Features Agronomy Fertility and Nutrients
Petiole nutrient recommendations may need revising

Potato growers use analysis of petiole (leaf stem) samples to monitor the nutrient
status of potato crops throughout the growing season. While it is a useful technique
for monitoring nutrient deficiencies that were not identified in spring soil
samples, Shelley Woods, a soil and water research scientist with the Irrigation
Branch, a division of Alberta Agriculture, Food and Rural Development at Lethbridge,
says that the range of petiole values used in Alberta may not be appropriate.

"Current standards for petiole nutrient status used in southern Alberta
were developed in the northwest US where soil, climate and day-length conditions
differ significantly," explains Woods. "We are looking to see if there
should be changes made in Alberta recommendations."

Woods became interested in refining petiole recommendations when she noticed
that petiole analysis results from previous southern Alberta Russet Burbank
studies suggested that the current recommendations for phosphorus (P) and potassium
(K) may be somewhat high, especially early in the growing season, and that nitrate-nitrogen
(N) concentrations may also need fine-tuning.

In that study, the lowest application rate used in the experiment (50 pounds
per acre P2O5) usually translated
to a small increase in yield (less than one ton per acre), but for the larger
application rates, there were no discernible effects on yield. However, the
increased fertilizer P inputs gave increased levels of petiole P, yet the increased
P content in the tissue did not translate into higher yield. That got Woods
interested in the new project looking at petiole analysis in Alberta.

With funding from the Potato Growers of Alberta and McCain Foods, Woods initiated
a three year project in 2004, to determine the optimal petiole nutrient concentrations
for Russet Burbank potato crops in southern Alberta. She is also looking at
the relationship, if any, between petiole nutrient concentrations and tuber
specific gravity. In 2004 and 2005, various rates of N, P and K fertilizers
were applied to a field of grower- managed Russet Burbank potatoes, near Taber,
Alberta. Petiole samples were collected and analyzed seven times in each growing
season. Tuber samples were collected in late September, graded for marketable
yield and analyzed for specific gravity.

"I would like to have one more year of data before providing firm recommendations
on petiole guidelines, especially since 2004 and 2005 had very different weather
patterns," says Woods. "These new results, along with the ones from
previous studies, indicate that climate may have a large impact on potato petiole
nutrient concentrations."

Looking at early trends, petiole standards may be too high for southern Alberta,
essentially meaning that farmers may be applying too much fertilizer. Specifically,
early season petiole P standards may be too high, while late season petiole
P standards may also be high. Petiole K results were inconclusive because of
high soil test K at the study site. However, increased rates of K had little
effect on potassium concentrations in the petioles and no effect on yield.

"Phosphate petiole concentrations kept coming out low and potassium was
quite low, but despite that, we never noticed any detectable effect on yield,"
explains Woods. "That tells me the standards may be set too high for southern
Alberta growers."

The big rains in southern Alberta in the spring of 2005 caused some additional
challenges in analyzing the petiole results for nitrogen. With several large
rainfall events, Woods says her spring applications of nitrogen were likely
leached below the rooting zone, which made results harder to interpret.

Other research being conducted by the Potato Growers of Alberta (PGA) is also
calling petiole standards into question. In research on potassium, the PGA found
potassium fertilizer had little effect on petioles and no effect on yield. They
also sent identical petiole samples to several laboratories and found significant
differences in nitrogen, phosphorus and potassium results, creating another
puzzle for growers to deal with when using petioles to manage fertility.

Woods hopes to collect her final year of data in 2006 and if all goes to plan,
may have new petiole standards available in late 2006 or early 2007. For growers
planning fertility programs in 2006, her best advice is business as usual, and
to work closely with their crop advisors and soil test laboratories to refine
their existing fertility program. -30-