Sulfate of Potash

Minimizing Storage Loss

Published in the January 2013 Issue Published online: Jan 25, 2013
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Potato storage losses in the U.S. have averaged about 7.5 percent over the past several years, according to the National Potato Council's Statistical Yearbook. To put that in perspective, if 7.5 percent of the 2011 potato harvest (427,406,000 cwt) was lost, that would be about 3.2 million cwt of potatoes.

"Mean Number Chart. This data was compiled by Dr. Samuel Essah at Colorado State University. It shows how potatoes treated with Sulfate of Potash had fewer sprouts per tuber after almost 200 days in storage. Essah's data showed that using Sulfate of Potash versus other sources of potassium improved not only yields but crop quality in terms of several key storage parameters, such as higher specific gravity and fewer external defects.
"Figure 1. Effect of source of potassium fertilizer application averaged over form of application on percent tuber weight loss of Rio Grande Russet potato in storage.
"Figure 1. Effect of source of potassium fertilizer application averaged over form of application on percent tuber weight loss of Rio Grande Russet potato in storage.
"Colorful varvar ieties. These varieties-reds, whites and yellows-were grown in southern Kern County, Calif., and all had Wolf T Trax DD DD DDP Micronutrients added to the dry pre-plant fertilizers. All micronutrients were added for crop removal rates based on tons per acre.

Postharvest losses can be due to a variety of things, ranging from fungal diseases, bruising, in-storage shrink or sprouting, to name a few. But because more than 70 percent of the total fall U.S. potato crop is placed into storage for year-round use, it's essential to do everything possible to minimize storage loss for fresh, process and chip markets.

Key Nutrients

Proper nutrient management of the potato crop can have a significant influence on how well potatoes endure storage with minimal damage and loss. Two key nutrients in your fertilization program are potassium and sulfur. It's important not only to supply these nutrients to the plant, but to choose the right source among many choices available, to optimize plant health and potato yield, quality, taste, color and texture.

University research has shown that potassium is vitally important to the production of top-quality potatoes-and potatoes demand a significant amount of the nutrient. Potassium is taken up by potato plants more than any other nutrient. A 500 cwt crop removes as much as 280 lbs. of potassium per acre annually. If that nutrient is not replenished, future crops can suffer in quality and lack of uniformity in terms of yield, specific gravity, storage capability and increased external defects.

Why is potassium important in potato production? According to the Potash Development Association (PDA), potassium is involved in regulating the amount of water in the plant and helps plants withstand water stress during periods of drought. Potassium also plays a key role in maintaining the turgidity of plant cells and in the movement and conversion of sugars into starches within the tuber. In addition to boosting potato yields, potassium also contributes to crop quality by reducing blackspot bruising, increasing specific gravity and improving chip and fry color.

Timing of fertilization is critical as well. Potassium is taken up by potato plants more than any other nutrient. During the six weeks after plant emergence, potatoes take up two-thirds of the season's potassium requirement.

Often referred to as the "fourth macronutrient" in potato production, sulfur is critically important in the production of amino acids and protein, which creates the photosynthetic energy needed to build starch in the tuber. This process is vital during tuber initiation and bulking. A study by Dr. Neal Kinsey and Dr. Charles Walters found that, "Based upon years of cumulative experience with sulfur testing, almost every soil tested does not have enough sulfur to produce top yields unless it is specifically added as part of the fertility program."

Sulfate of Potash

There are several sources of potassium and sulfur to choose from in potato production, but beware-some contain unwanted elements such as chloride or excess salts. These elements can build up in the soil and contribute to lower yields, lower crop quality and lower profitability. Chloride can also decrease the specific gravity levels in potatoes, which lowers crop quality in terms of chip and fry color. Lower specific gravity levels can also lead to more storage losses due to shrinkage.

Chemical analysis shows that Sulfate of Potash contains the most efficient balance of nutrients to optimize potato yields, quality and storability. Sulfate of Potash is the richest source of low-chloride potassium, providing equivalent potassium content of 50 percent K2O, commonly referred to as potash.

In addition, Sulfate of Potash is a 100 percent natural source of potassium. Research has shown that Sulfate of Potash increases specific gravity in potatoes compared to Muriate of Potash, which contains 47 percent chloride. The high chloride level of Muriate of Potash impacts overall quality and storability. Because it contains less than one 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.

Sulfate of Potash has the lowest salt index per unit of K2O among all major sources of potassium. This helps avoid many of the problems that salt poses to crop development and soil systems, including poor germination, nutritional imbalances, seedling injury, leaf burn, stunted root and shoot growth, reduced soil structure and decreased microbial activity.

Recent work by Colorado State University has shown that using Sulfate of Potash versus Muriate of Potash not only improved yield, but resulted in improved crop quality in terms of several key storage quality parameters such as delayed sprouting, higher specific gravity and fewer external defects.

Sulfate of Potash is 17 percent sulfur, in readily available sulfate form. It's important to provide sulfur in this form-plant roots can only absorb sulfur when it occurs as sulfate. The conversion of elemental and organic sulfur can take weeks or even months.