Fertilisers, Mulches, Aggregates And Soil

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Updated: November 1, 2020

At first glance this heading should be self-explanatory. However, these words mean different things to different people. Let’s talk more about fertilisers, mulches, aggregates and soil.

Some gardeners often use liquid seaweed thinking it’s a general- purpose fertiliser for their plants – not so. Others spread mulch on their garden beds thinking it too is a form of fertiliser or decoration – again, not so. If you ask the general public what aggregates are, they might say they’re very special soil… you guessed it – not so.

Soils are so varied and complex, it seems inaccurate to call it just ‘soil’. There are clay soils, sandy soils and loam soils all requiring different and specific attention.

Fertiliser is like medicine – the right amount works wonders but too much is lethal. Therefore, choose the right fertiliser for the job. Lawns, trees, shrubs, potted and edible plants all have different nutrient requirements for a healthy life. For example, if I want plenty of flowers and fruit for my citrus I would choose a fertiliser with plenty of potassium to provide lots of flowering and in turn, lots of fruit. Likewise, if I want a healthy lawn I would make sure my chosen fertiliser has plenty of phosphorus in it, which develops a healthy root system; and that means it will need less watering during summer. Also, the next time you plan to use a fertiliser, firstly ask yourself what time of year is it? In the cooler months plants often lay dormant and can’t take up all the nutrients you thoughtfully provide them. In the spring and summer plants love being fed and will reward you with plentiful fruit, growth and health.

Did You Know: Indigenous or native plants may die if fed phosphorus, which is found in all general-purpose fertilisers. Natives naturally occur in phosphorus-impoverished soils and are extremely efficient at utilising the small amounts of available phosphorus.

All fertilisers contain three primary nutrients that are always labelled on the packaging and always appear in this order: nitrogen, phosphorus and potassium (NPK).

Tip: Remember these letters by keeping “up, down and all-around” in mind when reading fertiliser labels. The first letter promotes ‘up’ (rapid growth,) the second promotes ‘down’ (root development), and the third letter promotes ‘all-around’ (overall health). Nitrogen (N) promotes rapid growth and lush green leaf. Phosphorus (P) helps develop healthy root systems. Starter lawn fertilisers have a high phosphorus count for this reason. Potassium (K) boosts the overall health of grass and plants and helps with disease resistance, drought protection, flower production and cold tolerance.

Mulch is a layer of organic or inorganic material covering the surface of a soil. We use these products to conserve moisture, often to improve the health of that soil, reduce weed growth and enhance the visual appeal of the area – about 70mm in depth of any type of mulch will do just fine. Organic mulches that break down quickly whilst improving soil structure include pea straw, sugar cane and composts. Bark mulches and wood chips are used for budget makeovers and large areas where soil conditioning is not important.

Tip: Mushroom compost breaks down quickly and adds to soil structure the fastest. An annual spreading is a good idea because organic mulches are quickly depleted as they decompose. Should you be looking to create a space with more aesthetic appeal, yet still want to conserve water, spread a layer of inorganic mulch such as river pebbles (white, black, brown), rocks of different types, shapes and sizes, coloured scoria (lightweight volcanic stone) or even blue metal (also used as an aggregate) over an area. Inorganic mulch is especially good for shaded areas and seldom used pedestrian walkways, like side passages.Phosphorus (P) helps develop healthy root systems. Starter lawn fertilisers have a high phosphorus count for this reason. Potassium (K) boosts the overall health of grass and plants and helps with disease resistance, drought protection, flower production and cold tolerance.

TIP: Adding coloured glass mulch to potted plants adds plenty of bling!

Aggregates are rock-like materials consisting of a collection of particles ranging in size from < 0.1mm to > 50mm. They include gravel, crushed rock (blue metal), sand, recycled concrete and the use of waste slag from the manufacture of iron and steel.

Significant refinement of the production and use of aggregate occurred during the Roman Empire, which used aggregate to build its vast network of roads and aqueducts. The invention of concrete, which was essential to architecture utilising arches, created an immediate, permanent demand for construction aggregates. These days it is used for fills, backfills, drainage and filtration applications, the base, subbase, and/or surface of roads in several forms.

It increases the volume of concrete, thus reducing the cost. In roads and railway ballast it is used to help distribute the load and assist in ground water running off the road. Specialty lightweight aggregates include clay, pumice, perlite, and vermiculite.

Importing poor soil to a job site could make your whole project look miserable and cost you money if you need to replace plants.

If buying in bulk, know something about the company you are buying from. Always ask for the pH value of the load. Without good soil, plants will struggle to survive, and your clients will be constantly feeding and watering them to compensate unwittingly for the poor soil. If the soil profile has clay in it, chances are that the clay is not far from the surface. If we dig into that clay for planting purposes it creates somewhat of a well and does not allow water to freely drain off and drowns the plants. Therefore, for clay soils we always need to add to the existing soil depth and retain the imported growing medium by way of timber/concrete sleepers, steel/plastic/timber edging and/or garden walls depending on the depth, the design of the space, and our customers’ budget.

If the soil profile is predominantly sandy we need to add organic matter to help retain moisture and increase the size of gaps in the medium to allow for more oxygen to penetrate the profile for better access to it by plant roots.

TIP: It’s best to raise or lower soil pH slowly over the course of a year or two and compost moderates soil pH whilst improving soil structure. Organic matter is quickly depleted as it decomposes.

Loam is the best soil profile. It rarely occurs in nature and consists of roughly equal parts sand and silt with a little less clay. Loams warm up quickly in spring but don’t dry out quickly in summer. Sand provides good aeration and drainage and silt helps clay and sand mix together more readily. Clay tends to be higher in nutrients than the other soil components. As a result, loam is loose and crumbly in our hands or friable.

Even if proper nutrients are present in the soil, some nutrients cannot be absorbed by grass and plants if the soil pH is too high or too low. Most plants require a soil pH of 6.5 to 6.8 to absorb phosphorus, which helps develop healthy root systems.

Organic fertilisers such as manures (cow, sheep, horse) and composts stimulate beneficial soil microorganisms and improve the structure of the soil. Soil microbes play an important role in converting organic fertilisers into soluble nutrients that can be absorbed by plants. In most cases, organic fertilisers and compost will provide all the secondary and micronutrients plants need.