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Field Research Brief No. 9
May, 19S5


TITLE: P Fertilization and Maintenance, Siting la'

RESEARCHERS: Mike Wade and Al-jabri

OBJECTIVES: 1. Determine initial F rates necessary to reach a
critical soil test value and optimum crop yield,
2. Determine per crop rates necessary to maintain
established soil levels,
3. Compare banding vs. broadcasting of mainentance
aDulicat ions.

SOIL: inceptisol, ECk, cropped for 7 years after bulldozer
clear ing without lime or fertilizer inputs

TREATMENTS: Factorial, RCB, 4 replications, 4.5 X 7m plots

Initial Rates Fer Cro~ Rates
(2X per year)

1. 0 k::g P/ha 1. none
2. 20 2. 20 kg P/ha oand
0 3. 20 kg P/ha broad-
4. SO cast
5. 160 "
6. 320

CROP: (2nd) Soybeans, cv Willis, 20 X 40cm X 3 seeds/hole,
planted 27 Jan 1985, harvested 27 April 1985
Pesticides: Carbofuran 20 kg/ha (35) at planting,
and endosulfan at 1 kg a.i./ha sprayed
weekly far 5 weeks starting at pod set
Base fertilizers: 150 kg KC1/ha + 100 kg MgS04/ha
Inoculum: U.S. commercial soybean inoculum

RESULTS: Despite a three-week drought commencing right after
emergence the beans grew and.produced well. Insect damage was
kept to a minimum by the pesticides used as described above. The
bean yield as affected by treatments can be seen in Figure 1.
When a maintenance dose of 20 kg F/ha was broadcast and
incorporated prior to planting (the second such dose this year)
optimum yields were obtained regardless of initial P rates,
including zero. Banding of the same amount of maintenance P was
not as efficient, as there was still considerable influence of the
initial F rates. A linear response through the 160 kg/ha initial
rate is seen. The residual plots, those getting only the initial
applications and no maintenance, show a steeper, slightly
curvilinear response to the 160 kg/ha rate also.

Both maintenance treatments gave considerably less sloping
responses than for the previous crop, corn. It would appear that








the 20 kgi/ha rate is not only maintaining but probably increasing
soil P. Hopefully pending soil analysis can detect this increase
in available P. The plots will be analyzed for soil P again at
the end of the year (August) to help determine a more accurate
maintenance rate.
Figure 2 shows that the critical level of extractable F was
dependent upon management practice. The residual and banded
maintenance treatments show a relationship that has a critical value
of 17 ppm (mod. Olsen) whereas the broadcast maintenance plots
indicate 9 ppm ext. P as sufficient for obtaining plateau yields.
Apparently this is due to the increased effectiveness of the per-
crop broadcast applications. Because of the relatively dry weather
during this crop period, it is suspected that the banded mainte-
nance applications may have resulted in a concentration of roots
around the band thereby limiting soil volume exploration, which
in turn would result in more soil moisture stress. Root
development measurements were not made but perhaps this is
Potential area for more research.











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