Potatoes
Potatoes often serve as a test plant to compare various K forms. Although research results do vary across varieties and locations, it is generally accepted that SOP is the recommended and preferred K source to improve potato quality and it also serves as an excellent SO4-S source.

In one of the first comparative studies ever performed in the U.S. in 1956-57 at the University of Maine, Aroostook County Farm (Murphy and Goven, 1958), field trials indicated that Russet Burbank tubers grown with SOP had considerably more surface area russeted than tubers grown with KCl With KCl only 21 percent of the potatoes had russeting extending over more than one-half of the surface area of the tubers. In contrast, the corresponding proportion with SOP was 69 percent.

Research results vary across all varieties and locations where potatoes are grown. Generally, however, intensive production of potatoes shows that SOP is often the recommended K source causing less of a problem with tuber quality and also providing S (Roberts and McDole, 1985).

Both potato yield and market quality requirements for potatoes are used as guidelines for establishment of nutritional requirements. Selecting a low salt-index source of K for high yield potatoes can help to reduce the risk of salt injury and maintain high specific gravity in the tuber (Murphy and Goven, 1965; Rowberry and Ketcheson, 1978; McBride and Jackson, 1985).

Where higher starch and dry matter contents in tubers are desired, SOP seems to be the preferred source over MOP (Beringer et al, 1990). As shown in Table 1, the yield of tubers and tuber:shoot ratios were increased more by K2SO4 than the same rate of K applied as KCl The higher starch content of the tubers grown on SO4-treated soil was believed to be due to a retardation of the effects of Cl rather than from a response to S.

Table 1.
The influence of K source on potato yield and quality.

A 3-year study by Tindall and Westermann (1991) resulted in SOP producing greater yields than MOP for most comparable treatments. This effect is attributed to a Cl-NO3 antagonism in nutrient uptake and not to S deficiency.

Idaho research by McDole et al. (1978) illustrated the benefit of K fertilization on total yield and additional percentage of U.S. No. 1 tubers. Both SOP and MOP produced similar significant yield increases, but SOP had the additional advantage of having a higher tuber specific gravity than MOP. Recent research in Idaho confirms the advantages of SOP when fertilizing potatoes (Tindall and Westermann, 1995). The SOP application produced a higher percentage of larger tubers and a trend toward higher specific gravity (Table 2). The Idaho researchers recommend preplant K fertilizer applications where possible. Higher rates of K should be split into a fall and spring application. Growers who monitor K petiole concentrations should fertigate with K if concentrations drop below 7.2 percent in the fourth mature petiole. The total rate per individual injection should not exceed 30 lb K2O during tuber growth.

Table 2.
Effect of preplant broadcast MOP and SOP on potato tuber yields.

McBride and Jackson (1985) found that both SOP and MOP appreciably increased total yield and the percentage of tubers weighing over 6 ounces, the most marketable size. Also, both sources reduced the incidence of tuber hollow heart and brown center. They noted that specific gravity was higher with SOP compared to MOP, while the MOP treatment tended to increase yields more than
SOP.

Rykbost et al. (1991) conducted a 3-year nutrient experiment at the Klamath Experiment Station in southern Oregon evaluating various combinations of N, P, K and S. They concluded that the most economical fertilizer combination for the three years was 240 lb/A of N, 60 P205, 60 K2O and 40 S. Potassium sulfate was included in the blend when the treatment combination called for both K and
S.

Beringer et al. (1991) concluded from a MOP versus SOP study that SOP accelerated the development of tubers and resulted in higher total dry matter yields. They suggest that potatoes fertilized with SOP have greater phloem loading rates and faster translocation of assimilates than those fertilized with MOP.

Many acres of potatoes are grown for the chip industry. A 3-year study was conducted in Michigan at the Montcalm Research Station on high peat soils to determine the effect of K rate and source on chipping yield and quality on two varieties, Spartan Pearl and Norchip (Chase et al., 1990). At the 100 lb/A rate K, SOP produced greater yields and had a higher specific gravity than the MOP treatment. Results were similar for both varieties.

The role of K and SOP in reducing the incidence of scab, black rot and other diseases which attack potatoes has been a factor in the continued importance of this material in potato production.

A University of Wisconsin (Panique, 1998) 3-year study conducted at three locations with Russet Burbank and Atlantic potatoes showed that the K source was important with respect to the hydrometer values (specific gravity) and grade A tubers. SOP improved specific gravity readings and the percentage of US #1 potatoes.