Saskatchewan Research
Yield Response of Spring Wheat
to Seed-Placed Phosphorus
B y R.P. Zentner, C . A . Campbell, and F . Selles
A 24-year Saskatchewan
study has shown spring wheat continues to respond postively to
seed-placed phosphorus
(P) in spite of marked increases in available soil P due to
fertilization.
Yield increases were related to spring and summer climatic
conditions.
W H E A T Y I E L D R E S P O N S E to seedplaced P has been observed f o r many
years i n the Canadian Prairies. This popup e f f e c t is p a r t i c u l a r l y notable when
soils are cool and wet i n early spring.
W i t h soil P levels rising f r o m the more
frequent use o f P fertilizers over the past
40 to 50 years, there is some question as to
whether crop response to starter P has
been decreasing due to this buildup o f soil
available P.
For 24 years, we have monitored yield
and bicarbonate-extractable soil P (OlsenP) annually on twelve cropping systems
on a m e d i u m textured, O r t h i c B r o w n
C h e r n o z e m at S w i f t C u r r e n t , Saskatchewan. These data provide an opport u n i t y to reassess c r o p
response to starter P under
conditions o f soil P buildup.
Two o f the 12 crop rotations
were examined i n this study.
B o t h systems were f a l l o w wheat-wheat ( F - W - W ) , one
receiving nitrogen ( N ) plus P
and one receiving only N .
Results
The initial level of soil test
P (Olsen P) at the start of the
study (1967) was 17 l b / A and,
a f t e r 24 years o f c r o p p i n g
w i t h o u t P f e r t i l i z e r application, the P test level did not
change ( F i g u r e 1). T h e P
removed i n the grain, the only
major source of P export f r o m
the system, averaged about
7 lb P 0 /A/yr. This amount is about equal
to that being generated through P mineralization f r o m soil organic matter and physiochemical transformation of less available
forms of inorganic P, because the unfertilized system has not decreased over the
24-year period.
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Addition o f P fertilizer, at an average
annual rate o f 13 lb P 0 /A/yr, increased
soil test P by about 0.8 l b / A each year o f
the study. The P exported i n the grain i n
the fertilized rotation averaged about 8 lb
P 0 / A / y r o f P. Therefore, P 0 input was
5 l b / A / y r greater than export, suggesting
that about 36 to 38 percent o f the extra
fertilizer P had entered the Olsen P fraction o f the soil.
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5
5
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Figure 1. Average annual application rates of 13 lb P 0yA
increased soil test P by about 0.8 lb/A each year of
the study.
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The authors are research scientists with Agriculture Canada, located at Swift Current Research
Station, Saskatchewan, Canada.
Better Crops/Fall 1992
Phosphorus application produced an
average of 2.8 b u / A more grain f o r wheat
grown on fallow, and 2.0 b u / A more grain
for wheat grown on stubble (Figure 2).
These increases were consistent over the
24-year period and occurred as frequently
i n the latter half of the study as the first
half. Thus, the gradual build-up i n available soil P due to f e r t i l i z a t i o n d i d not
appear to dampen the response to P f e r t i l izer. This shows that, although frequent
use of fertilizers w i l l increase available P
levels i n prairie soils, farmers may still
experience significant yield response to
small applications of P fertilizer placed
w i t h the seed.
The variability i n yield response was
closely related to the influence of spring
weather c o n d i t i o n s . For example, f o r
wheat grown on fallow, yield reponse to P
was d i r e c t l y r e l a t e d to t e m p e r a t u r e
between emergence and 3-leaf stage but
was depressed when soils were very wet at
seeding. The positive relationship
between yield and temperature may be
because bare fallow soils are
often moist and therefore cool
i n early spring. Because they
have been fallowed, they are
likely to have sufficient available P but, at low temperature,
p l a n t r o o t g r o w t h w i l l be
slowed, as w i l l available P
uptake and translocation
w i t h i n the plant.
The literature suggests that
fertilizer P placed close to the
seed w i l l be extremely important because early spring is
the period when wheat makes
m a x i m u m use of fertilizer P
Although prairie soils may have higher
levels of soil available P resulting f r o m
decades of P fertilizer use, seed-placed P
can s t i l l o f f s e t some y i e l d v a r i a b i l i t y
related to c l i m a t i c conditions and can
result i n moderate to substantial y i e l d
increases. Under excellent growing conditions, seed-placed P can aid plants i n
reaching higher yield capabilities leading
to higher profit potentials. •
Figure 2. Phosphorus application produced an average of
2.8 bu/A more grain for wheat grown on fallow and
2.0 bu/A more grain for wheat grown on stubble.
early season drought. Leaf development
failure may also serve as a P deficiency
symptom, based on results f r o m a greenhouse study evaluating several hard red
spring wheat varieties (Table 1).
Summary
M o s t spring wheat varieties w i l l
develop eight leaves on the plant m a i n
1992
Summary
1991
Plant Development. . . from page 27
Better Crops/Fall
I n contrast, the i m p o r t a n c e o f s o i l P
increases rapidly f r o m about four weeks
after emergence. The negative relationship
between yield response to P and precipitation at seeding may be the result o f excessive water due to the combination of high
levels of stored water plus high early May
precipitation. For wheat grown on stubble,
response to P was greater when soil temperatures were low at about 3- to 4-leaf
stage. This type o f response is similar to
that obtained i n growth chamber studies
by other scientists.
stem. Data indicate that varieties d i f f e r i n
P response f o r both tiller and leaf development. The significance of this lies w i t h the
fact that leaf development loss hasn't been
recognized as a small grain production
management concern. L i k e tiller and head
count numbers, small grain adventitious
roots and mainstem leaves are easy to
count and evaluate and are related to crop
yield p o t e n t i a l s
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