FERTILIZATION OF POTATOES

NITROGEN

Total Nitrogen Requirement = Coefficient x Yield Goal in Cwt./Acre

Potatoes are different than most crops in that varieties differ greatly in their nitrogen requirements. Long season varieties tend to need more nitrogen than short season varieties, but there are other differences that a just due to the variety in question. Below is a table of various coefficients for some varieties. Varieties also differ greatly on when you should apply the nitrogen.

Variety or Cultivar	Nitrogen Coefficient
Norkotah Standard	0.53
Norkotah selection 3	0.31
Norkotah selection 8, TX112, TX278, TX223	0.40
Nugget	0.31
Centennial	0.40
Russet Burbank	0.55
Sangre and other Reds	0.30
Century	0.28
Alphas	0.37
Crestone	0.28
Yukon Golds	0.36
Misc. Long Season	0.45
Misc. Short Season	0.40

Potatoes as a rule are not good foragers of nitrogen in the soil. Normally one would use a nitrogen budget to subtract soil available nitrogen, but in most cool areas if you depend on potatoes to extract nitrogen form soil residues, you'll be sorry. Its best to apply everything the potato needs to survive. In addition, usually rotation crops with potatoes such as small grains leave very little nitrogen in the soil in any case. In warm areas with varieties with large root systems, it may work better to subtract ½ of the available soil nitrate from the total nitrogen needs, but do this with caution. Also irrigation water nitrates are inefficiently used by a potato crop so don't expect much help from the water, although salty water can very significantly hurt a potato crop's yield.

Nitrogen Timing: Usually ½ to 3/4 of the total nitrogen goes on at planting, but there are exceptions. Some varieties like russet nugget and russet burbank are sensitive to early nitrogen (it delays the tuber set) and these much less can be applied early. Usually all the phosphorus is applied at this point where it can be incorporated well. Banding the pre-plant is common in the San Luis valley of Colorado since it greatly improves the efficiency of phosphorus availability in our phosphate fixing soils. I prefer 2 liquid fertilizer bands to each side of the seed piece and a little below. Some dry fertilizer forms can be harmful in large amounts needed in potato production. This is especially true of urea, never use urea in potato production - your yields will suffer. Total amount of nitrogen in the pre-plant bands should not exceed 110 unit/acre or salt problems will occur. In varieties that need high early amounts of nitrogen, extra nitrogen can be applied in dry form right before hilling and incorporated in the hilling operation. After the pre-plant and topdress applications at hilling, additional nitrogen can and should be applied through the irrigation water. These fertigations allow nitrogen to be spoon fed to potatoes mid-season when there needs are high. Some varieties, however, with problems in skin set, it is very important to stop nitrogen applications early to insure a mature tuber at harvest. Thus, in some varieties no injected nitrogen through the sprinkler is recommended. The timing of the nitrogen injections optimal can vary with variety and this is noted in the following table. Each variety and farm has to experiment to find the right fertilizer plan. Monitoring yields and potato petiole analysis can be used to refine these fertility plans. The table below summarizes some fertilizer plans for various varieties. These are based on a 450 cwt./Acre yield goal with good irrigation efficiencies so little nitrogen is leached below the root zone.

Variety or Cultivar	Pre-Plant Nitrogen	Nitrogen at Hilling	Injected Nitrogen
Norkotah Standard	100	40	80 -- 100 (spread out)
Norkotah selection 3	80	40	20 (mid-season)
Norkotah selection 8, TX112, TX278, TX223	100	40	40 (mid-season)
Nugget	80	0	60 (late)
Centennial	80	20	80 (mid-season)
Russet Burbank	100	0	140 (after tuber set)
Sangre and other Reds	100	40	0
Century	100	40	0
Alphas	100	20	40 - 60 (mid-season)
Crestone	100	20	0
Yukon Golds	100	40	20 (early)
Misc. Long Season	100	40	60-80 (mid-season)
Misc. Short Season	100	20	40 (mid-season)

Use a soil sufficiency method to determine any needed phosphorus:

Melich test results ppm	Olsen Sodium Bicarbonate results ppm	Phosphate Requirement in Units/A P2O5
Above 70	Above 35	180
50 - 70	26 - 32	200
40 - 50	21 - 26	200
30 - 40	15 - 21	220
20 - 30	10 - 15	240
10 - 20	5 - 10	260

Phosphorus availability varies a lot in SLV soils. Soils high in excess lime and with high pH values fix the phosphorus rapidly. These conditions also make it highly recommendable to band the phosphorus applied pre-plant. Adjustments should be made to the above recommendations as indicated:

-Soils with pH's of above 8.9 add 50 units/A to the above amounts.

-Soils with pH's of 8.5 to 8.9 add 30 units/A to the above amounts

-Soils with pH's of 8.0 to 8.5 add 10 units/A to the above amounts

-Soils with excess Calcium (excess lime) add 30 Lbs./A to the above amounts.

-On heavy soils add 20 units./A

Most SLV soils have sufficient potassium levels for potato crops but areas near Center, in the Carmel area and the west end of the valley may need some additional potash applied. Use the following table to determine amounts needed:

K test value in ppm	less than 100	100-150	150-250	250-350	350-
K needed in Units./A	180	150	120	60	30

Potatoes are not a heavy user of sulfur but some is always good. Often sulfur is over applied for the plants nutritional needs for other purposes. Sulfur applied in the band can acidify the band (especially elemental forms of sulfur) so I try to fit in as much sulfur as possible in the pre-plant band to increase the availability of phosphorus to the potato plant. In addition, some forms of nitrogen and sulfur fertilizers are much better for potato production. An example of this is in dry forms amonium sulfate is much better for potatoes that urea. Sulfur is a mobile nutrient like nitrogen which can move with soil water. Sulfur availability should be 1 unit of sulfur to 4 units nitrogen, but apply at least 40 units/A sulfur at a minimum.

Zinc is the most common micro-nutrient problem in the San Luis valley of Colorado. Use the table below to calculate your zinc fertilizer needs:

Zinc test value in ppm	< 0.5	0.5 - 1.0	1.0 - 2.5	> 2.5
Zinc Needed in units/A	5.5	4	3	1

Apply the zinc with the pre=plant band or with a liquid nitrogen injection early in the season.

Copper is often needed on Colorado potato fields. A soil level less than 1.0 ppm can benefit from applied copper. Spoil applied copper has not shown a good plant response so its best to apply it as a copper fungicide (combine with manex for good late blight control) or foliar copper.

Boron can often show up low on a soil test since it is a mobile nutrient like nitrogen. Most irrigation water in the western U.S. contains enough boron in the well water to supply a crops needs. In addition care must be taken when applying boron since the difference between deficiency and toxicity is small. A boron level in the soil of less than 0.5 ppm could be low if no boron is found in the irrigation water.

Magnesium

Magnesium problems are not common and fields irrigated with water with greater than 8 ppm magnesium will not need any additional magnesium. Soil levels of less than 40 ppm may need additional magnessium.

Other micro-nutrients: Problems with other micro-nutrients are not common and related to specific local conditions. Often micro-nutrient problems are related to an imbalance of other nutrients. In this case, applying the micro-nutrient in shortage will not solve the problem, since the imbalance of other nutrients still exists. Examples of these situations include:

• Lime induced iron chlorosis where excess lime in the soil blocks iron use in the plant. Tolerant varieties are the best solution.
• Manganese problems resulting from excess levels of potassium
• Zinc problems form excess phosphorus
• Iron problems form low copper and high manganese levels in soil.

Potatoes do not tolerate much soluable salts in te soil and irrigation water. If the Ec of the soil is above 1.5 or the Ece of the irrigation water is above 1.0 yield will be limited.

Colorado State's Potato Fertility Guide, click here.