Potatoes demand a tremendous amount of potassium-potato plants use more potassium than any other nutrient. Photo by Tyler J. Baum.
To maximize profitability in potato production, it's essential to produce high-quality potatoes. That means managing dozens of factors that affect potato quality. In that complex equation, proper fertilizer management is just as important as managing weed and insect pests.
One of the key nutrients in potato production is potassium. First and foremost, potatoes demand a tremendous amount of potassium-potato plants use more potassium than any other nutrient. A 500 cwt crop removes as much as 280 lbs. of potassium per acre annually. If it's not replenished, the next crop can suffer in quality and lack of uniformity in terms of quality, such as specific gravity, yield, storage capability and increased external defects.
According to Dr. Samuel Essah, assistant professor in potato production and storage at Colorado State University, there are a number of reasons why potassium deficiency occurs in potato production. Soil type has a direct relationship with potassium deficiency in many cases.
"When we have low pH soils, we have low cation-exchange capacity (CEC-the degree to which a particular soil can adsorb and exchange cations), then that's associated with low K and that's why we have deficiency," he explains. "In sandy soils, potassium can leach out due to precipitation or irrigation-these soils are commonly deficient in potassium."
Essah points out that heavy clay soils can also be potassium deficient, because they contain a mineral called illite. This causes potassium to fix to the soil, which means it is not readily available to the potato plant.
Another factor that can cause soils to be potassium deficient is the presence of magnesium. One example is serpentine soils, or soils that have been created from ultramafic rocks such as serpentinite. Typically, these soils have low concentrations of essential nutrients such as nitrogen, potassium and phosphorus.
"Basically, potassium and magnesium compete with each other for fixation sites on the soil molecule," explains Essah. "So, if soils are rich in magnesium, they will be deficient in potassium and vice versa. It's important to understand this relationship, because if you put too much magnesium in the soil in the fall, it will displace the potassium in your soil, leading to potassium deficiency and poor-quality potatoes."
Potassium deficiency manifests in a variety of highly visible symptoms in potato plants. In young plants, potassium deficiency is indicated when plant leaves are dark green to bluish in color, and usually glossy in appearance.
"If potassium deficiency is severe, you see bronzing on the leaves," says Essah. "The underside of the leaf becomes brown or yellow on the outer edges. Eventually the leaves dry up and senesce.
"At the top of the potato plant, in the upper foliage, you can see the leaflets are small and cupped and crowded," explains Essah. "Light green spots develop between the veins of the leaves on the younger leaves. If the potassium deficiency is not corrected, plants are shorter and stunted. This is because of the poor root growth in plants with potassium deficiency. The plants have shorter stolons and thus produce smaller, poor-quality tubers."
Potassium is involved in regulating the movement of water within the potato plant and in aiding photosynthesis. Potassium also helps move sugars from the plant leaves, where they are produced, and convert them into starches within the tuber. In addition, potassium deficiency affects the ability of the plant to access and process other nutrients properly.
"There are several reasons for this," says Essah. "One, a potato plant with potassium deficiency has a poor root system, so the plants cannot normally take up other nutrients. In addition, potassium affects translocation within the plant and the tubers-it causes imbalances in other nutrients. The plant is dysfunctional. If we have less potassium, then there is too much increase of magnesium and this is toxic to the plant."
The best way to prevent potassium deficiency is properly timed soil sampling and smart fertilizer management and application.
"I like to do soil testing just before planting, not in the fall," recommends Essah. "If you test in the fall, you may not get an accurate reading, due to winter precipitation and snow melt. These sources of moisture can leach the potassium from the soil before you plant."
Proper timing of soil testing and a pre-plant potassium application is the best way to avoid seeing the signs of potassium deficiency in potatoes, and losing profits due to poor-quality potatoes. Another important decision is choosing the most effective source of potassium, depending upon soil type and the results of soil testing.
"It's important to understand that the source of potassium you choose is important-it will affect the quality of potatoes as well as the overall yield," says Essah.
Of all the sources of potassium from which to choose, Essah and other leading potato researchers advise that Sulfate of Potash, also known as SOP (K2SO4) is the most effective potassium/sulfur combination available. Sulfate of Potash is the richest source of low-chloride potassium, providing equivalent potassium content of 50 percent K2O, commonly referred to as potash. It's a natural source of potassium and sulfur, another important nutrient for potatoes.
Recent work by Essah has shown that using Sulfate of Potash versus Muriate of Potash not only improved yield, but resulted in improved crop quality measured by delayed sprouting, higher specific gravity and fewer external defects.
"In my studies, we saw less sprouting and less tuber damage during storage in potatoes fertilized with Sulfate of Potash, or K2SO4," explains Essah. "Potatoes fertilized with Sulfate of Potash also maintained their weight better during storage. In addition, you have more tubers, and larger, more uniform tubers compared to [fertilizing with] potassium chloride (KCl). You have higher starch content with K2SO4 than KCl, and that's what the grower wants. Fertilizing with K2SO4 produces potatoes with higher specific gravity versus KCl.
"When you use K2SO4, you reduce external defects, the tubers are smoother and more appealing and of higher quality than with those fertilized with KCl. The major advantage is tuber quality, high specific gravity, high starch content, good french fries and good chips. In addition, tubers fertilized with Sulfate of Potash are better for the fresh market because they have fewer external defects. In most cases, you get more tubers and more premium grade tubers with K2SO4 than with KCl."
Essah says another reason to consider Sulfate of Potash is its low salt content. Research has shown that Sulfate of Potash increases specific gravity in potatoes compared to Muriate of Potash, which contains 47 percent chloride. Conversely, Sulfate of Potash has the lowest salt index per unit of K2O among all major sources of potassium. Because it contains less than 1 percent chloride, Sulfate of Potash produces potatoes with higher specific gravity levels, meaning more starch and less water in the harvested crop. That can significantly lessen storage losses due to shrink.
"The main reason Sulfate of Potash is superior to potassium chloride is the chloride in the potassium chloride," Essah says. "That chloride increases the salinity of the soil, so the soil health is reduced because the roots become hydrated. That increases the water content in the tuber, which decreases the specific gravity and thus the quality of the tubers. K2SO4 is highly recommended in highly saline soils and in any soil that is sulfur deficient."
Learn more about Essah's studies on potato fertilizer management and crop production at www.colostate.edu. To learn more about the benefits of using Sulfate of Potash and improving potato quality and quantity, visit www.textbookpotatoes.com. PG