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Soil Blends Part 2

by editor

I hope you’re enjoying our conversation and finding my advice is assisting you on the journey towards achieving the design intent.
During our last discussion, we touched on the first soil classification in AS 4419:2018, ‘Landscape soils on grade’. Now let’s take a look at low density soils designed for use on an artificial base such as planters, roof gardens and other locations where the blend is not in contact with natural soil.

Low density soils (on slab) should always possess an effective drainage system, which we’ll discuss later. They should always be installed at the correct depths and have similar physical characteristics to avoid potential layering problems. Incompatible soil layers result in poor drainage from water being held or ‘perched’ at the interfaces of the differing soil types. The layered horizon restricts water from filtering further down the soil profile.

Along with drainage, we’ll discuss the perils and nightmares of layered soils in a few months’ time.

Low density soils as the name suggests have low bulk density rates of somewhere between 300-800 kilograms to 1000 litres (m3). As you’ll note in the diagram, they are comprised of ‘B’ subsurface area (or B horizon) and ‘A’ surface area (A horizon). Australian standards avoid using the term ‘horizon’ within their classification because this has a specific technical meaning when applied to natural, undisturbed soil profiles. Let’s stick with using A and B terms because this is common lingo in the trade. Low P meaning phosphorous content is for native and exotic phosphorous sensitive plants such as proteas and kin.

As you can see in the classification chart, ‘A’ horizon soils possess a significant quantity of organic matter. This ingredient is intended to create the structural framework along with improving the water and nutrient holding capacity. It is also there to reduce bulk density or weight of the media and to encourage air filled porosity within the top 30cm of the artificial soil profile. They must never be used at depths beyond 300mm.

As I mentioned during our last chat and for similar reasons, I’m just as wary of using high organic matter in low density soils as much as I am with soils on grade. Remember that when we incorporate high levels of organic matter into the soil profile, we are relying on a structurally unstable material around our root zone. Althoughplants can initially have astonishing growth rates, once the organic matter breaks down or decomposes, our soil volume can dramatically reduce. This results in surface slumping and exposed root balls. Unless the reduction in media is replenished, the initial bounty of growth is retarded due to the loss off surface roots to drought.

Other important considerations we need to have if we’re using high organic matter soil is the likelihood of compaction.

Here’s a common scenario: once the material has been blown, craned or carried in. We go about working on the media while we’re irrigating, backfilling, planting and mulching the planter or roof garden.

Much like we have talked about in previous conversations, when we compact it we are restricting precious oxygen diffusing through the soil profile. Alas, we are reducing vital gaseous exchange and here we go again, we’re creating the anaerobic conditions which are ideal for growth inhibition and diseases.

Yet another concern we should have for high levels of organic matter is hydrophobia (water repellence). I’m sure all of us have experienced irrigation failure once or many times in an artificial soil profile such as pots and planters. When organic matter dries completely, it is very difficult to rewet. Given that virtually all gardens now rely on drip irrigation, rehydrating a water repellent media requires lashings of wetting agents, drenching with a sprinkler or better still, the nursery person’s weapon of choice, a dram watering wand. Bringing the soil back to readily absorbing water takes plenty of effort and can help to be avoided if we reduce such high levels of organic matter.

Given the problems I’ve just outlined with the highest or 3rd category of soil. If you are specified to employ such products within a project then I suggest you arm yourself with the comments above and urge the specifier to use the 2nd category of soil or as I prefer, just stick with the ‘B’ horizon soil throughout the entire planter profile.

B horizon soil is primarily mineral based, which is made from products such as sands, clay, volcanic rock and ash. It is intended to create a stable structural media that has very little organic matter so is not subject to decomposition and slumping. The product has excellent aeration and depending on the supply source and ingredients, it has a reasonably high water and nutrient holding capacity.

If you’re unsure, all of these important qualities can be confirmed by checking the supplier’s independent soil analysis report.

Sticking with installing only B horizon can save a lot of immediate and longterm headaches. This can be particularly important when the soil is placed by blowing or crane services. Much like when concrete is delivered, things can get busy. We can be faced with time pressures from suppliers that want to get their product in place and leave as soon as possible, trucks are waiting and we may only have the crane for a limited amount of time.

Remember the troubles associated with installing A horizon at depths greater than 300mm? Now, consider the likely reality when you’re cut and dumping bulk bags of A and B horizons in situ or relying on a blowing contractor to fill areas with sufficient B horizon to accommodate the final 30cm topping of A horizon. Managing either of these exercises while also ensuring you have ordered sufficient quantities of the two blends is a hassle and could possibly deliver you with inconsistent growth rates due to the variation of the soil conditions.

I find that sticking with B horizon throughout the entire project and all soil profiles:

  1. Reduces the headaches I’ve mentioned.
  2. Provides better infiltration rates by avoiding layering.
  3. Simplifies irrigation methods by having consistent water holding capacity.
  4. Avoids compaction and probable phytotoxic conditions.
  5. Delivers a mineral-based soil with a structural framework, which doesn’t slump or degrade over time.
  6. Provides excellent aeration and drainage efficiency.
  7. Is suitable for use beneath lawns and within planters.
  8. With appropriate irrigation and nutrition, will deliver overall consistent growth rates of plants and turf.

So, I’ve argued and demonstrated the benefits of sticking with B horizon throughout the project.

Before we close this conversation, we need to consider plant nutrition of the soil we have installed.

Generally speaking, B horizon carries less nutrient and depending on the manufacturer, the soil may lack the ability to hold and release nutrients within the root zone. This is known as cationic (pronounced cat-ion) exchange capacity.

Nutrient holding ability can be solved by either blending horticultural zeolite at a rate of 500 grams per m² through the top 100-150mm of the soil profile or blending a 30-50mm thickness of compost through the top 200mm of soil.

Just briefly, Zeolite is a naturally occurring mineral with a highly porous atomic structure, which behaves in a similar manner to humus. The porous structure allows it to attract, hold and release water soluble nutrients to roots. We’ll discuss zeolite in more detail when we talk about growing media for turf. It’s amazing stuff.

The simplest method of delivering nutrients to the soil is by applying controlled release complete fertiliser such as Osmocote along with composted manures to stimulate biological activity

As I’m based within the Sydney basin, I am fortunate to have access to specialised soil and sand blends that are produced by companies such as Benedict Sands, Australian Native Landscapes and Scapeworks. I have no financial gain from commenting on these suppliers’ products, I’m just very impressed with the quality and consistency. I’ll leave it to you to source suppliers in your state of a similar calibre.

In case you’re thinking that I am against organic matter, I’m not. I just believe in my experiences and the science that we can use too much.

The next time we chat, we’ll look even deeper into AS 4419 and the specialised soil blends that are available to us.

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