LETHBRIDGE – Commercial soil testing has been around for more than 40 years in Western Canada, but many fertilizer recommendations are based on research from the United States.
Western Canada does not have a good soil test calibration system for soil response to fertilizer, particularly for micronutrients, says Tom Jensen, head of the International Plant Nutrition Institute in Saskatoon.
“You are getting recommendations from research in the U.S., and there is very limited response on micronutrients,” he said during the Southern Alberta Research Association’s crop diagnostic school held in Lethbridge July 7.
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“We probably know more about copper than anything because the work has been done.”
That does not mean farmers should abandon soil testing because local information is improving all the time. However, few people test their soil on a regular basis because they think it is time consuming and complicated.
Jensen recommended taking 15 to 20 samples from a field regardless of the acreage. Samples need to be well blended and shipped to a laboratory as soon as possible.
Samples should be collected as close to seeding as possible but that is not always realistic. It can also be done in early spring or late fall because many people make fertilizer decisions in the winter.
“The closer you can sample before freeze up, the better,” said Ross McKenzie, a research agronomist at Agriculture Canada’s research centre in Lethbridge.
However, a mild winter in southern Alberta could cause organic matter in the soil to convert to nitrogen, which means survey results may vary.
“Nitrogen levels will increase over the winter if we have a mild winter,” McKenzie said.
Some producers freeze samples, but that could rupture soil bacteria and affect nutrient release. It is better to cool samples to 0 to 5 C so the bacteria slows down and then ship within 48 hours. If it is not possible to ship samples within two days, try spreading the soil on clean paper or aluminum foil and air dry it before sending it to the lab.
Samples should come from three depths.
McKenzie suggested collecting from the top six inches, six to 12 inches and 12 to 24 inches. Assessing phosphorus is best done from the shallow depth samples.
Samples from 48 inches deep can help assess the needs of certain crops with deep rooted systems and trace nitrogen movement through the soil profile.
“If you are in a glacial soil with lots of rocks, good luck getting down to 48 inches,” Jensen said.
Laboratories grind the samples and use different extraction solutions to measure nitrogen, phosphorus and potassium and perhaps other elements such as sulfur.
Lab reports on the major nutrient levels are ranked in pounds per acre or kilograms per hectare while micronutrients are assessed in parts per million.
As a result, one acre of land six inches deep weighs two million pounds and a mathematical formula is applied to figure out how much of a particular nutrient is needed. If the math is wrong, an incorrect level of fertilizer could be applied.
Alberta Agriculture offers an on-line program called the Alberta Farm Fertilizer Information and Recommendation Manager to simplify these calculations when basing fertilizer decisions on soil results, crop type and costs.
McKenzie recommended using the same lab all the time for consistent results.
A single field can be variable depending on low or high spots or drainage. If results seem unusual, ask the lab to rerun the tests.
“If you sample your fields every year or every other year and you know certain fields are high in phosphorus and one year it is not, talk to the lab and ask them to redo those samples,” he said.
Jensen said it is also possible for producers to make a sampling error, so redo the sample if results are odd.
Because many labs are better equipped to do analysis than provide fertilizer recommendations based on local conditions, McKenzie said farmers need to develop their own plans based on as much western Canadian information as they can find.
“You want to depend on yourselves to develop a fertilizer management plan rather than have a soil test lab make recommendations for you,” he said.
