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Abstract: Field experiments were conducted over eleven site-years where five K rates (0, 93, 187, 280, and 373 kg K ha−1) as KC1 or K2SO4 were band-applied at planting to potato (Solanum tuberosum L. ). Significant yield increases up to 332 kg K ha−1 were observed in five of eleven site-years when soil test K ranged from 75 to 110 mg kg1. The increase in tuber yield was associated with an increase of tuber size (170 to 370 g) and above in the US#1A category. Lack of yield response at the other site-years may be due to the high soil test K (125 to 180 mg kg−1). Statistically significant differences in total tuber yield were not evident between the two sources of K fertilizer studied; however, there was a tendency for a significant rate x source interaction (p > 0.15) in five site-years where K2SO4 increased tuber yield more than KC1 at rates up to 280 kg K ha−1. Above this rate, tuber yield decreased for K2SO4 but remained stable for KC1. Based on the tuber yield data and initial soil test K from the controls of each site-year, data from this study suggest that 104 mg K kg−1 is a critical pre-plant soil test level. A reduction in specific gravity with increasing applied K was evident in most of the site-years of this study, although decreases were generally not as marked when K2SO4 was used. A significant decrease in hollow heart with increasing rate of K fertilization was observed in four of eleven site-years; however, statistically significant yield responses to added K were found at only one of these sites. The incidence ofRhizoctonia solani was generally not affected by K rate; however, there was a tendency in some site-years for a higher disease incidence when KC1 was used instead of K2SO4 Potassium rate slightly decreased stem numbers per seed piece, averaging 3.7, 3.6, 3.5, 3.4, and 3.3 across all experiments, for the 0, 93, 187, 280, and 373 kg K ha−1 rates, respectively.

Potato tubers from field plots of calcium experiments in 1983 and 1984 were examined for the presence of internal brown spot. Presence of sub-apical necrosis on sprouts was also determined after storage in both years. Peel calcium concentration of 1983 tubers ranged from 0.226% to 0.130%; the range in percentage of these tubers with internal brown spot after 7 months storage was from 5.0% to 42.5%. The incidence of internal brown spot as well as sub-apical necrosis of sprouts was negatively correlated with tuber peel calcium levels (r = −0.754 and r = −0.792, respectively). Peel calcium concentration of 1984 tubers ranged from 0.224% to 0.104%; as in 1983, tuber calcium concentration was negatively correlated with the incidence of internal brown spot and sub-apical necrosis. Sub-apical necrosis of sprouts was present on 86.3% of the 1984 tubers with internal brown spot, and 65.3% of tubers with sub-apical necrosis on sprouts showed internal brown spot symptoms. The presence of sub-apical necrosis on tuber sprouts may serve as an indirect indicator of low calcium concentration in tubers.

Abstract: On low cation exchange capacity, sandy soils where soil test Ca is low, potato tuber (Solanum tuberosum L.) Ca levels may not be optimal for maximum yield or quality; therefore, this study evaluated the effect of several levels of applied Ca on potato yield and quality on four soil types in Wisconsin. Calcium was applied to a Plainfield loamy sand, a Keiner loamy sand, a Pence sandy loam, and an Antigo silt loam at five rates in 1984 (0, 84, 252, 420, and 588 kg Ca ha−1) and at six rates in 1985 (0, 56, 112, 168, 224, and 336 kg Ca ha−1) as a band of CaSO4 in the hill. In 1984, on the Plainfield soil, three rates of sidedress Ca(NO3)2 (0, 84, and 168 kg Ca ha−1) were factorially combined with the five CaSO4 treatments to evaluate the efficacy of Ca(NO3)2 as a concomitant Ca and N source. In general, total yield was not affected by CaSO4 additions; however, the addition of approximately 100 kg Ca ha−1 as Ca(NO3)2 increased total yield 3.6 Mg ha−1. On the low-CEC, low-Ca soils (Plainfield and Kelner), improved tuber grade and/ or US1A size grade were detected with the addition of up to approximately 300 kg Ca ha−1 as CaSO4. The percent of US1A tubers increased 5 to 10% and the yield of US1A prime size (170–370 g) tubers increased 3.4 to 8.4 Mg ha−1 above control values. These improvements were accompanied by increased periderm Ca concentrations, even though leaflet Ca concentrations were not affected by Ca applications. The use of 100 kg Ca ha−1 asCa(NO3)2 in combination with CaSO4 was more effective than CaSO4 alone. On higher soil test Ca soils (Pence and Antigo), responses were inconsistent and periderm Ca was generally not increased.