Most soils cropped to potatoes contain large amounts of potassium (K). The intended market use of the harvested tubers influences application this nutrient more than N or P. K promotes larger sizing of potato tubers solely by increasing water accumulation in tubers resulting in a lowering of dry matter content. This lowering in the percent dry matter (specific gravity) can eliminate tubers from the chipping market. However, percent dry matter is less important for most fry-stock varieties for which tuber length and blocky shape, > 12 oz weight, are premium. For table-stock, dry matter content is unimportant. For these two markets, more K may be desired. For seed-stock, the target is to keep tubers smaller, 4-8 oz, than the other markets so less K is desired. K is most available in soils with pH greater than 6.0; availability gradually decreases as pH decreases below this level.
Table 1 shows the effects of K levels on potato tuber yields (US#1 grade), dry matter content and sugar content of tubers. For table-stock and fry-stock, as with P fertilization, K fertilization is applied to increase tuber size grades. Dry matter content is decreased most by potassium chloride then by potassium nitrate and even less by potassium sulfate. Table 2 shows suggested K fertilizer rates based on soil K test levels.
DEFICIENCY and EXCESS SYMPTOMS (Tables 3 and 4):
Deficit of K is most likely in leaching soil types especially sandy soils. Early symptoms of K deficiency are a dark greening or bluish greening of foliage. Leaves appear glossy. Tiny (1/25th inch), light green spots develop between the veins of larger leaves. In the upper canopy, leaf margins curl down and leaflets are small, cupped and crowded. They become crinkled and bronzed on their upper surface, and the lower surface has brown speckles, superficially similar to early blight. Older leaves turn bronze then brown (necrotic) and die early. The key symptom is the overall bronzing of the canopy. A severe deficiency results in short plants with shortened internodes, poor root growth and shortened stolons. The stem end of tubers harvested from K deficient plants have small (about 1/10th inch) sunken lesions that, upon drying, hollow out, surrounded by corky tissue. Tubers are predisposed to black spot bruising and are disease-susceptible. Sugar levels are high and dry matter low.
Excess K primarily affects dry matter content, specific gravity, in tubers since K stimulates water accumulation in tubers. Potassium fertilizer application influences the usability of tubers for French fry and potato chip processing differently. For French fries and table-stock, excessive K levels results in most of the tubers to be useable. While, for potato chips, it may result in rejection due to low specific gravity, too much water in tubers. These differences are related to the relative importance of the tuber size and dry matter content. Potato chip processors desire a higher dry matter content while French fry processors and the fresh market want longer tubers and count-cartons, respectively.