How much seed-placed fertilizer is safe under direct-seeding conditions?
The following tables indicate the approximate safe rates of urea (46-0-0) with the seed of specific crops if seedbed soil moisture is good to excellent (soil moisture at or near field capacity).
All rates are in pounds actual nitrogen (N) per acre (divide by 0.46 to get lbs. of 46-0-0 per acre)
Table 1. Granular Nitrogen - Cereal Grains
|
|
1 inch spread 1
(Disc or knife) 2
|
2 inch spread 1
(Spoon or hoe)
|
3 inch spread 1
(Sweep)
|
|
Soil texture
|
Row spacing
|
Row spacing
|
Row spacing
|
|
6"
|
9"
|
12"
|
6"
|
9"
|
12"
|
6"
|
9"
|
12"
|
|
SBU 3
|
SBU 3
|
SBU 3
|
|
17%
|
11%
|
8%
|
33%
|
22%
|
17%
|
50%
|
33%
|
25%
|
|
Light (sandy loam)
|
20
|
15
|
10
|
30
|
25
|
20
|
40
|
30
|
25
|
|
Medium (loam to clay loam)
|
30
|
25
|
20
|
40
|
35
|
30
|
50
|
40
|
35
|
|
Heavy (clay to heavy clay)
|
35
|
30
|
30
|
50
|
40
|
35
|
60
|
50
|
40
|
Table 2. Granular Nitrogen - Canola and Flax
|
|
1 inch spread 1
(Disc or knife) 2
|
2 inch spread 1
(Spoon or hoe)
|
3 inch spread 1
(Sweep)
|
|
Soil texture
|
Row spacing
|
Row spacing
|
Row spacing
|
|
6"
|
9"
|
12"
|
6"
|
9"
|
12"
|
6"
|
9"
|
12"
|
|
SBU 3
|
SBU 3
|
SBU 3
|
|
17%
|
11%
|
8%
|
33%
|
22%
|
17%
|
50%
|
33%
|
25%
|
|
Light (sandy loam)
|
10
|
5
|
0
|
20
|
15
|
10
|
30
|
20
|
15
|
|
Medium (loam to clay loam)
|
15
|
10
|
5
|
30
|
20
|
15
|
40
|
30
|
20
|
|
Heavy (clay to heavy clay)
|
20
|
15
|
10
|
40
|
30
|
20
|
50
|
40
|
30
|
1 Width of spread must be checked under field conditions. It will vary with air velocity, ground speed, seeding depth, surface crop residue, and other seeding conditions including soil texture and moisture content.
2 Some openers give less than one inch spread.
3 SBU (Seedbed Utilization) is the amount of the seedbed over which the fertilizer has been spread. Thus, it is a reflection of the relative concentration of fertilizer. Calculated, %SBU is the width of spread divided by the row spacing multiplied by 100. For example, if a seeding implement has 9-inch row spacing and spreads the seed and fertilizer over 3 inches, the SBU would be (three divided by nine multiplied by 100 which equals) 33 per cent. Generally, the higher the SBU the more fertilizer that can SAFELY be seed-placed. Some openers may disperse the fertilizer vertically (variable depth) and/or provide a degree of separation from the seed (i.e. double-shoot) - SBU only applies to seed-placed fertilizer (i.e. single-shoot seeding systems).
Crop injury from seed-placed fertilizer is relative to rate, soil texture, and SBU.
Saskatchewan Agriculture – Oct 2001
More about SBU [Author’s compilation based on review of numerous studies & recommendations]:
- The nitrogen (N) rates in the above tables are conservative, however they should be considered along with the N in safe rates of seed-placed starter fertilizer (i.e. 11-51-0-0) and sulfur blends (i.e. 21-0-0-24).
- Ammonium nitrate (34-0-0) is less damaging to seed than urea (46-0-0). Ammonium toxicity is the main mechanism of impaired germination and seedling damage. Excess salinity in the seed row is also a major cause of crop injury. Ammonium nitrate has a higher salt index than urea.
- For cereal crops, suggested safe N rates can usually be increased by about 25 per cent when ammonium nitrate is used. This recommendation should be approached with caution when seedbeds are dry. Oats is slightly more tolerant of seed-placed N than barley, which is slightly more tolerant than wheat.
- Flax is less tolerant than canola to seed-placed N. For canola, ammonium nitrate is as damaging to seedlings as urea. There is little information available on the relative effect of ammonium nitrate versus urea on flax.
- Reduce the rate of seed-placed fertilizer when seedbed moisture is less than ideal. The combined salt effect of granular fertilizer (dry blends) will likely be more severe with low SBU and dry soil, than under moist conditions.
- By design, separation and placement of liquid fertilizer is different than granular, however the agronomic guidelines for seed-placed N (i.e. 28-0-0 or UAN) are similar to the dry granular blends.
- Anhydrous Ammonia (82-0-0 or NH3) is unsafe near the seed.
- Grower experience and innovation continuely change and challenge the manner in which fertilizer-N and other crop nutrients can be successfully applied with direct seeding.
- Field-checks help growers learn about SBU. Variable rates and no-fertilizer strips will provide clues to adjust fertilizer application accordingly. Evaluate seed germination and crop emergence about 3 weeks after seeding.
