Fertilizer Facts: February 1997, Number 13

Residual Soil Nitrate Responses to Early and Late-Season Nitrogen Applications in Irrigated Spring Wheat

Mal Westcott, Rick Engel, Jeff Jacobsen, Grant Jackson, and Bob Stougaard
Western Ag Research Center, Land Resources & Environ. Science Dept.,  Land Resources & Environ. Science Dept., Western Triangle Ag Research Center and Northwestern Ag Research Center, Montana State University

Spring wheat grain protein is consistently increased by 0.5 to 2% with the application of late-season N under irrigated conditions when early-season N fertility is optimum for grain yield. An effective and efficient approach to N fertilization for yield and protein is to make an early-season N application targeted for a yield potential in consideration of soil analysis, followed with an application of N at heading to increase grain protein. This kind of fertilization program, because it relies on N application late in the season, may lead to high levels of residual soil nitrate following harvest and therefore pose an environmental risk to ground water. Our objective was to document the effects of late-season N applications on residual soil nitrate following irrigated spring wheat production at several sites and with varying rates of early-season N fertilization.

Irrigated studies consisted of growing three hard red spring wheat varieties (Len, Newana, and Hi-Line) with four rates of early-season N applied as granular urea at rates ranging up to 150 to 300 lbs N/a depending on soil N analysis and yield potential for the site. Three sites were included in 1994 and four sites in 1995 (Tables 1 and 2). At heading, half of each N rate plot received an additional 40 lbs N/a as granular urea and the other half remained without further N application. Grain yield and protein responses are reported in Fertilizer Facts #11. After grain harvest, soil samples were taken to a depth of four feet. These samples were analyzed for nitrate concentration in two-foot increments in 1994 and one-foot increments in 1995.

Nitrogen fertilization had no significant effect on residual soil nitrate levels at Conrad in 1994 (Table 1). At Bozeman, early-season application of 120 lbs N/a resulted in elevated residual nitrate levels in the upper two feet of the soil profile and the 180 lb N/a rate caused increases throughout the sampling depth of four feet. These initial fertilization rates were in excess of the optimum for yield and protein. Late-season N application did not increase residual soil nitrate levels at this site. At Kalispell, late-season N application significantly elevated residual soil nitrate levels in the upper two feet when initial N application rates were excessive; at optimum or sub-optimum initial N rates, the effects of late-season N on residual soil nitrate were slight.

 

Table 1. Effects of initial N fertilization rate and late-season N application on residual soil nitrate (NO3-N) following harvest of irrigated hard red spring wheat at four locations in 1994.





Location

Initial
N rate



N applied at heading		         Soil NO3-N
          Soil Depth (ft)

0 - 2

2 - 4
  ------- lbs N/a ------- -------- ppm --------
Bozeman 0


60*

120

180

 0
40

0
40

0
40

0
40

4.6
4.4

4.9
5.4

7.0
11.0

12.9
11.9

 4.8
6.1

5.1
6.2

5.9
6.6

12.0
9.7
Conrad 0

20

95*

170

 0
40

0
40

0
40

0
40

12.0
3.3

3.9
7.9

2.3
2.8

5.6
4.0

 2.4
2.0

2.7
5.4

2.7
2.5

4.5
4.6
Kalispell 0

50*

100

150

 0
40

0
40

0
40

0
40

4.4
7.7

3.8
6.5

4.9
10.0

5.6
10.1

 3.2
3.6

3.9
4.2

3.7
3.8

3.7
4.7

*Denotes optimum N fertilization for grain yield and protein.

Results were similar in 1995 (Table 2). At Bozeman and Huntley, both initial N and late-season N significantly increased residual soil nitrate levels, but only in cases where initial N rates were excessive. At Kalispell, late-season N had no significant effect on residual soil nitrate regardless of early-season N, but the two highest rates of early-season N did increase nitrate in the lower two feet of the sampled soil profile. Only at Corvallis was there a consistent and significant effect of late-season N; at the 1 to 2 foot soil depth, late-season N increased residual soil nitrate levels by about 3 ppm regardless of early-season N application rate. However, it is worth noting that this effect was small compared to the greater effects of excessive early-season N, which increased soil nitrate concentration by 4 to 9 ppm at the 1 to 2 foot depth compared to the optimum rate of 60 lbs N/a.

 

Table 2. Effects of initial N fertilization rate and late-season N application on residual soil nitrate (NO3-N) following harvest of irrigated hard red spring wheat at four locations in 1995.

      Soil NO3-N
 

Initial
N rate

N applied at heading		 Soil Depth (ft)
Location 0 - 1 1 - 2 2 - 3 3 - 4
  -------- lbs N/a ------- ----------------- ppm ---------------
Bozeman 0


60*

120

180

 0
40

0
40

0
40

0
40

4.7
5.1

3.9
3.8

3.8
8.8

5.4
13.9

 5.2
6.3

5.8
5.8

6.9
7.4

6.5
8.9

 5.5
5.7

4.7
5.4

6.5
6.6

6.3
7.4

 3.7
3.8

3.8
3.7

4.1
5.3

5.4
9.1
Corvallis 0

60*

120

180

 0
40

0
40

0
40

0
40

2.9
3.3

5.2
6.6

5.5
6.6

7.5
6.4

 3.8
6.6

7.5
10.7

11.6
14.7

16.4
18.5

 3.4
4.3

5.1
5.5

6.7
9.1

7.7
9.7

 3.0
3.6

3.9
4.8

5.6
6.4

5.7
6.3
Huntley 0

20

95*

170

 0
40

0
40

0
40

0
40

2.0
1.7

1.9
2.2

1.5
2.7

4.8
7.9

 1.7
1.3

1.6
1.6

1.2
1.7

2.1
2.7

 1.5
1.3

2.4
1.5

1.1
1.5

1.7
2.3

 1.5
1.8

2.3
1.4

1.4
1.9

1.9
2.0
Kalispell 0 

50 

100*

150 

0
40

0
40

0
40

0
40

1.4
1.7

1.5
1.3

1.3
1.7

1.6
1.6

 2.3
3.2

1.8
2.5

2.9
3.4

2.5
5.8

 1.9
1.6

1.1
1.3

2.4
3.6

3.7
2.7

 2.3
2.2

1.1
1.2

2.0
1.8

3.3
3.0

*Denotes optimum N fertilization for grain yield and protein.

Fertilizer Facts:


Edited by Jeff Jacobsen, Extension Soil Scientist