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GardenSafe: Soil quality indicators

What's in your soil, Victoria?

Soil is a complex mixture of minerals, organic matter, gas, water and living organisms. Its composition is a product of your local environment and geology, and is altered by agriculture, industrial activity and home gardening practices. The balance of your soil mixture can influence soil quality and the types of plants that can grow well in it.

GardenSafe assesses soil from 3 locations in your garden: your front yard, your back yard and your veggie patch. We focus on veggie patch soil as this is where your edible plants grow, and where you may have increased exposure to soil particles from working in the garden and consuming home grown produce.

GardenSafe assesses a suite of garden soil quality indicators in your soil using a combination of RemScan mid-infrared spectrometry (soil composition, soil texture, total organic carbon) and X-Ray fluorescence spectrometry (nutrients; phosphorus and potassium).

Garden soil quality indicators

Garden soil quality indicators provide a snapshot of the condition of your soil. These results are unlikely to impact human health but can influence how well your plants grow.

If your soil quality indicators are outside of the optimum range desired, you can change your soil composition by adding organic matter (compost, leaves etc.) or commercial products (fertiliser, lime etc.).

Soil composition

Soil is made up of three different particles:

Particle typeParticle size
Sand2 mm to 0.05 mm
Silt0.05 mm to 0.02 mm
ClayLess than 0.02 mm

The proportion of each of these particles determines the soil texture.

Soil texture

Soil texture is the proportion of sand, silt and clay in your soil. Soil texture influences:

  • Water movement and retention

  • Soil fertility

  • Soil erodibility

  • Accessibility for plant roots and burrowing insects

Garden soil with equal proportions of sand, silt and clay is best for general veggie growing. This mixture, called loam, is light, nutrient rich and retains sufficient water for plants to thrive.

Very sandy soil has good aeration but does not hold water or nutrients well. In contrast, heavy clay can be nutrient rich and retain water well but is less aerated and drains poorly due to the smaller particle size.

Soil texture varies across Victoria as it is influenced by local geology. Soil from the north west is naturally sandy, while pockets of heavy clay occur to the south.

Plants that are native to your region should grow well in your natural soil, but your veggie patch soil may require some attention for optimal production. Adding organic matter (like compost, manure and leaf litter) or new purchased soil from a reputable provider can help to support soil texture for vegetable growing.

Total organic carbon

As plants, animals and microorganisms decay, they release carbon into the soil. Carbon is an energy source for plants and organisms, necessary for their growth. Carbon also supports soil structure by preventing compaction, improving water and nutrient retention, and buffering soil pH.

GardenSafe’s total organic carbon measurement tells you how much organic carbon (organic matter) is in the soil.

Organic matter typically makes up 2 – 10 % of the total soil composition, although this content can be higher. Compost, manure, mulch and leaf litter are excellent sources of organic carbon, and be used to improve soil.

Plant nutrients

The nutrients nitrogen (N), phosphorous (P) and potassium (K) are essential for plant growth. These nutrients are the basis of fertiliser, each supporting a different plant function.

GardenSafe reports the total concentration of two nutrients: P and K, in your soil.

NutrientAbbreviationBenefitsGardenSafe common range (mg/kg)
NitrogenNPromotes growth of green foliageNot assessed by GardenSafe
PhosphorusPHealthy roots, fruits and flowers3000 - 3500
PotassiumKSupports overall plant wellbeing4000 - 5500

The ideal balance of N-P-K depends on the veggies you are growing. For example, N is important for producing leafy greens like lettuce and kale. For tomatoes, however, too much N and not enough P will grow tall, leafy plants with limited fruit.

Commercial fertilisers contain a balance of these nutrients. Fertilisers for specific crops (e.g., tomatoes) can help to maintain the correct balance. Organic matter, including manure, compost and leaf litter can assist in improving the natural balance of N, P and K in your soil.

The table above shows the common range of phosphorus (P) and potassium (K) concentrations measured in 54 GardenSafe soil samples. These values represent the range for which 95% of soil concentrations can be expected to occur (95% confidence interval). Soil nutrient results above or below this range do not indicate cause for concern.

Macquarie University
Northumbria University
Environmental Protection Authority (Victoria)

The 360 Dust Analysis program is a global research initiative to collect and analyse data on contaminants of concern that may be harmful to human health in homes and gardens..






Macquarie University, NSW, Australia.

Environment Protection Authority, VIC, Australia.

Northumbria University, Newcastle, United Kingdom.

IUPUI, Indianapolis, United States.

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