To sample, or not to sample (soil). Who’s right?

Some of your neighbours have stopped soil sampling. Others are sampling more than ever.

In any year, at most 10 per cent of the fields are soil sampled,” says Tom Jensen, a director in the North American program of the International Plant Nutrition Institute. “Some people say 20 per cent of farmers do some soil testing, but they may only do it every couple to three years.”

Farm consolidation is one factor that’s leading to the decline. Soil sampling can be a prohibitively time-consuming procedure for 10,000-acre farms, and these operators may opt instead for a standard fertilizer recommendation for each crop type, rather than each field. Nutrient rates are also adjusted based on target yields and what each crop is expected to remove.

“We’ve seen a decrease in the amount of soil sampling,” Jensen says. “A lot of our farms have shifted over to what I would call yield-based removal replacement, with a lot of experience from their local area thrown in.”

But it isn’t the same story on every farm. On precision farms, in particular, growers are actually doing more soil sampling than they used to, Jensen points out. They’re dividing their fields into sub-fields based on soil management zones derived from data gathered from multi-year satellite images, yield monitor maps, and other digital information.

“We have some fields that are being sampled more intensively and more regularly than they ever were,” says Jensen.

Even so, some farmers continue using the same blend at the same rate every year, notes Jeff Schoenau, soil scientist at the University of Saskatchewan.

“They may be missing out,” Schoenau says. “There are opportunities to make more efficient use of the fertilizer dollars that they’re spending by allocating their fertilizer to where it could be put to best use in different fields, or even in different parts of the same field.”

“Most of our farmers don’t overfertilize, but they may misfertilize sometimes, and overfertilize in one part of their field and underfertilize in another,” agrees Jensen. “On average they’re doing a good job, but with precision agriculture, they can do some fine tuning and adjust those rates to do a little better job.”

The best combination of rate, time, placement and source of fertilizer can vary from farm to farm and from field to field, Jensen says. He agrees too that it can even vary between different parts of a single field.

Don Flaten, a nutrient management specialist at the University of Manitoba’s soil science department, also believes soil testing is underutilized.

“There are a lot of farmers who are soil testing in some years and on some fields, but there are very few farmers who soil test every field every year,” Flaten says.

Even though soil testing is not a perfect, stand-alone way to determine fertilizer recommendations, it’s a very important tool that could and should be used more widely, Flaten says. “Expanded use of soil testing to every field and every year could certainly help to refine our nutrient management practices.”

Soil testing is a valuable investment and its cost is more than offset through higher yields and lower fertilizer bills, Flaten argues.

Schoenau says it will pay off. “Knowledge is power in terms of making more informed decisions regarding input use like fertilizer.”

But Jensen notes his institute’s research reveals the U.S. is ahead of Canada when it comes to embracing soil testing, which he links to its greater adoption rate of precision farming and resulting increased sampling. The highest rate of adoption is in the U.S. Midwest for corn and soybean acres.

“When you get into the small grains cereals — lower yields, generally lower net returns — there’s less need or desire to use precision farming and more soil sampling,” Jensen says.

Farmers in Canada, he predicts, are unlikely to catch up. With limited heat units and rarely adequate precipitation in all three provinces at the same time, the Prairies — the northern fringe of North American agriculture — just don’t have the same yield potential as do areas to the south.

Jensen also believes more farm consolidation is on its way, resulting in even less soil sampling on some farms.

But on smaller-size operations, he sees the move toward variable-rate precision farming growing.

“We’re in somewhat of an early adopter phase,” Jensen says. “It will become a significant part of a lot of farm operations.”

The next generation of agriculture school grads could well be influential in making that happen, Jensen adds.

“We certainly encourage our students to use this as one of the tools in a nutrient management crop nutrition program,” adds Flaten. “I would hope that the majority of our graduating students would be making very good use of precision farming and associated increases in soil sampling.”

Fertilizer benefits

Soil testing allows farmers to observe trends in their fields, Flaten says. Long-term monitoring of a given field helps them identify whether their soil is experiencing decreasing mineral nutrient fertility, as well as variability.

Soil’s ability to supply nutrients can vary a great deal, depending on type of soil, field management, crops produced, growing conditions and environmental conditions after harvest.

“Different conditions can make substantial differences in the amount of leftover nitrogen that’s in the soil from one year to the next,” Flaten points out. “To get the most out of a soil-testing program, a regular historical record should be developed for each field on an annual basis.”

Flaten says he’s at least as concerned about the potential to lose yield due to nutrient deficiency as he is about spending more on fertilizer than what’s required.

“The gain from making sure that you have enough nutrients to reach your yield potential is a much larger financial benefit than the cost of overfertilizing,” Flaten says.

Soil testing leads to more reliable, accurate fertilizer recommendations that more closely match the actual amount of nutrients that are required by plants, agrees Schoenau. “There’s certainly some potential gains to be realized. One of the challenges we have here in Western Canada is the variability in the growing season from year to year, and that very much influences the crop demand for the added nutrient,” he says. “So one of the big challenges in fertilizer recommendations is coming up with what a person feels is a realistic target yield or anticipated yield and therefore crop nutrient demand. And it can change so much on the Prairies, as we swing from wet years to dry years.”

Soil sampling not only provides economic and agronomic benefits, but environmental ones as well.

“You don’t want to apply more fertilizer nutrients than what the crop needs because there is the potential for the excess to be lost or wasted. And if nutrient gets lost, it can get lost to the air as a gas or potentially enter into water, and that poses environmental concerns,” Schoenau says.

Most sampling occurs following harvest after soils have cooled. Sampling too early, for instance at the end of August or early September, could result in a poor representation as rains and warm temperatures can increase levels of available nutrients.

“Toward the end of September and into early October is a good time for soil sampling, and you get a pretty good idea of what the crop will have available in spring,” Schoenau says. But there may be changes if winter snowfall is very high and spring conditions are saturated, particularly with nitrogen prior to the actual seeding period. “So some people will actually do a check in the spring just to see if things have changed much.”

Flaten adds cold Prairie winters usually mean the risk of overwinter nitrogen loss is much less than in warmer and more humid parts of Canada and the U.S.

“Some exceptions apply, (such as) fields or areas of fields that are prone to substantial flooding in early spring, where residual nitrate that’s present in the fall may be lost by denitrification or leaching before the crop is planted,” Flaten says.

The other advantage to fall sampling versus spring is easy sampling down to two feet in unfrozen soil, as well as sufficient time for soil sampling, analyses, and recommendations to be completed and sent to producers. Also, farmers are able to complete their fertilizer purchases and deliveries well before planting.

“As a result,” Flaten says, “fall sampling is widely recommended across all three Prairie provinces, even though it’s not recommended in Ontario, Quebec or Atlantic Canada.”

This article first appeared as “Choice sampling” in the September 2015 issue of Country Guide

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