March 2015
The Power of Potassium By Melanie Epp, Spudman Correspondent

As most growers know, healthy potato crops require an incredible amount of resources, but in particular, potassium. In fact, potatoes use more potassium than any other nutrient, said Samuel Essah, associate professor and Extension specialist, potato crop management and physiology, Colorado State University. If nutrients are not managed properly, quality and uniformity can suffer immensely. But proper management isn’t just about how much you use; it’s about proper timing and the actual source itself.

It’s important to understand that the source of potassium you choose is important,” Essah said. “It will affect the quality of potatoes as well as the overall yield.”

Potassium has many functions in potatoes, including decreasing internal blackening and hollow heart, Essah said. Not only that, but it also extends potato shelf life, decreases storage losses and improves chip and fry color. Potato tubers rich in potassium are more resistant to mechanical damage, and more tolerant to external stress, like frost, drought, heat and high light intensity.

“Stress from disease and insect damage are also reduced with adequate supply of potassium,” Essah said. That, and sugar translocation and starch synthesis are also improved, which effectively increases the production of large-sized tubers.

When not enough potassium is available to the plant it decreases photosynthesis, which reduces dry matter and starch formation, Essah said. “There is a reduction in plant growth and all the symptoms of potassium deficiency begin to show.”

Determining deficiency

How do you know if your soil is potassium deficient? What does potassium deficiency look like? Early symptoms appear as a dark greening or bluish greening of plant foliage, Essah said. Leaves will appear glossy, and tiny, light green spots will develop between the veins of larger leaves.

In the upper canopy, the leaf margins of potassium deficient plants will curl down and leaflets will appear small, cupped and crowded. Older leaves will turn brown and die off early. “The key symptom is the overall bronzing of the canopy,” Essah said. “Severe deficiency leads to short plants with poor root growth and shortened stolons and premature senescence before adequate tuber bulking.”

Potassium deficiencies appear most often under certain conditions. Essah said he sees deficiencies in low pH soils with low cation exchange capacity, and in sandy soils where leaching can occur. Deficiencies can also occur under drought-like conditions, in fields where leaching is caused by heavy rainfall or intense irrigation.

Heavy clay (illite) soils with high, unavailable potassium-fixing power, and soils with low potassium reserves and magnesium rich soils can also be problematic for growers. Finally, potassium deficiency can also be a problem on serpentine soils that are high in exchangeable magnesium and on peat, muck or other soils that release potassium too slowly during the peak growth period.

Avoiding deficiency

About 240 to 250 pounds of potassium (K) is needed to produce 400 to 500 cwt potatoes,” Essah said. “Growers should sample and analyze their soil early before planting to know how much potassium is in the soil before planting. This will help growers to know how much K fertilizer is needed for the growing season.”

Essah recommends that growers apply all the K needed pre-plant, since it’s usually more effective than applying most or all of the seasonal K supply through fertigation.

“Applying over 50 percent of the K requirement during tuber bulking reduces tuber yield and specific gravity compared to pre-plant,” he said.

Essah said that petioles should also be analyzed for K concentration. A petiole concentration of 0.3 to 1.8 percent shows deficiency. A concentration of 3.0 to 11.5 percent, however, does not.

Leaf blades should also be analyzed. “One percent K in leaf blade is the critical level,” Essah said. ” And 0.3 to 0.9 percent means deficiency, while 1.5 to 5 percent means no deficiency.”

Petioles and leaf blades should be tested every two weeks until vine kill. “The mere addition of fertilizer is no proof that nutrients become available for better plant growth,” Essah said.

Choosing the right source

There are plenty of sources to choose from, but Essah and other leading researchers believe that sulfate of potash (SOP) is the most effective potassium/sulfur combination available to growers.

Essah recommends SOP as a good source of K fertilizer to most potato growers, particularly because the use of SOP results in higher tuber dry matter and starch content than equivalent quantities of potassium chloride (KCI). Also, he says SOP results in high tuber specific gravity, and it produces high-quality potatoes that are good for the production of french fries and chips. Finally, he says external defects, such as growth cracks, knobs and misshapes are also reduced where and where SOP is used. Growers should be aware, though, that SOP contains 7 percent sulfur, and be sure to credit it in the sulfur fertilizer budget.

Applications of SOP are particularly preferable in sulfur-deficient soils, and under conditions of soil salinity and/or where saline irrigation water is in use. In fact, to date, there are no conditions under which SOP is not a preferred source of K. “Current research knowledge on SOP indicates that it is the preferred source of K under most soil conditions,” Essah said.

Jeff Miller, president and CEO of Miller Research, an independent research company, has done work with SOP under contract with Compass Minerals. Under that contract, he conducted trials that compared SOP to muriate of potash (MOP) to KCI under conditions where soil composition was about 75 percent sand. In terms of performance, he says that he did not observe a large difference in product performance.

According to Miller, one advantage of using SOP is the reduced salt index (46.1) compared to MOP (116.3). “Soils with a high salt content may benefit from using SOP instead of MOP,” he said. “SOP has typically cost more than MOP, but this increase in cost may be offset by the value of using a fertilizer with a lower salt index. One idea is to use a 50/50 blend of SOP/MOP. This offers the benefit of a lower salt index fertilizer than using MOP alone and results in an application that is not as expensive as using SOP alone.”

“Both SOP and MOP have worked similarly,” Miller said. “MOP is often the product of choice because it costs less. I have used both, but I choose to use SOP for a number of reasons.”

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