Cold Prairie Soils: Friend of the Farmer and of the Environment

John Heard, Soil Fertility Specialist

As I prepare to write annual articles on fall nitrogen (N) fertilizer application and management, I often review past articles. The first article I wrote in Manitoba was 13 years ago and although basic principles are unchanged, new awareness and management options exist. Now there is increased awareness of the environmental threat of nitrous oxide which is emitted when nitrate-form nitrogen is denitrified. And now growers have increased access to more stable forms of nitrogen and a guide to soil temperatures in their area.

Those practices that minimize losses of nitrogen remain based on applying nitrogen in the right place, at the right time using the right form. The main management objective is to maintain N in the ammonia form which is not lost in the spring by leaching in sandy soils or denitrification under flooded or water logged soils. The process by which ammonium-form nitrogen is converted to nitrate form is biological by bacteria, requiring heat. In some areas inhibitors are applied with nitrogen to reduce bacteria activity, whereas our traditional Prairie strategy is to simply freeze them off until spring. So the logical application period is late fall prior to 5 months of frozen soils with no risk of loss until spring thaw. Many areas in North America that still apply fall nitrogen cannot claim to have the security of frozen soil.

The basic guidelines for fall N applications follow:

Ammonium forms of nitrogen (anhydrous ammonia, urea and ammonium sulphate) are preferred since they are not readily lost by leaching and denitrification. Ammonium nitrate (34-0-0) and liquid UAN (28-0-0) are less preferable, especially on soils subject to leaching or saturation in the spring.
Banding N is more efficient that broadcast application. Banding ammonium fertilizer places the fertilizer at concentrations beyond the tolerance of soil microbes, which delays the denitrification process for 2-3 weeks. Banding should be done to a 3-4" depth and in spacings of 12-18". Narrower banding does not concentrate the N as much to delay denitrification and wider spacing may leave streaking in spring crops unless secondary tillage distributes the bands. However, N efficiency is actually superior if fall bands are not disturbed or mixed in the spring.
Delay applications until late fall when soils cool below 10 degrees C, when soil microbes are less active in converting ammonium forms of N to nitrate. Soil temperatures should be measured at the 4" depth. Delaying fall fertilizer application will also minimize the N uptake by weed and volunteer crop growth. Weed growth encouraged by nitrogen may consume valuable soil water and nutrients. Farmers will start N applications earlier than this guideline in order to avoid being shut out completely by an early freeze-up or snowfall. In such instances, banding ammonium forms will minimize their losses.

Manitoba Agriculture, Food and Rural Initiatives measures and presents soil temperature data in the province. A provincial network of 31 remote weather stations continuously measures soil temperatures at the 2” depth. This network serves as a guide as to the rate of soil cooling and the associated risk of ammonium conversion to nitrate form. Updated charts of these soil temperatures can be viewed at the website http://tgs.gov.mb.ca/climate/SoilMoisture.aspx

But when cold soils alone are not sufficient to curb N conversion and losses on soils that become waterlogged or leached in the spring, growers should consider other options:

N sources including inhibitors such as DCD in Super U or controlled release products like ESN. These will be discussed in detail at the 2009 Manitoba Agronomist Conference by Dr. Clain Jones from Montana State University.
Change application timing until spring (before or at planting) or in-season applications.

So when buried waterlines freeze and burst this winter, rest assured that your nitrogen is in place and not moving. Provided of course, that you are employing the right fertilizer management.

For further information, contact your GO Representative.