Abstract

Response of wheat grain yields to fertilization with nitrogen (N), phosphorus (P) and potassium (K) has been determined in numerous soil fertility experiments around the world.  Experiment 502 was established in 1971 to evaluate the effect of long-term N, P and K fertilization in continuous winter wheat.  Yield increases due to applied N (80 pounds N/acre) have averaged between 15 and 20 bushels/acre/year.   No response to applied P or K has been seen in any year, since soil test P and K levels were high when this experiment was initiated in 1970.  Soil test P levels have declined somewhat where no P has been applied, however, sufficiency levels still exceed 100%.  Soil organic C levels have increased with increasing N applied when compared to the check.  Grain yields were maximized at the 80 lb N/ac/yr rate.  In the first ten years of the experiment, N applied preplant and incorporated had no effect on grain %N.  However, in the last 20 years, grain %N increased linearly up to 100 lb N/ac. 

Materials and Methods

Experiment 502 was established in the fall of 1970 under conventional tillage on a Grant silt loam (fine-silty, mixed, thermic Udic Argiustoll).  Wheat has been planted for 28 continuous years in 10 inch rows at seeding rates of 60 pounds per acre.  The variety 'Nicoma' was planted from 1971-74, 'Triumph 64' from 1975-1976, 'Osage' in 1977 and 1979, 'Triumph 64' in 1978, 'TAM W-101' from 1980-91, Karl 92 from 1993 to 1994 and Tonkawa from 1995 to present.  Changes in management, application dates and fertilization are reported in Table 1.  The experimental design employed is a randomized complete block with four replications.  Fertilizer treatments used in this experiment and average grain yield means for selected periods are reported in Table 2.  Results from surface (0-6 inches) soil samples collected in 1995 are reported in Table 3.  Individual plots at this site are 16 feet wide and 60 feet long.  The center 10 feet of each plot was harvested for yield using a conventional combine.  In addition to wheat grain yield measured every year (exception was 1973 where crop failure resulted due to lack of rainfall), periodic soil and grain samples were taken for further chemical analyses.

Results

In the first ten years of the experiment, grain yields increased by an average of 14 bushels/acre/year when 80 pounds of N as ammonium nitrate was applied preplant.  Similarly, applied N over the last twenty years of the experiment resulted in yield increases up to 20 bu/ac.  No increase in grain yield could be attributed to P or K in any year of the experiment.  Soil test P levels have declined somewhat where no P has been applied, but sufficiency levels still exceed 100%.  Soil organic C levels increased with increasing applied N when compared to the check (Table 3).  Soil pH and K have changed very little over the 28 years that these treatments have been evaluated.  In the first ten years of the experiment N applied preplant and incorporated had no effect on grain %N.  However, in the last 20 years, grain %N increased linearly up to 100 lb N/ac. 

Table 1. Treatment  applications and experimental management for continuous winter wheat Experiment 502, Lahoma, OK, 1971-2015.

Year	Variety	Fert App Date	Planting Date	Harvest	Seeding Rate, lb/ac
1971	Nicoma
1972	Nicoma
1973	Nicoma
1974	NIcoma
1975	Triumph64
1976	Triumph64
1977	Osage
1978	Triumph64			6/13/1978
1979	TAM W-101			6/28/1979
1980	TAM W-101			6/24/1980
1981	TAM W-101		10/31/1980	6/18/1981	65
1982	TAM W-101			6/28/1982
1983	TAM W-101		10/18/1992	7/1/1983
1984	TAM W-101			6/21/1984
1985	TAM W-101		10/30/1984	6/13/1985	75
1986	TAM W-101		10/21/1985	6/11/1986	74
1987	TAM W-101		10/28/1986	6/18/1987	68
1988	TAM W-101	8/31/1987	10/2/1987	6/20/1988	67
1989	TAM W-101	10/10/1988	10/14/1988	6/19/1989	70
1990	TAM W-101		10/13/1989	6/20/1990	65
1991	TAM W-101	8/2/1990	10/15/1990	6/6/1991	65
1992	TAM W-101	9/9/1991	9/26/1991		63
1993	Karl	8/24/1992	10/1/1992		76
1994	Karl	9/14/1993	9/28/1993		75
1995	Tonkawa	8/5/1994	10/28/1994	6/19/1995	62
1996	Tonkawa	8/31/1995	10/10/1995	6/21/1996	69
1997	Tonkawa	9/4/1996	10/3/1996	6/13/1997	66
1998	Tonkawa	9/11/1997	10/17/1997	6/12/1998	70
1999	Tonkawa	9/3/1998	10/9/1998	6/30/1999	73
2000	Custer	9/8/1999	10/12/1999	6/13/2000	75
2001	Custer			6/15/2001
2002	Custer		11/21/2001	6/25/2002
2003	Custer		10/8/2002	6/17/2003
2004	Custer		10/15/2003	6/11/2004
2005	Overley		9/24/2004	6/15/2005
2006	Overley
2007	Overley		10/2/2006	6/26/2007
2008	Overley
2009	Endurance	9/23/2008	9/30/2008	6/18/2009
2010	Bullet	9/15/2009	10/7/2009	6/11/2010
2011	Bullet	10/1/2010	10/6/2010	6/6/2011
2012	Billings	9/27/2011	10/24/2011	5/22/2012	90
2013	Billings	10/4/2012	10/11/2012	6/24/2013
2014	Ruby Lee	10/9/2013	10/24/2013	6/17/2014	70
2015	Iba	10/3/2014	10/21/2014	6/22/2015
2016	Iba
2017
2018
2019

Table 2.  Treatment structure of long-term wheat Experiment 502, wheat grain yield and percent N in the grain for selected periods, Lahoma, OK, 1971-2000.

N, P₂O₅, and K₂O applied as ammonium nitrate (34-0-0), triple superphosphate (0-46-0) and potassium chloride (0-0-60), respectively. * K₂O applied  as sul-po-mag (0-0-22). SED - standard error of the difference between two equally replicated means, CV - coefficient of variation.

Table 3.  Treatment structure of long-term wheat Experiment 502, and surface (0-6 inches) soil test analyses from samples collected in the summer of 1995, Lahoma, OK.

N, P₂O₅, and K₂O applied as ammonium nitrate (34-0-0), triple superphosphate (0-46-0) and potassium chloride (0-0-60), respectively. * K₂O applied  as sul-po-mag (0-0-22). SED - standard error of the difference between two equally replicated means, CV - coefficient of variation.