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Timing and rate of application crucial in shallow-rooted plants

Stress conditions – moisture, nutrient availability, soil properties, limited root systems – can play a crucial role in tuber development, so managing the application timing of nutrients is vital to set and bulking, Darryl Warncke told Michigan potato growers at the Great Lakes Fruit, Vegetable and Farm Market EXPO in Grand Rapids last December.

Warncke challenged potato breeders to consider the root system when working on new crosses. Plants with weak root systems require more precise nutrient applications for growth.

“The root system that does exist is fairly shallow-rooted,” he said. “Potato breeders really need to develop more rigorous root systems.”

Applying the correct of amount of nutrients is most important during tuber bulking, Warncke said. During bulking, potato plants can use between 800 and 1,200 pounds of biomass per acre. For optimum nutrient availability, soil pH should be between 6.2 and 6.8. Above 6.8 and below 6.2, phosphorus availability is reduced. A low pH also reduces the availability of calcium and magnesium, but below a soil acidity of pH 5.5 the probability of common scab is reduced.

“You need to be aware of the nutrient requirements that the crops have.”

As a general guide, Warncke said potatoes accumulate about 13 pounds of nitrogen, 13 pounds of phosphate and 63 pounds of potassium in every 100 cwt. of tubers. Because plant root systems are not efficient – especially for uptake of phosphorous – nutrients should be applied in bands around the seed piece. Warncke recommends applying the fertilizers 2 inches on either side and 2 inches below the seedpiece to ensure the nutrients are taken up into the plant. The bands of fertilizer at planting should contain up to 60 pounds of nitrogen, all the phosphate the plants will require and up to 100 pounds of potassium per acre.

Phosphates can be difficult to regulate, as the nutrient is not absorbed into the plant quickly. But Warncke said with the addition of ammonium, early phosphate uptake could be enhanced. Soil sampling is necessary to maintain a phosphate level of about 150 ppm.

Broadcasting phosphate fertilizer will increase the chance of runoff and the nutrient will likely not be taken up into the plants, Warncke said. Because phosphate is prone to runoff, the most effective application is with band placement.

Nitrogen, on the other hand, is very mobile, Warncke said. When applied in the soil, it is converted to nitrate form and can be lost to leaching or denitrification. The best practice is to apply nitrogen at planting and do two later applications.

“Having adequate N available early in the growing season is important for good vegetative growth prior to tuber set and bulking,” Warncke said. “A general guide for N application is to apply one-third at planting or emergence, one-third at hilling and one-third broadcast and/or irrigated in during tuber sizing.”

For Michigan growers, Warncke said nitrogen applications should be completed by mid-July, because nitrogen applied later in the season won’t be effective and would increase the risk of contaminating the groundwater.

The rate of nitrogen application varies depending on the variety and the yield estimates. For a 300 cwt. per acre yield, Warncke recommends 150 pounds of nitrogen per acre. Up to 250 pounds per acre should be applied for a yield of 600 cwt. per acre. Longer season varieties can require an additional 40 pounds per acre, Warncke said.

The type and texture of a field’s soil will vary the amount of potassium needed. If a double band of nutrients is used, up to 100 pounds per acre can be applied. For a single band, limit the amount to 60 pounds per acre. Broadcasting potash prior to planting is recommended for high amounts, Warncke said, but too much potassium can affect the specific gravity, which is an important aspect for many of Michigan’s chip growers.

“Potatoes accumulate large amounts of potassium, but applying a high amount can reduce tuber density – specific gravity – especially with potassium chloride,” Warncke said.

Timing and application amount depends heavily on growers monitoring their soil and plants. Testing for pH and biomass nutrients will help to limit stress conditions and provide the plants with the proper nutrients for bulking without creating larger issues such as runoff or groundwater contamination.

Originally posted Saturday, Apr. 7, 2007

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