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How to determine soil health

Noelle
Valued Contributor

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It’s no secret that being able to grow happy, productive plants depends on the health of the soil in your garden. But how do you work out whether your soil is good, or not?

 

The most productive soil is a humus-rich loam – a workable or friable mix of fine, medium and coarse particles mixed with organic matter that absorbs and holds moisture and nutrients, drains freely and allows roots to grow deep into it.

 

Working out if your garden soil fits that profile is not always easy and is usually gained from years of gardening experience. Expensive laboratory tests on soil samples taken from random locations around your property will give you a detailed scientific analysis and snapshot of the soil characteristics. But there are much simpler and less costly ways to diagnose soil health, including these basics:

 

 

We’ll look at each in turn and give you some handy tips so you can gain an informed picture of the state of your soil, and what you could do to improve it if need be.

 

Earthworms

 

Finding earthworms in your soil when you dig or fork it over is a good indication that your soil is reasonably healthy. Worms digest organic materials such as leaf litter, compost and aged manure in the soil so their presence usually means your soil contains plenty of organics.

 

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These and other soil organisms thrive in loam soils that have a mix of particle sizes, a neutral to mildly acid pH level, absorb and retain moisture but don’t stay wet for long periods and are easy to “work” (dig or fork over).

 

Soils devoid of worms are generally not considered all that healthy. They are often extremely acidic, lacking in organic matter, poorly textured so they either don’t hold moisture (sands that water drains straight through) or hold too much moisture (clay or clay-loam that doesn’t drain well).

 

Adding compost and aged manures to light sandy soils and treating clays with gypsum will improve soil texture markedly over a period of time (at least a couple of years or longer). Once the soil is in better condition, worms will start to appear.

 

Smell

 

A healthy garden loam has a noticeably “earthy” smell that is quite pleasant and without any whiff of rotting material or manure, whereas heavy clay often have a rank odour reminiscent of decay or mould. A light sand will have almost no smell at all.

 

While we don’t recommend sticking your nose right into a sample, you should be able to detect that rich earthy smell when soil is freshly dug or moistened.

 

If your soil has a strong and nasty odour, you should investigate how well it drains after watering or rainfall. Soils that stay wet for extended periods, even loams, will stink as any organic matter present will rot.

 

Colour

 

Soils vary considerably in colour depending on the amounts of different clays, sands and loams in them as well as their geological origins. Volcanic soils, for example, are often rich red while those arising from basalt are usually more grey to black.

 

While these attributes will affect colour to some degree, as a general guide healthy loams are frequently black or dark brown while clays are yellow to orange and sands grey to white. Loams will almost always include small percentages of clay and sand but usually not enough to affect colour.

 

Feel or texture

 

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Picking up handfuls of soil around the garden and running it through your fingers will give you a reasonably good indication of its overall health and ability to grow plants successfully. There are also manual tests for more precisely determining its texture.

 

Start by collecting representative samples of soil from different areas of your garden. If it is obvious from appearance that you have quite different soils across your property, then collect samples of each type and keep them separate.

 

Break up large clumps before passing each sample through a 2mm soil sieve to remove stones and other debris. Spread the samples out to air dry for a few days. When your samples are dry, do the following for each:

 

  1. Collect enough to comfortably hold in the palm of your hand

 

  1. Add enough water to be able to work the sample into a ball and knead it for a couple of minutes. Add more water if required until the ball stops sticking to your fingers.

 

  1. Take note of how the soil feels. If you can’t form a ball at all, your sample is very sandy.
  • gritty = sandy
  • silky = silty
  • spongy = loam
  • sticky = clay.

 

Most soils will be a mix of two or more types, but this basic guide will be good enough for home gardening.

 

To improve the texture of sandy soils, add plenty of organic matter in the form of compost or aged manures. You can also grow green manure crops like lupins or oats and turn them into the soil before they flower.

 

Clays and clay loams will respond well to gypsum which will help the fine particles clump together to form a workable soil.  Add at the rate of a good handful per square metre to bare soil and allow the action of seasonal rains to wash it into the soil. Don’t be tempted to fork it in as it works better by being absorbed slowly.

 

For a more detailed explanation of determining soil texture, refer to WA Agriculture’s website.

 

pH value

 

The acidity or alkalinity of a soil is referred to as its pH value. The scale is from 1 (extremely acid) to 14 (extremely alkaline), with 7 being neutral. Most natural soils have pH values ranging from around 5 (medium acidity) to 7.5 (mildly alkaline).

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The vast majority of plants thrive in a mildly acidic soil – from 5.5 to 6.5, with a few requiring a slightly more acidic pH such as rhododendrons, azaleas, magnolias, camellias and even roses. However, there are some plants that prefer an alkaline soil including lavenders, sweet peas, geraniums, gerberas and hibiscus. Vegies include the brassicas (cauliflower, cabbage, broccoli), celery and cucumbers.

 

The pH of your soil can affect the availability of various nutrients to plants. The optimum value for most of the major, minor and trace elements being readily available is around 5.5. Below or above this value may mean some of them are less freely available, which means you will almost certainly need to use complete or all-purpose fertilisers to ensure your plants are able source what they need.

 

Testing your soil’s pH is reasonably straightforward using an inexpensive reagent-type test kit or a pH meter. As you did when testing texture, take samples from various locations around your garden and follow the instructions on the packaging of whichever means you choose as your testing device.

 

It is important to note that reagent-type test kits which use a colour chart to read pH values can give inaccurate readings when the soils being tested contain high levels of organic matter. The result is often a false alkaline (pH above 7) reading.


If your soil has been heavily mulched over a number of years or you’ve added large volumes of compost, the result could be quite misleading. In such instances, if pH is critical to your gardening efforts, you may need to send a sample to scientific testing laboratory for an accurate reading.

 

To “sweeten” highly acidic soils, apply garden lime or dolomite at the rate of a good handful per square metre to fallow soil and allow rains to carry it into the soil. Alkaline soils can be acidified reasonably quickly with sulfur or, if you’re not in a great hurry, add lots of compost and aged manures.

 

Water absorption and retention

 

The rate at which water soaks into your soil is another indication of how healthy it may be. If water is rapidly absorbed or the soil dries out very quickly after watering or rainfall, it is likely you have a light sand or sandy loam. These light soils are good for some vegetable crops like carrots where there’s plenty of irrigation water available but are not ideal for general gardening. Adding lots of compost and aged manure as well as mulching with an organic material like sugarcane or lucerne hay will improve these soils over time so they will still absorb water well but will retain it for longer.

 

Heavy clay loams and clays have the opposite problem – water is slow to be absorbed and soils remain wet to waterlogged for long periods after rain or heavy watering. Strange as it may seem, adding organic matter will help improve these soils too, by increasing the air space between soil particles.

 

If you are uncertain of how well your soil absorbs moisture, try the following:

 

  • Choose a spot you suspect may be problematic and dig a hole about 300mm square and up to 450mm deep in the topsoil layer only, making sure you do not dig into underlying clay.

 

  • Add enough water to two-thirds fill the hole.

 

  • Observe how long it takes for the water level to drop to the base of the hole.

 

Anything under 30 minutes indicates your soil is highly absorbent and probably tending to sandy loam or sand. If the water level drops steadily over the next couple of hours, the soil is mid-scale – quality loam. The other extreme, where water takes a long period to soak in (12 hours or more) then you have a heavy soil or clay that is not readily absorbent but once wet will stay that way for a long time.

 

Drainage

 

While most permanent plantings (trees, shrubs, climbers and groundcovers) are capable of sending anchoring roots down into the sub-soil, they will not thrive if the topsoil (where the feeding roots may be) is poorly drained.

 

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How well a soil drains depends not only on the “lie of the land” (ie slope) but also the texture of the top layer and its depth. While your property may have terrific loamy soil, if it is only 150-200mm deep with underlying clay, chances are you will have a drainage problem especially if your land is flat.

 

Soil that remains wet for long periods, especially over winter, will often become smelly as well as being impossible to cultivate.

 

The cure is to put in some form of drainage to carry excess water away from garden beds and lawn areas. There are various systems available with some easier for the DIY-er to install than others. It’s critical to lay underground pipe or hose on top of the subsoil and with a slight fall across your property for best results. And any form of underground drain system must be connected into a stormwater pit or sump on your property.

 

If you need any assistance with testing or improving the health of the soil in your garden, don’t hesitate to join in the gardening discussion here on Bunnings Workshop.

 

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1 Reply
EricL
Bunnings Team Member
Bunnings Team Member

Hello @Noelle,

 

Thank you so much for sharing your knowledge about soil health. I'm sure our members and those who are planning to begin their own garden will find this guide very useful. Soil health is definitely a topic to consider before planting anything in the ground.

 

Again thank you so much for sharing your wisdom about soil health and the techniques you can use to improve your soil.

 

Eric

 

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