Soil Testing: How to Take a Soil Sample and Read the Report (Save on Fertiliser)

Fertiliser is one of the biggest costs on most farms,

and much of it is applied by habit or by guess.

Some fields get more phosphorus than they will use for years, while others run short of potassium or zinc and nobody knows why the yield is stuck. A soil test replaces the guess with a measurement. It tells you the soil's acidity, its main nutrients and its organic matter, so that you can put fertiliser and lime where they pay, and save money where they do not. Applying only what the crop needs also keeps excess nutrients out of rivers and groundwater.

In this video: when to sample, how to take a sample that truly represents your field, what the main numbers on the report mean, and how to turn the report into a fertiliser plan. The method is the same on every continent, although the details of the tests and the recommendations come from your local laboratory.

For most fields, test every two to four years, and more often for intensive or high-value crops.

Sample at the same time of year each time, so that results can be compared, ideally after harvest and before you plan the next crop's fertiliser. That leaves time to get the report back, and to apply lime well ahead of sowing.

Avoid sampling straight after applying fertiliser, manure or lime, because the results will be distorted;

wait at least a couple of months. And avoid very wet or waterlogged soil, which is hard to sample and mix evenly.

A laboratory analyses only a few grams of soil.

That spoonful has to stand for a whole field, so how you collect the sample matters more than anything the laboratory does.

Start by dividing the farm into sampling areas that are as uniform as possible.

Separate areas that differ in soil type or colour, slope, drainage, past crops, or the way they have been fertilised or irrigated. Each area gets its own sample. As a rough guide, one sample should represent no more than a few hectares of uniform land. A field with a sandy hill and a heavy, low corner needs two samples, not one.

Leave out the spots that are not typical.

Field edges and headlands, the ground near trees, old fence lines and roads, where manure or fertiliser was heaped, burnt patches, waterlogged hollows and termite or ant mounds.

If such a patch is large and important, sample it separately.

Walk across each area in a zig-zag or a W pattern, and take fifteen to twenty small sub-samples, called cores.

For most crops, sample the plough layer, from the surface down to about fifteen to twenty centimetres. Take every core to the same depth, because nutrients are usually richest near the surface, and a few shallow cores can make the field look better than it is.

A soil probe or auger is the easiest tool. If you only have a spade, dig a V-shaped hole to the sampling depth, then cut a slice about two to three centimetres thick from one side, and keep the central strip of that slice from top to bottom.

Collect the cores in a clean plastic bucket.

Avoid galvanised or brass buckets and tools, because they can add zinc or copper to the sample. When you have all the cores, break up the lumps, remove stones, roots and residue, and mix thoroughly.

Then take about half a kilogram of the mixed soil for the laboratory.

If the sample cannot reach the laboratory the same day, spread it out to dry in the shade, at room temperature.

Do not dry it in the sun or in an oven, which changes some results.

Put it in a clean bag, and label it clearly:

your name, the field name or number, the sampling depth, the date, the last crop and the crop you plan to grow next. The laboratory needs the next crop to make its recommendation.

Use a laboratory that is accredited, or that takes part in a proficiency testing scheme, and stay with the same laboratory over the years.

Different laboratories may use different extraction methods, and their numbers are not always comparable.

Quick test kits are useful for checking pH in the field, but for fertiliser decisions, a laboratory test is far more reliable.

Start with pH, the measure of acidity, because it controls how available every other nutrient is. Most crops grow best between about six and seven and a half.

Below about five and a half, phosphorus becomes locked up, and aluminium and manganese can reach toxic levels, so roots stay short and yields fall. The cure is lime, and the report will often give a lime requirement in tonnes per hectare. Lime works slowly, so apply it months ahead of sowing, and work it into the soil where you can.

Above about eight, the opposite happens.

Iron, zinc and manganese become less available, and phosphorus combines with calcium.

Here you choose tolerant varieties, add organic matter, and correct micronutrients directly.

Electrical conductivity, often shown as EC, measures salts.

A high value warns of salinity, which damages sensitive crops and calls for better drainage and careful irrigation rather than more fertiliser.

Next come the nutrients.

Phosphorus, potassium and often sulphur, zinc and boron are reported with a rating such as low, medium or high, or a sufficiency range. Compare these ratings, not raw numbers between laboratories, because the same soil gives different numbers with different methods.

Olsen phosphorus, for example, is common on neutral and alkaline soils, while Bray or Mehlich tests are common on acidic soils.

Organic carbon, or organic matter, tells you about soil structure, water holding and the soil's ability to supply nutrients over the season.

It rises slowly, with residues, manure, compost and cover crops.

Nitrogen is the exception.

Available nitrogen changes quickly with weather and soil biology, so in many regions a single soil test is not a reliable guide. Nitrogen rates are usually based on the crop's expected yield, the previous crop and the organic matter, and in some regions on a nitrate test taken just before the crop's main demand.

Now turn the numbers into a plan.

The basic idea is simple: what the crop needs, minus what the soil can supply, is what you add.

Where a nutrient tests low, the recommendation may include extra to build the soil up over a few seasons.

Where it tests medium, you replace roughly what the crop removes.

Where it tests high, you can often cut that nutrient back sharply, or skip it for a season, and that is where soil testing pays for itself.

Fix the pH first, because no fertiliser works well in strongly acidic soil. Then apply the nutrients that are short, in the right form, at the right time and in the right place.

Keep every report, by field, and compare them over the years.

The trend tells you whether your fertiliser plan is building the soil, holding it steady, or mining it.

On larger farms, the same principles extend to precision agriculture.

Instead of one sample per field, samples are taken on a grid or by management zone, and fertiliser and lime are spread at variable rates.

To recap.

Test every two to four years, at the same time of year, before planning fertiliser. Divide the farm into uniform areas, and leave out untypical spots.

In each area, take fifteen to twenty cores to the same depth, in a zig-zag, with clean, non-galvanised tools.

Mix, dry in the shade, label, and send about half a kilogram to an accredited laboratory.

Read pH first, then the nutrient ratings, and treat nitrogen separately.

Then fix the pH, add what is short, and cut back what is high.

You can keep your soil test results and get crop recommendations in Laisha, at laisha.ai. Thank you for watching.