Clay

Mineral particles with a diameter of less than 0.002mm; being plate like in shape. There are two main scales:

1. The International Scale which categorises clay particles as being than less than 0.002mm in diameter.
2. The US (Udden–Wentworth) scale which categorises clay particles as between 0.00098mm to 0.0039mm, although this scale is not used within turf care industry classifications.

There are different types of clay mineral (being hydrous aluminium silicates) which form clay particles. These can be classified into mineral groups, mostly according to their layered structure, being 1:1 or 2:1. Kaolinites; Smectites; Hydrous Micas – Illites and Vermiculites, being the major groupings.

Clay minerals are formed due to primary minerals, which are present in rocks, being weathered and then reformed into two basic structural units. Whilst sand is typically composed of a simple structural unit, quartz (SiO2), clay is a more complex structure, typically being built from two different components:

1. Silicon tetrahedron, (= 4 corner points. There is one silicon molecule connected to 4 oxygen molecules), which is then built into layers or ‘sheets’, and

2. Aluminium (alumina) octahedron, (= 6 corner points. There is one aluminium molecule connected to 6 oxygen molecules), which is then built into layers, or ‘sheets’.

The different types of sheets can be combined to form different types of clay minerals.

A clay mineral termed 1:1 is one octahedral sheet bonded to one tetrahedral sheet, with an example of this type being Kaolinite.

A clay mineral termed 2:1 is one octahedral sheet, which is typically sandwiched between two tetrahedral sheets, with an example of this type being Vermiculite.

There is another clay mineral termed 2:2 (which is actually an older term), but also termed 2:1:1, think of it as 2:1 plus another layer (octahedron) added onto the sandwich. The additional layer added onto the 2:1 layer is a sheet of cations, commonly Mg, Al and Fe. This layer is also termed the interspace (between two 2:1 sheets) into which is included this additional sheet – hence 2:1:1 or 2:2 (2 tetrahedral sheets and 2 octahedral sheets in the structure). This additional layer is commonly referred to as the brucite-like layer, due to its closer resemblance to the mineral brucite. An example of this type is Chlorite.

Clay particles are mainly negatively charged and thereby attract positively charged ions (cations), such as ammonium, calcium, magnesium and potassium, contributing significantly to a soil’s cation exchange capacity. Negatively charged ions, such as phosphates, are also retained within some areas of the clay particles where localised positive charges exist.

Clay particles influence soil pH by acting as a buffer to rapid change, due to their ability to retain and release hydrogen ions. This buffering effect will often provide for a soil pH level which is suitable for optimum nutrient availability within a soil. However, there is also the possibility this could potentially lead to a nutrient imbalance within a soil solution due to the relatively strong retention of some nutrients on the clay particles.

Being able to retain many nutrients reduces the likelihood of leaching from clay soils, but also attracts and retains water molecules due to the very small size of clay particles and their electrostatic charge. This can make soil water available for longer periods of time during dry and warm conditions, but also increases soil compaction from play and machinery use and waterlogging potential during colder wetter periods. This can reduce available soil oxygen and rooting potential, so only small percentages of clay particles would typically be included within rootzones for sports turf pitches.

Sandy soils, which might appear to provide good drainage potential in theory, can, with small amounts of clay become sealed on the surface and suffer from a low drainage capability, especially when the soil structure is poor and when machinery is used in unsuitable (moist to wet) soil conditions, causing compaction.

Clay particles significantly influence soil structure due to their ability to bind together forming soil aggregates. These can provide natural soils with a good soil structure, although where soils with high clay contents are used for sports then the soil structure will be destroyed, resulting in poor drainage and frequent waterlogging, with consequential match cancellations, which can be common on football and rugby pitches which are marked out on these soil types.

Tennis and cricket pitches have relatively high percentages of clay particles within their rootzones, however, with these sports being played on grass mainly over the spring and summer periods and with good maintenance practices, the surfaces are playable for the majority of the time, which would not be the case if play was to take place over the wetter autumn and winter periods.