Scott Fraser at Narrabeen Sports High School. Photo: Stephanie Salter
The Loam Ranger – SESL in the community: Narrabeen Sports High School
In response to a phone call from a teacher at Narrabeen Sports High School looking for a lab that might be able to show some “science in practice” to students, Scott Fraser, who lives near the school, was invited to take the Year 9 science class on a field trip in the school grounds for a lesson in soil and water sampling.
Under Scott’s direction, students collected soil samples from the school’s playing fields and water samples from Narrabeen Creek. The samples were analysed back at the lab and at a couple of other labs, who also donated their time to the school.
The results should prove useful to the school. The key finding was that the soil of the playing field is heavily compacted and could be improved to support a top-quality playing surface. An ideal soil for a playing surface has an even particle size distribution: that is, the particles are all more or less the same size. This prevents them from sticking together and setting like concrete. (Conversely, the ideal distribution for concrete has a range of sizes, for precisely the same reason.) The soil at the school has a wide range of particle sizes, causing it to set hard and thus limit the growth of the turf. Over 40% by mass of the soil particles are <0.1 mm across (very fine sand, silt and clay), much higher than the ideal range for new sports fields of 8% to 12%. This sort of composition leads to poor drainage. For example, the predicted return-to-play period after heavy rain exceeds 72 hours.
To improve the ability of the field to support play, it will be critical to reduce compaction. Regular treatment with a mechanical soil aerator several times a year in combination with topdressing with coarse sand over several years will gradually improve drainage.
Nutrient analysis showed that the soil nitrogen, potassium, sulphate and copper levels are low, but the phosphorus level is high. The electrical conductivity of the soil is very low, indicating that the soil has a low capacity to hold onto most nutrients. Therefore, a slow-release fertiliser is appropriate.
The creek water analysis gives most cause for concern. Coliform bacteria, including E. coli counts, exceed the ANZECC recreational water quality guidelines for both primary contact, such as swimming, and secondary contact, such as boating and fishing. Creek water quality is outside the school’s influence, so unfortunately, the students will have to limit their field activities to visual observations.