Potato fertilization on irrigated soils
Optimum potato growth depends on many management factors, including sufficient supply of nutrients. Potatoes have a shallow root system and a relatively high demand for many nutrients (Table 1). A comprehensive nutrient management program is essential for maintaining a healthy potato crop, optimizing tuber yield and quality, and minimizing undesirable impacts on the environment.
Irrigated potatoes are usually grown on coarse-textured soils low in organic matter. Typically, these soils are sandy loams or loamy sands, low in native fertility, and quite acid. The crop's high nutrient demand coupled with low native fertility means that potatoes often have high fertilizer requirements. Over the years, however, continued fertilizer applications can build up the soil test levels of certain nutrients. Base your nutrient management program on soil test recommendations, plant tissue testing, variety, time of harvest, yield goal and the previous crop in the rotation.
Nutrient removal by the potato crop
The amount of nutrients removed by a potato crop is closely related to yield (Table 1). Twice the yield will usually result in twice the removal of nutrients. The vines take up a portion of the nutrients needed for production. The rest goes to the tubers and is removed from the field with harvest. The purpose of Table 1 is to provide relative uptake of essential elements for potato production. Do not use the table as a basis for fertilizer recommendations.
Fundamental to any effective nutrient management program is a reliable soil analysis and soil test interpretation. Take samples in the top eight inches, representative of the area you will fertilize. The soil test will help to determine whether the crop needs lime or nutrients and the rate of application. A typical soil analysis for potatoes should include pH, organic matter, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), zinc (Zn), and boron (B). Soil nitrate tests are not reliable for nitrogen (N) recommendations on irrigated sandy soils, because nitrate can move rapidly and fluctuate widely. We recommend nitrate testing for the finer textured soils and drier conditions of western Minnesota. You can test sulfur (S) can on sandy soils if you suspect a problem. Copper (Cu) and manganese (Mn) soil tests are reliable only for organic soils in Minnesota. Iron (Fe) deficiencies are more related to soil pH than to soil test levels. Tissue analysis (see next section) is an alternative method of monitoring the adequacy of Cu, Fe, and Mn. These nutrients are not likely to be limiting on the acid, sandy soils commonly used for potato production.
While the actual soil test results should be fairly similar from one lab to the next, interpretations may vary widely. For most accurate fertilizer recommendations, base soil test interpretations on local or regional research.
You can use plant tissue analysis or tissue testingto: 1) diagnose a nutrient deficiency or toxicity, 2) help predict the need for additional nutrients (primarily nitrogen), and 3) monitor the effectiveness of a fertilizer program. Optimum nutrient ranges provide the basis behind tissue analysis. If the level of a nutrient falls outside its sufficiency range, then take corrective measures.
Tissue test the petiole (leaf stem and midrib) of the fourth leaf from the shoot tip. Younger or older tissue will have different nutrient concentrations and can lead to erroneous interpretations. For sampling, collect approximately 40 leaves from randomly selected plants. Strip and discard the leaflets. Petioles are then sent to a laboratory for analysis. We base most diagnostic criteria for tissue analysis on a sample taken during the tuber bulking stage. Samples taken too early in the season or soon after a fertilizer application may not accurately reflect the nutritional status of the crop because the roots have not taken up applied fertilizer. In general, tissue analysis should begin about one week after final hilling and at least four days after a fertigation. Nitrogen is an exception to the rule because sufficiency ranges have also been developed for the vegetative and tuber maturation growth stages.
You can use whole leaves for analysis, but you'll need different diagnostic criteria for interpretations. Petioles are generally preferred as the tissue to use for predictive purposes, because they more accurately reflect the immediate nutritional status of the plants and whether they are currently taking up sufficient nutrients. Nutrients are ultimately transported from the petiole to the leaflets and the whole leaf provides a more integrated nutrient status since nutrients tend to accumulate in the leaflets. Therefore, leaves are better indicators of the cumulative nutritional status of plants and whether nutrient uptake has been adequate up to the present. Table 2 presents a comparison of nutrient sufficiency ranges for petioles vs. whole leaves. Note that K requirements are much higher in petioles compared to whole leaves. Also note that we use total N for whole leaves but nitrate-N for petioles. Most N in petioles is in the nitrate form and measurement of nitrate-N is a more straightforward procedure than total N. However, there is little nitrate-N in leaflets and total N provides a more accurate measurement of N status for whole leaves.
Depending on the analytical procedure, total N may not be an accurate measurement because of variable conversion of nitrate-N to ammonium-N during sample digestion. Many laboratories do no not take precautions to account for nitrate-N in total N analysis. This is not a problem when nitrate-N is a small percentage of total N. However, whole potato leaves contain a lot of nitrate in the petiole and if the procedure used does not consistently convert nitrate to ammonium-N, you will end up with variable results. If you analyze whole leaves, make sure that the total N reported includes complete conversion of nitrate to ammonium-N in the analytical procedure.
Rather than sending samples into the lab for nitrate analysis, diagnostic criteria have been developed for nitrate analysis of the petiole sap. This provides a quick procedure to determine the N status of the plant without having to wait for results from a laboratory. Sap nitrate analysis is primarily used for irrigated potatoes because the water status of the plant is more uniform. It provide inconsistent readings in non-irrigated soils because sap nitrate concentrations can fluctuate with the water status of the plant. Table 3 provides petiole sap nitrate-N sufficiency ranges for Russet Burbank potatoes at different growth stages. Other potato varieties may differ slightly in their sufficiency ranges, but Table 3 is still a suitable starting point for determining the need for additional N.
One of the more important chemical properties affecting nutrient use is soil pH. Many soils used for potato production have become more acid over time due to use of ammonium fertilizers and leaching of cations from the root zone. Acid conditions are generally better for reducing common scab (Strepotmyces scabies), which is most widespread when soil pH is above 5.5. Use of liming amendments is often avoided to minimize scab. Controlling scab in this manner, however, can result in a soil pH that will cause nutrient imbalances. Once soil pH drops below 4.9, nutrient deficiencies and toxicities become more common. In particular, Mn and aluminum (Al) toxicity and P, K, Ca, and Mg deficiencies may occur in these low pH soils. The problem may not be prevalent through the entire field, but may occur in smaller areas where the soil consists of higher sand or lower organic matter content. In some cases, grid sampling a field for pH may be useful to identify areas that need correction. If you need to take corrective measures, lime the soil to a pH of 5.5 during a year in the rotation when potatoes are not grown. We also recommending using scab-resistant varieties to maintain desirable pH range.
Potatoes have a relatively shallow root system with most roots located in the top 2 feet of soil. We recommend using banded fertilizer two to three inches below and two to three inches to the side of the tuber at planting. For most efficient fertilizer use, select a practical yield goal. Reasonable yield goals are usually set at 15 - 20 percent higher than a grower's average for the past 5 years.
Of all the essential elements, N is the one most often limiting for potato growth. Ensuring adequate N is necessary to achieve high yields, but too much N can also cause problems. Excessive N can reduce both yield and tuber quality and has the potential to leach to groundwater on well-drained sandy soils.
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.
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. We do not recommend nitrification inhibitors, as most are not registered for potatoes. 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 20 to 40 lb N/A per application for mid/late season varieties and up to 30 lb N/A for early season varieties.
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 planting. Ammonium sulfate, diammonium phosphate, monoammonium phosphate, poly ammonium phosphate (10-34-0), and urea are the preferred N sources for starter fertilizer. For sidedress applications, use urea, ammonium nitrate, urea-ammonium nitrate solutions, ammonium sulfate, or anhydrous ammonia. Urea-ammonium nitrate solutions are generally used for fertigation.
Take care not to band high amounts of ammonium fertilizer close to the seed, as ammonia toxicity may result, especially on high pH soils. Ammonium nitrate is a quickly available N source and used frequently on early maturing varieties. It is also the most susceptible to leaching. Advantages of urea compared with ammonium nitrate are lower cost and delayed potential for leaching. Disadvantages of urea are that it is hygroscopic (attracts water), it must be incorporated after application or ammonia volatilization losses may occur, and its slow conversion to nitrate in cool seasons may reduce yields. 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.
Substantial reductions in nitrate leaching can occur 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.
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. 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.
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 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. Applying significant amounts of K during the tuber bulking phase can also reduce yields. Potassium is a relatively immobile nutrient in medium- and fine-textured soils, but it does leach in sandy soils, particularly when they are acid 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. 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.
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, magnesium, and sulfur
Potato production on acid sandy soils low in organic matter may require addition of one or more of the secondary nutrients (Ca, Mg, and S) for optimum tuber yield and quality.
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 you are storing the potatoes 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.
Similar to Ca, inadequate Mg can occur on acid sandy soils that are not periodically limed. High rates of K fertilizer, which are often required for potatoes, can also induce Mg deficiencies since K and Mg compete for uptake. Table 8 gives Mg recommendations for potato based on a soil test. Magnesium sulfate or potassium-magnesium sulfate are the most common Mg sources available. They can be broadcast and incorporated prior to planting or banded in the row at planting. As with Ca, another alternative is to apply low rates of lime during a non-potato year in the rotation. An application of 1000 lb dolomite/A will meet both the Mg and Ca recommendations for low testing soils.
On many soils, soil organic matter will be enough to meet S requirements. Rainwater and irrigation water contain some sulfate and can also provide a significant proportion of the S needed for growth. Sulfate readily leaches through sandy soils, so yield reductions from S deficiency are most common on sandy, low organic matter soils. Table 9 gives S recommendations for potato based on a soil test. The S soil test is only reliable for sandy soils. Sulfate-S is the form taken up by plants, so ammonium sulfate, potassium sulfate, magnesium sulfate, and calcium sulfate are common sources used to supply S. They can be broadcast and incorporated prior to planting or banded in the row at planting. With ammonium sulfate, be sure to account for the N it contains in meeting the crop N requirement. Elemental S is not an immediately available form. Soil bacteria must oxidized it to sulfate before plants can use it. The oxidation to sulfate has an acidifying effect on the soil, but the effect is small at the rates required to meet recommendations.
Most soils contain sufficient amounts of zinc (Zn), boron (B), copper (Cu), manganese (Mn), iron (Fe), chlorine (Cl), molybdenum (Mo), and nickel (Ni) to meet plant needs. However, in some areas micronutrient shortages occur and may limit yields. Calibrated soil tests for mineral soils are only available for Zn (Table 10) and B (Table 11). Soil tests for Cu and Mn are only reliable for organic soils. You can use tissue analysis to monitor micronutrient status (Table 2).
A 5-year Minnesota study on irrigated sandy soil found increases in potato yields with B and Zn applications, but not with Mn or Cu applications. In acid soils, Fe, Mn, and Cu should be available in adequate amounts to meet crop needs. Pesticide sprays often contain enough Cu and Zn to meet plant demands for these nutrients. In extremely acid soils (pH less than 4.8), Mn toxicity may be a problem. Tissue Mn levels greater than 1,000 parts per million are often associated with stem streak necrosis. Potato responses to Mo and Cl have not been reported in Minnesota. Little research has been done on Ni, but required amounts are very low and soil deficiency is probably very uncommon.
If soil or tissue tests show the need for a micronutrient, you can use foliar applications during the growing season. However, with B, we recommend soil application because B applied to the foliage is not readily transported to the tuber. Excessive B applications can be toxic. You can band soil-applied micronutrients with the starter fertilizer.
Reviewed in 2018