Getting down to the grassroots

Brazilian football star Vinicius Junior is quite upset with the playing field at the Met-Life stadium,  New Jersey (also known as New Jersey-New York stadium). He and his teammates have  complained that the field felt dry and is not suited to their style of playing.  French player Adrien Rabiot  too is not pleased  with the touch and feel of the  grass.   And to think that FIFA worked hard for almost a decade and spent $60 million dollars to lay  the  perfect the pitch   in all the  16 stadiums spread over  Canada, USA and  Mexico!    

Courtesy: FIFA

That has not been an  easy task at all.  The 16 stadiums are so   diverse  in terms of weather and geography, that  the grass that flourishes in  Miami may not grow at all  in Seattle.  Added to that a few stadiums have their roofs covered  while  others are open air.  Critical to quality is resilience of the grass, the ability of  the turf  to endure the continuous  stomping, kicking, jumping, running and rolling   by twenty-two  athletic players.   Apart from being an excellent shock absorber the turf must  ensure consistent mechanical behaviour of  the  ball such as roll, rebound and bounce. Rolling resistance determines how far the ball travels horizontally on  the pitch, which is largely influenced by surface friction which in turn is governed by the    grass height and moisture content.  According to FIFA regulations, the ball must roll to a distance of 6.0 to 10.6 meters before coming to a halt. Similarly, dropped from 2.0 meters, a ball must rebound to a height between 0.6 to 1.0 meter.   When a ball  impacts the turf at an angle, the bounce  must closely match the impact angle and speed.  Unevenness (divots, bald patches) or varying moisture levels interfere with all these parameters. 

Search and research for a perfect grass pitch began in earnest in 2018.  Turfologists   Dr John Sorochan of  University of Tennessee  and Prof  John Rogers III of Michigan State University  and their research teams took up the challenge.  Commercial growers and stadium groundskeepers were roped in  because their  field  knowledge  was essential to  choose the right grass type and define the appropriate maintenance  protocol (light, watering, ventilation, draining etc) for ensuring  grass health throughout the tournament period.  

Plastic fibres supporting the grass roots  
Courtesy:Inside FIFA.com

Aesthetic parameters such as  uniformity in  color, texture and thickness/density are equally important.   A patchy, dried or yellowed turf doesn't match either the professionalism or the  standard of FIFA.  In color preference, market survey showed that  dark green tops the list.  Consolidating  all the  requirements of performance and aesthetics  Sorochan and Rogers suggested Hybrid Turfgrass systems.  Here  natural grass  is reinforced with  tiny plastic filaments  (polyethylene, nylon or polypropylene)  as scaffolds.  It was presumed that  such a turf of  95% natural grass  and 5% plastic filament  not only enhances the look and feel of the turf  but also provides a  durable , resilient  playing surface that prevents deep divots.

Cool season  blend of  84% Kentucky bluegrass and 16% perennial ryegrass was selected for enclosed stadiums  and   Bermuda grass for warm  open-air arenas.   The  Kentucky Bluegrass- Perennial Ryegrass combination has unique characteristics. Bluegrass is durable and fast spreading while ryegrass sprouts quickly.  Bermuda grass grows aggressively, is sturdy,  draught resistant and self repairing. The ultimate target  was  to have a lush green  soft turf  which can  not only withstand a stampede but self-heal as well. 

Ground preparation for hybrid turf

The  selected species of grass can be  grown on a layer of  sand spread atop a plastic sheet so that  roots grow laterally and  intertwine locking the sand bed into a dense, hardy mat.  These can then be  rolled into  bales off the plastic sheet   and  transported to wherever required. The bales are unrolled over   specially prepared ground.  The grass mat sits over a layer of  several inches of firmly pressed sand. Below that upto a depth of 45 centimeters or more are  several layers of soil and gravel mixtures and drainage membranes.  An elaborate underground system is in place to ensure aeration, watering, and efficient  drainage.  In stadiums with domes or limited sunlight  arrays of  artificial grow lights are provided.      Once in place,  plastic fibers are stitched into the ground about 18-20 cm deep using special heavy duty sewing machines.  These fibres    become anchors for the roots below  and  the blades above.  Alternately   a meshwork  sporting sufficiently long(tall?)  polythene fibrils can be  laid out at the lower root zone level, filled with soil and sand  and then seeded with the selected  grass.  As the grass grows  roots entangle with the plastic filaments and get reinforced. 

During the  research and development phase Sorochan and Rogers   experimented with various  types of  turfs.     Robotic cleats, ball-launching machines, and stadium simulators were used  to mimic  on-field  actions and reactions of the ball.  Based  on these elaborate and extensive  experiments a critical grass height of   22 millimeters was recommended.  Even a 5-millimeter  variation  drastically altered  the behaviour  of the  ball.  By the way the  alternating light and dark green stripes that we see on the ground is just an optical illusion created by mowing the grass in alternate directions. 

TAIL PIECE

Considering   FIFA expects  to rake up an overall revenue  of roughly $10+ billion from this  World Cup, the $60 million spent for the perfect pitch seems to be spare change.   

REFERENCES:

1. Grasses for Sports grounds and its influence of playing quality : A review

2. FIFA Certified Football Turf: The standard for 2026 World Cup Stadiums

3. The Scientific Quest for a Perfect World Cup Field