strength), to withstand hundreds of ball impacts, and so the shrinkage will result in a large blocky structure, with no crumbling or dust after lengthy periods of continued impact on a dry wicket. However, cohesion that is too high may result in a concrete hard surface with very large distances between cracks, which will cause very poor conditions for grass growth and difficulty in rewetting of the wicket.

Stable clay aggregates
An Emerson test shows how well clay particles stick together in soil aggregates. The type of clay minerals, the type of cations adsorbed to the clay, and the concentration of soluble salts all affect the clay aggregate stability. The test results show the rate of dispersion of a soil. This is a quick and easy test that gives an indication of whether problems with water infiltration are likely. In a dispersing clay cracks may close so quickly at the surface that not enough water infiltrates to wet the layer below.

Testing of Wicket Soils and Specifications
How can the physical properties of a cricket wicket soil be tested in the laboratory?

Mechanical (particle size) analysis A set of sieves is used to measure particles above 0.053mm in size with a hydrometer being used for particles smaller than this. The test results should be compared against this standard: 50-70% of particles below 0.002mm (2µm); clay. 20-50% of particles in the 0.002-0.250mm range; fine silt to fine sand. 0-10% of particles in the 0.250-1.0mm range; medium and coarse sand.

There should not be any particles bigger than 1.0mm. If the clay content is too high or low, the silt content too high or the coarse sand too high this soil will probably not be suitable. However, the other tests should still be carried out on a new source of wicket soil as this first test only shows the proportion of each particle range, and not how it will behave in the field.

Cracking, shrinkage and cohesion tests
To test the cracking, shrinkage and cohesion there are very specific tests. For these tests wet and remoulded soil is used. Once the soil has been prepared for the tests the samples are left to air dry for a week or more before the measurements are taken.

The cracking is measured on 100mm diameter, 10mm deep Petri dishes. Upon drying the soil should break into pieces - two to five pieces per dish is preferable. If the soil breaks into eleven or more pieces the soil is too crumbly. This test gives an indication of a good or bad cracking pattern. The cracking pattern in the field will be different because the soil is deeper and the cracks will not be as close together.

This apparatus is used to measure the crushing strength, or cohesion, of a soil

Shrinkage is measured on small, prepared cores of about 25mm in diameter and 25mm to 40mm in length. Shrinkage from field capacity to air dry should be between 8% and 15%, preferably 10%. If the soil does not shrink enough or shrinks too much it is not suitable. The crushing strength of the clay, or the effective cohesion, is measured on small balls of soil, 5mm to 25mm in diameter. The individual balls of clay are placed in a laboratory press and a steadily increasing amount of pressure applied. A reading of pressure is taken at the point where the ball splits. Crushing strength should be between 0.8 and 1.6MPa. If the crushing strength is less than 0.8MPa the soil is likely to be too crumbly, and if above 1.6MPa the soil is too strong and the cracks will be a long way apart.

Each simple test cannot be evaluated in isolation, the combination of different test values is important in order to gain an overall picture of the likely performance of the soil in the field.

Topdressing
It is vital to know what your wicket consists of in order to make informed management decisions, including the application of topdressing. The wicket must be top dressed with a soil that has similar characteristics to the soil that is already there. This does not just mean the same clay content, although that is important, but also the same shrinkage, cracking and cohesion characteristics. If you topdress with a soil with different characteristics the two soils will not bind together, will swell and shrink at different rates, and you will end up with splitting, plating or layering problems which can be difficult to rectify.

Sampling Soil from a Wicket
It is essential to be careful when sampling and to ensure that the sample you send to a laboratory for testing is representative of all the soil in the wicket. Do not take samples from the edges of the wicket table where there could be contamination from soil, for example loam, from the rest of the field. Several soil cores should be taken at random from each wicket and the holes backfilled with compatible cricket wicket soil and compacted.

Impact of Wicket Preparation
Soil testing can choose a material that has the correct properties for a good cricket wicket, but it should be pointed out that in many cases it is impossible to find a soil that meets all the specifications. Cultural practices and preparation can also have a very big impact on how any soil behaves in a wicket. Remember that it is still possible to prepare an acceptable wicket from soil outside specification.

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