Home Feature Articles Soil Blends

Soil Blends

by editor

Considering the deluges our eastern seaboard is getting, it’s quite poignant that drainage and aeration was our last conversation. If you’re based in the areas of these prolonged rainfall events, I hope your drainage was suitable and aeration was maintained.

As we journey on our quest to achieve designed intent, it’s time we burrow into the realm of blended soils. Although it’s generally preferable to amend and work with the native topsoil on site, often can be the case, that we need to import soil blends to mimic or construct a new soil profile. If the site has been stripped of topsoil, when we’re installing in planters or developing roof gardens, we’ll find ourselves in the need to import appropriate growing media from a reputable and reliable supplier.

During my work through NSW, I call into landscape suppliers to take a look at what’s being sold as ‘garden soil’. The variations and qualities are quite significant and, in some cases, quite concerning. Many products look satisfactory from an initial glance, but upon closer inspection, they can be a worry. The stockpile is still steaming (indicating the organic matter is still composting), the blend may be poorly graded with inappropriate compost or concentrated clumps of organic matter, such as spent mushroom compost. The PH can vary from acids of 4 up to steaming alkaline composts of 9. I dread to think what would be found if a full chemical analysis was taken of the dubious soil mixes we’ve all probably seen.

I’m sure that most suppliers don’t intentionally sell questionable growing medias. Often is the case that the merchant doesn’t have a full understanding of the science that goes into the creation of landscape soil blends. Afterall, his principal line of products are more often sands and aggregates.

It’s a big relief that each year our industry matures that little bit more and the good old days of ‘she’ll be right’ materials are gradually becoming fewer. Once again, it all gets back to education. When you find a sand and gravel merchant selling questionable growing media or worse still, you have experienced the ill effects of installing the poor material, it’s very important to discuss with and hopefullyeducate the merchant about the importance of getting the soil blends right so they are fit for purpose. Quite a few of the smaller regional merchants blend their own materials from questionable resources with little understanding that different plants have different needs.

Another issue observed through most supplier yards is the size of the stockpile and the drainage capability of the storage bay. As horticulturists and landscape professionals perhaps three questions on our minds should be:

  1. How often is the stockpile, which is rich in organic matter being, turned to enable healthy levels of gaseous exchange?
  2. Does the storage bay have a base surface that enables drainage? We know the outcomes when organic matter is steeped in water with no oxygen.
  3. So, what’s going on in the bottom of the storage bay of that expensive media that you’re about to invest in?

Whenever you’re buying landscape soil blend, always ask the loader to take the material from the top of the pile. The deeper you go, the less air there is!

Naturally, as we’re supplying these products to the consumer, we have duty of care to ensure that the product we are supplying is fit for purpose and complies with Australian Standard 4419.

If you’re unaware of AS 4419:2018. The objective of this Standard is to provide manufacturers with a set of requirements which will ensure that soils can support plant growth and to give users, such as growers, landscape designer and consumers, assurance of the suitability and quality of soils.

The requirement of the standard is to specify soils that are suitable for majority of plants. It does not cover soils for plants with special requirements such as cricket pitches, orchids, cactus, succulents and other specialised sporting turfs. So how can we learn more about landscape soil blends?

As a practicing and consulting horticulturist there are two bibles and one document I regularly consult with and refer to. The document being the afore mentioned AS4419:2018 and the bibles being:

  1. Soils for landscape development – authors Simon Leake and Elke Haege.
  2. Growing media for ornamental plants and turf – authors Kevin Handreck and Neil Black.

I strongly recommend investing in these valuable tools.

Hopefully this article will arouse your interest in this important topic, and you will dig deeper into learning about constructing effective soil profiles. It’s a whole lot more than backing the truck up and tipping a load of blend into the yard. As the science and understanding of horticulture in the built environment increases so too will be the expectations put upon us to create quality soil profiles that enable plants to perform and achieve the designed intent.

As I only have you for 15 minutes or so, let’s leap into physical properties of the standard soil that is hopefully available for you to collect from the supplier tomorrow morning. We can chew into the other three classifications the next time we speak.

So putting chemical analysis aside, let’s take a look at the first classification of landscape soils and what goes into it.

The mineral content of these soils is generally sand, gravel, ash and occasionally minor elements of silt and clay. Silt and clay content often depends on the standard of the sandy loam that the manufacturer started with.

Looking at the table on page 18, this class is broken down into three different grades being low to high in organic matter content.

Let’s look at what organic matter does for this specific soil and plant ecology, then we can unpack the quantities of organic matter and what it will mean for your project.

The organic matter in these soil blends provides nutrients, assists with moisture retention, improves aeration by separating the mineral particles and will eventually decompose to valuable humus compounds. Essentially, the organic matter content in the soil is delivering the structure to an otherwise unstable sandy soil.

Looking at the percentages of organics, I would always be wary of using a product at the high end. 15–25% OM is a significant amount of organic matter to be situated more than 10cm underground.

The amount of oxygen the organisms in this soil will require to survive will come at a cost to the plants. Coupled with the likelihood that once the material has been spread out, foot traffic will compact it and further restrict gaseous exchange. If you can keep this soil aerated and draining freely into the sub soil then the plants will most likely grow quite rapidly. Keep in mind that as the high level of organic matter decomposes, you will see the soil levels contract to the point where some root balls may become visible.

When importing soils for backfilling over 30cm in depths, my preference is to spread a sandy loam with no organic matter present and fork this into the sub soil to ensure against layering, my next step is to import a soil blend with a low or medium organic matter content and fork that into the loam to ‘key’ the next layer into the lower. Be very careful spreading landscape soil blends at depths more than 30cm. When doing this it’s very important to keep the soil well aerated. Use boards and loosen areas you’ve walked on with a fork. Sure, there’s exceptions to rules and no doubt there are occasions when bulk backfilling of landscape blends hasn’t created the dire outcomes that can happen. Please keep in mind that during these very wet times, organic matter will be starved of oxygen. When this happens, it’s no longer a benefit to your cause.

I’m looking forward to discussing soil blends further in our next chat.

Related Articles