KS-23F
NITROGEN AND PHOSPHORUS FERTILIZATION
OF IRRIGATED CORN AND GRAIN SORGHUM
Alan Schlegel
Summary
Long-term research shows that phosphorus (P) and nitrogen (N) fertilizer must be applied
to optimize production of irrigated corn and grain sorghum in western Kansas. In 2004, N and P
applied alone increased corn yields about 95 and 30 bu/A, respectively; however, N and P
applied together increased yields up to 173 bu/A. Averaged across the past 10 years, corn yields
were increased more than 100 bu/A by N and P fertilization. Application of 120 lb N/A (with P)
was sufficient to produce ≥95% of maximum yield in 2004, which was consistent with the 10year average. Phosphorus increased corn yields from 72 to 131 bu/A (average about 100 bu/A)
when applied with at least 120 lb N/A. Application of 80 lb P2O5/A increased yields 5 to 9 bu/A
compared to 40 lb P2O5/A when applied with at least 120 lb N/A. For grain sorghum in 2004, N
and P applied alone increased yields about 43 and 17 bu/A, respectively; however when applied
together yields increased up to 63 bu/A. Averaged across the past 9 years, sorghum yields were
increased more than 50 bu/A by N and P fertilization. Application of 40 lb N/A (with P) was
sufficient to produce >90% of maximum yield in 2004, which was consistent with the 9-year
average. The benefit from P decreased at the higher N rates. Application of K had no significant
effect on sorghum yield in 2004 or long term.
Introduction
This study was initiated in 1961 to determine responses of continuous corn and grain
sorghum grown under flood irrigation to N, P, and K fertilization. The study was conducted on a
Ulysses silt loam soil with an inherently high K content. No yield benefit to corn from K
fertilization was observed in 30 years and soil K levels remained high so the K treatment was
discontinued in 1992 and replaced with a higher P rate.
Procedures
Initial fertilizer treatments (1961) for both corn and grain sorghum were N rates of 0, 40,
80, 120, 160, and 200 lb N/A without P and K; with 40 lb P2O5/A and zero K; and with 40 lb
P2O5/A and 40 lb K2O/A. In 1992, the corn treatments were changed with the K variable being
replaced by a higher rate of P (80 lb P2O5/A). All fertilizers were broadcast by hand in the spring
and incorporated prior to planting. The soil is a Ulysses silt loam. The corn hybrids were
Pioneer 3225 (1995-97), Pioneer 3395IR (1998), Pioneer 33A14 (2000), Pioneer 33R93 (2001
and 2002), DeKalb C60-12 (2003), and Pioneer 34N45 (2004) planted at about 32,000 seeds/A in
late April or early May. Sorghum (Mycogen TE Y-75 in 1996, Pioneer 8414 in 1997, and
Pioneer 8500/8505 from 1998-2004) was planted in late May or early June. The crops were
irrigated to minimize water stress. Furrow irrigation was used through 2000 and sprinkler
irrigation since 2001. The center 2 rows of each plot were machine harvested after physiological
maturity. Grain yields were adjusted to 15.5% moisture for corn and 12.5% for grain sorghum.
1
KS-23F
Results
Corn
Corn yields in 2004 were considerably higher than the 10-year average (Table 1). Nitrogen
alone increased yields up to 95 bu/A while P alone increased yields about 30 bu/A. However, N
and P applied together increased corn yields up to 173 bu/A. Only 120 lb N/A with P was
required to obtain ≥95% of maximum yields which is consistent with the 10-year average. Corn
yields were 2 bu/A greater with 80 than with 40 lb P2O5/A, compared to the 10-year average of 4
bu/A. However, with N rates of 120 lb N/A or greater the higher P rate increased yields about 7
bu/A in 2004.
Grain Sorghum
Grain sorghum yields in 2004 were slightly less than the 9-year average (Table 2). Nitrogen
alone increased yields up to 43 bu/A while P alone increased yields 17 bu/A. Nitrogen and P
applied together increased sorghum yields up to 63 bu/A. Only 40 lb N/A was required to obtain
>90% of maximum yields which was consistent with the 10-year average. At the higher N rates
(160 lb N/A or greater), the response to P was less than at the medium N rates (40 to 120 lb
N/A). Sorghum yields were not affected by K fertilization, which has been the case throughout
the study period.
2
KS-23F
Table 1. Effect of N and P fertilizers on irrigated corn. Tribune, KS, 1995-2004.
Nitrogen P2O5
- - - - lb/A - - - 0
0
0
40
40
40
80
80
80
120
120
120
160
160
160
200
200
200
0
40
80
0
40
80
0
40
80
0
40
80
0
40
80
0
40
80
ANOVA
N
Linear
Quadratic
P2O5
Linear
Quadratic
NxP
MEANS
N, lb/A
0
40
80
120
160
200
LSD0.05
P2O5, lb/A
0
40
80
LSD0.05
1995
1996
1997
Grain Yield
1998* 2000 2001
2002
2003
2004
Mean
- - - - - - - - - - - - - - - - - - - - - - bu/A- - - - - - - - - - - - - - - - - - - - - - 22
27
26
34
68
65
34
94
93
39
100
111
44
103
100
62
106
109
58
64
73
87
111
106
95
164
159
97
185
183
103
185
195
110
180
190
66
79
83
86
111
114
130
153
155
105
173
162
108
169
187
110
185
193
49
55
55
76
107
95
95
155
149
92
180
179
101
186
185
130
188
197
131
152
153
150
195
202
149
205
211
143
204
224
154
203
214
165
207
218
54
43
48
71
127
129
75
169
182
56
177
191
76
186
188
130
177
194
39
43
44
47
69
76
53
81
84
50
78
85
50
80
85
67
79
95
79
95
93
107
147
150
122
188
186
122
194
200
127
190
197
141
197
201
67
97
98
92
154
148
118
209
205
103
228
234
136
231
240
162
234
239
63
73
75
83
121
121
97
157
158
90
169
174
100
170
177
120
173
182
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.008
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.007
0.133
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
25
56
74
83
82
92
7
65
102
139
155
161
160
10
76
104
146
147
155
163
12
53
93
133
150
157
172
11
145
182
188
190
190
197
10
48
109
142
142
150
167
15
42
64
73
71
71
80
8
89
135
165
172
172
180
9
87
132
178
188
203
212
11
70
108
137
144
149
158
6
39
83
84
5
92
148
151
7
101
145
149
9
91
145
143
7
149
194
204
7
77
147
155
10
51
72
78
6
116
168
171
6
113
192
194
8
92
144
148
4
*Note: There was no yield data for 1999 because of hail damage. Hail reduced yields in 2002.
3
KS-23F
Table 2. Irrigated Sorghum Fertility. Effect of N, P, and K fertilizers on irrigated sorghum yields, Tribune, KS,
1996-2004.
N
P2O5
K2O
- - - - - lb/A - - - 0
0
0
40
40
40
80
80
80
120
120
120
160
160
160
200
200
200
0
40
40
0
40
40
0
40
40
0
40
40
0
40
40
0
40
40
ANOVA
(P>F)
Nitrogen
Linear
Quadratic
P-K
Zero P
vs P
P vs PK
N x P-K
MEANS
Nitrogen
P2O5K2O
0 lb/A
40
80
120
160
200
LSD0.05
0
0
40
0
0
40
0
0
40
0
0
40
0
0
40
0
0
40
1996
1997
1998
1999
2000
2001
2002
2004
Mean
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - bu/A - - - - - - - - - - - - - - - - - - - - 74
81
77
74 - - - -77- - - - - -76- - - - 73
80
57
74
77
79
74
100
101
73
103
103
79
94
99
85
92
91
86
109
95
75
83
104
114
121
100
121
130
91
124
128
118
116
119
107
126
115
77
76
91
118
114
111
125
130
102
125
128
118
131
124
121
133
130
85
84
83
117
114
94
113
123
76
102
105
100
116
107
113
110
120
87
83
88
116
114
97
116
120
82
116
118
96
118
115
104
114
120
81
83
92
124
119
110
138
134
98
134
135
118
141
136
132
139
142
81
82
82
120
121
97
127
131
86
132
127
116
137
133
113
136
143
93
93
92
140
140
108
139
149
97
135
132
122
146
135
131
132
145
73
74
60
112
117
73
103
123
66
106
115
86
120
113
100
115
123
81
82
85
118
118
96
121
127
86
119
121
107
125
120
113
124
127
0.003
0.002
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.018
0.001
0.001
0.001
0.116
0.001
0.001
0.001
0.001
0.001
0.227
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.694
0.121
0.689
0.727
0.185
0.436
0.045
0.649
0.186
0.741
0.482
0.803
0.061
0.619
0.058
0.920
0.030
0.008
0.022
0.014
77
92
93
91
89
97
9
80
113
117
114
118
116
10
76
108
122
118
124
128
8
81
105
110
95
108
115
13
82
106
111
105
110
113
7
80
112
127
122
132
138
8
79
108
119
115
129
131
9
88
124
132
121
134
136
10
68
96
100
96
107
113
11
79
107
115
109
117
121
7
105
74
94
79
96
95
7
100
113
116
7
103
118
117
6
90
107
109
9
91
111
112
5
104
126
125
6
94
122
123
6
131
132
7
105
111
7
115
116
5
0 lb/A
40- 0
40-40
LSD0.05
2003
4