Soil nutrient analysis is an important step in 4R nutrient management. Fall tests done as close to ground freeze-up as possible will provide a good indicator of soil nutrient reserves in the spring. These reserves might be higher than you expect after a dryer-than-normal growing season.
The following article was written by Warren Ward, an agronomy specialist with the Canola Council of Canada.
The principles of 4R nutrient management include a couple of jobs that could be done in the fall. One is soil testing – a relatively easy practice that doesn’t require a large investment. The other is fall banding of nitrogen – if fall application is part of your fertilizer system.
Soil analysis is useful every year, but may be especially useful this year if yields were less than expected. Results should show higher nutrient reserves and lower required fertilizer rates for 2022.
SOIL TEST TIPS
Collect fall soil samples once the soil has cooled to at least 10°C. Cool soils reduce the microbial activity that can mobilize nutrients. By waiting until this activity slows down, the soil test result will be a more accurate indicator of nutrient levels next spring.
One composite sample per field can provide a general impression of soil nutrient levels. For the composite, take 15-20 sub-samples from the most productive areas – not hill tops, not low spots, not saline areas. Before blending these sub-samples to make the composite, divide each core into two or three soil depths and put them into separate pails. Three suggested depths would be zero-to-six inches, six-12 inches and 12-24 inches. Two depths would be zero-to-six and six-to-24. With the 15-20 sub-samples separated by depth, blend those samples to create one composite sample per depth. Submit each depth in its own sample bag.
For more precision, sample based on common zones within the field. Zones are generally based on productivity differences, often the result of soil characteristics, drainage or elevation. Three zones could be hilltop, mid-slope and low-lying areas. Follow sampling methods described above for each zone. This could mean up to nine samples per field (three depths for three zones), but this method can provide meaningful results for fields with higher levels of soil variability, and can point to the value of variable-rate application. This approach may also provide some insight into yield variability exacerbated by drought conditions in 2021.
Variable-rate systems can be complicated, but they don’t have to be. For a low-tech option, growers could dial down nitrogen rates while passing over low-producing areas like saline areas and hill tops, or areas that have higher levels of residual nitrogen following a challenging year.
4Rs FOR FALL FERTILIZATION APPLICATION
The four Rs are “Right Source at the Right Rate, Right Time, Right Place”.
Right Source. When making a fall application of nitrogen, the best sources are urea or anhydrous ammonia. Urease and nitrification inhibitors can reduce the risk of loss. Ask the fertilizer supplier about options. In fall, avoid nitrogen sources that contain nitrate, such as UAN, as there are higher losses associated with this more mobile form of nitrogen.
Right Rate. This is where the soil test comes in. For canola, we encourage a rate specific to the needs and yield potential of each field. If fertilizer blends specific to each field are not logistically possible, apply the blend at different rates to match the yield goal for each field.
Right Time. While spring is the ideal time from a 4R perspective, fall application can be a good plan B if logistics make it a challenge to apply enough nitrogen in spring. Make fall applications after soil temperatures have cooled to less than 10°C on well-drained sites and even cooler in high-moisture areas. Cool temperatures significantly reduce losses by reducing the rate of change from ammonium to the more loss-prone nitrate form.
Before fall applications this year, note that soil disturbance will reduce the snow-trapping capacity of stubble heading into winter. This moisture could be valuable after a dryer-than-normal year. Also, if soils are too dry to provide a proper seal of the band, you may need to wait for better conditions.
Right Place. 4R recommends sub-surface bands to reduce losses that can occur with surface applications. This is especially true for fall applications. Band at least two inches deep and space bands no more than 18” apart for cereals and oilseed crops.
Before banding, consider soil moisture. Banding in dry soils can also increase gassing-off losses because soil does not provide a proper seal on the band. For anhydrous ammonia application into dry soils, go deeper (four inches or so) into moisture to reduce losses. Banding in wet soil conditions can also increase losses if the soil does not close well over the band.
Surface spreading of nitrogen fertilizer in the fall can lead to major losses. Broadcast nitrogen should be incorporated, which will require greater soil disturbance, or include urease and nitrification inhibitors (as described under “Right Source”) to reduce those losses. Once soils freeze or become permanently snow covered, nitrogen should no longer be broadcast, even when using an enhanced source.
CANOLA’S 4R GOAL
Canada’s canola industry has a goal to see 4R practices used on 90 per cent of canola acres by 2025.
Fertilizer Canada administers the 4R Nutrient Stewardship Program that formally tracks 4R acres. To get their acres counted, farmers have to work with a 4R designated agronomist. The agronomist helps the farmer with the planning process required for 4R, and the agronomist then submits these acres to Fertilizer Canada.
What’s in it for the farmer? Through the use of 4R Nutrient Stewardship, farmers can improve nutrient efficiency and get more return from the investment. For more 4R tips and links, including companies with 4R designated agronomists, please read “How to get acres counted as 4R” at canolawatch.org.
(Above photo credit: Agvise Labs)
