What was it all about?

Maintaining turfgrass quality under limited water supply is particularly problematic in sandy soils, which have a relatively low water retention and are highly dependent on frequent summer irrigation. This is a problem in the face of future water restrictions, which are expected to hit hard in coming years due to climate change and population growth.

Running from 2013 to 2016, this project investigated the use of soil amendments to improve the water retention of sandy soils and decrease turf water requirements. While there has been the perception that the use of amendments in these soils can reduce deep water drainage and lead to more efficient use of irrigation water, this was the first time use has been independently tested.

Field experiments were conducted at the University of Western Australia’s Turf Research Facility in Perth, to determine the effect of various amendments on soft-leaf buffalo grass quality and growth. Amendments had different grain/particle sizes and included bentonite, kaolinite, spongelite, ReadyGrit and zeolite. They were either incorporated into topsoil individually or in combination with compost, and were irrigated two to three times a week.

The research found that finer-grained soil amendments in the topsoil (bentonite and kaolinite clays and compost) did increase water-holding capacity and reduce deep drainage when compared to the control plots and plots with differently textured amendments. However, even in the control plots none of the irrigation water drained beyond the root zone, and, by retaining most water in the topsoil, plots with finer-grained amendments appeared more likely to lose irrigation water through soil evaporation. The researchers note that because of this, under both irrigation rates, turf in plots with amendments did not differ in colour from turf in control plots.

Subsequent investigation found that placing amendments lower in the soil profile – 5 to 15cm deep – did improve turf colour under limited irrigation, with modelling confirming deeper placement below the soil surface can further reduce the loss of irrigation to evaporation.

The researchers concluded that best water saving is achieved by incorporating fine-grained amendments (clays, compost) at a rate of 5 or 10 per cent (wt/wt), in a band of at least 10cm and deeper than 3cm below the soil surface. The project team notes that it is only in situations with an increased chance of irrigation water drainage (for example, watering with large volumes, or watering turf with shallow roots) that incorporation of amendments in the topsoil will be beneficial for turfgrass water use efficiency.

They recommended further research to validate the effects of deep banding on turfgrass water use efficiency in a field situation, and establish optimal amendment type, placement depth and incorporation rates.