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Check soil for nutrient stratification

SASKATOON – Nutrient stratification is happening in a field near you, says Elston Solberg, a consultant with Agri-Trend Agrology.

“Stratification is a layering of certain soil properties near the soil surface. Reduced tillage and straw retention amplifies these situations, as does manure application and other things,” Solberg said at Agri-Trend’s recent Farm Forum conference in Saskatoon.

“Immobile nutrients are most affected. If we continually pull immobile nutrients from the subsoil to the surface, then drop the straw on the surface, those nutrients will stay near the surface. This will get amplified over time and if we keep doing what we’re doing, it’s going to get worse.”

Mobile nutrients include nitrogen, sulfur, chloride and boron. Immobile nutrients include phosphorus, potassium, copper, zinc and manganese.

Soil pH and organic matter are characteristics that are also immobile.

“From my perspective, we need to start thinking about what your crop is really seeing,” he said.

“What does that mean in terms of yield potential and future issues.”

In an effort to answer these questions, Agri-Trend has developed a research initiative with 24 of its consultants from Alberta, Saskatchewan, Manitoba and Ontario.

The project tested a variety of sampling depths: zero to one inch; one to two inches; two to four inches; four to six inches; six to nine inches; nine to 12 inches and 12 to 24 inches.

Using as an example a field from southern Alberta, Solberg said the pH level in the top inch is higher than in the next inch. The straw laid on the surface has a liming quality, with the calcium in the straw, while much of the fertilizer is applied in the two to four inch zone, which has an acidifying effect on the soil that lowers the pH level.

Solberg said in a typical year about one-third of potassium that is applied goes into the grain while the rest goes into the straw.

Agri-Trend president Rob Saik said plants need potassium in a dry year.

“Potassium goes a long way to help control water loss and increase water use efficiency. What we’re seeing in soils is that potassium is not evenly distributed.”

He said the potassium level is fine in the zero to six inch soil sample, but a more detailed analysis will show most of the potassium is in the top inch.

Saik said all the potassium in the straw that’s spread behind the combine during harvesting can be leached out of the straw in as little as 52 weeks and end up on the soil surface.

“Potassium is not very mobile, so it starts to accumulate and you end up with potassium right at the soil surface,” Saik said. “When a drought hits, how much root activity is there at the surface? Not much. What’s the No. 1 nutrient for regulating water loss in a crop? Potassium. Where is the potassium? It’s at the surface and there’s not much at the lower levels where the root activity is.”

Agri-Trend also analyzed soil magnesium levels.

“Compare the magnesium graph. What competes with potassium for uptake? Magnesium. So not only do you have no potash down lower, you have higher levels of magnesium,” Saik said.

Potassium-to-magnesium ratios fall abruptly in the lower soil levels. A good ratio is 0.25 to 0.35, but Saik said most of the soil in his example is far below that.

“Either low levels of potassium or high levels of magnesium are causing problems. It’s no wonder farmers see their bottom leaves die off, which is classic potassium deficiency, yet people are saying there’s no way we have a potassium deficiency. Well, you don’t if you’re in the top two inches.

“If you look at the pH on this field, for the most part it’s in the neutral range. But not at the surface. At the surface, there’s a dramatic decrease in pH.”

The level drops to 5.5 in the one to two inch level from approximately 5.8 in the zero to one inch depth.

Saik said farmers need to be aware of this because few herbicides can persist with a low pH.

“As we do more continuous cropping, you have more organic acids released at the surface and it’s changing the composition of your soil,” he said.

A field near Vegreville, Alta., was also used as an example. It had been direct seeded with all the straw retained for the past 16 years.

The field showed high organic matter in the top six inches, plus low pH in the same zone.

“What do these two mirror images mean? If you know about herbicide residues, you know this combination with certain herbicides is dynamite. If you know that going in, it makes crop rotations or herbicide rotations a lot easier,” Solberg said.

“With phosphorus, the top three inches we’re swimming in it, to the point where if you’re an environmentalist or regulator you may be having a heart attack. Get to the next three inches and where did all the phosphorus go? With potash, the top three inches again is swimming, while the next three inches drops dramatically. With magnesium, there’s not much at the surface, but it’s loaded down deeper.”

Solberg said if a plant’s roots are in the subsoil growing in an ocean of magnesium, it’s hard to find the small amount of potassium. That’s why agronomists use the potassium-to-magnesium ratio as an indicator of potassium availability.

While this research project has only been done in a couple of dozen locations so far, Solberg said the company is finding similar results in fields sampled across the Prairies.

Solberg said nutrient stratification is different from field variability, which involves different levels of nutrients or soil characteristics such as pH across the width of a field.

Stratification refers to different levels of nutrients or soil characteristics from the surface to various depths.

In a field with a pH varying from 5.5 to 7.2 across the width of a field, he said that’s a lot of difference.

“The pH scale is logarithmic. That’s a huge difference, in the order of 15 to 17 times difference, from a plant’s perspective.”

Phosphorus levels in one field ranged from 18 parts per million to 193 ppm. Percent base saturation of potash is best between three and five percent in most soil types. In one sample the field was 1.3 to 8.3.

“What does that mean to a plant?” Solberg said.

“These are some of the questions we need to ask. Depending on where you are in the field as a plant, you’re seeing night and day differences. Plants see variability across fields that is huge.”

Looking at potash levels at two depths, Solberg said that if it’s the middle of July and it’s been dry for two or three weeks, roots are down six to 12 or 18 to 24 inches, the plant isn’t receiving much potash and it may have problems.

Typically copper will be lower in the top six inches and the levels will increase with depth.

Nutrients vary across fields: some may be excessive, others deficient. A balanced fertility program limits the variability and often enhances yield.

“We have some research in place that is helping us understand what a plant sees when it’s growing in a field. There’s huge variability across the field and there’s huge variability in a vertical direction – in the rooting zone – of a field. It’s a three dimensional issue we have to grapple with.”

Immobile nutrients concentrate themselves in the top one to two inches of the soil. In most of Western Canada, water is the most limiting factor for crop production. When water becomes scarce, root systems harvest water and nutrients at deeper depths, putting them into environments that are devoid of phosphorus and potassium.

“We’ve only looked at a small snippet of information, we’ve only looked at the immobile nutrients. There’s lots of other things we could look at, but if we consider not only the variability we see across fields and how that affects crop growth, then consider the variability we see vertically in fields, that might help us find solutions that make sense,” Solberg said.

“The solutions will be entirely different, depending on what the plant is seeing in every different place. The one conclusion I’ve come to is that stratification is real. How do we deal with those two issues? Maybe we deep band in the fall, at six or eight inches, some phosphorus and potassium. Maybe we invert the soil profile.”

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