A soil test tells you the pH, the amount of organic matter and the availability of key nutrients in the sampled layer. It is a guide to what the soil can supply, not a guarantee of what the crop will take up, and it should be repeated on a regular cycle so that trends can be seen. The value of the test depends on three things: the quality of the sample, the method of the laboratory and the way the result is interpreted.
Many growers treat the soil test as a yearly chore, and the report is filed without much attention. A better approach is to treat it as the first step in a plan. The result shapes the rate of fertilizer and lime, the choice of product and the timing of applications, and the record of successive tests shows whether the programme is working.
Standard analyses
- pH, which controls nutrient availability and the lime requirement.
- Electrical conductivity, which indicates the salt level and the risk of salinity.
- Organic matter, which indicates soil biology and the nutrient supply from the soil.
- Available phosphorus, potassium and, where relevant, sulphur, calcium, magnesium and micronutrients.
The laboratory will usually report the results against interpretation bands, which show whether each nutrient is low, adequate or high for the crop. The bands are set by research in the region, so they differ between countries and between crops. The grower should ask which bands are used, and whether the recommendation is based on a calibrated model for the crop.
How to sample
Divide the field into uniform zones, based on soil type, slope, drainage and past management. Collect 15 to 20 cores per zone at a consistent depth, typically the top 15 to 20 centimetres for annual crops. Mix the cores thoroughly in a clean container, and send a representative sub-sample to the laboratory, with a clear label for each zone.
- Sample at the same time of year each cycle, so that seasonal changes do not confuse the trend.
- Avoid sampling soon after fertilizer, lime or manure is applied, because the reading will reflect the input.
- Use the same sampling depth and the same zones each time.
- Keep the sample dry and cool until it reaches the laboratory, and send it promptly.
Sampling errors are the most common reason for a misleading result. Taking all the cores from one corner of the field, sampling at a different depth, or mixing zones together can produce a result that does not represent the field. Spending time on the sampling is therefore worth more than spending on the laboratory method alone.
How often
Most programmes test every two to three years for the main nutrients and pH, and more often for intensively cropped or changing fields. Fields that receive large amounts of lime, or that have shown a rapid change in pH, may need a test every year until the trend is clear. The right interval depends on the field's history and the risk of change.
Reading the report
Read the report with the field history in mind. A result that shows low phosphorus on a field that has been heavily fertilized may reflect the sampling, the fixation in the soil, or a recent change in the crop's removal. The report gives a starting point, and the adviser can help interpret it in context.
Keep the reports together, and compare them with the fertilizer and lime records. Over several seasons the pattern shows whether the soil is building or losing fertility, and whether the programme is delivering the nutrients the crop needs. The record is one of the most valuable assets on a farm.
Sharing results with advisers
Share the field history with the laboratory and the adviser: previous crops, fertilizer and lime rates, and any recent problems. The recommendation is only as good as the information behind it, and a history note often changes the advice. The adviser can then recommend a rate that fits the crop and the soil, rather than a generic figure.
Keeping track of sampling costs
Soil testing costs money, and a grower may be tempted to reduce the number of samples. The better approach is to test fewer fields more thoroughly, rather than many fields poorly. A well-sampled field gives a result that can be trusted and used, while a poorly sampled field may waste the cost of the laboratory and mislead the plan.
Some growers share the cost of testing through a cooperative, which reduces the cost per field and gives a consistent sampling method across members. Consistency across a district also helps advisers see regional trends.
Keeping the record useful
The record of soil tests is most useful when it is easy to read. A simple table with the date, the field, the sampling zone, the key values and the decision taken is enough for most farms. Add a column for the weather and the crop, so that unusual results can be explained later.
Store the record in a form that will last, such as a spreadsheet with a backup, and check that the entries are complete each year. A record that is kept well for ten years becomes one of the most valuable tools a grower has, because it shows the true history of the soil.
Sampling variable fields
Some fields are variable enough that a single composite sample misleads. Where the soil changes across the field because of old channels, slopes, past manure heaps or uneven levelling, the sampling should follow the variation. Zones based on yield maps, soil colour or electrical conductivity maps help place the samples where the soil differs.
Variable sampling costs more, but it gives a better basis for variable rate fertilizer and zone liming. The additional cost is often repaid by avoiding over-application in one zone and under-application in another.
Checking the laboratory
A laboratory should be able to explain its methods, its calibration and its quality control. Ask for the reference samples it uses, how often it runs them, and whether it takes part in a proficiency scheme. A laboratory that can answer these questions is more likely to give results that can be relied on across years.
If results from two laboratories disagree, do not average them. Check which method each one uses, send a split sample to both, and use the one whose method suits the crop and the region.
Common questions
Can I test soil myself? Simple field kits can give a rough guide, and they are useful for a quick check. Laboratory analysis is more reliable for the nutrients and pH that decide the fertilizer plan, so use the kit as a guide between laboratory tests.
What if the test results disagree with the crop? Check the sample, the sampling depth and the laboratory method. Then look at the other factors in the field, such as drainage, pests and disease, which can mask the nutrient status.
How do I compare results across years? Use the same laboratory, the same method and the same sampling zones. Changing any of these makes the trend hard to interpret, and it can produce a false change in the record.
- University extension soil testing guides in your region.
- International Plant Nutrition Institute (IPNI), soil sampling resources.
- FAO, public materials on soil analysis and fertilizer recommendations.
- Brady, N.C. and Weil, R.R., The Nature and Properties of Soils (academic reference on soil analysis and interpretation).
Recommendations depend on local soil, climate and crop conditions. Follow the product label and local regulations, and consult a qualified agronomist for field decisions.