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Add soil sampling to the fall “to do” list

Testing soil in the fall gives you time over the winter to plan your spring nutrient needs

extracting a soil sample from a field

As if there wasn’t already enough on the fall “to do NOW” list, experts advise adding soil sampling to the fall work load, if it’s not already part of the farm management plan.

“The reality of the situation is if you don’t know what you have to start with, you won’t know how much or what to fertilize with,” says Ray Dowbenko, senior specialist, agronomic services at Agrium. “You will, in fact, be flying blind.”

John Heard, crop nutrition specialist with Manitoba Agriculture, Food and Rural Development, agrees with Dowbenko. “The real value of soil testing is not just to determine how much fertilizer to buy,” says Heard. “It’s an important audit tool to track the nutrients in your soil and the effectiveness of your fertility program.”

Soil testing is an important tool used to gauge soil fertility and plan a suitable fertilizer regime to maximize the yield potential of the crop. But the results are only as good as the sample.

“The reality of soil testing is that most farmers are not doing it themselves,” explains Heard. “Most now are using the services of independent agronomists or a fertilizer company or supplier. These professionals are properly equipped with truck-mounted samplers, and as a result, if a grower has consistency in his service provider for both sampling and testing, he will reap the benefit of long-term tracking of his fertility program.”

Soil sample timing and placement

“Fall is a good time to soil test,” says Dowbenko. “In spring, most growers deal with some serious time crunches and if test results are delayed, or fertilizer supply is interrupted, there is little time to get a comprehensive fertility plan in place.” Heard agrees. “Farmers have to differentiate between the best time to soil test and the most practical time to soil test,” he says. “Just before the crop is about to use the nutrients in the soil might be the best time to test, but it’s certainly not practical. Fall testing in cool soils gives the grower time to formulate a comprehensive fertility program, as well as take advantage of both fall buying and application opportunities.”

Soil samples are best taken when soil temperature has dropped to 10 C or lower. “Right after combining, and before any fall tillage operations, is a good time to take soil samples,” says Dowbenko. “The soil has usually cooled sufficiently that any changes due to bacterial activity and mineralization are at a minimum.”

Farmers can work with local agronomists or fertilizer dealers to draw up a sampling plan to ensure soil samples are taken in sufficient volume in enough areas of the field to produce a representative view of the field’s fertility status. It’s very important that the person most familiar with the topography has input into the sampling plan. “The key is to know where to avoid sampling,” says Heard. “For example, a saline area might reveal very high levels nutrients like sulphur simply because crops won’t grow there very well, if at all. A sample like this can easily taint an otherwise good soil testing program.” Heard advises to leave out problems areas or sample them separately.

Lab tests

The laboratory will conduct a soil test based on your instructions. Generally, a typical soil test package will include tests for nitrate-nitrogen, available phosphorus and potassium and extractable sulphur, as well as pH and salinity. Additionally, a micronutrient scan can be requested to determine levels of elements such as copper, zinc or boron, amongst others. “Farmers should stick with a lab once they find one they are happy with,” says Heard. “Different labs can conduct tests using different methods which would make long term monitoring more difficult. It’s the apples to apples analogy.”

“Understanding and interpreting soil test recommendations is very important to designing the fertilizer management plan for your farm,” says Dowbenko. “Think of the recommendations as a guideline — a starting point with which to engage your advisor or local dealer in a discussion on your fertilizer plan for the upcoming season. The laboratory does not know how you farm, the moisture levels you typically experience or what your growing season is like.”

Heard recommends using a “rotational fertilization strategy.”

“Farmers have been achieving very high yields, and we know some of our crops in particular are greedy phosphorus users, especially canola and soybeans,” says Heard. “We are exporting more phosphorus generally than we are applying, so it’s important farmers take into consideration both the soil test values and the yields they are achieving now.” Many farmers have been scaling up their fertilizer programs, but modern seeding equipment does not always allow the full recommended or desired rate of phosphorus to be applied with the seed for seed safety reasons. “A rotational fertilization strategy comes into play in a situation like this,” explains Heard. “Some nutrients can be applied in excess of the needs of the current crop, for instance wheat, but remain available for a canola, soybean or flax crop in a following year.”

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