March 2013
Feed Your Seed: Nutritional guidelines help determine ‘just right’ rates By Bill Schaefer

In the potato catalog of best management practices, nutrient management is at the top of the list. Growers must contend with reaching that perfect nutritional medium for the health of their crop: How much do you supply without supplying too much?

There’s a number of variables growers must contend with; climate, water, variety, soil conditions and more, but when all is said and done, nutrient management remains the most vital of management factors resulting in quality potatoes and good yields at the end of harvest.

The first step in determining your crops’ nutrient needs is soil management and testing. John Taberna, owner of Western Laboratories, said that growers should make sure they have a crop adviser you trust.

You have to have faith in your crop adviser to do a good job,” Taberna said.

Taberna, who refers to himself as a soil scientist, said that for a good soil analysis he wants from 9 to 11 samples.

“You don’t want to take a good and a bad area because when you mix it up you create a soil that is not in the field,” Taberna said. “You have to take it from what is typical from that field,”

Taberna advocates taking fewer samples to avoid getting a poor analysis.

“The more soil you take, the harder it is to get a homogeneous mixture before you put it into the bag to send it to the lab,” Taberna said.

Taberna advocates taking a little less but taking samples from the good areas in your field and then thoroughly mixing the samples together.

Taberna recommends avoiding shallow samples, and making sure your probe goes down 10 to 12 inches. Shallow samples tend to test high in phosphorous and zinc.

“It’s not going to be representative of the soil the plant lives in,” Taberna said.

Taberna advises to pre-irrigate the field prior to sampling, adding that the best time to take soil samples is following harvest.

At the 2013 Idaho Potato Conference (IPC), Jeff Stark, director of the University of Idaho potato variety development program, recommended that growers take representative samples from each unique soil type or management zone and submit the samples to private or university labs within 48 hours.

Stark said that some of the factors affecting nutrient requirements include:

1) Yield potential, based on long-term averages
2) Residual amount of each nutrient in the soil
3) Nutrient concentrations in the irrigation water
4) The amount and type of previous crop residue
5) Soil type and physical condition
6) Manure and compost additions

The most effective early season placement for most nutrients is either in-row application at planting or band application at mark-out.

“Early in the year the root system is not extensive on a potato plant. So you want the nutrients where the early roots are going to get to it,” Stark said.

Later in the growing season you’ll want to do a broadcast application to get the nutrients up to a sufficiency level.

Stark’s advice to growers when doing petiole tissue analysis is to sample the fourth petiole and take 40 to 50 petioles per zone or field. Stark said to sample under consistent conditions. Petiole nitrate conditions differ from morning to afternoon.

“You want to sample each field at about the same time of day as you go through your site,” Stark said.

Neither phosphorous nor potassium, particularly phosphorous, are very mobile in soils Stark said.

Phosphorous is the second most critical nutrient, following only nitrogen.

Stark said that phosphorous is essential for energy transfer and is critical for root growth. He advocated using in-furrow and banding early in the season with foliar applications on as determined by petiole testing.

“Set yourself up as best you can with soil-applied nutrients and then monitor through a petiole sampling system whether or not you have adequate nutrients. Use foliars to correct deficiencies,” Stark said.

Bryan Hopkins, associate professor in the department of plant and animal sciences at Brigham Young University, discussed phosphorous use efficiency at the 2013 IPC.

Hopkins reminded growers that plants have to drink their nutrients, thus the nutrients have to be liquid form in the soil.

“The real battle we have with phosphorous is that it doesn’t want to be in liquid form,” Hopkins said. “It wants to be in solid form. We get very little efficiency with phosphorous because it’s so insoluble.”

Hopkins said that previous studies show a wide variance, from 1 percent to 34 percent, in the rate of recovery of phosphorous in the first year of application.

Hopkins advocates that growers keep applying phosphorous in bands to the side and keep the bands intact.

“Because of the root architecture, it’s more important to keep it (phosphorous) concentrated in a band,” Hopkins said.

Hopkins said that his research has shown that the use of humic acids pre-mixed at a 10:1 ratio aids in the solubility of phosphorous resulting in increased yields and increased specific gravity. He recommended using Carbond P produced by Land View in Rupert, Idaho.

Potassium is the third most important mineral in your portfolio of potato nutrients.

Samuel Essah, assistant professor of potato management and plant physiology at Colorado State University’s San Luis Valley research center, is a strong proponent for the use of sulfate of potash (SOP) versus muriate of potash (MOP) for meeting potassium requirements.

Essah said that his studies have shown that MOP has more chlorine than SOP and MOP tends to leave salt residue in the soil resulting in yield reduction and lower specific gravity along with higher overall yields and higher percentage of #1 potatoes. Additionally SOP will delay sprouting compared to MOP, Essah said.

Finally, the early outlook for nutrient costs in 2013 appears to be stable according to Paul Patterson, University of Idaho Extension agriculture economist.

“Prices are fairly stable,” Patterson said. “We’ve got a good supply of all the major components, whether it’s urea, nitrogen, phosphate or potash. The supply demand should indicate that the price of fertilizer should be stable and comparable to last year.”

Patterson said that there shouldn’t be as great a demand for fertilizer in the Midwest because much of the nutrients were left in the ground due to the drought there.

Like any economist, Patterson did hedge his prediction, with one proviso — water levels on the Mississippi River. Fertilizer prices could be impacted if barge traffic becomes restricted due to low water levels on the Big Muddy.

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