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Features Agronomy Fertility and Nutrients
Balancing nutrient needs

Michigan specialist advises on primary and secondary nutrient levels.

November 14, 2007
By Margaret Land


16aIn life, it is advised that you not sweat the small stuff. But as a potato
producer, that would be a very big mistake, says Dr. Darryl Warncke, a soil
fertility specialist with Michigan State University. "Potatoes are sensitive
to stress conditions," he explains.

These stresses can range from soil moisture levels to physical soil properties,
he adds, and can also include deficiencies in soil micronutrients, which are
required by potato plants in order to grow.

Potatoes have high nutrient requirements, accumulating 33 pounds of nitrogen,
13 pounds of phosphate and 63 pounds of potassium in each 100cwt of tubers.
The plants rely on their root systems to aid in the uptake of nutrients from
the soil, explains Warncke. How quickly and at what quantity the nutrients are
transferred depends a lot on the size of the root system, which is a problem
for potato plants.


"As crops go, potatoes have one of the weakest root systems going,"
says Warncke. "Potato breeders need to focus on producing a more vigorous
plant with a larger root system. It would really help us out."

Also playing a role in nutrient uptake is soil pH. For optimum nutrient availability,
Warncke suggests soil be in the 6.2 to 6.8 pH range. This is supported by the
fact that phosphorous availability decreases when soil pH drops below 6.0 or
rises above 6.8 and micronutrient availability decreases when the pH rises above
6.8, he adds.

Unfortunately, in potato production, a soil pH between 5.5 and 7.0 is less
favourable for the development of common scab, Streptomyces scabies.
"I suggest keeping pH in the optimum range and grow varieties resistant
to scab," says Warncke (see Common scab: an ever increasing problem,
page 24).

Nitrogen management
Nitrogen has a large effect on tuber yield and quality and is quite mobile in
the soil. In light of this, Warncke recommends that nitrogen applications be
split across the potato growing season, thus matching the application to the
needs of the plant and minimizing the risk of loss through leaching.

"If you load nitrogen at the front of the growing season, you may lose
it," he warns. "You can also end up with too much top growth."

Table 1a. Nitrogen recommendation
guidelines for potatoes (Michigan).
Potential yield (cwt/ac) Suggested N rate (lb/ac)
300 150
350 165
400 180
450 195
500 210
550 225
600 250
For long season potato varieties, add 40 pounds N per
Table 1b. Potato nitrogen recommendations
Expected marketable yield (t/ha) Actual N (lb/ac)
15 45
20 67
25 116
30+ 178
For early crop on mineral soil, 62lb N/ac.
For early and main crops on muck soil, 53lb N/ac.

Warncke advises that one-third of the season's nitrogen be applied at planting
in a band application two inches below the seed piece and two inches on each
side. The second one-third application of nitrogen should be done at hilling
and the final third applied via broadcast or irrigation when the tubers are
in the early stages of bulking. The total amount of nitrogen to apply for the
season will depend on the expected yield of potatoes (see Tables 1a and 1b).

For early varieties, Warncke suggests all nitrogen applications be completed
by June 18. For late varieties, all nitrogen should be applied by July 15. Phosphorous

Phosphorous is an important nutrient for root growth but it is not very mobile
in the soil; its availability is reduced as soon as it has contact with the
soil. Because of this, Warncke recommends a banded application of phosphate
at planting, following the same two inch spacing used for banded nitrogen applications.
Adding ammonium to the soil at that time can enhance early phosphate uptake,
he adds.

Table 2. Phosphorous recommendation
guidelines for potatoes (Michigan).
Soil test (ppm) Potential yield (cwt)
400 500 600
Pounds P2O5
per acre
40 227* 240* 253*
80 52** 65** 78**
120 52** 65** 78**
160 42*** 52*** 62***
200 0 0 0
* = buildup, ** = maintenance, ***
= drawdown

In order to maintain optimum levels of phosphorous in the soil, Warncke suggests
that about 50 pounds of phosphate per acre be applied each season in cases where
tests show phosphorous levels in the soil are low or at optimum (see Table 2).

"Applying less than crop removal will result in the reduction of the soil
phosphorous level," he says, adding even in cases where phosphorous levels
are high, producers may want to add 25 to 40 pounds of phosphate per acre "if
they're not comfortable with applying none."

Potassium management
Potatoes have a high requirement for potassium, utilizing about 63 pounds of
the nutrient per hundredweight of tubers produced. However, applying high rates
of potassium has been associated with reduced tuber density (a lower specific
gravity). In order to remedy this, Warncke recommends potato producers apply
potash to fields the fall before they are planted. This should not be done on
sandy soils though, he warns, due to leaching issues. A soil test should be
done in order to establish existing potassium levels in the soil. How much additional
potassium to add will depend on what yield of potatoes the producer feels the
soil can produce (see Table 3).

Table 3. Potassium recommendation
guidelines for potatoes (Michigan).
Soil test (ppm) Yield goal (cwt/ac)
350 450 550
80 194 257 300 204 267 300 214 277 300
100 189 252 300 189 252 300 194 257 300
120 142 189 236 189 252 300 189 252 300
140 0 0 0 47 63 79 142 189 236
160 0 0 0 0 0 0 0 0 0
CEC = Cation Exchange Capacity
Maintenance: 300cwt = 189lb K2O/ac; 400cwt = 25lb
K2O/ac; 500cwt = 315lb K2O/ac.
Recommendation is capped at 300lb K2O/ac.

Calcium management
Generally, calcium levels in the soil are adequate for potato production, but
Warncke does warn that marginal calcium levels can result in internal brown
spot (IBS) under hot and dry soil conditions. He recommends that soils testing
lower than 350ppm of calcium receive an application of calcium through gypsum
or lime (if pH is acidic). For broadcasting, Warncke suggests producers apply
about 200 pounds of calcium per acre if broadcast, or about 50 pounds of calcium
per acre in a banded application.

He does not recommend that calcium be foliar applied. "You may end up
with increased calcium concentration in the leaves but it will not be transferred
to the tubers," he says.

Magnesium management
Potatoes are quite responsive to magnesium, especially in sandy soils, says
Warncke. If soils tests show less than 40ppm of magnesium, he suggests producers
apply about 10 to 20 pounds of magnesium per acre in a banded application or
about 50 pounds per acre through broadcast. If a deficiency is found during
the growing season, magnesium can also be foliar applied at a rate of about
two pounds actual magnesium per acre.

Sulphur management
Sulphur may be beneficial
for nitrogen utilization and plant growth, especially in sandy soils, says Warncke.
The best way to introduce sulphur to the soil is through an application of gypsum
(calcium sulphate), potash, magnesium sulphate or ammonium sulphate, usually
in the area of about five pounds per acre, he says, adding applications should
not exceed 10 pounds per acre.

Editor's note
Nutrient requirements, application methods, timing and placement
all vary with individual grower situations and may be dependent on soil
type, variety requirements and weather conditions.

For all nutrients, growers are advised to work with their supply outlets
and crop consultants. Micronutrient applications may also be warranted
and consideration should be given to leaf sampling and analysis, again
in consultation with advisors.


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