Nitrogen (N)

The amount of N fertilizer for potato production varies with the intended market -- table-stock, chip-stock, fry-stock and seed-stock. Amount and application timings also varies with the variety grown requiring varietal specific fertilizer management. Published variety profiles are available for many commonly-grown potatoes. Varietal-specific petiole nitrate-N curves are used by most growers to guide  them during the season for N fertilization.

General recommendations given here for N fertilizer are based on residual soil nitrate-N levels. Soil samples should be taken from the 0 to 12-inch depth plus any additional depth of rooting for the specific variety being grown. The best way to determine levels is through soil testing. Leguminous crops, e.g., alfalfa, in the previous rotation, organic matter, and manure or other organic waste may affect N fertilization rate for potatoes. Manure and other organic waste may, however, carry potato pathogens, e.g. common scab, which can lower marketable quality of potatoes. Nitrogen in the presence of adequate phosphorus and potassium stimulates canopy growth, leaves and branches. It needs to be present from emergence to flowering to promote rapid canopy growth. The optimal pH range for nitrogen availability is 6.0 to 8.0. There is a gradual decrease in availability below and above this range.

Table 1 shows the effects of N levels on potato tuber yields (US#1 grade), dry matter content and sugar content of tubers. For table-stock, N fertilization is applied for yield because dry matter content is not a concern. Dry matter content is critical for potato chip production. Less N is applied for varieties going to this market. Ammonium nitrate effects dry matter content more than ammonium sulfate. In French frying, tuber size is most important with yield; so for this market, more N is usually applied but dry matter content is carefully monitored to avoid going too low. Longer season varieties usually need more N to maintain a longer vegetable growth period. Table 2 shows N fertilizer rate recommendations based on soil nitrate-N levels. Total N should be between 150 to 200 lb/acre but maybe higher or lower depending on the potato variety and the market.

DEFICIENCY and EXCESS SYMPTOMS (Tables 3 and 4):

Factors that influence the deficiencies and excessiveness of nutrients are: soil pH, availability, presence in relation to other nutrients, ion exchange, organic matter, etc. Tuber bulking is the  stage of potato growth and development at which the most nutrient uptake occurs. Over 90% of the nutrients taken up end up in the tubers. Visual identification of symptoms can be misleading as many are similar  to pathogenic symptoms. A chemical analysis of plant tissue is recommended when a deficiency or excess is suspected.

When potatoes are grown under deficient or excessive N, harvested tubers are smaller, have higher sugar levels, and lower starch content than desired. Tuber maturation is also affected. Without  proper N fertilization, plants are more susceptible to diseases. Excessive N can also delay tuber initiation and promote overgrowth of vines. The amount of N fertilizer for potato production varies with the  intended market -- table-stock, chip-stock, fry-stock and seed-stock. Amount and application timings also varies with the variety grown requiring varietal specific fertilizer management.

Nitrogen deficiency symptoms in the canopy are initially characterized by a general yellowing (chlorosis) of older lower leaves. These, in time, will turn brown (necrosis) and die. Young leaves tend to be green and yellow as they mature. Leaf veins stay green while the rest of the leaf turns yellow. Severe deficiency results in slow canopy growth even some stunting, an erect stature and small pale leaves. Tuber appearance doesn't seem affected but there are yield losses associated with very small tubers. Overly mature tubers are harvested and tubers are more susceptible to disease. Process quality is poor  due to high sugar and low dry matter contents.

Too much nitrogen results in poor root development. Leaves may roll and deform ("nutrient leaf roll"). Excess N around tuberization may delay tuber initiation and growth, and excess N  during mid to late bulking can delay tuber maturity and canopy senescence. Tubers are smaller than optimal but many are still marketable, however, there is a yield loss. Tubers are immature, and tend to bruise easily and be more susceptible to diseases. Dry matter decreases with increasing nitrogen and sugar levels increase. This is especially possible with application during bulking. In general, care is needed when applying N after full bloom. Phosphorus fertilizer application will help to improve quality (e.g., skin maturity and dry matter content) of tubers at harvest when potato crops have excessive nitrogen fertility  levels.

FERTILIZER PLACEMENT: BROADCAST OR BAND?

Studies on cut Russet Burbank seed-pieces in Idaho showed that ammonium sulfate (120-360 lb N/acre) banded six inches deep and six inches to the side resulted in more growth cracks and culls than when broadcasted and worked into the soil six to eight inches. Harvested tubers from fields in which the starter fertilizer was broadcasted had a higher percentage of US#1, greater yield of US#1 and more larger  US#1 tubers compared to banded application. However, in California, another group reported no difference in yield between band or broadcast application of ammonium sulfate (60-240 lb N/ac). In New York,  researchers reported that seed-piece contact with urea (150-300 lb N/acre) may delay emergence and early growth. Urea was banded in the furrow. Ammonium nitrate did not have adverse effects when in contact with  the seed-piece. The recommendation was to use an ammoniated fertilizer with mono ammonia phosphate (MAP). Placement is best two inches to the side and slightly below seed-piece. With this placement, urea also  did not injure the seed-piece. This recommendation also comes from research in Maine where no difference between broadcast and band application was observed with super-phosphate. In work done in Ohio using ammonium sulfate and sodium nitrate, injury to cut seed-pieces was observed when in contact with the fertilizer. Whole (single drop) seed-pieces were not affected.

The effect of contact with nitrogen in the soil seems to be related to water movement from the seed-piece or sprout and not due to some toxicity.  Injury is worst the more soluble the nitrogen or phosphorous form and the drier the soil.

Injury symptoms to watch out for:

1. Retardation of sprout growth,
2. Inhibition of soil moisture absorption by seed-piece (shriveling at cut surface)
3. Severe injury and possible death of sprouts growing into a fertilizer band (especially under drought conditions),
4. Prevention of wound healing of fresh cut surfaces by contact with fertilizer.

Injury symptoms after emergence due to contact:

1. Delayed sprout emergence
2. Weak, skinny plants
3. Poor stand due to seed rot or sprout death

Conclusion

There is no difference between broadcast and band application except in amount of fertilizer.  Use an ammoniated fertilizer (ammonium sulfate/nitrate/phosphate) and avoid urea if banding.  Place band to the side of the seedpiece (2 inches) and slightly below but never above. There is no need to apply more than 200 lb N/acre broadcast.

Resources

Pavlista, A.D. 1995. Potato Production Stages: Scheduling Key Practices. EC95-1249. University of Nebraska Cooperative Extension, Lincoln, NE.

Pavlista, A.D. and J.C. Ojala. 1997. Potatoes: Chip and French Fry Processing. In Processing Vegetables: Science and Technology, Eds. Smith, Cash, Nip and Hui. Technomics Publ. Co. Inc., Lancaster, PA.

Pavlista, A.D. and J.M. Blumenthal. 2000. Potatoes. In Nutrient Management of Agronomic Crops in Nebraska. Eds. R.B. Ferguson and K.M. De Groot, Publ. Univ. Nebraska Cooperative Extension (EC00-155), Lincoln, NE.

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