Abstract

In 1968, experiment #222 was initiated at the Agronomy Research Station in Stillwater, Oklahoma.  This trial was established to evaluate long-term winter wheat grain yield response to applied nitrogen (N), phosphorus (P) and potassium (K).  Applications of sulfur (S) and magnesium (Mg) (Sul-Po-Mag) were also compared.  In the first decade of the experiment, few responses to applied N, P and/or K were found.  Following twenty years without fertilization, average check plot yields were 20 bu/ac.  It was not until the third decade of this experiment that a dramatic response to applied N was observed.  A tendency for increased yields with applied P and K (5 vs 6 and 8 vs 9) was present for the 1989-98 time period, but, this was not significant.  Over the 31 years that these treatments have been evaluated, only applied N has produced a significant increase in grain yields.  In many regards this continuous wheat data demonstrates the difficulty of evaluating P and K fertilizers since no response could be induced without fertilization following 31 years.  Nitrogen applied at an annual rate of 80 pounds per acre was adequate to produce near maximum yields in all three decades evaluated. Considering these results it can also be concluded that if there was no response to applied K (as KCl), there would also be no response to applied Cl.  Also, no response was seen to applied S and/or Mg as K,MgSO₄.  Grain %N increased with increasing rates of applied N in all three decades evaluated.

Materials and Methods

Experiment #222 was established in 1969 under conventional tillage on a Kirkland silt loam (fine, mixed, thermic Udertic Paleustoll).  Wheat was planted for 22 continuous years in 10-inch rows at seeding rates of 60 pounds per acre.  Since 1992, winter wheat has been planted in 7.5-inch rows.  The variety 'Scout 66' was planted from 1968-73, 'Triumph 64' from 1974-1977, 'Osage' from 1978-80 and 'TAM W-101' from 1981-91, 'Karl' from 1992-1994, 'Tonkawa' from 1995 to 1998 and ‘Custer’ since 1999.  Changes in management, fertilization and application dates are reported in Table 1.  The experimental design employed was a randomized complete block with four replications.  Fertilizer treatments used in this experiment and average grain yield means over selected periods are reported in Table 2.  Surface soil (0-6 inches) test analyses from samples collected in 1995 are also reported in Table 2.  Individual plots at this site are 20 feet wide and 60 ft long.  The center 10 feet (1969-1995) and 6 ft (1996-present) of each plot was harvested with a conventional combine the entire 60 ft in length for yield.  In addition to wheat grain yield measured every year from this experiment, periodic soil and grain samples were taken for further chemical analyses.

A significant response to applied N was not seen until the second decade of the experiment (Table 2, 1979-1988).  Since that time, applied N has resulted in significant yield increases.  Other than applied N, limited response to applied P or K has been found in this experiment.  However, a recent trend for increased yields as a result of applying K has been observed, especially at the high rates of applied N.  Considering these results it can also be concluded that if there was no consistent response to applied K (as KCl), there would also be no response to applied Cl.  Also, no response was seen to applied S and/or Mg as K,MgSO₄ (treatment 13 versus 3). 

The fertilizer treatments evaluated have resulted in relatively small surface soil pH (0-6 inches) changes following 27 years (1969-1995) of continuous winter wheat.  Soil test P declined with increasing applied N, a result of increased depletion.  Surface soil organic matter levels have not changed dramatically, however, organic matter levels have increased at the high N rates when compared to the 0 N checks.   

References

Ranney, R.W. 1969. An organic carbon-organic matter conversion equation for Pennsylvania surface soils. Soil Sci. Soc. Amer. Proc. 33:809-811.

Table 1.  Treatment  applications, and experimental management for continuous winter wheat Experiment 222, Stillwater, OK, 1969-2015.

Year	Variety	Fert App	Planting	Harvest Date	Seeding Rate	Topdress
		Date	Date			Date
1969	Scout 66				60
1970	Scout 66				60
1971	Scout 66				60
1972	Scout 66				60
1973	Scout 66	10/3/1972	10/9/1972		60	3/16/1973
1974	Triumph 64				60
1975	Triumph 63	8/29/1975			60
1976	Triumph 64				60
1977	Triumph 64			6/15/1977	60
1978	Osage		9/9/1977	6/14/1978	60	3/29/1978
1979	Osage			6/29/1979	60
1980	Osage			6/25/1980	60
1981	TAM W-101			6-31-81	72
1982	TAM W-101	9/18/1981	9/22/1981	6/14/1982	60
1983	TAM W-101	9/29/1982		6/21/1983	90	3/1/1983
1984	TAM W-101	8/31/1983	10/5/1983	6/25/1984	72
1985	TAM W-101	8/23/1984	10/2/1984	6/12/1985	72	3/8/1985
1986	TAM W-101		10/7/1985	6/12/1986	72	2/18/1986
1987	TAM W-101	8/20/1986	10/17/1986	6/15/1987	72	3/6/1987
1988	TAM W-101		9/17/1987	6/14/1988	72
1989	TAM W-101	8/18/1988		6/20/1989	60
1990	TAM W-101	8/29/1989	10/11/1989	6/13/1990	60
1991	TAM W-101				60
1992	TAM W-101	9/10/1991	9/30/1991	6/17/1992	75	2/3/1992
1993	Karl	9/16/1992	10/12/1992	6/17/1993	95	2/3/1993
1994	Karl	9/22/1993	9/27/1993	6/8/1994	95
1995	Tonkawa	8/30/1994	9/29/1994	6/20/1995	90	2/24/1995
1996	Tonkawa	10/9/1995	10/10/1995	6/11/1996	70	3/13/1996
1997	Tonkawa	9/5/1996	10/3/1996	6/19/1997	70	2/17/1997
1998	Tonkawa	9/29/1997	10/3/1997	6/10/1998	68	1/30/1998
1999	Tonkawa	9/3/1998	10/13/1998	6/15/1999	68	2/15/1999
2000	Custer	9/5/1999	10/7/1999	7/6/2000	68	2/17/2000
2001	Custer		11/20/2000	6/12/2001
2002	Custer		10/1/2001	3/3/2002
2003	Custer		10/14/2002	6/23/2003
2004	Custer		10/6/2003	6/16/2004
2005	P2174		10/21/2004	6/7/2005
2006	Endurance		1/7/2005	5/26/2006
2007	Endurance		1/3/2006	6/8/2007
2008	OKField		10/9/2007
2009	OKField	9/16/2008	6/25/2008			3/9/2009
2010	GoLead	10/1/2009	3/3/2010	6/29/2010	90	2/22/2011
2011	Centerfield	9/1/2010	9/30/2010	6/9/2011
2012	Endurance	9/15/2011	10/13/2011	6/8/2012	90	3/7/2012
2013	OK9935C	9/26/2012	11/16/2012	6/28/2013		3/11/2013
2014	Doublestop-CL	10/10/2013	10/22/2013	6/18/2014	75	3/21/2014
2015	Iba	9/26/2014	10/20/2014	6/8/2015	75	3/16/2015

Table 2.  Soil fertility treatment effects on wheat grain yields, Experiment #222, Stillwater, OK 1969-2000, and surface (0-6 inches) soil test results from 1995. 

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.  pH 1:1 soil:water, P, Mehlich III extraction, OM = organic matter = OC*1.8+0.35 (Ranney, 1969)

Table 3.  Soil fertility treatment effects on wheat grain %N, Experiment #222, Stillwater, OK 1969-2000.

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.  pH 1:1 soil:water, P, Mehlich III extraction, OM = organic matter = OC*1.8+0.35 (Ranney, 1969)

<----- NORTH

View of Experiment #222 looking south east, April 2005

View of Experiment #222 looking south, January 2006

222, Stillwater, OK