Soil testing has long been a recommended practice before applying nutrients, and variable-rate fertilizing has made it more essential than ever.
But the difficulty with soil testing is that so much can go wrong.
Everything from how a sample is handled to the choice of a lab can impact results and recommendations.
However, producers have more control over the entire process than they might think, said agrologist Len Kryzanowski, director of environmental strategy and research with Alberta Agriculture and Forestry. It’s a matter of making good decisions in the field, handling samples with care, and making sure your choice of lab fits your soil testing needs.
Have a strategy
Start the sampling process with a strategy that considers topography; previous and current field management; and problem areas in a field.
“The most important step in my mind is that soil sample collected in the field,” said Kryzanowski. “You want representative samples of the area you’re managing. If you’re managing a whole field in a uniform manner, then the common practice of random composite sampling — where soil samples are collected from a number of locations within a field and combined to produce one sample per depth — is feasible.
“However, within what you feel is a uniform field, there may be areas that have hidden factors below the surface that may influence crop productivity.”
Although sampling the top six inches of soil is a common practice, Kryzanowski urges producers to collect deeper samples. The agronomic benefits of knowing subsurface nutrient levels will outweigh the additional cost, he said.
“We recommend that soil samples be collected down to a depth of two feet. There are people who just get the top surface sample or combine the first and second depths, thinking it will save them lab analysis costs.
“If you just do a surface test and ignore the subsurface, the lab will tend to estimate what’s down below. That estimate may be right — but more often it’s going to be wrong and will affect your nutrient recommendation.”
Implements count when testing deep below the surface.
“We recommend using a sampling tube or a core when collecting samples to separate soil depths. Corkscrew or auger types of sampling tools are not recommended; you tend to get too much mixing of the soil depths with those tools.”
Sample lots, but with purpose
Generally speaking, the more samples a grower takes, the better their chances of finding a representative soil sample. However, the process must be strategic. Kryzanowski recommends dividing farmland into 15 to 20 management zones based on factors that limit or improve crop productivity.
“Management zones may be reflective of past management or knowledge of the crop capabilities in that part of the field,” he said. “They may be based on topographic factors such as high, mid-, and low slope positions. Getting that representative soil sample for those areas and extending it to the entire field is very critical.”
Benchmark sampling is another tool in the tool box.
“A benchmark site is an annual measurement in which producers go back every year to the same area and track nutrient levels from year to year to see if they’re increasing or decreasing over time. With GPS you’re now able to go back to a very specific location every year. A field can have several benchmark sites to reflect the field variability.”
Don’t fear fall sampling
Collecting samples in the spring just before applying fertilizer is considered a best practice. However, it also comes at the busiest time of the year for labs.
“It’s the ideal time but it’s also the most challenging time in terms of getting lab results back and trying to make a decision from those results,” said Kryzanowski.
Research by Kryzanowski and others has shown that post-harvest, late-fall sampling can be just as reliable as spring testing — perhaps more.
“If you’re closer to the fall or winter just before freeze-up, you get a soil test similar to what you find in the springtime.”
Mid-season, summer sampling isn’t recommended except when required to identify field-related crop problems.
“When you see a field area giving you poor crop growth, you take soil samples from that area and an adjacent healthy growth area and try to identify the possible soil difference that may be causing the problem.”
Handle with care
If possible, growers should get their samples to the laboratory within a few hours of collecting the sample. Moist soil samples are still biologically active and can alter the soil test nitrogen levels, but overall offer the most accurate nutrient content information. If that’s not possible, the best thing to do is nothing — at least when it comes to artificially drying the sample.
“Traditionally we’ve told people to spread the sample out in an aluminum pan,” said Kryzanowski. “Don’t try to artificially dry it because if you add too much heat you might lose some of the nitrogen value. If you can get it to the lab within a few hours, ship it off in a cool state.”
Providing pertinent information with the sample is critical.
“You’ll want to include the crop that was grown on that soil last year and what crop is going to be grown in the coming season. If a field has been receiving fairly significant amounts of manure, that’s an important piece of information the laboratory needs to know.
“You’ll also want to mark where the samples came from so when you look at the results you know which ones are from which field or part of the field. Those all provide supplemental information for the lab to judge the results.”
Grill your lab
Picking a laboratory for soil testing can get tricky. Different labs use different methodologies and not all methodologies are calibrated to the soil in a producer’s area. Moreover, a method that works in one province may not be as reliable in another.
Other parties may be involved as well. Some producers use fertilizer dealers or hire consultants to do their soil samples for them, for example. Those companies may use a particular lab — possibly one from out of province — that uses methods that don’t meet the province’s recommended nutrient levels.
Also, some labs don’t include their methodologies in the final report to the producer. And methods are always being re-evaluated. For example, at one time most labs would test macronutrients individually. Today, many use multi-nutrient extractants which may test up to four macronutrients at once.
“The advantage of using multi-nutrient extractants is it can save a lot of time in terms of lab processing, with the farmer getting the results back much quicker,” said Kryzanowski.
But that can compromise the numbers coming out.
“The original soil test method was calibrated with field or greenhouse research samples and crop yield response data. New soil test methods may not have this same calibration, or were calibrated against the original soil test method but not with the research samples and crop yield response data. As a result, errors can be introduced in the calibrations.”
The bottom line is to ask a lot of questions.
“Ask the lab how it selected the methods it’s utilizing, the basis for its calibration, how up to date it is with the research work going on in the universities or at provincial and federal agencies. Knowing what methods a lab is using is part of the business process.”