Links between ground conditions and injury – Part 2

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My agronomic research initially looked at hardness on sand vs soil profiles.

(To read part 1 from last week, click here).

Soils become excessively hard when they dry out, but sands don’t – in fact they become softer as they dry out. My tests showed that if the fines content (clay, silt and very fine sand component) of a rootzone was less than around 17%, it would not become excessively hard when droughted, even if bare. In addition, the fines content of a rootzone needs to be less than around 11% to provide a sufficient infiltration rate for drainage (e.g. greater than 30mm/hr). So the conclusion was that re-constructing fields in sand could solve any hardness problems, even in drought. However, as many turf managers are aware, sands can become very shifty in drought if they lose their grass coverage. My tests showed that rootzones with a fines content less than around 15% have undesirably low shear strength. So if grass coverage is lost in high-wear zones such as goalsquares, the sand rootzone can be easily kicked out, causing depressions that are dangerous and extremely difficult to manage. Consequently, sand-based grounds should have less traffic put on them than soil-based grounds. This is a hard concept for parks managers and football clubs to understand – they have just spent $500,000 on a new construction, and you’re telling them to schedule LESS play on it, not more! The bottom line is, sand-based football grounds can drain well and will not become excessively hard when droughted – but you need to control the usage, and don’t allow any high-wear zones to become denuded of grass.

My agronomic research then looked at the influence of grass coverage on hardness. It may seem obvious, but in fact there is very little research on this, especially in drought conditions. I set up two zones (one grassed, one bare) on the edge of a sports oval at Ballarat University. The soil was a fine clay, which I compacted several times prior to the trial, and the area was unirrigated. I measured Clegg hardness and soil moisture over an extended drought period, as shown on Figure 3:

Figure 3: Relationship between Clegg hardness and soil moisture in plots with-grass and without-grass. The soil moisture probe was unable to penetrate when soil moisture fell below 3%.


The graph shows that, obviously, soil moisture content has the greatest influence on hardness in this soil type. However, it also shows the benefit of having grass coverage. The presence of grass (the green dots) resulted in Clegg hardness in this soil only reaching around 150-160g when it was very droughted. Without grass, in contrast, hardness reached 250g when it was dry. If the Clegg safety threshold was set at 160g, then the presence of grass cover would allow this field to remain open. A lack of grass coverage would cause it to be closed.

I tested the effect of several other agronomic variables on hardness, but I concluded that the most practical way to eliminate excessive hardness on soil-based fields in drought was to maintain full grass coverage, over the whole ground and through the whole season. Sounds simple, but in a drought it isn’t simple at all. It clearly couldn’t be done with perennial ryegrass.

Look back at the couchgrass area in the goalsquare in Figures 1 and 2. That’s a natural invasion of a local couchgrass, which is extremely drought and wear tolerant, and capable of sustaining a full sward of grass even in severe drought. Councils that successfully converted grounds from perennial ryegrass to couchgrass through the Millennium Drought ended up with excellent grounds, despite the water restrictions. However, many councils didn’t adopt couchgrass, either because the conversions were too disruptive, or because of a warning from medical research that couchgrass had a higher risk of ACL injury than perennial ryegrass, due to its excessively high rotational traction. That is the topic of part two of this report.

Acknowledgements: Phillip Ford would like to thank PGG Wrightson Turf (Australia), Strathayr and Rocla Quarries for their material assistance in this project.

References: available on request