Another dimension to sports pitch performance
Performance Quality Standards.
Pitchcare Journal. 20 September 2013
Further to the current Performance Quality Standard (PQS) there are a number of additional factors that deserve consideration. Established to set a standard for natural grass pitches, the PQS does seem to be inadequate in certain criteria. As it was developed to ensure funding produces pitches of sufficient quality for club and community use, it would be expected that the standard covers the essential requirements in a grassed football pitch.
In essence, we are striving to achieve a standard that, while being within the budget of the controlling authority, provides a firm grassed playing surface that can support regular use throughout the wet winter months. Conditions will vary considerably during the year and it seems the best time for assessment is in the middle of winter when the pitches are in full use.
Vital criteria that deserve attention appear to be:
The degree of ground cover will deteriorate with the progress of the playing season. Irrespective of the level of construction and the cultural practices undertaken in maintenance, grass cover will tolerate a limited amount of wear. The density of grass growth is far more important than ground cover. The latter is quickly lost with heavy wear but it is the density of plant shoots that determine the durability of a grassed surface – and this should be specified. The sward height is of little value when the density of grass cover is low. Density can only be achieved with regular and timely mowing, adequate nutrition and good soil and growing conditions with adequate moisture. Mowing as we know is the most influencing factor and for best results cannot be programmed by the calendar – the frequency ensuring that the grass growth is never set back – and in particular with no more than a third of the plant height ever be removed in a single mowing. The preferable sward height depends to a large extent on the growing conditions. There would be the tolerance in sward height with the current desired height for football around 25mm while in rugby the preferred height appears to be nearer 50mm than 75mm.
A pitch sown to a recommended grass seed mixture would be expected to be dominated with the species sown and relatively free of foreign grasses and broad leaved weeds. Prior to site clearing all existing grass and weed cover are sprayed with glyphosate as failure to do this can result in the regeneration of indigenous grasses such as Yorkshire fog and other pasture species. Summer seeding without irrigation can often limit success with germination and brings the possibility of foreign grass and weed encroachment. Apart from being unacceptable in the turfgrass sward these invading grasses and weeds do not provide the density and durability of the preferred ryegrass cultivers sown. Limits in the content acceptable should be clearly specified and restricted.
There is a misunderstanding between infiltration and drainage. Apart from premier football pitches where sandy rootzones are installed, the vast majority of pitches are limited to the local topsoil that is available and infiltration rates are very low and below the PQS standard. Infiltration rates measured with a double ring infiltrometer would not be expected to exceed 1mm per hour in compacted sandy loam or clay loam topsoils which contain a wide range of particle sizes and especially high silt and clay content. Hence this limitation becomes a vital factor.
On the other hand, with adequate provision for the removal of surplus surface water even sandy loam or clay loam topsoiled pitches can remain firm and playable within 24 hours of heavy rainfall. This achievement, though not covered in the PQS can only be attained with the installation and maintenance of secondary drainage provisions. Furthermore, amelioration with sand in the surface layers, even if adequate to create the desired particle size distribution for improved infiltration, does not improve the drainage potential if adequate drainage provisions below the impermeable layers are not effective.
The procedure of laser grading is accepted as an essential requirement in the final preparation of a pitch. A tolerance of 25mm above and below the desired level is generally accepted over the entire pitch but it is the depression in level over short distances that brings problems with surface water collection and initiates the formation of soft muddy areas. These depressions as a result of inadequate grading can occur over a distance of 5 to 10m and some cannot be identified over a 2m straight edge. Hence depressions that hold water remain a concern whatever distance they are measured over. It is significant too that the flatter the final surface the greater is the possibility of minor depressions developing – and this results in soft wetter areas. Dressing out depressed areas on graded pitches is laborious and not easily attained
Slope on the pitch is probably the greatest contributor to pitch performance in the wet winter months. Hence it is puzzling to note that reference is only made in the PQS to maximum slopes in the direction of play and across the pitch. This immediately implies that a level pitch is acceptable. A level pitch promotes no surface drainage and on sandy loam and clay loam topsoiled pitches water is bound to collect in isolated depressions and develop wet muddy areas. Installing secondary drainage into level areas does assist in removing surplus surface water but there remains the tendency for water to persist on the surface after drainage is complete – particularly after heavy rain.
Creating an acceptable cross-play gradient ensures run-off of rainfall that cannot be retained in the grassed surface and drainage installation. Graphic evidence is seen in the collection of surface water in swales running down the sides of adequately sloped pitches.
So what cross-pitch gradient is needed? There is little documentation on the subject but WJ Adams and RJ Gibbs in their publication Natural Turf for Sport and Amenity maintain the need for surface gradient on slit drained pitches to be in the region of 1:67 to 1:100 diagonally. K McIntyre in his publication Problem Solving for sports grounds stresses the need to shed excessive water during heavy and prolonged rain and recommends at least 1:70 across play. In cut to fill construction slopes of up to 1:50 are hardly evident to the eye and eliminate the formation of puddles..
If we are to judge the performance of a pitch in the simplest terms the assessment must centre on the performance of the pitch during the wet months of the year. It seems logical to consider the essential criteria that would determine whether the pitch construction and maintenance is adequate enough to provide use when required. In my view the main criteria must be:
- grass cover – density of vigorous plant shoots with reference to sward height
- purity of grass cover – acceptable percentages of foreign grasses and weeds
- drainage potential – ability to provide a firm surface within 24 hours of heavy rain
- slopes on the pitch – essential to promote surface water run-off
- evenness of grade – adequate to prevent the collection of water in puddles
In contrast the main short-comings of the current PQS must hinge around the following
- ground cover is relative to sward height with no reference to plant density
- hardness will vary with the moisture at the time in the surface layers
- infiltration rate in compacted sandy loam and clay soils will not reach 5mm/hour
- evenness over 2m straight edge will not isolate all depressions that collect water
- slope must be defined with both maximum and minimum gradient acceptable
In essence, the current PQS will not establish whether the pitch will perform in the winter months to the level that would have been expected when securing funds for development. While annual maintenance is critical in sustaining the aspired level of use the inherent limitations in construction cannot be overlooked. Sadly, failures can often mean that no effort short of reconstruction becomes imperative if the pitch is to be fit for the purpose initially envisaged.