Testing Sand Carpet Sportsfields for Contamination

Clogged surface pores from excessive organic matter

It is important to test sand carpet sportsfields due to the inevitable build-up of organic matter and/or very fine particles (typically from earthworm activity) that clog the pore space between the sand grains. The less pore space, the slower the drainage and higher the surface moisture retention. This leads to softer, wetter conditions that can effect:

• The amount of use
• The quality of the playing surface
• Maintenance/renovation requirements
• The rate of asset depreciation and, hence, the need for replacement

How does sand carpet drainage work?

In order to understand the need for testing, an appreciation of how a sand carpet works is required.

The main components are:

New sand carpet layer with very little contamination

• Grass species at the surface
• Surface sand layer (sand carpet) typically between 40-100mm thick
• Narrow, closely spaced sand/gravel filled drains (slit drains)
• Larger wider spaced drains (lateral drains)
• Main drain to provide an outlet for the lateral drains

Water makes its way through the grass canopy and into the sand carpet layer. Some water might percolate into the underlying soil, but the vast majority moves downhill across the soil surface until it reaches the permeable backfill above a slit drain. The water then enters the slit drain and moves sideways through the gravel in the base of the slit to the lateral drain and, from there, ultimately, to the main drain.

The rate limiting step in well-constructed sand carpets drainage systems is typically through the surface sand carpet layer where the bulk of the roots develop. The sand carpet can become contaminated by the combination of:

• Live and decaying plant parts (i.e. leaves, roots, stems, stolons, rhizomes) = organic matter
• Earthworm castings (silt/clay from underlying soil brought to the surface)
• Fine particles from sand topdressing (very fine sand/silt/clay)

Severe surface organic matter accumulation due to lack of renovation and sand topdressing

• Wind-blown material (dust)

Why is organic matter so important in sand?

Over time, the organic matter increases and changes its characteristic from "plant parts" to become finer decayed material which holds moisture and blocks pore space between the sand grains. This blocking of pore space by decayed organic matter does not happen in soil. Soil drainage is reliant on soil structure (aggregation), and organic matter contributes to the aggregation process.

Earthworm casting typically occurs at the surface and, so, the silt/clay particles brought up and mixed in with the decaying organic matter increase the blocking of the pore space.

If the amount of removal via coring, dethatching, fraise mowing, etc) and/or dilution (via sand topdressing) is not intensive enough, the fines content increases so that surface layer is no longer sand dominated. Consequently, drainage declines and surface moisture retention increases.

It is not possible to visually assess the level of fines contamination accurately. An objective measure of fines contamination is a more useful management tool that allows changes over time to be tracked accurately.

How do we measure the contamination?

Different degrees of sand carpet contamination as measured by the settlement test

Firstly, you need to know how many fines there are in your sand topdressing applications. This is determined by a particle size distribution test (PSD). Normal best practice is to ascertain this when selecting and monitoring your sand topdressing. Ideally, you want a "clean" topdressing sand (i.e. one with less than 2% very fine sand and less than 1% silt and clay).

Secondly, previous NZSTI research has shown that, whilst there are many different factors and characteristics that can be measured, the best correlation to the drainage rate has been with, what is termed, the sand contamination test. This is sometimes referred to as the "settlement" test. With this test we measure the amount of fine particles present and express this as a percentage of the total rootzone (typically the top 40mm of the sand carpet is measured).

The results are compared to known values of contamination from past research to provide a rating of the level of contamination (see table).

Regular sampling (annual is recommended) measures the contamination and allows changes to be tracked over time. Differences between "good" and "bad" areas can also be measured to help diagnose the cause of problems. This helps decision making for the expensive but necessary renovations and, in addition, quantifies the ageing process of sand profiles. As such, it is a valuable tool when planning for eventual replacement.

When managing sand carpet sports fields, testing the level of contamination by fines provides valuable information that allows renovation programmes to be accurately planned based on objective data. The alternative of a best guess based on visual observation, or what "we have done in the past", almost certainly won't result in a programme that is ideally suited to the specific situation. A renovation programme based on objective data will help to ensure longevity of good drainage performance.
Article by kind permission of the NewZealand Sports Turf Institute. www.nzsti.org.nz