Wheat School: Fertilizer placement and long-term soil acidification


Nitrogen and phosphorus fertilization is an important step in seeding a wheat crop and getting it off to the right start. However, over time fertilizer placed at the same depth can cause soil acidification.

Initially set up in 1967, with various rotations and fertility treatments, long-term research plots were the basis for interesting research results from Dr. Barbara Cade-Menun, research scientist at Agriculture and Agri-Food Canada (AAFC) at Swift Current, Sask. Phosphorus in particular is Cade-Menun’s research focus, but interesting proof after P stopped being added to the plots in 1995 is the topic of this Wheat School episode.

Using wheat plots that had added N and P fertilizer, and plots that did not have added N and P, Cade-Menun and her research team noticed that the no-P plots were still getting enough P, despite no yield differences and no additions of P since 1995.

“One thing we noticed was that there was a really big difference in pH, but the no-nitrogen and no-phosphorus plots had the highest pH at about 6.5 and the ones that were getting nitrogen and phosphorus dropped down to about 5.5,” says Cade-Menun.

A soil pH drop by that much causes a lot of issues for P, because it binds tightly to the soil at low pH. Cations and exchangeable aluminum is also a concern at pH levels that low, as is a drop in calcium availability.

Cade-Menun says the research is being expanded to other crops, and is underway at sites in Manitoba and Quebec, where preliminary results show the same thing is happening. She adds that this isn’t a new concept — research shows to expect about half a pH unit for every 1000 kg of N added as ammonia. (Story continues below video)

Long-term soil acidification happens in the surface soils, because that’s where fertilizer is applied at the time of seeding. Especially in zero-till operations, the fertilizer will sit at depths of anywhere from three to 10 cm, depending on depth of A horizon.

“If you’re collecting your soil samples and monitoring for pH, and you’re doing the whole plough layer, you’re not going to see it because it will get diluted down,” says Cade-Menun; so she recommends shallower soil sampling.

The soil pH changes happen exactly where the fertilizer is placed, so at the same time as sampling soils for fertility recommendations, Cade-Menun says to keep an eye on soil pH, especially where there are differences in crop growth.

Ten years from now, Cade-Menun says the issue will not improve on its own, so as higher rates of fertilizer are added, lime recommendations might also have to be part of the package, as is the case in the Northern Great Plains of the States.

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