Soils blends Part 3


I hope you’re enjoying the brief introduction to AS 4419:2018 and it is helping to enrich your understanding of this important facet when creating soil profiles within the built environment.

Thanks to the representatives and volunteers from our industry guilds, research centres and government authorities who have not only helped to develop soils that are fit for purpose “on grade” and “on slab” that we generally use from day to day, they have also provided us with the specifications to create growing medias for specific and unique situations where the demands for our craft comes to the fore.

This is the area that intrigues me most. It’s the nitty gritty end of AS4419. These include the third classification of soils for turf and lawns, then followed by the fourth classification, wetlands and one of my favourites, structural soils. These final classifications tend to focus more on particle sizes of the blends and a little less on organic matter contents.

Let’s first take a look at growing medias for turf and lawns. This classification is separated into three sections. The first being underlay, the second being sports fields and the third, top dressing.

Most of us are quite familiar with using turf underlays. They are generally a blend of sand and soil, commonly known as 70/30, 80/20 and 90/10. The ratios indicate the sand to soil quantity, with some blends containing small amounts of composted organic matter to increase water and nutrient holding capacity. The acceptable percentage range of organic matter content in turf underlay is between 2% and 10%.

Note the inconsistent sizes or grades of the stone. The gap graded stone mixed with the soil beside it creates a stable foundation to pave over whilst allowing tree roots to exploit necessary resources for healthy growth.

Providing there is no organic matter present then these blends are useful for bulk filling at depths greater than 250mm.

An important element of the sand soil blends that can be overlooked or often misunderstood is the importance of inconsistent particle sizes. When soils are blended or tested for suitability, they undergo sieve analysis to measure the various particle sizes. This test involves several layers of sieves with different grades of sieve opening sizes. The finest sized sieve lies on the bottom of the stack with each layered sieve stacked above in order of increasing sieve size or gradient. When a sample of sand or soil is placed on the top sieve and sifted, the individual particles of the material are gradually separated onto the final layer that the particles will not pass through.

You may have heard the term ‘gap- graded’ before. This means the media has a range of particle sizes missing i.e. ‘gaps’ within the gradient sieve spectrum. It’s these types of medias that work best for us as the gaps within the material tend not to make it pack down hard, thus leaving adequate pore spaces for air, water and of course, roots. Engineers like well graded or uniform soils to pack down hard and build upon whereas we like poorly or ‘gap’ graded soils to grow in. Another issue we face with inappropriate gradings is the potential of the particles within the blended media separating after stockpiling or worse still, after placement or filling. If all of the fine particles within the blend segregate and settle, a layering situation occurs. As I’ve mentioned previously, the drainage and air filled porosity dramas with a layered or separated soil can be very problematic.

The importance of utilising an appropriately gap graded media within turf rootzones brings us to the blends for sports fields. These are sand based and are designed to withstand high foot traffic, provide excellent drainage and air-filled porosity. The load bearing capacities of gap graded sands also greatly reduce the blends from migrating or being pushed around during high impact activities such as football. Drainage is also improved with the addition of washing the silty soil from the turf prior to laying. Again, the benefits of removing the silty turf farm soil is to eliminate the dramas of layered horizons. We’ll talk about washed turf in a few articles time. For now, let’s focus on these blends and in particular, the sports fields media.


Many years ago, the United States Golf Association developed an optimal, gap graded sand blend for putting greens, fairways and top dressing. The blend was designed for financial and turf health reasons. The loss of revenue golf courses experience from wet weather closures can be astronomical. By creating a growing media that has a high infiltration rate of somewhere around 50mm an hour enables courses to be played on soon after heavy rainfall. USGA specified sand is available throughout most of Australia. You can order it with the addition of organic matter and zeolite to improve the nutrient and water holding capacity as well. Considering the price of it is comparable to generic turf underlay. I’m a fan of using it in gardens with busy kids, dogs and areas susceptible to high foot traffic. We’ll cover the topic of improving water efficiency of sand profiles during the chat we’ll have about turfing in the future.

Our last category covers wetland and structural soils. In brief, wetland soils are intended to be inundated with water, which makes them prone to anaerobic conditions. These blends aren’t readily available and require pre ordering. When it comes to this media, I suggest you invest in the book ‘Soils for Landscape Development’ from Elke Haege and Simon Leake. This will enable you to be better informed on what’s required for this challenging application.

So now we come to structural soils. These are medias that have been designed to increase soil volumes for trees within heavily built environments such as roadways, carparks and other areas that require a stable growing media for tree roots to inhabit and exploit resources beneath concrete and other impervious structures that must remain stable. The soil can be built upon without the risk of settling or compaction. Structural soil is basically a blend of blue metal with a loamy soil mixed through for the roots to obtain moisture and nutrients from. The particle or aggregate sizes are what’s important. Railway cobble with sizes ranging from 37 – 63mm is used as the principal aggregate. This is then blended with a loamy soil for moisture and nutrient retention. The importance of gap grading is vital with structural soils. If the grades aren’t correct then the loamy soil will segregate from the large aggregates and settle at the bottom of the profile, thus making the media useless for the tree to exploit. Basically, we’re aiming for voids of around 8mm between the blue metal cobbles. These voids have been deemed to be sufficient for feeder roots to grow through without expanding to the point where the structure above will be lifted or adversely affected. Pretty amazing stuff in my opinion and an excellent example of the importance of science within our profession.


Another element within AS 4419 that I haven’t touched on yet is the presence of plant propagules within soils.

Propagules are vegetative structures such as rhizomes, corms, bulbs and seeds to name a few. They’re most likely to be weeds and obviously spell trouble. The standards state that all soils ‘should’ not contain plant propagules rather than ‘shall’ not. The difference between should and shall is that should is a recommendation whereas shall is mandatory. Buying from a familiar and reputable supplier is probably the safest way to avoid the hassle of inheriting weeds in your soil blends. Apart from the merchant’s reputation of selling a quality product, there is no legislated buyers protection against weeds within the standards.

Our ability to successfully grow plants amongst buildings, on structures, on disturbed lands and other challenging environments is what separates us as horticulturists and landscape practitioners from the numerous other trades within the building industry who presume our work is just a matter of dumping soil and sticking a load of plants in. Be proud of the science you already know, what you are continuing to learn and the important role we play in the construction process.

Let’s never forget that without our profession, the buildings look stark, wildlife seems missing, summer temperatures are much higher, the wellbeing of the community is much poorer and the disconnect mankind has with nature continues to grow.

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