Experiment 505, Long-Term Application of Nitrogen Using Different Sources and Times in Winter Wheat, Lahoma, OK

Abstract

Environmental concerns relative to nitrogen applications in grain crop production systems have become increasingly important. Recommendations are based on projected yield potentials (yield goals) and estimated economic returns. Various sources of nitrogen are available to farmers in wheat production systems, however, few have ever been evaluated over a long period of time. In 1971, Experiment 505 was initiated to compare sources and rates of N application on wheat grain yield. Few differences between N sources were found in this experiment. Wheat grain yields increased significantly when N was applied at low annual N rates (30-60 pounds/acre), becoming greater with time. In recent years, split applied N has resulted in increased yields (14 vs 8 and 24 vs 18) when compared using the same N source and total N rate (30-30 split versus 60 lb N/ac applied preplant). Grain % N continued to increase beyond the N rate required for maximum yield for most N sources. The highest rates of N (120-240 lb N/ac) are associated with decreased yield and critically low (<5.0) soil pH. Similar to results reported for other long-term winter wheat experiments, applied N tended to increase soil organic C when compared to the check, especially at rates in excess of that required for maximum yield.

Materials and Methods

In the fall of 1970, Experiment 505 was established under conventional tillage on a Grant silt loam (fine-silty, mixed, thermic Udic Argiustoll). For 28 years, wheat has been planted in 10 inch rows in this North Central Research Station experiment. The experimental design employed is a randomized complete block with four replications. Variety changes and associated fertilizer application, planting and harvest dates are included in Table 1. Fertilizer treatments used in this experiment are defined in Table 2. The sources of nitrogen included anhydrous ammonia (AA), ammonium nitrate (AN), urea, (UR), and sulfur coated urea (SCU) applied at rates of 0, 30, 60, 120 and 240 pounds of N/ac. Additional topdress treatments were also evaluated using AN, UR and SCU as defined in Table 2. Sources of AN, UR, and SCU were broadcast and incorporated prior to planting in the fall while AA was injected preplant with knife spacings of 18 inches. Topdress applications were broadcast without incorporation on the dates listed in Table 1. Phosphorus and potassium were applied as triple superphosphate and potassium chloride at rates of 60 pounds P₂O₅ /acre and 60 pounds K₂O/ac respectively. Individual plots at this site are 16 feet wide and 60 feet long. The center 6-10 feet (depending on year) of each plot was harvested with a conventional combine for yield. In addition to wheat grain yield measured every year, periodic soil and grain samples were taken for further chemical analyses. Results from surface (0-6 inches) soil test analyses from samples collected in 1995 are reported in Table 3.

Table 1. Treatment applications, and experimental management for continuous winter wheat Experiment 505, Lahoma, OK, 1971-2000.

Year	Variety	Fertilizer Application Date	Planting Date	Harvest Date	Seeding Rate lb/ac	Topdress Dates
1971	Nicoma		10-13-70
1972	Nicoma			5-8-72
1973	Nicoma		9-15-72	6-18-73
1974	Nicoma	9-20-73	10-19-73	6-12-74	55	11-19-73, 1-7-74, 2-12-74
1975	Triumph	9-10-74	10-11-74	6-18-75	58	9-9-74, 10-2-74, 1-14-75
1976	Triumph	8-22-75	10-20-75	6-12-76	60	11-19-75, 1-12-76, 2-17-76
1977	Osage	8-18-76	10-12-76	6-9-77	67	9-21-76, 11-17-76, 12-30-76
1978	Triumph	9-1-77	10-12-77	6-14-78	70	11-17-77, 1-3-78, 3-13-78
1979	Osage	9-11-78	10-4-78	6-28-79	70	12-4-78, 1-22-79, 3-9-79
1980	TAM W-101	9-30-79	10-24-79	6-25-80	60	11-15-79,1-15-80,2-15-80
1981	TAM W-101	10-2-80	10-31-80	6-12-81	65	12-3-80,1-7-81,2-17-81
1982	TAM W-101	9-30-81	10-23-81	6-23-82	65	12-8-81,1-4-82,3-2-82
1983	TAM W-101	8-24-82	10-18-82	7-1-83	70	11-17-82,1-7-83,3-1-83
1984	TAM W-101	8-31-83	10-28-83	6-19-84	67	11-16-83,1-6-84,2-15-84
1985	TAM W-101	9-10-84	10-30-84	6-13-85	75	13-3-84, 1-25-85, 3-6-85
1986	TAM W-101	8-12-85	10-4-85	6-9-86	74	11-11-85, 1-15-86, 2-24-86
1987	TAM W-101	8-4-86	10-20-86	6-17-87	68	12-4-86, 1-6-87, 3-5-87
1988	TAM W-101	8-31-87	9-25-87	6-20-88	67	11-13-87,2-2-88,2-15-88
1989	TAM W-101	9-29-88	10-13-88	6-19-89	70	11-18-88,1-5-89,2-21-89
1990	TAM W-101	8-25-89	9-25-89	6-18-90	75	11-16-89,1-4-90,2-15-90
1991	TAM W-101	8-3-90	10-12-90	6-5-91	50	11-19-90,1-11-91,2-14-91
1992	TAM W-101	9-9-91	9-27-91		73	11-25-91,1-3-92,2-13-92
1993	Karl	8-24-92	10-1-92		76	11-17-92,1-5-93,3-9-93
1994	Karl	9-14-93	9-28-93		75	11-15-93,1-4-94,2-15-94
1995	Tonkawa	8-5-94	10-28-94	6-19-95	62	11-16-94,1-3-95,2-15-95
1996	Tonkawa	8-31-95	10-13-95	6-21-96	63	11-16-95,1-3-96,2-15-96
1997	Tonkawa	9-4-96	10-3-96	6-13-97	66	1-24-97,2-16-97
1998	Tonkawa	9-11-97	10-17-97	6-19-98	70	11-18-97,1-13-98,2-12-98
1999	Tonkawa	9-3-98	10-8-98	7-3-99	73	11-25-98,1-4-99,2-15-99
2000	Custer	9-7-99	10-12-99	6-15-00	75	11-19-99,1-3-00,2-14-00

Table 2. Treatment structure of long-term Experiment 505, grain yield and %N in the grain, Lahoma, OK, 1971-2000.

		Time of Application
Trt.	Preplant	Nov 16	Jan 1	Feb 15	Source	1971-1980		1981-1990		1991-2000		1971-2000
	lb N/ac					bu/ac	%N	bu/ac	%N	bu/ac	%N	bu/ac	%N
1	0				-	24.2	1.81	24.6	2.33	20.4	2.51	23.1	2.18
2	30				AA	35.6	1.95	38.5	2.67	40.2	3.13	38.1	2.50
3	60				AA	38.0	2.12	35.8	2.73	37.5	3.24	37.1	2.60
4	120				AA	36.9	2.70	36.4	2.84	26.8	3.27	33.4	2.87
5	240				AA	37.5	2.68	33.2	2.78	11.6	3.23	27.4	2.83
6	0				-	25.9	1.84	32.5	2.26	27.2	2.50	28.5	2.15
7	30				UR	34.3	1.94	38.1	2.31	32.1	2.55	34.8	2.22
8	60				UR	40.6	1.95	42.9	2.62	40.8	2.89	41.4	2.42
9	120				UR	38.5	2.60	40.7	2.89	40.8	3.03	39.9	2.81
10	240				UR	38.1	2.69	37.9	2.88	30.2	3.04	35.4	2.84
11	30	30	30	30	UR	37.6	2.48	35.3	2.97	38.9	3.26	37.3	2.85
12	60	60	60	60	UR	38.7	2.63	36.3	2.94	34.3	3.29	26.5	2.89
13	15	15	15	15	UR	39.0	2.01	41.0	2.66	38.8	2.89	39.6	2.47
14	30			30	UR	39.9	2.08	44.1	2.79	43.3	3.00	42.5	2.57
15	60			60	UR	39.2	2.52	39.5	2.95	40.8	3.15	39.8	2.83
16	120			120	UR	37.7	2.63	37.2	2.82	25.8	3.15	33.6	2.81
17	30				AN	31.5	1.85	39.7	2.42	34.6	2.56	35.3	2.24
18	60				AN	36.1	1.87	42.1	2.68	39.5	2.67	39.2	2.38
19	120				AN	40.4	2.34	41.1	2.91	42.5	3.00	41.3	2.71
20	240				AN	37.6	2.67	39.4	2.99	25.2	2.97	34.1	2.86
21	30	30	30	30	AN	38.2	2.51	38.6	2.98	40.3	3.16	39.0	2.84
22	60	60	60	60	AN	38.3	2.71	37.5	2.96	19.2	3.09	31.6	2.88
23	15	15	15	15	AN	38.3	2.10	39.9	2.88	39.4	3.07	39.2	2.63
24	30			30	AN	38.9	1.98	43.3	2.81	44.1	2.92	42.1	2.52
25	60			60	AN	39.9	2.28	36.9	2.91	39.2	3.16	38.6	2.73
26	120			120	AN	38.1	2.71	39.7	2.80	22.9	3.10	33.5	2.82
27	30				SCU	31.5	1.79	35.3	2.32	29.3	2.36	32.0	2.13
28	60				SCU	39.8	2.01	41.9	2.58	41.0	2.86	40.9	2.43
29	120				SCU	41.7	2.43	41.0	2.84	41.6	2.97	41.4	2.72
30	240				SCU	38.4	2.72	39.1	2.89	12.5	3.01	30.0	2.85
SED						2.86	0.13	2.88	0.16	4.54	0.09	3.85	0.14
CV, %						11	8	11	8	19	4	15	8

*All plots received 60 lbs P₂O₅ (0-46-0) and 60 lbs K₂O (0-0-60) applied broadcast preplant. AA-anhydrous ammonia, UR-urea,
AN-ammonium nitrate, SCU-sulfur coated urea.

Table 3. Treatment structure of long-term nitrogen source-method-rate wheat Experiment 505 and surface (0-6 inches) soil test analyses from samples collected in 1995, Lahoma, OK.

*All plots received 60 lbs P2O5 (0-46-0) and 60 lbs K2O (0-0-60) applied broadcast preplant. AA-anhydrous ammonia, UR-urea, AN-ammonium nitrate, SCU-sulfur coated urea. SED - standard error of the difference between two equally replicated means, CV - coefficient of variation.