Test Smarter to Spend Less

Monday, September 22, 2025

How Soil Tests Help You Buy the Right Fertilizer at the Right Time

By Scott Gillespie

Soil testing isn’t expensive – especially compared to fertilizer. The cost of collecting and analyzing a basic sample shouldn’t be more than a few dollars per acre when spread across a quarter section.

Testing every field every year helps build field- and crop-specific baselines. If budgets are tight, you can reduce costs by focusing on the acres ahead of a specific crop or by rotating through a percentage of your fields each year.

Soil testing pays in two main ways: Avoiding under- or over-application. A blanket blend for all your wheat acres may be convenient, but it can short some fields while overloading others.

Timing your buying decisions

After several years of drought, phosphorus levels tend to increase. Knowing that can justify rate reductions – or even skipping an application when fertilizer prices spike.

I’ve been reading soil tests and making fertilizer recommendations for a few decades now.

I’ve seen reports from labs across Western Canada and beyond. While I prefer labs that are calibrated for Prairie soils, I’ll still take any test over no test. A single test gives a snapshot. Multiple years tell a story.

What I look at first

I always start with organic matter. It gives a ‘report card’ on how the soil’s been treated. There is no perfect number. The best comparison is to similar fields in your area, or whether you have been able to increase, or at least maintain, levels over the years. Electrical conductivity (EC) tells me if salinity is a problem now – or could be in the future. Soil pH helps tie it all together. If it’s between 6 and 8, you’re likely in the safe zone. Lower than that? You might need lime. Higher? It could indicate sodicity, which requires further testing to determine if the problem is manageable.

Next, I look at phosphorus (P). It reveals more than just fertility. If it’s excessively low, the soil has been mined of nutrients. If it’s high, it’s likely been manured or composted regularly. Most Prairie soils aren’t potassium (K) deficient, but it’s still useful to track in case yours is the exception. Sulphur (S) is essential for all crops, but especially so for canola and mustard. Finally, I check nitrate-nitrogen (NO3). It helps plan your N application and shows whether the last crop ran out of gas or had enough to finish strong.

Soil health tests

Soil health tests aim to measure biological activity or long-term system function. They’re promising, but they’re not yet calibrated for Prairie crops like traditional chemical tests are.

Standard chemical soil tests are valuable because their numbers correlate with yield response. A low P test, for example, points to a likely response to added P. More importantly, rates can be adjusted based on fertilizer and crop prices – making the decision both agronomic and economic.

I got interested in soil health tests back in 2018 and did a deep dive. My verdict? I’ve never sent samples off or recommended them to producers. That hasn’t stopped others from trying them, though – I’ve been sent test results and asked to interpret them. I see these two problems most of the time:

Lack of context

Often there’s no benchmark for what the result should be. If a range is given, it’s usually not backed by published data. It’s based on internal calibrations that companies won’t share because they’re proprietary. By contrast, chemical soil test calibrations were developed using public funding and are widely available.

Vague recommendations

Most results come back with generic advice: ‘increase diversity’ or ‘add cover crops’ – with no guidance on which species, rates, or timing to use. Cover crop work in the Prairies is still in its infancy, and I’ve yet to see a peer-reviewed study in the scientific literature showing improved soil health test scores – let alone yield responses – from using these new practices.

One group that I think is doing good work is the Soil Health Institute (SHI). They spent their first eight years of existence cataloguing hundreds of tests and narrowed them down to three with real-world potential. To make their list, tests had to measure something not already covered by chemical tests, track meaningful changes, and be practical to implement. Their top three are:

• Organic carbon;
• Carbon mineralization potential;
• Aggregate stability.

If those don’t mean much to you yet, that’s okay. The next step is to figure out whether these indicators can provide consistent, repeatable data when a farmer implements new practices. That will take time. Remember – it took decades to develop and calibrate the chemical tests we rely on today. In Alberta, we were still refining the best phosphorus test into the 1980s and 1990s.

Precision soil testing

Once you’re comfortable with treating each field separately, you can move on to testing the production zones within each field separately. An early idea was to lay a grid over a field and make 5-, 10-, or 20-acre fields within a field, similar to the gridding of the Prairies into 640-acre sections. While a good idea, the lab costs and the time needed to do this proved to be impractical.

Now we have tools like satellite imagery, yield maps, topography, and other soil mapping tools to help us focus on where variability exists. Instead of dozens of zones, we can often narrow it down to a few.

In my experience, zone sampling pays off when you’re dealing with high-value crops or highly variable land. However, it’s only worth it if you’re actually going to use the data. Having the maps and not having the machinery to vary rates, or the people who are willing to put in the effort to do it, just gives you more pretty maps on your computer.

Flat land can be variable, too! You might be surprised. If you’ve been collecting yield data for years or have had the chance to see satellite imagery of your fields, then you’ll know this. Use this existing data to target the fields that would most benefit from variable-rate application.

Correcting soil pH with lime is one situation where the costs of mapping and sampling almost always pays, versus flat rating. In this case, you get the lime where it is needed at a high enough rate to do some good, and you avoid applying it where it is not needed.

Final thoughts

So, is now the best time to soil sample? Yes! Soil samplers are a tough group of individuals. I did my time early in my career. I learned lots from seeing the soil. But now I just want to see the numbers. While the weeks just before freeze up are the ideal time, it’s not logistically possible for all the sampling in Western Canada to be done at that time.

Book your time early and get some samples done this fall. You’ll be happy to have them when deciding on when to buy your fertilizer and planning for the 2026 crop. BF