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Establishing a safe rate

Urea is the dominant source of granular nitrogen fertilizer used on the prairies. Unfortunately, for growers who want to apply their N requirements in or near the seedrow, this product can have an adverse effect on seed germination and crop establishment at higher rates of application. The risk of germination damage from urea placed in close proximity to the seed also varies significantly with crop type and soil conditions.

Know the seedbed utilization factor (SBU) for your seeder.

Original guidelines quite restrictive
The original guidelines establishing ‘safe’ limits for seedrow applied urea were developed for relatively narrow double-disc openers that placed the seed and fertilizer close together. Under these conditions, researchers suggest the amount of urea-based nitrogen that could be placed in the seedrow for cereal crops be limited to a maximum of 20 to 25lb/ac of N. However, as growers adopted seed openers that could place seed and fertilizer in wider bands, it has become obvious that these original seedrow N guidelines were too restrictive.

Table 1. SBU ranges associated with some typical prairie seed openers.
Opener style
Percentage SBU
Knives
5 to 10
Disc
7 to 10
Hoe
12 to 15
Spoons
15 to 30
Small sweeps
25 to 40
Large sweeps
40 to 60
Discers
50 to 70
Note: Values shown are only estimates. Actual SBU values that are determined under field operating conditions should be used for estimating 'safe' rates of seedrow N applications.

Increased seedbed utilization
Researchers working at Westco Fertilizers determined that the rate of urea N that could be safely placed in the seedrow was directly proportional to the percentage of the total available seedbed utilized (SBU) for placement of seed and fertilizer. In other words, for a given soil, the wider the seedrows and the narrower the spacing between seedrows, the greater the amount of urea N that would be tolerated with the seed. Refer to the table listing the various types of openers and their estimated ranges of potential seedbed utilization (see Table 1).

Calculating your SBU
The percent SBU typical of your seeding equipment can be easily calculated if you can determine the effective width of seed and fertilizer spread that is achieved by your opener during field operation. For example, an opener could spread seed and fertilizer over a width of 3.5 inches with a space between the seed openers of nine inches. The SBU for this opener can be determined by multiplying the seed spread by 100 (i.e. 3.5in x 100 = 350) and then dividing that value by the seed opener spacing (i.e. 350 divided by 9in = 39 percent).

Similarly, for an opener that has an effective spread width of two inches and a row spacing of 10 inches, the SBU is 20 percent. Keep in mind that the degree of seed spreading can be influenced by soil conditions as well as the speed of seeding.

Table 2. Maximum allowable seed-placed urea N (lb/ac) for cereals as influenced by SBU and seedbed soil textural category under ideal seedbed moisture conditions.
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Light texture
Medium texture
Heavy texture
Percentage SBU
Sandy loam-light loam
Loam-clay loam
Clay-heavy clay
10
15 to 20
25 to 30
27 to 32
20
22 to 27
30 to 35
34 to 39
30
30 to 35
37 to 42
42 to 47
40
35 to 40
45 to 50
49 to 54
50
40 to 45
52 to 57
57 to 62
60
45 to 50
59 to 64
65 to 70
70
50 to 60
68 to 73
72 to 77
Adapted from Westco Seedrow Nitrogen Guidelines. J.T. Harapiak and N.A. Flore. December 1993.
Note: These guidelines do not apply to hulless barley.

Establish soil texture to determine ‘safe’ upper limit
To use this table of guidelines you must know the correct soil textural grouping for your land. Many growers still confuse soil colour with texture. In fact, colour has nothing to do with texture. You can have a dark or a light coloured soil in any of the three textural groupings included in the table. Basically, texture is a reflection of the clay content of your soil. The 'Light Texture' category is not a reflection of soil colour, but rather of a low clay content and a higher sand content. The higher the clay content, the more N can be tolerated in the seedrow (see Table 2).

Guidelines apply to favourable seeding conditions
Soil conditions within a field can vary quite considerably. These guidelines were developed assuming that the crop is being seeded under the following favorable conditions:

  • excellent seedbed moisture,
  • soil is free of lime (i.e. eroded knolls),
  • soil is free of excessive salts (i.e. saline/alkali patches),
  • seeding depth is not excessive,
  • seed is of good quality,
  • minimal field variability in texture and organic matter content.

‘Safe’ rate significantly influenced by field variability
Keep in mind that if the field being seeded is not fairly uniform with respect to topography as well as soil chemical and physical factors, then it is the poorest part of the field that is generally most vulnerable to seed germination damage. Therefore, from a seed safety point of view, unless you are prepared to vary the rate of seedrow applied N within a field, it will in effect be the poorest or most vulnerable part of the field that will establish the practical upper ‘safe’ limit for the field.

The ‘safe rate’ will depend on the conditions at any location in the field.

Reduce rates by 50 percent if seedbed moisture is less than ideal
The guidelines contained in the table only apply to seedbeds with ideal moisture conditions. If the seedbed moisture is less than ideal, the ‘safe’ upper limit for application of urea N with the seed must be decreased by 50 percent (i.e. cut in half) in order to avoid the risk of incurring significant stand damage and delayed crop maturity.

Caution warranted with more sensitive crops
Oilseed crops such as canola and flax are generally much more sensitive to damage from seedrow applied nitrogen. Under ideal seedbed moisture conditions, the ‘safe’ rate for these two crops, is probably in the range of 10 to 20lb/ac lower than the rates indicated in the table for cereal crops. Other more sensitive crops include canary seed, mustard and hulless barley.

Growers have exceeded guidelines after experimentation
I must admit that these published guidelines are in fact deliberately cautious because of the many unknowns that exist in making a recommendation from a distance. Some growers working soils with a lesser degree of field variability have in fact been experimenting, with some success, by exceeding these guidelines under favourable seedbed moisture conditions. However, such a step should only be considered with caution and only after some experimentation in a small, representative part of a field. Check the crop carefully for signs of seed damage, delayed emergence, loss of plant stand, excessive tillering and delayed maturity in the strips with higher rates of seed applied N. -30-

John Harapiak has approximately 40 years of western Canadian based fertilizer related experience. He will continue to contribute stories to Top Crop Manager. He can be contacted by e-mail at: jharapiak@shaw.ca.


January 15, 2008
By John Harapiak

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