Potato fertilization on irrigated soils (2024)

N application rate primarily depends on the cultivar and date of harvest, expected yield goal, amount of soil organic matter and the previous crop. Table 4 shows the effects of these factors on N recommendations for irrigated potato production. If using manure, include that in your estimate for meeting the total N recommendation. Irrigation water may contain significant amounts of nitrate-N. Include it as part of the N applied to the crop.

Different potato varieties and differences in harvest date will have a pronounced effect on yields and yield goals. Because of earlier harvest and lower yield, early maturing varieties generally require less N than later maturing varieties. We still use the yield goal concept to guide N recommendations for potatoes, with variety and harvest date, until a more complete measure of the N supplying capacity of the soil is available. Currently N recommendations are also adjusted for the amount of soil organic matter, with higher rates for low organic matter soils than for medium to high organic matter soils, which have a greater capacity to release plant-available N. We base yield goal on the total yield obtained rather than the marketable yield, but the two are generally well-correlated. An overestimation of the yield goal will result in excessive applications of N, which can potentially result in nitrate losses to groundwater.

High rates of N can also affect potato yields and tuber quality. Too high a rate of N will delay tuber initiation and maturity leading to excessive vine growth at the expense of tuber growth. Delayed maturity can result in tubers with lower specific gravity. Excess N can also increase brown center and the incidence of knobby, misshapen, and hollow tubers. High N will induce vigorous foliage, which can lead to an increase in vine rot diseases. On the other hand, lack of N can increase early blight infestations. Controlling early blight with proper use of fungicides will, in some years, reduce the N requirement. In other years, fungicides can increase yield potential. Hence, when blight is under control, the N requirement is the same or higher. Generalizations on foliar disease incidence and N requirement are difficult to make.

Previous crop can also affect N needs. Legumes in a crop rotation can supply significant N to subsequent crops, as shown by the recommendations in Table 4. Account for the N supplied by legumes to avoid a build up of soil N, increase the potential for nitrate leaching and reduce tuber yield and quality.

Group 1 crops:

• alfalfa (poor stand)¹
• alsike clover
• birdsfoot trefoil
• grass-legume hay
• grass-legume pasture
• red clover
• fallow

Group 2 crops:

• barley
• buckwheat
• canola
• corn
• edible beans
• flax
• grass hay
• grass pasture
• millet
• mustard

Group 2 crops continued:

• oats
• potatoes
• rye
• sorghum-sudan
• sugarbeets
• sunflowers
• sweet corn
• triticale
• vegetables
• wheat

Application timing

Efficient use of N requires matching N applications with N demands by the crop. Nitrogen applications in the fall are very susceptible to leaching. Nitrogen applied early in the season when plants are not yet established is also susceptible to losses with late spring and early summer rains. Peak N demand and uptake for late season potatoes occurs between 20 and 60 days after emergence. Uptake is highest during the tuber bulking phase. Optimum potato production depends on having an adequate supply of N during this period.

Apply some N at planting for early plant growth. Then, apply the majority of the N in split applications beginning slightly before (by 10 days) the optimum uptake period. This assures that adequate N is available at the time the plants need it. Starter fertilizer should contain no more than 40 lb N/A for full season varieties. Split applications will encourage more N uptake compared to large applications applied before emergence. Incorporate any N applied through the hilling stage to maximize availability of the N to the potato root system.

Plan the majority of N inputs from 10 to 50 days after emergence. Late applications of N can delay maturity and lead to poor skin set. Just as N fertilizer applied too early in the season can potentially lead to nitrate losses, so can N fertilizer applied too late in the season. Nitrogen applied beyond 10 weeks after emergence is rarely beneficial and can lead to nitrate accumulation in the soil at the end of the season. This residual nitrate is then subject to leaching.

For determinate early harvested varieties like Red Norland, higher rates of N in the starter may be beneficial (up to 60 lb N/A). These varieties tend to respond to higher rates of N upfront, but the total amount of N required is generally lower because of early harvest and lower yield potential (Table 4). Late application will also tend to delay maturity and reduce yields, particularly if the goal is to sell for an early market. In many cases it is not possible to know when the exact harvest date will be because it depends on market demands and weather conditions during the season. Because of these unknowns it is important to have some flexibility in both rate and timing of N application.

We have seen increases in N use efficiency when some of the N is injected into the irrigation water after hilling (fertigation). Because the root system of the potato is largely confined to the row area during early growth, we do not recommend fertigation until plants are well established and potato roots have begun to explore the furrow area between rows. This is usually about three weeks after emergence. Post-hilling N applications are most beneficial in years with excessive rainfall pre-hilling. Base fertigation timing on petiole nitrate-N levels (Tables 2 and 3) as discussed in the Tissue analysis section. If you need N, inject 10 to 30 lb N/A per application for mid/late season varieties and up to 20 lb N/A for early season varieties. Application of late-season nitrogen can lead to misshapen tubers in some cultivars.

General guidelines for N application timing for mid/late season varieties are:

• Band starter N at planting
• Apply 1/3 to 1/2 of the recommended N at or around emergence
• If fertigation is not available, apply the remainder of the recommended N at final hilling
• If fertigation is available and final hilling is done 10-14 days after emergence, apply 1/3 of the recommended N at final hilling and fertigate the remainder based on petiole analysis
• If fertigation is available and final hilling is done at emergence, begin fertigating 14-21 days later and apply the remainder of the recommended N based on petiole analysis

General guidelines for N application timing for early season varieties are:

• Band starter N at planting
• Apply 1/3 to 2/3 of the recommended N at or around emergence
• Apply the remainder of the recommended N at final hilling
• If fertigation is available, apply any additional N based on petiole analysis and anticipated harvest date

Each fertilizer N source used for potatoes has advantages and disadvantages, depending on how they are managed. Because leaching rains often occur in the spring, avoid fertilizer sources containing nitrate (ammonium nitrate and urea-ammonium nitrate solutions) at or before planting. Ammonium sulfate, diammonium phosphate, monoammonium phosphate, and poly ammonium phosphate (10-34-0) are the preferred N sources for starter fertilizer. For sidedress applications, use urea, ammonium nitrate, urea-ammonium nitrate solutions, ammonium sulfate, or anhydrous ammonia. Because of its explosive nature, ammonium nitrate is not generally used for potato production in the upper Midwest. Urea-ammonium nitrate solutions are the main sources used for fertigation. Irrigation water may contain elevated nitrate levels and should be taken into account as a fertilizer N input if the nitrate-N concentration is greater than 10 ppm.

Take care not to band high amounts of ammonium fertilizer close to the seed, as ammonia toxicity may result, especially on high pH soils. Urea is the most common N source used for sidedressing. Urea is susceptible to ammonia volatilization if left on the soil surface. Therefore, it must be incorporated or irrigated in within 12 hours after application. Coating urea with a urease inhibitor, such as NBPT or NPPT, reduces the need for immediate incorporation.

Ammonium sulfate also provides sulfur and is the most acidifying N fertilizer. On a nitrogen basis, the cost of ammonium sulfate is double that of urea. However, if sulfur is also needed, then ammonium sulfate is an economical source to use. Anhydrous ammonia may be beneficial in delaying the potential for leaching losses. However, positional availability of the N in relation to the hill may be a problem with sidedress applications. Specialty N sources such as calcium nitrate can be effective, but are many times the cost of urea.

Reductions in nitrate leaching can occur under some conditions with slow-release N sources. Slow-release N sources include polymer coated urea that can be formulated to release N over various time intervals. These slow-release sources can also be applied earlier in the season without the fear of nitrate leaching losses. The main disadvantages of slow-release N fertilizer are delayed release to ammonium and nitrate when soil temperatures are cool and the higher cost of many of the products compared to conventional quick release N fertilizers. However, there are some newer slow-release fertilizers on the market that are more affordable. The cost savings of being able to make a single N fertilizer application is another factor to consider. Minnesota research with ESN, a relatively low cost slow-release N fertilizer, has shown promising results with a single ESN application at emergence, compared to quick release urea applied using standard split application practices.

Dicyandiamide (DCD), a nitrification inhibitor, can also slow down the conversion of ammonium to nitrate, but limited research with potatoes has not shown a reduction in nitrate leaching relative to use of urea alone. Nitrapyrin is another nitrification inhibitor, but it is not registered for use on potatoes and should not be used for this crop.

Phosphorus is important in enhancing early crop growth and promoting tuber maturity. Minnesota research has also found that P plays an important role in regulating tuber set with higher tuber numbers when P nutrition is high. We recommend banded P applications at planting, because P movement in the soil is limited. Placing P close to the seed piece is especially important early in the season when soil temperatures are cool and root systems are undeveloped. We have not seen benefits to in-season application of P on acid sandy soils in the upper Midwest. Soil pH affects P availability, which is reduced under both acid and alkaline conditions. Availability is highest at slightly acid to near neutral conditions, so the practice of growing potatoes at low pH to reduce scab can limit P uptake if it drops too low (see the Soil pH section).

Experiments conducted over a 6-year period in Minnesota revealed a consistent response to banded P fertilizer applied at rates of 100 to 150 lb P₂O₅/A in lower P testing soils (Bray P less than 25 ppm). We found inconsistent response to P fertilizer in high P testing soils (Bray P greater than 25 ppm). In about 50 percent of the studies, we found a positive response to P on high testing soils. In some cases, the positive response may have been due to low pH (5.3 or less), which tends to tie up P. In the other 50 percent, the P response was not significant. In addition, response to P fertilizer is cultivar dependent with some cultivars especially responsive to P fertilizer, even on very high testing soils. On average, some P fertilizer appears to be necessary for potatoes to reach maximum yields on the sandy soils of central Minnesota. Tuber yields affect P requirements due to greater P uptake with higher yields (Table 1). Table 5 presents phosphorus fertilizer recommendations for potato based on soil test levels and yield goal. Keep in mind that some cultivars can be highly responsive to P fertilizer even at lower yield goals.

Common granular sources of P fertilizer include monoammonium phosphate or MAP (11-48 to 52-0) and diammonium phosphate or DAP (18-46-0). Research comparing these two P sources on potatoes in Minnesota found no difference between them in yield, although there are potential advantages and disadvantages to each. When MAP dissolves it initially results in an acid reaction in the soil, while DAP results in an alkaline reaction. For this reason MAP is often used on alkaline soils and DAP is often used on acid soils, although crop response to the two is usually similar. At equivalent P fertilizer rates, MAP has a lower N content than DAP. It is often the recommended P source to minimize early season N application on sandy soils vulnerable to nitrate leaching.

Ammonium polyphosphate (10-34-0) is the most commonly used liquid P fertilizer and is suitable for banded application in potatoes. A variety of related liquid products are available and suitable, although they have lower P contents. Orthophosphate P, as found in MAP and DAP, is the form of P taken up by plants. A large proportion of the P in liquid fertilizers is polyphosphate P. This should not be a factor in selecting a P source because polyphosphate is quickly converted to orthophosphate in the soil. The two forms of P have been found to have equal effects in numerous studies.

Potatoes take up significant quantities of K (Table 1), a nutrient that plays important roles in tuber yield, size and quality. The plant needs high K to prevent blackspot bruising and shattering and attain good storage quality. However, you may reduce specific gravity if K fertilization is too high because it increases tuber water absorption. In-season K applications have a greater effect on lowering specific gravity than preplant or planting applications. Potassium chloride (0-0-60) can have more of an effect than potassium sulfate (0-0-50) at equivalent K rates. Potassium is a relatively immobile nutrient in medium- and fine-textured soils, but it does leach in sandy soils, particularly when they are acidic and low in organic matter. Excessive Mg fertilization can inhibit K uptake and induce a K deficiency, especially when soil K is low.

Soils tests are very useful in predicting K responsive soils. We base K recommendations for potato on a combination of soil test level and yield goal (Table 6). On low K testing soils, which require high K fertilizer application rates, we recommend both broadcast and banded applications. At least half of the K should be broadcast and incorporated before planting and the remainder banded at planting. On higher testing soils you can band all the K at planting. Because of concern over chloride (Cl^-) leaching to groundwater, spring application of K is preferred over fall application to minimize leaching of Cl^-and to some extent, K.

Potassium source generally has no effect on total yield. Potassium chloride is the most economical K source, but it has a high salt index and may cause salt problems if banded at rates higher than 200 lb K₂O/A. Potassium sulfate has a lower salt index and may produce slightly higher percentages of large tubers, but is more expensive. It is more competitive if S is also required. Potassium-magnesium sulfate (0-0-22-18S-11Mg) is also more expensive than potassium chloride, but is a good option to supply at least part of the K when both S and Mg are required.

¹Do not apply more than 200 lb/A K₂O in the band at planting.

Deficiency symptoms

Symptoms of phosphorus deficiency are stunted growth and a dark green or purpling of the leaves. Potatoes may develop these symptoms in the early spring when soil temperatures are cool. Potassium deficiency symptoms include scorching of the margins of older leaves.

Calcium deficiency is rare in many agricultural soils, because they have high native Ca levels or are periodically limed to maintain soil pH. Sandy soils, however, do not maintain high Ca reserves. Plus, the practice of growing potatoes at low pH to reduce scab means that they are rarely limed (see the Soil pH section). Under these conditions soil Ca can fall to levels that reduce tuber quality and tuber yield.

Calcium plays an important role in maintaining tuber quality in storage and reducing internal tuber disorders like brown spot and hollow heart. Low Ca in tubers is often due to inadequate transport of Ca to the tuber caused by water or temperature stress. This may be a localized Ca deficiency with adequate Ca levels occurring in leaves and the soil testing high in Ca. We recommend adding Ca on high testing soils only if the potatoes you are storing have had storage problems in the past.

Table 7 provides Ca recommendation for potato based on a Ca soil test. Calcium sulfate (gypsum) and calcium nitrate are two Ca sources that can increase tuber calcium concentrations. You can apply gypsum at or before planting. Incorporate calcium nitrate into the hill as a sidedress application after emergence. Calcium nitrate is also the N source in this case, so application rates should not exceed the N requirement. If the recommended Ca rate is high, you may need additional Ca from another source. An additional alternative is to apply low rates of lime during a non-potato year in the rotation. Dolomitic lime will supply both Ca and Mg. Because transport of Ca from other parts of the plant to tubers is poor, be sure to place Ca in the zone of tuber formation. That way tuber or stolon roots can take it up directly from the soil.