| Experiment 505: Wheat Grain Yield Response to Nitrogen Source and Timing, Lahoma, OK | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
AbstractEnvironmental 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 MethodsIn 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.
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