Features Agronomy Fertility and Nutrients
Yield benefit of phosphorus fertilizer

Phosphorus fertilizers are applied annually to almost all cereal and oilseed
crops grown on the Canadian prairies. Their benefit to crop yield was first
demonstrated in studies conducted from 1928 to 1930 and their use expanded rapidly
from the early 1940s until the late 1960s. Since 1975, the import of P in fertilizers
to the three prairie provinces has been approximately equal to the export of
P in grain.

The widespread use of P fertilizers has likely contributed to a reduced response
of P fertilizer. A recent review of fertilizer trials in Saskatchewan found
that prior to 1970, P fertilizer increased wheat yield by an average of 26 percent
(874 trials). After 1970, P fertilizer increased wheat yield by an average of
only 11 percent (252 trials).

To evaluate the current P responses in Alberta, small plot fertilizer trials
were conducted from 1991 through 1993 at 154 locations across the province.
About 27 percent of locations were on Brown (Aridic Boroll) or Dark Brown (Typic
Boroll) soils, 45 percent were on Black (Udic Boroll) soils and 27 percent were
on Gray (Boralfic Boroll) soils.

About 80 percent of the locations were on stubble (recrop) land and 20 percent
were on fallow land. At each location, fertilizer responses were determined
for barley, spring wheat and canola (at some locations, only one or two crops
were tested).

The replicated field trials included the treatments zero, 13, 26 and 39 pounds
per acre P2O5 applied as mono-ammonium
phosphate (MAP). Treatments were applied with the seed except at the highest
application rate for canola. To avoid germination damage in canola, application
was split with 13lb/ac P2O5 seed-placed
and 26lb/ac P2O5 banded prior
to seeding. Nitrogen (N) and any other required fertilizer nutrients were banded
prior to seeding. The best-rated crop varieties were used in each region.

Soil samples were obtained during the previous fall at locations in southern
Alberta or just prior to trial establishment at locations in central or northern
Alberta. Available soil P was determined using the modified Kelowna method (0.15
M NH4F, 1.0 M CH3COONH4,
0.5 M CH3COOH). At maturity, crop yields were collected
and expressed on a dry weight basis. Yield data from each experimental site
was subject to an analysis of variance and only those experimental sites that
had a coefficient of variation of less than 20 percent were used in the analysis.

In total, fertilizer responses were recorded at 143 barley sites, 141 wheat
sites and 108 canola sites. Two thirds of the cereal sites and just under half
of the canola sites had a significant (p < 0.05, LSD) yield increase due
to P application (see Table 1). The average increase in yield due to application
of P fertilizer was 10 percent. This is similar to the 11 percent yield response
reported for 252 trials conducted across Saskatchewan between 1970 and 1991.

Differences in average P fertilizer response were small among crop types (see
Table 1). The only significant difference was a slightly smaller percentage
yield gain for wheat than for barley or canola.

Soil type significantly affected P fertilizer response, depending on crop type
(see Table 1). The grain yield increase due to P fertilizer, expressed as percent
of unfertilized check, was significantly greater for barley and canola on Gray
soils relative to Black or Brown soils, but was similar for wheat.

One factor that contributes to differences in P fertilizer response among crop
types is P acquisition strategy. Canola roots lower the pH within the rhizosphere
more effectively than cereal roots, allowing canola to deplete acid-soluble
P fractions more effectively than cereals. This strategy is likely to be most
effective in calcareous soils and may partially account for the less frequent
and smaller response of canola in Brown and Black soils than in the more acidic
Gray soils. A second factor that may contribute to differences in crop response
to P fertilizer is the 'starter' effect of P fertilizer. The importance of starter
fertilizer in cool soils may account for the greater P fertilizer response in
Gray soils, which are found in regions characterized by cooler spring (May-June)
air temperatures.

An alternative method of presenting fertilizer recommendations that is useful
when factors other than fertility greatly influence crop response is based on
probability diagrams. Using the response functions determined for each site
and fertilizer-to-grain cost ratios, the probability (percentage of sites) of
an economic increase in yield as a function of soil test and fertilizer rate
can be determined (see Figure 1). It is important to remember that this applies
to the application year only, and does not take into consideration any residual
P from the applied fertilizer that will be available in future years. The probability
of an economic increase in yield due to application of the first 13lb/ac P2O5
was high for all crops and soil test levels, declining from close to 100 percent
when soil test P was less than 10 parts per million (ppm) to about 60 percent
when soil test P was more than 30ppm. Crop type and soil zone had little influence
on the probability of an economic increase with the first increment of fertilizer
addition.

With the second increment of 13lb/ac P2O5,
a wider variation in the probability was observed. The probability of an economic
P fertilizer response at the lowest soil test level was least for barley and
canola in Brown soils. The probability of an economic increase in yield with
the third increment of 13lb/ac P2O5
was low for all soil test levels and crops. Maximum profits for fertilizer P
applied in that year were achieved if sites were fertilized at rates that provided
an approximate 40 percent probability of an economic increase. At this level,
rates of required P fertilizer ranged from about 20lb/ac P2O5
at high soil test P levels to about 35lb/ac P2O5
at low soil test P levels.

Since this work was completed in 1993, soil test summaries indicate that about
60 percent of Alberta soils test medium or below in soil P and would likely
respond to fertilizer P application. An evaluation of crop production and fertilizer
use in 2001 found that fertilizer additions accounted for 87 percent of crop
removal in the province. These results indicate that any change in soil P since
this study have been minor.

The average yield response to P fertilizer at the 154 locations included in
this study was 10 percent. This increase was much smaller than reported in early
studies on the Canadian prairies, likely due to a gradual increase in residual
soil P fertility caused by the regular application of P fertilizer. Despite
this weak response, the application of 20 to 35 pounds P2O5
per acre generally provided optimum returns in the application year. The probability
of a profitable yield benefit declined with increasing fertilizer rate or soil
test P level.

The results of this study support the use of soil testing to establish the
availability of soil P and develop suitable fertilizer management practices.
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*Dr. McKenzie (ross.mckenzie@ gov.ab.ca)
and Mr. Middleton are with the Crop Diversification Division, Alberta Agriculture,
Food and Rural Development, in Lethbridge. Mr. Kryzanowski is with the Crop
Diversification Division, Alberta Agriculture, Food and Rural Development, in
Edmonton. Dr. Heaney is with Norwest Labs in Edmonton, and Dr. Bremer, Mr. Solberg,
Mr. Coy and Mr. Harapiak are private agronomic consultants in Alberta.

Reprinted from Better Crops with Plant Food with permission
of PPI/PPIC.

Top Crop Manager welcomes submissions for technical stories.
Contact Peter Darbishire at: email peterd@aiscommunications.net telephone (519)
235-2400