Sports Turf Research Institute - Rootzones, Sands & Top Dressing Materials for Sports Turf March 20 2014
Sports turf is a dynamic system with the properties of the turf varying according to the amount of play and level of compaction to which it is subjected. A build up of compaction will tend to reduce the permeability of the surface layer and this will be exacerbated if finer particles of mineral matter migrate to the surface as a result of wear and fine organic residues accumulate from the breakdown of grass material. On fine turf constructions, the accumulation of a thatchy layer of organic material will have an important effect on the impact and roll of golf balls and the speed of bowls running across a green. Maintenance operations are essential to control the potential deterioration of turf resulting from compaction and organic matter production and sand has a vital function in any maintenance programme.
WINTER GAMES PITCHES
The main uses of sand in the maintenance programme for a winter games pitch is to dilute accumulations of fine mineral and organic matter at the surface to retain the permeability of a pitch and to provide a firmer surface in wet weather. In these circumstances pure sand, with no soil amendment, is the preferred top dressing material. Sand or a sandy compost is also important to repair divots kicked out by the players, thus restoring the levels of the surface.
The advantages of sand are clearly seen in Fig. 13 which shows the effects of different rates of sand on a pipe drained sandy loam soil, the same soil with slit drains at 600 mm centres and a sand carpet construction. The sand rates of 0, 4, 8 and 16 kg/m2 per year correspond to annual applications of 0, 25, 50 and 100 tonnes of sand per year to a pitch of 6,250 m2. The advantages of higher grades of construction are evident with the sand carpet pitch retaining more cover during wear than the slit drained and particularly the pipe drained pitch. The sand carpet pitch also has a dry, firm surface where ball bounce, for example, is good. However, players find ball rebound values less than 15% unacceptable and the wet, muddy surface on some of the pipe drained and slit drained plots gave very low values indeed.
Sand application is important on all types of construction: where topsoil is present sand makes the surface layer less plastic in wet conditions giving a firmer and drier surface and on sand constructions it can be important in maintaining surface permeability and diluting organic material which can accumulate in the areas of lower wear, e.g. on the wings. On a slit drained construction the absence of protective sand dressings causes the slits to be sealed by play preventing rainfall from entering the slit system and causing waterlogged surfaces. In the example shown in Fig. 13 the grass cover on the sub-plots with no sand was zero compared to 43% where the equivalent of 100 tonnes per year was used and the wet, muddy surface on the slit drained plots with no sand gave ball rebound only 1% compared with 30% on the area with the highest rate of sand.
The results for the slit drained plots with no sand are similar to those of the pipe drained area, in other words the advantages of a £20,000 slit drainage scheme can disappear in less than 1 ½ seasons of wear because of the lack of adequate sand dressing. Indeed this trial and observations on actual pitches show that even a few months of play on a slit drained pitch with no sand dressing can negate the value of the drainage work.
Fig. 13: The effect of sand top dressing rate on ground cover and ball rebound on pipe drained, slit drained and sand carpet constructions receiving simulated football-type wear. (Adapted from Baker & Canaway [1989, 1990] – data for December 1988.)
GOLF AND BOWLING GREENS
On areas of fine turf the main purpose of top dressing is to preserve a true and level surface and to dilute the build up of thatch. Ideally an annual application of 5-6 kg/m2 of top dressing material will be used, with this quantity being divided into perhaps three applications to avoid excessive amounts of material on the surface at any one time. If the playing surface is on a heavy soil with poor drainage or has excess thatch, larger quantities should be applied in conjunction with a programme of hollow tinning so that the drainage and aeration of the surface layer can be improved.
The composition of the top dressing material may vary depending on the type of construction and the availability of topsoil materials or composts for the top dressing. Where a green has been constructed with a special rootzone mix it is important to use a top dressing material which matches the mix in terms of the quantity of sand and the sand type. This will preserve a continuity in the profile of the green. In the extreme case of a pure sand construction the size and uniformity of the sand should match that of the rootzone sand.
Where a golf green has been developed from a native soil which may have a relatively high silt and clay content, it is sensible to use a relatively light top dressing material in which the clay content has been diluted by sand to below 5% and the total silt and clay content should not exceed 10%. The use of a sand dominated by the medium sand size fraction (0.25-0.5 mm diameter) is preferred for the preparation of this type of top dressing mix.
Sands with a high content of fine material should be avoided as these can clog the surface and excessively coarse sands are not popular as it is difficult to work sand grains above 2 mm in diameter into the turf and coarse particles such as these will damage mowers and are unpopular with golfers and bowlers.
Consistency in the use of top dressing materials on fine turf is important and for instance, use of pure sand for perhaps a year followed by reversion to a sand-soil mix can form a root break. This thin layer of sand can have significant effects on the vertical movement of soil moisture and the penetration of grass roots will tend to reflect the moisture distribution within the soil.
In fine turf areas, the free lime content of the top dressing material can be critical. If the sand contains large quantities of shell or other calcareous material the pH of the surface layer will increase. This can have important effects on weed invasion, earthworm activity, turfgrass disease and the composition of grass species within the turf. In particular annual meadow-grass (Poa annua) will tend to invade the turf at the expense of the fescue (Festuca) and bent (Agrostis) species.
With regard to the build up of lime in a soil in relation to acidification by fertilisers it has been concluded that, to introduce a safety margin, the top dressing mix should contain no more than 0.5% calcium carbonates. If the soil component of the top dressing mix were lime-free a slightly higher lime content in the sand could be tolerated, e.g. a mix of 1 part soil:2 parts sand would still have a total lime content of <0.5% if the lime content of the sand were no more than 0.75%. There are, however, constraints on the ratio of soil to sand in a top dressing mix as the final blend of material must satisfy the physical requirements as well as those of lime content. If the total lime content will exceed 0.5% an alternative source of sand must be considered.