MICRONUTRIENTS
Potatoes do require small amounts of various micronutrients but soils used for potato production are rarely deficient in these. Some zinc (Zn), up to 5 lb/acre, is occasionally added. Boron (B) or magnesium (Mg) on occasion may be added, 1-3 lb/acre, when leaf stripping is seen. Calcium is often added to improve potato tuber quality.
BORON (B)
The function of boron in plants is not well understood. It is involved in carbohydrate transport, and cell membrane and cell wall development.
Deficiency is very rare in crops. The main deficiency symptom on vines is bushiness. This results from death of the growing points and growth of lateral buds. Branching occurs due to loss of apical dominance. Leaves may exhibit leaf roll like symptoms. Roots are short or stunted, and thick.
Tubers are small with surface cracking. Localized brown areas appear under skin near stolon end. There's brown vascular discoloration.
Even small amounts of added boron can be toxic to many varieties. It is normally associated with arid and semiarid environments, or where municipal compost is applied. Symptoms are a yellowing (chlorosis) of mature leaves and on margins of young ones. The yellowing can turn brown (necrosis).
The optimal pH range for boron availability is 5.0 to 7.0 (slightly acid). There is a gradual decrease as pH becomes less than 5.0, more acid, and also as pH increases from 7.0 to 8.5, becomes alkaline. Between soil pH 8.5 and 8.75, availability of boron increases rapidly with increased alkalinity. Then, above pH 8.75, there is no further change in boron availability.
Summary of main symptoms for BORON:
VINE -- bushy, leaf roll, short roots;
TUBER -- small, vascular discoloration;
EXCESS -- toxic, leaf browning.
CALCIUM (Ca)
This nutrient is key in cell wall strength and cellular adherence.
Deficiency in potatoes is most severe in acid (pH<5) sandy soil. Vine symptoms include spindly stems and small, upward rolling, crinkled leaflets. Leaflets have yellow margins that may turn brown. In severe cases, leaves are wrinkled and have a rosette appearance, and roots don't grow.
Tubers have vascular discoloration near the stolon end and flecks in the core. Calcium deficiency has been associated with some physiological disorders such as internal heat necrosis. Severe deficiency causes extremely small tubers to form. In seed tubers, multiple lateral sprouts and internal sprouting may develop.
No excess symptoms are reported.
The optimal pH range for calcium availability is 7.0 to 8.5. There is a gradual decrease as pH becomes less than 7.0, acidifies, and also as pH increases above 8.5, more alkaline. This pH affect on availability of calcium is the same as for magnesium.
Summary of main symptoms for CALCIUM:
VINE -- spindly, small rolled and yellow leaves, short roots;
TUBER -- vascular discoloration, heat necrosis, hollow heart (possible);
EXCESS -- none.
Relating calcium to cementing (click here) or to crusting (click here).
IRON (FE)
Iron plays a function in photosynthesis. It does not seem to have an adverse affect on yield or quality even when plants show severe deficiency.
Iron is highly insoluble in water and, when applied foliage, must be in a chelated form. Deficiency is usually observed in water-logged soils and in alkaline soils. The degree that symptoms appear varies greatly between potato cultivars; some show very little such as Red Pontiac while others show a considerable amount such as Red Norland. Symptoms appear on the young foliage. Leaves begin with inter-veinal chlorosis similar to other micronutrient deficiencies. Necrosis does not appear. Iron-deficiency may appear first on the base of leaflets and expand to the whole leaflet and leaf. Leaf tips and edges, and veins remain green.
No excess symptoms are reported.
The optimal pH range for iron availability is below 6.25 (acid). There is a gradual decrease in availability as pH increases until about pH 8.25 above which pH has little effect.
Summary of main symptoms for IRON:
VINE -- inter-veinal yellowing to whitening with no effect on leaf margins;
TUBER -- none;
EXCESS -- none.
Relating iron to cementing (click here) or to crusting (click here).
MAGNESIUM (Mg)
An essential part of chlorophyll (much like iron to hemoglobin in blood), magnesium is a part of energy transfer.
Magnesium is highly water soluble and leaches readily. Therefore, deficiency is more common than other micro-nutrients. Readily-drained, acid, sandy soils are most prone to deficiency. Symptoms occur on older leaves which appear pale, light green beginning at the tip, moving to margins and then between veins. Yellowing becomes most severe at center of leaflet. Leaves become thick and brittle, and roll upward. Roots become stunted. High potassium lowers magnesium uptake and increases deficiency.
No excess symptoms are reported.
The optimal pH range for magnesium availability is 7.0 to 8.5. There is a gradual decrease as pH becomes less than 7.0, acidifies, and also as pH increases above 8.5, more alkaline. This pH affect on availability of magnesium is the same as for calcium.
Summary of main symptoms for MAGNESIUM:
VINE -- inter-veinal yellowing of older leaves, leaf thickening, leaf brittleness, short roots;
TUBER -- none;
EXCESS -- none.
MANGANESE (Mn)
Manganese activates many enzymes including metabolism, energy transport and fatty acid synthesis.
Deficiency is rare and occurs on alkaline (pH>8) calcareous soil. Young leaves (upper canopy) lack luster and are light green between the veins that turns to yellow and then to white. Older, lower leaves are least affected. In severe deficiency, brown spots may appear along veins of younger leaves.
Excess manganese occurs on light acid (pH<5) soils due to the nutrient's solubility. Potato is very sensitive. Stem streaking is the main symptom. Early toxicity symptom is a dark flecking on stems and petioles starting at the base of the vine and moving up. Symptoms progress fast after flowering and stems turn dark brown and become brittle. Plants are stunted and leaves are pale yellow-green with yellowing between veins and brown margins. Stems become dry, hang and break off. Plants die early.
The optimal pH range for manganese availability is 5.0 to 6.5 (slightly acid). There is a gradual decrease as pH becomes less than 5.0, more acid, and a very gradual decrease as pH increases above 6.5.
Summary of main symptoms for MANGANESE:
VINE -- inter-veinal yellowing to whitening;
TUBER -- none;
EXCESS -- stem streaking then brittleness, vine stunting, leaf yellowing.
ZINC (Zn)
Zinc as manganese activates many enzymes. It is also essential for synthesis of auxin, a key hormone controlling cell growth.
Deficiency results usually on alkaline soils or in the presence of excessive phosphorus. Symptoms include stunting, brown spots on stems and petioles, and leaf malformation. Young leaves roll up as with leaf roll virus and have a tip burn. The leaf roll is called "fern leaf" and is upward and cupping; leaves are also thick, brittle and puckered. Older leaves have gray-brown areas and bronzing along margins.
Application of zinc containing fungicides readily prevents deficiency.
No excess symptoms are reported.
The optimal pH range for zinc availability is 5.0 to 7.0 (slightly acid). There is a gradual decrease as pH becomes less than 5.0, more acid, and also as pH increases from 7.0 to 8.5, becomes alkaline. Availability of zinc remains unchanged above pH 8.5.
Summary of main symptoms for ZINC:
VINE -- stunting, spottiness on stem, leaf mis-shaping with tip burn and brittleness;
TUBER -- none;
EXCESS -- none.