by J.D. Davis, R.D. Davidson and S.Y.C. Essah* (9/14)
- Proper nitrogen management is one of the most important practices for high-yielding, high-quality potato production.
- Apply nitrogen fertilizers at rates based on expected crop yields minus credits for residual soil nitrates, nitrogen mineralized from soil organic matter, previous legume crop residues and manure, and nitrate-nitrogen present in irrigation water.
- Apply phosphate fertilizers at rates based on soil test results.
- Most Colorado soils contain sufficient potassium for potato production.
Adequate soil fertility is one of the requirements for profitable potato production. Potatoes are mainly grown on sandy soils, and nitrogen (N) is the most yield-limiting nutrient. Phosphorus (P) is the next most limiting nutrient, while zinc (Zn) and occasionally iron (Fe) may also be deficient in some Colorado soils.
Use cultivar-specific management practices whenever possible because potato cultivars vary significantly in root density, maturity, and their response to timing of N applications and environmental conditions. Optimum yield and quality should result from using such practices.
The value of a soil test to predict nutrient availability during the growing season depends on how well the sample collected represents the area sampled. Take surface samples to determine available nutrients to a depth of 1 foot. A good sample is a composite of 15 to 20 soil cores taken from an area uniform in soil type. Sample areas with major differences in soil properties or management practices separately.
Thoroughly air dry all soil samples within 12 hours after collection by spreading the soil on any clean surface where the soil will not be contaminated. Do not oven dry the soil because this can change soil test results. Place the air-dried soil in a clean sample container for shipment to the soil test laboratory. Carefully complete the laboratory’s information form and send it in with the soil sample. This form provides information so fertilizer application suggestions can be tailored to your specific situation. Take soil samples for NO3-N analysis every year for optimum N fertilization of crops. To analyze the availability of the other nutrients, pH, and organic matter content, it may be sufficient to take samples every three to four years.
For more detailed explanations of the importance of taking proper soil samples, see or contact the Colorado State University Soil, Water and Plant Testing Laboratory is in Room A319, Natural and Environmental Sciences Building, Colorado State University, Fort Collins, CO 80523; (970) 491-5061; www.soiltestinglab.colostate.edu.
Base nitrogen rates for potatoes on known cultivar requirements for the expected yield on each field. Give credit for the level of available NO3-N in the soil, as determined by soil tests. Other credits for N include the amounts expected to become available during the season from mineralization of soil organic matter, previous legume crops (see Table 1),
application of animal manure, and NO3-N in irrigation water. Subtract these credits from the total N needs to determine the suggested N fertilizer rate for the expected yield.
|Table 1: Nitrogen credits for previous legume crops.|
|Legume crop||lb N/A credit*|
|Alfalfa > 80% stand
60 – 80% stand
0 – 60% standDry beans
|100 – 140
60 – 100
0 – 60
|*For the second year, use 1/2 of the first year N credit.|
Other factors that affect N rates are cultivar, plant population, planting and harvesting dates, crop residues incorporated into the soil, soil type, and leaching losses from irrigation.
Irrigation water may contain NO3-N which is available to plants. The amount of N contained in one acre-foot of irrigation water is 2.7 pounds of nitrogen for each ppm of NO3-N. However, subtract only the NO3-N in irrigation water applied before tuberization.
Table 2 suggests N rates for potatoes grown in eastern Colorado for an expected yield of 400 cwt/A. Fertilizer rates decrease with increasing levels of NO3-N the top foot of soil. Suggested N rates in this table do not account for the other N credits listed above. Subtract these credits from the N rates in Table 2 to determine the N rate for the field.
|Table 2: Suggested nitrogen rates for potatoes in eastern Colorado, as related to nitrate-nitrogen in the soil (expected yield, 400 cwt/A).|
|ppm NO3-N in soil*||Fertilizer rate, lb N/A|
|0 – 18||180|
|19 – 24||170|
|25 – 30||160|
|31 – 36||150|
|*Concentration of NO3-N in the surface 0 – 1 foot sample depth.
-Subtract 30 lb of N/A for each percent soil organic matter above 1.0%.
-See Table 1 for N credits from previous legume Crops.
-To adjust N rate for expected yields from 300 to 400 cwt/A, subtract 30 lb of N/A for each 50 cwt/A below 400 cwt/A..
Suggested N rates for the primary cultivars of potatoes grown in the San Luis Valley are given in Table 3. This table gives the total suggested N rate at an expected yield of 400 cwt/A. The suggested N rates for preplant N and subsequent applications through sprinkler systems during the growing season also are included. Suggested N rates in this table do not account for the N credits discussed above. Subtract these credits from the N rates in Table 3 to determine the N rate for the field. The usual rate of N applied through sprinkler systems is no more than 20 pounds per acre per application.
|Table 3: Suggested nitrogen rates for split application to potatoes in the San Luis Valley, according to cultivar (expected
yield, 400 cwt/A).
|Cultivar||Fertilizer rate, lb N/A|
|Total rate||Pre-plant||Sprinkler applied|
|Atlantic||180||80 – 90||90 – 100|
|Centennial Russet||190||90 – 100||80 – 100|
|Chipeta||140||60 – 70||60 – 80|
|Ranger Russet||170||70 – 90||80 – 100|
|Russet Burbank||200||90 – 110||80 – 100|
|Russet Norkotah Sel 3||160||80 – 90||40 – 70|
|Russet Nugget||130||60 – 80||60 – 70|
|Sangre||150||90 – 100||40 – 60|
|Rio Grande Russet||170||80 – 90||70 – 80|
|Canela Russet||160||80 – 90||70 – 80|
|– Subtract 8 lb of N/A for each ppm of soil NO3-N above 5 ppm in soil prior to planting.
– Subtract 10 lb of N/A for each percent soil organic matter above 1.0 percent.
– See Table 1 for N credits from previous legume crops.
– To adjust N rates for expected yields from 300 to 400 cwt/A, subtract 30 lb of N/A for each 50 cwt/A below 400 cwt/A.
Methods and Timing of N Applications
Proper N management is the most important practice needed to obtain high yields of high quality potatoes. The N supply early in the season must be adequate for vegetative growth. However, excessive levels of soil N before or at tuberization can delay tuber initiation, reduce yields and decrease specific gravity in some cultivars.Cultivars, like Rio Grande Russet are extremely sensitive to excessive early season N applications. Growers must know the specific cultivar characteristics prior to applying early season N fertilizer. In addition, excessive N in late summer and early fall can delay maturity of the tubers and result in poor skin set, which can adversely affect tuber quality and storage characteristics.
Nitrogen needs of potatoes are best met by split applications of N fertilizers during the vegetative period. This involves applying some of the N fertilizer pre-plant or at planting, with the remainder of the crop’s N needs applied with irrigation water. However, apply all of the fertilizer before July 31 to avoid delaying tuber maturation. Some N may be band applied in combination with starter fertilizers, but the rate should be less than 40 pounds of N per acre if urea or diammonium phosphate (DAP) are applied.
Anhydrous ammonia is not suggested for application to potatoes under San Luis Valley conditions. Apparently, the rate of nitrification is decreased because of cooler soil temperatures, so NH4-N continues to be converted to NO3-N later in the growing season.
Potato roots quickly grow into the soil between the rows. Sidedress or topdress N fertilizers early in the growing season to avoid root pruning.
Application of N fertilizers with irrigation water is a convenient method and allows split applications to improve potato yields and quality, as well as optimizing N-use efficiency. Use in-season soil or petiole analysis to determine the N status of the growing crop. If the N status is low or growing conditions appear above average, apply additional N with the next irrigation. The maximum amount of N to apply with each irrigation is 20 pounds of N per acre.
Nitrogen fertilizers may be applied through sprinkler irrigation systems. All closed irrigation systems must be equipped with backflow prevention valves if N fertilizers or other agrichemicals are applied through the system. Apply N fertilizers in furrow irrigation systems only in fields where a tailwater recovery and reuse system is in place. However, application of N fertilizers in furrow irrigation water may not result in uniform application of N, so this method is not suggested.
Crop responses to applied P are most likely on soils with low or medium levels of extractable P, although lower P rates may be effective for potatoes on San Luis Valley soils high in extractable P because of cool soil temperatures in the spring. The main soil tests for extractable P in Colorado soils are the AB-DTPA and sodium bicarbonate (NaHCO3) also known as Olsen tests. Values for both tests are given in Table 4. Suggested P fertilizer rates are for preplant application related to soil test levels. Broadcast and incorporate high P rates with a portion band applied as starter fertilizer. Most growers in the San Luis Valley band apply most or all of the required P fertilizer to minimize P fixation in the soil.
|Table 4: Suggested phosphorus rates for potatoes (expected yield, 400 cwt/A).|
|ppm P in soil||Relative level||Fertilizer rate, lb. P2O5/A|
|0 – 3||0 – 6||very low||240|
|4 – 7||7 – 14||low||180|
|8 – 11||15 – 22||medium||120|
|> 11||> 22||high||60|
|NOTE: High P rates should be applied broadcast preplant, with a portion band-applied as a starter fertilizer.|
Placement of P fertilizers in the root zone is important because P is not mobile in soil. Broadcast incorporated applications are effective on low-P soils because broadcasting provides a greater probability for roots to come in contact with P fertilizer, so absorption of fertilizer P is greater. However, broadcast P fertilizers may be fixed rapidly in high pH, high lime soils causing some of the applied P to quickly becomes unavailable to plants. Band application at planting (starter fertilizer) is the most efficient placement method for P. Place ammonium phosphates as starter fertilizers below and to the side of the seed piece at planting, and rates should not exceed 40 pounds ofN per acre.
Most Colorado soils are relatively high in extractable K, and few crop responses to K fertilizers have been reported. Suggested K rates related to soil test values (AB-DTPA or NH4OAc) are given in Table 5. The main K fertilizer is KCl (potash). Broadcast application tilled into the soil prior to planting is the usual method. Use of KCl instead of K2SO4 may decrease specific gravity of potatoes.
|Table 5: Suggested potassium rates for potatoes (expected yield, 400 cwt/A).|
|ppm K in soil
AB-DTPA or NH4OAc
|Relative level||Fertilizer rate, lb. K2O/A|
|0 – 60||low||160|
|61 – 120||medium||80|
|121 – 180||high||40|
|> 180||very high||0|
The availability of soil Zn decreases with increasing soil pH, and most Zn deficiencies are reported on soils with pH levels higher than 7.0. Zinc deficiencies also are found on soils leveled for irrigation where the subsoil is exposed, on soils with very high levels of free lime, sandy soils, or soils low in organic matter.
Suggested Zn fertilizer rates in Table 6 for band applications are listed for use of zinc sulphate or Zn chelates, such as zinc EDTA. Band application of Zn fertilizers with starter fertilizers is more effective than broadcast application. Soil test values for extractable Zn by the DTPA soil test are similar to those by the AB-DTPA test shown in Table 6. Zinc fertilizers have measurable residual effects, and repeated annual applications will result in a buildup of extractable Zn. As soil test Zn increases to higher levels, decrease Zn rates according to test results.
|Table 6: Suggested zinc rates for potatoes.|
|ppm Zn in soil||Relative level||Fertilizer rate, lb. Zn/A|
|zinc sulfate||zinc EDTA|
|0 – 0.9||low||10||4|
|1.0 – 1.5||marginal||5||2|
|NOTE: Suggested Zn rates are for band application with starter fertilizers.|
Zinc deficiencies also may be corrected by foliar sprays of a 0.5 percent ZnSO4 solution applied at a rate of 20 to 30 gallons per acre, but several spray applications may be necessary. However, it is difficult to prepare this solution in the field, so ZnEDTA or other soluble Zn sources can be used. A surfactant (wetting agent) increases plant absorption of the applied Zn.
Iron deficiencies (chlorosis) are most likely to occur on highly calcareous soils (pH higher than 7.8) or on soils leveled for irrigation where the subsoil has been exposed. The Centennial Russet cultivar is more susceptible to Fe chlorosis than other cultivars. Foliar spray applications (Table 7) of a 1 percent FeSO4 solution at 20 to 30 gallons per acre are not always completely effective in correcting chlorosis, and several applications may be necessary.
Because FeSO4 solutions are difficult to prepare in the field, other Fe sources may be used. Inclusion of urea and a detergent increase effectiveness of applied Fe. Soil applications of most fertilizers generally are not effective.
|Table 7: Suggested iron spray applications for potatoes.|
|ppm Fe in soil
|Relative level||Spray application notes|
|0 – 3.0||low||Likely to be beneficial|
|3.1 – 5.0||marginal||May or may not be beneficial|
|> 5.0||adequate||Response not likely|
|NOTE: Soil applications of most Fe fertilizers are not effective.|
Most Colorado soils contain adequate levels of available sulphur (S), and soil tests for available S are not routinely performed. However, some sandy soils may require S applications. Irrigation water from most surface water and some wells often contains appreciable SO4-S, so irrigated soils usually are adequately supplied with S. However, some deep well waters are low in S, so analyze water samples for SO4-S if soils are low in organic matter and you suspect S deficiency.
There have been no confirmed deficiencies of boron (B), copper (Cu), manganese (Mn), or molybdenum (Mo) in potatoes in Colorado.
*J. G. Davis, Colorado State University Extension soil specialist and professor, soil and crop sciences; R.D. Davidson, Extension seed potato specialist and assistant professor, horticulture and landscape architecture, San Luis Valley Research Center; and S.Y.C. Essah, potato physiologist and assistant professor, horticulture and landscape architecture, San Luis Valley Research Center. Original authors included J.J. Mortvedt, former Colorado State University Extension soils specialist; P.N. Soltanpour, former professor, soil and crop sciences; and R.T. Zink, former Extension potato specialist. 3/96. Revised 9/14.
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