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  Gradation Index advert image shown if present

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This is often abbreviated to G.I. and is a numerical value based which identifies the uniformity of sand particles in a sample of sieved soil. A G.I. value is a ratio of the larger to smaller diameter particles and is based on the selected D values used. A uniform sand reduces the potential for the particles to be compacted, with a more open pore space being able to be retained, improving drainage and root growth potential. The lower the G.I. value the more uniform the particles and the more suitable they are for sports turf situations. A larger G.I. value will see much more interpacking of the particles, significantly reducing drainage and creating a much more compact rootzone.

An example might be given as D90 over D10 (expressed as D90 / D10), with particle sizes being 1.00mm and 0.25mm respectively. This example means that 90% of the sand particles pass through a 1.00mm diameter sieve, whilst 10% of the sand particles pass through the finer a 0.25mm diameter sieve. The Gradation Index for this example is therefore 1.00 ÷ 0.25 = 4.

A G.I. of no greater than 5, ideally less, has been suggested for topsoil sands for sports turf situations, with a good D90 / D10 for sand typically being 0.500mm and 0.125mm, although variations either side of these parameters are also used. The G.I. of D90 / D10 is the more commonly accepted ratio to use, although D95 / D5, D85 / D15, and others have been used depending on specific requirements. The USGA have a G.I. of 2 - 3.5 for a D60 / D10 for the rootzone mixtures used in golf green construction.

References:
• Adams, W.A. & Gibbs, R.J. (1994) ‘Natural Turf for Sport and Amenity: Science and practice', CAB International, pp4 & 21.
• Baker, S. (2006) ‘Rootzones, Sands and Top Dressing Materials for Sports Turf', STRI, pp9-11, 22 & 61.
• Stewart, V.I. (1994) ‘Sports Turf: Science, construction and maintenance', E&FN Spon, pp110 & 117-118.
• USGA Recommendations for a Method of Putting Green Construction (2018 Revision) https://archive.lib.msu.edu/tic/usgamisc/monos/2018recommendationsmethodputtinggreen.pdf accessed 16th June 2021