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Determining safe seed-placed fertilizer blend rates

Understand factors that affect germination damage to help determine safe, single-shoot fertilizer rates.

November 19, 2007
By Bruce Barker


The move to low disturbance seeding systems with wider row spacings affects
on safe rates of seedrow fertilizer. Much of the early research was done using
older press drills with disc openers on six inch row spacings. Now, with a variety
of opener options, and the use of starter blends, the issue of how much is too
much is worth revisiting.

"What makes seedrow fertilizer difficult to work with is that there are
so many variables," explains Adrian Johnston, Northern Great Plains Region
Director with the Potash and Phosphate Institute of Canada (PPIC) at Saskatoon,
Saskatchewan. "It is a difficult process to make recommendations on seed-placed
fertilizer. There is a high, high probability of emergence problems if conditions
aren't ideal, so the recommendations are conservative."

In developing seed-placed recommendations, researchers consider the many variables
that cause seedling burn and germination damage. The two main fertilizer factors
are fertilizer salt index and ammonia toxicity.


Fertilizer salt index is a rating provided to indicate how fertilizer products
differ in their salt content. The ratings are made relative to sodium nitrate,
a fertilizer that was common when concern was initially raised about how salty
fertilizers were. With a rating of 100 for sodium nitrate, potash (KCl) has
a rating of 116, urea a rating of 75, ammonium sulphate a rating of 69 and mono-ammonium
phosphate has a rating of 32. Fertilizer products can also be rated on the salt
index per nutrient, which can change the rating. Crops vary in their ability
to tolerate fertilizer salts, with cereals generally being the most tolerant,
oilseeds the least, and pulses somewhere between.

The other main fertilizer factor that affects seedrow fertilizer burn is ammonia
toxicity. Ammonia gas (NH3) in high concentration is
toxic to seedling roots, reducing nutrient and water uptake and leading to poor
growth. Soil conditions of high pH, large amounts of free lime (carbonates),
low cation exchange capacity, and dry conditions all increase free ammonia in
the soil. Fertilizer that produces ammonia gas, such as urea, can impact seedling
development more than other fertilizers such as ammonium nitrate. A liquid nitrogen
fertilizer source like UAN (urea-ammonium nitrate solution) has only part of
its nitrogen in a form (urea) that produces ammonia.

"The biggest advantage with solution fertilizer is the ability to direct
the fertilizer away from the seedrow," explains Johnston. "I've seen
research that shows if you put the same rate of UAN as urea right on top of
the seed, you will get the same damage. But if you direct the UAN solution away
from the seed, there is no damage."

Crop and environment also affect safety
Johnston outlines four main non-fertilizer factors affecting safe seedrow fertilizer.
Crops have varying tolerances to seed-placed fertilizer. Generally, cereals
have the greatest tolerance to high rates, while oilseeds are at the bottom.
Flax in particular is very sensitive to seed-placed fertilizer. Pulse crops
vary in their tolerance, with peas having low tolerance and lentils more.

Soil moisture also has a major impact on crop tolerance to seedrow fertilizer.
In fact, Johnston explains that most provincial fertilizer guidelines state
that safe rates should be reduced by 50 percent when soils are not at field
capacity – or basically full of water. On the other hand, receiving 0.5
inch or more of rain within five days of seeding will wash most of the fertilizer
salts and free ammonia away from the germinating seed, reducing crop damage
to near zero.

Soils higher in clay content can mean higher seedrow fertilizer rates. The
finer the soil (more clay), the greater the tolerance. The high surface area
of a clay versus a sandy soil binds free ammonia, preventing it from damaging
germinating seedlings. Clay soils also hold more water, diluting the fertilizer
salts in the seedrow.

Seedbed utilization (SBU) has mostly influenced changes in fertilizer placement
for the past 20 years. With the development of air-seeders using sweeps, farmers
drastically expanded the seedrow area. Then, with the movement to low disturbance
seeding with air-drills, seedbed utilization moved back towards disc openers,
although on wider row spacing than conventional press drills of the 1970s.

Seedbed utilization is the proportion of the seedbed used for seed and fertilizer
placement. For example, a two inch opener on a 10 inch row spacing has a SBU
of 20 percent. As SBU increases, the tolerance of the crop to high rates of
seedrow fertilizer increases, explains Johnston, because less of the seed is
in direct contact with the fertilizer. As a result, spreading seed and fertilizer
under a sweep affects crop emergence less than a narrow knife.

"The other factor which is difficult to measure, is the effect of air
delivery systems on tolerable seed-placed rates. Older research was done with
gravity-fed press drills, where the seed and fertilizer ended up in the same
area. With air systems, seed and fertilizer is spread out more in width and
height between the top and bottom of the furrow because of seed bounce,"
explains Johnston. "There isn't much research looking at that effect, but
it could increase tolerance."

What about safe rates of fertilizer blends?
Call a local crop consultant, or log-on to a provincial fertility site for information
on safe rates of seedrow N under ideal moisture conditions, based on SBU and
soil texture and crop type. And recommendations are available for safe rates
of seedrow P, or K, or S. But what happens if you want to apply them in a blend?
That is where it gets a bit tricky.

The Saskatchewan Agriculture and Food (SAF) guideline for a P-K blend recommends
that the total pounds of P205
plus pounds of K20 should not exceed the maximum rate
of seed-placed phosphate in their P recommendations, assuming good soil moisture
conditions. For sulphur, when ammonium sulphate is placed with the seed, SAF
recommends adding the pounds of N from other nitrogen fertilizer being placed
with the seed – and the total N should not exceed maximum safe rates of
seedrow urea.

So, according to those recommendations, if 35 pounds is the maximum rate of
actual N (76 pounds of urea), you should be able to add up the amount of actual
N in ammonium sulphate (20-0-0-24), urea and mono-ammonium phosphate (11-52-0)
to get the maximum allowable N rate in the blend. But what about crop sensitivity
to P? Is it additive as well? That is where the recommendations get fuzzy.

Johnston says that field research in Alberta at Bow Island looked at safe rates
of urea and phosphate fertilizer blends on canola. Those results were masked
by rainfall shortly after seeding, so various rates of N, P, K and S with different
SBU showed little damage to seedrow fertilizer blends.

Table 1. Fertilizer salt index ratings for common fertilizer
products. Based on a rating of 100 for sodium nitrate. Values in brackets
represent index per unit of nutrient.
Fertilizer Formulation Salt index
Potassium chloride 0-0-60 116 (1.94)
Ammonium nitrate 34-0-0 105 (3.0)
Urea ammonium nitrate 28-0-0 95 (2.3)
Ammonium thiosulphate 12-0-0-26 84 (7.0)
Urea 46-0-0 75 (1.6)
Ammonium sulphate 20-0-0-24 69 (3.3)
Anhydrous ammonia 82-0-0 47 (0.6)
Mono-ammonium phosphate 11-52-0 34 (2.4N/0.5P)
Source: PPIC.

Other research from a growth chamber study conducted by Les Henry at the University
of Saskatchewan looked at application of fertilizer blends on sandy loam and
clay loam soil. He used a gravity disc drill configuration with a one inch spread
on seven inch row spacing or about 14 percent SBU. On these soils, rates of
P205 and K20
up to 80 pounds per acre could be applied in the seedrow with spring wheat.
However, canola emergence at rates above 20 pounds of P205
or K20 severely reduced crop stand.

Henry's research considered two blends with a N-P205-K20-S
blend of 13-14-15-12 and 18-18-20-0 applied at 50, 100 and 200 pounds of product
per acre. Both blends could be applied at rates of up to 150 pounds of product
on both soil types in spring wheat. However, with canola rates, above 100 pounds
of product per acre resulted in significant plant stand reduction of greater
than 40 percent. For both crops, it is unlikely that these rates would provide
the needed fertility for a high yielding cereal or oilseed crop, so additional
N, and possibly P, would likely need to be banded.

Johnston says that the lack of field research on fertilizer blends means that
farmers will have to experiment on their own, using their knowledge of factors
affecting seedling damage. He doubts that research will ever be able to provide
definitive recommendations because of all the variables affecting seedling damage.

"From my perspective, it is a very serious challenge. Some growers are
applying seedrow fertilizer based on the tolerance of the crop to meet all their
crop requirements. There is a need to get away from that and consider alternative
fertilizer placements that avoid seedling damage and also meet the requirements
of a high yielding crop," says Johnston. -30-

Updating safe rates for phosphate

Jeff Schoenau, soil scientist, says the move to low disturbance direct seeding
systems is shifting seeding equipment to narrow openers and wider row spacings
– configurations that make it difficult to place all the recommended fertilizer
nutrients safely in the seedrow. As a result, he says that many farmers are
side-banding or mid-row banding the majority of the nutrient requirements, especially
nitrogen, while seedrow applying a starter phosphate fertilizer.

Table 2. General maximum safe seed-placed P205
fertilizer rate at 10 to 15 percent SBU in growth chamber trials at University
of Saskatchewan.
Crop P205
rate (kg/ha)
Wheat 40
Canaryseed 30
Pinto bean 30
Chickpea 25
Canola 20 to 25
Mustard 15 to 20
Flax 15 to 20
Bromegrass 15
Alfalfa 15
Pea 15
Source: J.J. Schoenau, University of Saskatchewan. Crop
Tolerance and Response to Seed-Row Phosphorus Fertilizer.

"We had limited information on crop tolerance to starter phosphate in
low disturbance configurations, especially for some of the newer specialty crops,"
says Schoenau.

In a new research study, he looked at how seed-placed fertilizer (12-51-0)
affected early plant P nutrition and growth for six crops with limited information:
canaryseed, mustard, chickpea, pinto bean, alfalfa and brome grass, in comparison
to wheat, canola, flax and pea. It was conducted in a controlled environment
chamber using loam textured soils that were P-deficient and a SBU of 10 to 15

The majority of the crops showed no negative impact on seed germination and
emergence with seed-placed P at rates of 18 pounds P205
per acre (20kg P205/ha) or less.
In the trials, he rated the crop tolerance to phosphate fertilizer as follows:

  • wheat (most tolerant) > canaryseed + pinto bean > chickpea + canola > mustard
    + flax > bromegrass > alfalfa > pea (most sensitive).

"We found that canaryseed was fairly tolerant to seed-placed P in the
study. Peas were quite sensitive, and mustard was more sensitive than canola,"
says Schoenau. "Pinto beans seem quite tolerant and chickpea showed a little
higher tolerance than expected."

Schoenau cautions that these results were determined under optimum growing
conditions and that the injury thresholds for the newer crops need to be further
evaluated in the field. He has initiated studies looking at P-K blends and the
use of a controlled release P fertilizer product, which may allow higher rates
to be safely seed-placed. -30-


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