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Nitrogen fertilizer enhances fertilizer-P uptake

Too much N in a dual N-P band can delay uptake.

November 26, 2007  By John Harapiak

The synergistic benefit of combining the application of N and P fertilizer
on crop response is a well established scientific fact. This was the reason
why recovery of fertilizer-P was enhanced by applying these fertilizer nutrients
in the form of mono-ammonium phosphates (MAP) such as 11-48-0, 11-51-0, 11-52-0
and so on. This practice was especially effective on fallow fields.

This 1960s dramatic response to the application of seedrow applied
fertilizer-P is becoming quite infrequent, due to the long history of P-fertilization
and the associated build-up in residual fertilizer-P supplies within the
soil. This development has made it practical for growers to explore other
options for supplying fertilizer-P to their crops.

Impact of changes in N rates
Fields being re-cropped had N requirements that usually exceeded the N that
existed in MAP, so growers began requesting blends of phosphate with either
ammonium nitrate or urea. Since only a limited amount of N fertilizer could
be safely applied within the seedrow, as rates of N application increased, in
order to avoid the risk of germination damage, other placement options were

Development of new placement options
An early option was to band N-P blends in a separate operation prior to seeding.
However, with the growing interest in using no-till/direct seeding systems,
other fertilizer placement options grew in popularity. As illustrated in Figure
1, these included such options as side-banding of the fertilizer either beside
each seedrow, between pairs of seedrows or in the mid-row position.


Perceived side-band limitations
A concern about the use of some side-band seeding openers was that under some
soil conditions, the amount of soil disturbance created was deemed to be excessive
for no-till. Other concerns included the fact that some of these openers had
a high horsepower requirement and loosened the seedbed excessively. In addition,
some of these openers failed to maintain an adequate separation of the fertilizer
from the seed, resulting in an increased risk of germination damage.

Figure 1. Illustration of some of the approaches used for applying
higher rates of fertilizer N and P away from the seedrow. At moderate rates
of fertilizer application, these placement options can effectively supply
the crop's fertility requirements. Illustration courtesy
of Westco Fertilizers.

Mid-row option and seed safety
An opener option that was developed as an alternative was to band fertilizer
in the mid-row position, using a dedicated opener to place fertilizer. In order
to reduce the cost of the openers used for applying fertilizer, these openers
were only inserted between alternate pairs of equally spaced seedrows. This
approach reduced the number of fertilizer openers required by 50 percent.

Fewer bands impacts concentration
As a result of using the mid-row approach, concentration of fertilizer contained
in each band is effectively doubled, compared to placing fertilizer beside each
seedrow. Figure 2 demonstrates why using this approach results in a significant
increase in fertilizer concentration within a band. At wider row spacing and
at higher N rates, the potential negative impact of this fact on crop recovery
of fertilizer-P is not well understood.

Possible three week delay of P-uptake
A problem can arise when the concentration
of N in dual N-P bands becomes excessive. Research at Westco and the University
of Manitoba has indicated that at a band spacing of 12 inches and at a N rate
of 80lb/ac to 90lb/ac, the P contained in the band would not be available for
crop uptake for a period of three weeks. This was due to the inability of roots
to penetrate the core of the band. This fact obviously could have serious implications
for P recovery from mid-row N-P bands.

Figure 2. Demonstration of the relationship between seedrow and fertilizer
spacing that exists in mid-row fertilizer band seeding systems. The concentration
of fertilizer within each fertilizer band is effectively doubled compared
to a side-banding set-up. For this reason, mid-row banding is most at risk
for creating 'Hot Bands'. Illustration courtesy of Westco
Figure 3. Illustration of the effect of higher band concentration of
N on the behaviour of plant roots. The roots will initially only penetrate
the outer perimeter of the expanding N enriched zone. Phosphate, which is
relatively immobile, is 'stranded' within the core of the dual N-P fertilizer
band and will not be taken up by the crop until the roots can penetrate
into the P-enriched zone. This delay in P-uptake can last for a period of
up to three weeks from the date of application. Illustration
courtesy of Westco Fertilizers.

Difference in N and P mobility
Several issues have been identified that can contribute to the delay in uptake
of P from highly concentrated fertilizer bands. First, the N is much more mobile
within the soil than is P, so it migrates outward from the zone of highest concentration.
That means the roots do not have to enter the core of the fertilizer band in
order to access the N they require. However, since P is relatively immobile
within the soil, it remains 'stranded' in the core of the fertilizer band, which
the roots are unable to penetrate for a period of time.

Toxic environment for roots
Furthermore, urea breaks down to create a band environment that contains large
amounts of ammonium-N (NH4+) and ammonia-N (NH3),
both of which can be toxic to roots at high concentrations. The conversion of
NH4+ to nitrate-N (NO3-) is
a two-step process. Unfortunately, the high pH environment interferes with the
completion of the second step. This fact results in the build-up of nitrite-N,
which can be quite toxic to roots. For this reason, the uptake of fertilizer-P
can be delayed if higher concentrations of N exist in N-P bands. As illustrated
in Figure 3, we refer to the existence of these high N concentrations as 'Hot

Impact of wider band spacing
If growers plant their crops using mid-row fertilizer bands where the seedrows
are spaced eight inches apart, the mid-row fertilizer bands will be effectively
spaced at 16 inch intervals. In this scenario, as illustrated in Table 1, uptake
of P from a dual N-P band will be delayed for a period of three weeks starting
at a N rate of 60lb/ac to 70lb/ac. Given a mid-row system where the seedrows
are spaced 12 inches apart and the fertilizer rows are spaced 24 inches apart,
P-uptake could be seriously delayed at a rate of 40lb/ac to 45lb/ac of N. In
order to avoid this potential problem, some starter-P should be placed directly
within the seedrow.

Greater risk in corn crops
Some corn growers have attempted to side-band higher amounts of the N and P
required, at the time of seeding, and have reported a poor response to fertilizer-P
applied in this manner. This is not surprising, based on the wider row spacing
commonly used in corn production. At a row spacing of 36 inches, a significant
delay in the uptake of fertilizer-P from a N-P band would start to occur at
a N rate of only 25lb/ac to 30lb/ac.


Table 1. Impact of fertilizer band spacing on N rate at
which roots will be unable to access P due to toxic environment of the bands.
Row spacing (inches) N rate (lb/ac)
12 80 to 90
16 60 to 70
24 40 to 45
36 25 to 30
48 20 to 23
Rates of N application at which a significant delay in
P-uptake can start to occur from dual N-P bands as influenced by fertilizer
band spacing. At these or higher N rates, root penetration of the fertilizer
bands will be restricted to the outer perimeter due to the 'toxic' nature
of the core of the fertilizer band. The lack of root penetration has been
attributed to the development of a condition referred to as 'Hot Bands'.
Normally, a period of three weeks will be required to overcome this problem.
Information adapted from research conducted at Westco Fertilizers.
*John Harapiak has more than 40 years of western Canadian based
fertilizer related experience. He will continue to contribute stories to
Top Crop Manager. He can be contacted by e-mail at:



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