How to take a soil test

Remove the guesswork from gardening by defining your soil's makeup.

Credit: Carolyn Herriot

Remove the guesswork from gardening by defining your soil’s makeup and getting to know the complex world beneath your feet

If we could travel underground to explore what’s going on in the soil, it would be like we’d discovered another planet teeming with alien life forms. Up until recently I gardened in complete ignorance of this world and how plants derive nourishment from it. I’d experimented with different ways of improving soil texture and fertility, and my composting skills were down to a fine art. But it wasn’t until I understood more about the intricate web of life in the soil that I saw the big picture.

Plants depend on amazingly diverse organisms, ranging from bacteria, algae, fungi and protozoa, to more complex nematodes and micro-arthropods (microscopic bugs) and creatures we are quite familiar with, such as earthworms, insects and small vertebrates. These combined organisms decompose organic matter, fix nitrogen from the air, store nutrients and make them available to plants, enhance soil tilth, and manage or destroy pests and pollutants. They are critical in creating, regulating and maintaining healthy soils and plant growth. Soil organisms follow seasonal and daily patterns, being subject to temperature and moisture fluctuations. The greatest activity occurs in spring, when temperature and moisture are optimal. When your garden bursts into growth, it is the increasing activity in the soil that triggers this spurt.

Taking a soil test

How do you know if your soil needs amending or if a particular plant will thrive in it? How can you best find the solution to problems with plants in your garden? There’s only one surefire way, and that’s by having the soil tested. Once you become familiar with your soil conditions, it’s much easier to answer these questions. If you have moved to a new garden, have established new beds or are struggling with a garden that is not growing well, a soil test will take the guesswork out of gardening.

Ideally you should have your soil tested every three years. Your local garden centre can put you in contact with a laboratory that conducts soil tests. Different tests provide you with varying amounts of information. The most basic test gives you the soil pH and macronutrient analysis (NPK), but by paying more you can get a detailed analysis. You can find out the levels of nitrogen, phosphate, potash, iron, zinc, sulfur, copper, manganese, magnesium, sodium, calcium, organic matter, soluble salts, lime and the cation exchange capacity (CEC), which indicates the nutrient holding capacity of the soil. This analysis will be accompanied by appropriate fertilizer recommendations, based on the results of the test.

How to take a soil sample for testing

You’ll need several random samples from the area you want to test. In total the laboratory will need about 250 to 500 ml (1 to 2 cups) of soil.

Step 1: With a shovel, dig a hole from the soil surface to the root zone. Remove a thick 2.5-cm (1-in.) slice from the smooth side of the open hole, including the surface crust of the soil. Remove the slice from the blade of the shovel, and place a 5-cm (2-in.) wedge of it into a clean container (plastic is best). Repeat this procedure at random across the area you want to have analyzed.

Step 2: Mix the representative slices thoroughly, removing all foreign matter such as roots, stalks, rocks, etc. Make sure some of the surface crusts are mixed into the final sample, as they contain soluble fertilizer salts that can affect plants.

Step 3: Place your sample in a zip-lock bag and identify it for the laboratory records.

Different levels of life in the soil

The primary level consists of plants—lichens, moss and algae—which are photosynthesizers fuelled by the sun’s energy to fix carbon dioxide into life-giving compounds.

The next level consists of decomposers—bacteria and fungi—that convert complex materials into nutrients and make them available to plants and other soil organisms.

Then there are the shredders—predators and grazers—represented by earthworms, nematodes and macro-arthropods (bugs such as cutworms, millipedes, weevils, beetles, etc.). Their activities enhance soil structure and control root-feeding pests and disease.

The highest level is represented by mice, birds and above-ground animals and insects. All these valuable organisms are most concentrated around roots, in plant litter, on humus and on the surface of soil aggregates. Tillage, insecticides, fungicides and herbicides have an enormous impact on non-target species in the soil web of life.

Disruption of the intricate relationship and balance between the pathogens and beneficial organisms opens the door to problems with pests and diseases. Enriching the soil with organic matter, avoiding synthetic chemical pesticides and fertilizers, and minimizing tillage while maximizing your use of mulch and green manures all help to sustain and protect this wonderful underground world on which all plant life depends. Fungi are decomposers that convert complex materials into nutrients, making them available to plants and other soil organisms (see picture below). Plant roots exist in a mutually beneficial relationship with mychorrhizal fungi, providing nutrients and water from the soil (see picture below). Carolyn Herriot owns The Garden Path Centre for Organic Gardening in Victoria ( and Seeds of Victoria. She is author of A Year On The Garden Path: A 52-Week Organic Gardening Guide (ISBN 0-9738058-0-3).