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Nutrient stratification in no-till is not a major concern

While immobile nutrients can become concentrated near the soil surface, nutrient uptake is not affected.

During the past several years, nutrient stratification under no-till has been making the coffee shop and meeting rounds. Is it happening? Does it impact nutrient uptake? Should I get out the cultivator, stir things up or start placing nutrients very deep? The answers can be found in a wide range of research conducted in the last decade. “I don’t think you need to worry too much about stratification,” says Jeff Schoenau, a soil scientist at the University of Saskatchewan. “You don’t need to get out the cultivator.”

The impact of tillage is largely on the immobile nutrients like phosphorus (P) and potassium (K). These less mobile nutrients have the potential to become stratified near the surface of the soil. Nutrients like nitrogen (N) and sulphur (S) are mobile in the soil, and tillage has little impact on their availability to plants.

Stratification happens!

Most research has shown that P and K accumulate at the depth they are
placed. While higher concentrations of P and K can be found near the
soil surface under no-till compared to conventional tillage, research
in both Saskatchewan and Alberta have found that there is no difference
in nutrient uptake by the plants.

Research shows nutrient stratification has not impacted nutrient uptake. Photo by Rick Taillieu, RTL

Adrian Johnston, a director with the International Plant Nutrition Institute at Saskatoon, Saskatchewan, has also been involved in assessing the impact of nutrient stratification on nutrient uptake by plants. He recalls that back in the 1980s, deep banding was a new technology and he was often asked how deep fertilizer should be banded. Back then, research on deep banding depth found that all depths pretty much gave the same results.

In long-term no-till studies, the results found that little stratification occurred for N and S, while immobile nutrients like P and K generally accumulated at the depth of fertilizer placement or crop residue placement. For crop residue placement under no-till, this generally means the top surface layer of zero to two inches.

Indeed, research confirms the stratification of P and K at the level of fertilizer or crop residue placement. Researcher Fernando Selles at Agriculture and Agri-Food Canada at Swift Current found that after 12 years of continuous no-till wheat, plant available P was concentrated in the surface two inch layer. In the Black soil zone at Brandon and Indian Head, AAFC researchers found no difference in nutrient stratification of P and K between tillage systems. This was attributed to P and K fertilizer placement in the seedrow for all tillage systems – accumulating where it is placed.

“Yes, we do see P and K stratification when we stop mixing the soil with tillage, and yes, it is in the surface layer, but I question how deep farmers in conventional systems till their soil. It isn’t much deeper than side-banded fertilizer placement. So have we really changed much with no-till and banding of nutrients? My feeling is no.
In fact, we likely get higher efficiency of nutrients from bands in the soil rather than when nutrients are mixed with tillage,” explains Johnston.

Stratification effect on nutrient uptake measured
Schoenau looked at P uptake to gain a greater understanding of the stratification issue. He looked at research on one year, five year and 25 year no-till fields compared to conventional tillage systems.

In the one year no-till fields, the supply of available P was generally greater than or equal to minimum-till fields, which were greater than conventional fields. This was attributed to higher soil moisture content under no-till which enhanced the ability of P to diffuse through the soil.

Looking at longer-term no-till fields in another trial, he found that the impact of P fertilizer on increasing P availability was greatest on the 25 year no-till soil compared to the five year no-till land. Schoenau says this suggests greater fixation of P fertilizer into less soluble forms in the short-term no-till soil. Significantly higher available N as nitrate was also found in long-term no-till.

Schoenau also looked at research from the opposite direction: what would happen to nutrient uptake if a long-term no-till soil was cultivated? Would nutrient uptake be
any better with no-till soils that were tilled? In this research, tillage was conducted on long-term (10 plus years) no-till fields in the Brown, Black and Gray soils. Several tillage treatments were compared, including no-till, spring heavy-duty cultivator with sweeps, fall and spring tillage, and fall and spring tillage plus a tandem discing. Soil
samples were taken from the zero to two inch, two to four inch, and four to six inch depths, and P uptake was also assessed.

Mixing of the soil with tillage did reduce P stratification to some extent in the top six inches; however, there was no large effect on P availability and uptake. “More uniform P might result in more uniform distribution of roots in the topsoil. However, mixing by tillage may also increase the contact between the fertilizer band and soil constituents responsible for P fixation, resulting in lower levels of crop available P,” explains Schoenau.

At the Tisdale site, tillage actually had a negative effect on yield. There, tillage incorporated large amounts of straw and chaff into the soil, resulting in N immobilization that caused a N deficiency, reduced yield and reduced P demand.

Johnston says that the current understanding in the scientific community supports the fact that ammonium-N, P and K do tend to accumulate near the soil surface of no-till treatments, compared to conventional tillage. However, he says that whether this poses a long-term problem is uncertain, and that the research to-date does not show a problem with nutrient uptake.

“We haven’t seen a serious problem. The accumulation of surface crop residues does an excellent job of maintaining higher soil moisture levels at the surface. This will keep roots active and in a position to access these accumulated nutrients,” explains Johnston. “Under dry conditions, a deficiency of P or K may occur if the roots can’t access these surface nutrients. This tells me that we should place an increased importance on in-soil banding of these nutrients, below the surface accumulations, rather than tilling the soil.”

Schoenau agrees, and says that the no-till systems evaluated had a generally positive effect on soil P availability over short and long-term no-till fields, with benefits accruing over time. “Stratification of P as a factor that affects P availability does not appear to be a major issue and tillage to address this does not seem to be warranted,” he says.

Peter Gamache, Team Leader with Alberta Reduced
Tillage Linkages (RTL), says that RTL farmer members have not mentioned nutrient stratification as a major concern in their direct seeded and no-till fields. He agrees with Schoenau’s and Johnston’s assessments that the benefits of no-till can help overcome a concentration of immobile nutrients near the soil surface.

“It’s important that researchers continue to monitor nutrient stratification to see if any production issues develop, and when you look at the benefits of no-till, such as improved water infiltration, better soil moisture near the surface, increased organic matter content and better water use efficiency, it is still the most sustainable production system on the prairies,” says Gamache. “Stratification is normal, it is how our prairie grasslands developed and our domesticated plants are similarly adapted.”

The next step in Schoenau’s research is looking at the different forms of P in the soil to see if tillage makes a difference in the amounts and forms of organic and inorganic P in the soil, and how that affects P uptake. In the meantime, the current research should help to answer those coffee shop questions and that, at this time, nutrient stratification does not seem to be a concern.