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Extension > Agriculture > Nutrient management > Nutrient/Lime guidelines > Potato fertilization on irrigated soils

Potato fertilization on irrigated soils

Carl J. Rosen and Peter M. Bierman

potato-plant

Optimum potato growth and profitable production depend on many management factors, one of which is ensuring a sufficient supply of nutrients. There are 14 soil-derived elements or nutrients considered to be essential for growth of plants. When the supply of nutrients from the soil is not adequate to meet the demands for growth, fertilizer application becomes necessary. Potatoes have a shallow root system and a relatively high demand for many nutrients (Table 1). Therefore, 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. High nutrient demand coupled with low native fertility often results in high fertilizer requirements for irrigated potato production. Over the years, however, continued fertilizer applications can build up the soil test levels of certain nutrients. Environmental concerns, especially for nitrogen leaching, are also an important factor in fertilizer use on irrigated sandy soils. A sound nutrient management program for potatoes to ensure optimum crop nutrition without adverse effects on water quality should be based on soil test recommendations, plant tissue testing, the variety grown and the 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. It should not be used as a basis for fertilizer recommendations.

Table 1. Uptake of soil nutrients by potato vines and tubers as a function of yield.

Tuber yield, cwt/A
Vines 200 300 400 500 600
Nutrient Nutrient uptake lb/A
Nitrogen (N)9086128171214252
Phosphorus (P)111217232835
Potassium (K)7596144192240288
Calcium (Ca)433.04.45.97.48.9
Magnesium (Mg)255.98.911.814.717.6
Sulfur (S)8.813.217.622.026.4
Zinc (Zn)0.110.700.110.140.180.22
Manganese (Mn)0.170.030.040.060.070.08
Iron (Fe)2.210.530.791.061.321.58
Copper (Cu)0.030.040.060.080.100.12
Boron (B)0.140.030.040.050.060.07

Soil testing

Fundamental to any effective nutrient management program is a reliable soil analysis and soil test interpretation. Samples should be representative of the area to be fertilized and generally should be taken in the top 8 inches. The soil test will help to determine whether lime or nutrients are needed and if so, what rate should be applied. 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. Nitrate testing is recommended for the finer textured soils and drier conditions of western Minnesota. Sulfur (S) can be tested for on sandy soils if a problem is suspected, but copper (Cu) and manganese (Mn) soil tests are reliable only for organic soils in Minnesota and 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, soil test interpretations should be based on local or regional research. University of Minnesota fertilizer recommendations for potatoes, based on soil test results, are provided in the sections on individual nutrients later in this bulletin. These recommendations, as well as soil test based recommendations for other vegetable crops, are also available in the University of Minnesota bulletin Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota.

Tissue analysis

Plant tissue analysis or tissue testing has been used for many years as an additional nutrient management tool to: 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. The basis behind tissue analysis is that maximum yield and quality are associated with optimum ranges of nutrients in the tissue sampled. If the level of a nutrient falls outside its sufficiency range, then corrective measures should be taken.

The most common tissue used for nutrient analysis in potato is the petiole (leaf stem and midrib) of the fourth leaf from the shoot tip. It is critical that this tissue stage is collected because younger or older tissue will have different nutrient concentrations and can lead to erroneous interpretations. For sampling, approximately 40 leaves from randomly selected plants should be collected and the leaflets stripped off and discarded. Petioles are then sent to a laboratory for analysis. Most diagnostic criteria for tissue analysis are based 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 true or potential nutritional status of the crop if uptake of applied fertilizer by roots has not yet occurred. 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 of sampling only during the tuber bulking phase, because sufficiency ranges have also been developed for the vegetative and tuber maturation growth stages.

Whole leaves can also be used for analysis; however, different diagnostic criteria need to be used 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 point in time. A comparison of nutrient sufficiency ranges for petioles vs. whole leaves is presented in Table 2. Note that K requirements are much higher in petioles compared to whole leaves. Also note that total N is used for whole leaves while nitrate-N is used 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.

Table 2. Suggested nutrient concentration sufficiency ranges in potato tissue collected from the 4th leaf from the top of the shoot during tuber bulking stage (3 growth stages for petiole nitrate-N).

Tissue sampled
Element Petiole Whole leaf (leaflets + petiole)
percent (%)
Total N 3.5 - 4.5
Nitrate-N
Vegetative 1.7 - 2.2
Tuber bulking 1.1 - 1.5
Maturation 0.6 - 0.9
Phosphorus 0.22 - 0.40 0.25 - 0.50
Potassium 8.0 - 10.0 4.0 - 6.0
Calcium 0.6 - 1.0 0.5- 0.9
Magnesium 0.30 - 0.55 0.25 - 0.50
Sulfur 0.20 - 0.35 0.19 - 0.35
parts per million (ppm)
Zinc 20 - 40 20 - 40
Boron 20 - 40 20 - 40
Manganese 30 - 300 20 - 450
Iron 50 - 200 30 - 150
Copper 4 - 20 5 - 20

A word of caution about total N needs to be made. 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, then variable total N results will be obtained. If whole leaves are being analyzed 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. Use of sap nitrate analysis may 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.

Table 3. Petiole sap nitrate-N sufficiency levels for Russet Burbank potatoes.

Time of season Stage of growth Sap NO3-N
ppm
Early Vegetative/tuberization
(June 15-June 30)
1200 - 1600
Mid Tuber growth/bulking
(July 1-July 15)
800 - 1100
Late Tuber bulking/maturation
(July 15-August 15)
400 - 700

Soil pH

One of the more important chemical properties affecting nutrient use is soil pH. Many soils used for potato production have become increasingly more acid over time due to use of ammonium containing fertilizers and leaching of cations from the root zone. Acid conditions are generally favored for potatoes in order to minimize the incidence of 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 corrective measures need to be taken, lime the soil to a pH of 5.5 during a year in the rotation when potatoes are not grown. Use of scab resistant varieties is also recommended so that pH can be maintained in the more desired range.

Nutrient management strategies

Nitrogen

Potatoes have a relatively shallow root system with most roots located in the top 2 feet of soil. Banded fertilizer 2 - 3 inches below and 2 - 3 inches to the side of the tuber at planting is an efficient application method for a portion of the nitrogen and most of the other required nutrients. For most efficient fertilizer use, a practical yield goal should be selected. 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. Application of fertilizer N is usually necessary to ensure profitable potato production, because soil N is largely tied up in organic matter and a relatively small amount of this organic N becomes available for uptake over the growing season. 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. This bulletin focuses on the crop production (yield and quality) aspects of N management and will discuss N rates, timing of N application, and sources of N. These factors also affect the environmental concerns surrounding N use, but for a more detailed treatment of practices that protect water quality see Best Management Practices for Nitrogen Use: Irrigated Potatoes.

Nitrogen rate

The rate of N to apply to irrigated potatoes 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 manure is used, then an estimate of N availability from the manure should be included as part of the N applied in meeting the total N recommendation. See Manure Management in Minnesota for information on calculating the N credit from manure. Irrigation water may contain significant amounts of nitrate-N and this should also be included as part of the N applied to the crop. See the Best Management Practices for Nitrogen Use: Irrigated Potatoes bulletin referred to above for information on calculating the N credit from irrigation water.

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 like Red Norland generally require less N than later maturing varieties, such as Russet Burbank. The yield goal concept is still being used to guide N recommendations for potatoes, in conjunction 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. Yield goal for potatoes is based 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.

In addition to environmental concerns due to excessive N applications, high rates of N can detrimentally 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, use of fungicides increases yield potential and hence the N requirement is the same or higher when early blight is controlled. 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. Failing to account for N supplied by legumes can lead to a build up of soil N, increase the potential for nitrate leaching, and reduce tuber yield and quality.

Table 4. Nitrogen recommendations for irrigated potato production.

Previous crop and organic matter (O.M.) level
Alfalfa (good stand)1 Soybeans Field peas Any crop group 1 Any crop group 2
Organic matter content2
Yield goal3 Harvest4 Low Med-High Low Med-High Low Med-High Low Med-High
cwt/A N to apply (lb/A)
<250 Early 0 0 80 60 60 40 100 80
250-299 25 0 105 85 85 65 125 105
300-349 50 30 130 110 110 90 150 130
350-399 Mid 75 55 155 135 135 115 175 155
400-449 100 80 180 160 160 140 200 180
450-499 Late 125 105 205 185 185 165 225 205
500+ 150 130 230 210 210 190 250 230
Crops in Group 1 Crops in Group 2
alfalfa (poor stand)1 barley grass hay sorghum-sudan
alsike clover buckwheat grass pasture sugarbeets
birdsfoot trefoil canola millet sunflowers
grass-legume hay corn mustard sweet corn
grass-legume pasture edible beans oats triticale
red clover flax potatoes wheat
fallow rye vegetables
1Poor stand is less than 4 crowns per sq. ft.
2Low = less than 3.1% O.M.; medium to high = 3.1 - 19% O.M.
3Yield in this table refers to total yield, not marketable yield
4Early = vines killed or green dug before August 1; Mid = vines killed or green dug August 1-August 31; Late = vines killed or green dug after Sept. 1

Timing of N application

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. Most nitrification inhibitors are not registered for potatoes and therefore cannot be recommended. 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.

Recommended timing is to apply some N at planting for early plant growth and to 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. Uptake of N by the crop (vines plus tubers) increases when split N applications are used compared with large applications applied before emergence. Nitrogen applied through the hilling stage should be incorporated into the hill 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 than indeterminate varieties, but the total amount of N required is generally lower because of early harvest and lower yield potential (Table 4). In addition, late application of N to these varieties will 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 as this will depend on market demands as well as weather conditions during the season. Because of these unknowns it is important to have some flexibility in both rate and timing of N application.

center-pivot

Center pivot irrigation of potatoes at Big Lake, Sherburne County, Minnesota.

Increases in N use efficiency have been shown 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, fertigation is not recommended 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 when excessive rainfall occurs before hilling. Fertigation timing should be based on petiole nitrate-N levels (Tables 2 and 3) as discussed in the Tissue analysis section. If N is needed, 20 to 40 lb N/A can be injected 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:

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

Sources of N

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, fertilizer sources containing nitrate (ammonium nitrate and urea-ammonium nitrate solutions) should be avoided 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, urea, ammonium nitrate, urea-ammonium nitrate solutions, ammonium sulfate, and anhydrous ammonia are used. Urea-ammonium nitrate solutions are generally used for fertigation.

Care should be taken not to band high amounts of ammonium containing fertilizer close to the seed piece 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. Further research needs to be conducted on the use of anhydrous ammonia for potato. 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 if slow release sources of N are used. 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 economical and the cost savings of being able to make a single N fertilizer application rather than multiple applications is another factor to consider. Minnesota research with ESN, a relatively low cost slow release N fertilizer, has shown promising results when a single ESN application at emergence is compared to quick release urea applied using standard split application practices.

Phosphorus

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. Banded P applications at planting are recommended, 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. In-season application of P has generally not been found to be beneficial 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). Inconsistent response to P fertilizer was found in high P testing soils (Bray P greater than 25 ppm). In about 50 percent of the studies, a positive response to P was found 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). Phosphorus fertilizer recommendations for potato based on soil test levels and yield goal are presented in Table 5.

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 and is often the recommended P source to maximize P application and 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 and a large proportion of the P in liquid fertilizers is polyphosphate P. However, this should not be a factor in selecting a P source because polyphosphate is quickly converted to orthophosphate in the soil and the two forms of P have been found to have equal effects in numerous studies.

Table 5. Phosphate recommendations for irrigated potato production.1

Soil test P level (ppm)
Yield goal2 Bray-P1 0-5 6-10 11-15 16-20 21-25 26-30 31-50 51+
Olsen-P 0-3 4-7 8-11 12-15 16-18 19-22 23-41 42+
cwt/A P2O5 to apply (lb/A)3
less than 200 75 50 25 0 0 0 0 0
200-299 100 75 50 25 0 0 0 0
300-399 125 100 75 50 50 50 50 50
400-499 150 125 100 75 75 75 75 75
500 or more 175 150 125 100 100 100 100 75
1For acid irrigated sands, responses up to 150 lb/A P2O5 have been observed on very high (41+ ppm) P soils.
2Yield in this table refers to total yield, not marketable yield.
3For most efficient application, apply phosphate fertilizer in a band 2-3 inches below and 2-3 inches to each side of the tuber at planting.

Potassium

Potatoes take up significant quantities of K (Table 1) and this nutrient plays important roles in tuber yield, size, and quality. High K is necessary to prevent blackspot bruising and shattering and attain good storage quality, but specific gravity may be reduced 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 and 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 and K recommendations for potato are based on a combination of soil test level and yield goal (Table 6). On low K testing soils, which require high K fertilizer application rates, both broadcast and banded applications are recommended. At least half of the K should be broadcast and incorporated before planting and the remainder banded at planting. On higher testing soils all of the K can be banded 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.

Table 6. Potassium recommendations for irrigated potato production.

Yield goal Soil test K level (ppm)
0-40 41-80 81-120 121-160 161-200 201+
cwt/A K2O to apply (lb/A)1
less than 200 150 75 50 25 0 0
200-299 200 100 75 50 25 20
300-399 300 200 100 75 50 25
400-499 400 300 200 100 75 50
500 or more 500 400 300 200 100 75
1Do not apply more than 200 lb/A K2O in the band at planting.

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

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 and 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 such as 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. Addition of Ca on high testing soils is recommended only if the potatoes are to be stored and storage problems have been encountered 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 be used to increase tuber calcium concentrations. Gypsum can be applied at or before planting. Calcium nitrate should be incorporated 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, additional Ca may be required 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, it is important to place Ca in the zone of tuber formation so that tuber or stolon roots can take it up directly from the soil.

Table 7. Calcium recommendations for irrigated potato production.

Calcium soil test Relative level Calcium to apply
ppm lb/A
0-150 low 200
151-299 medium 100
300+ high 0

Magnesium

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.

Table 8. Magnesium recommendations for irrigated potato production.

Magnesium to apply
Magnesium soil test Relative level Broadcast Row
ppm lb/A
0-49 low 100 20
50-99 medium 50 10
100+ high 0 0

Sulfur

On many soils S requirements are met from soil organic matter breakdown. 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. When ammonium sulfate is used, be sure to account for the N it contains in meeting the crop N requirement. Elemental S is not an immediately available form and must be oxidized by soil bacteria to sulfate before it can be used by plants. The oxidation to sulfate has an acidifying effect on the soil, but the effect is small at the rates required to meet recommendations.

Table 9. Sulfur recommendations for irrigated potato production.

Sulfur to apply
Sulfur soil test Relative level Broadcast Row
ppm lb/A
0-6 low 20-30 10-15
7-12 medium trial only trial only
12.1+ high 0 0

Micronutrients

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. Tissue analysis can be used 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, foliar applications can be used to correct deficiencies during the growing season; however, when B is needed soil application is recommended because B applied to the foliage is not readily transported to the tuber. Excessive B applications can be toxic. Soil applied micronutrients can be banded with the starter fertilizer. See Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota for micronutrient fertilizer sources.

Table 10. Boron recommendations for irrigated potato production.

Boron soil test Relative level Boron to apply
ppm lb/A
0.0-0.4 low 1
0.5-0.9 medium 0
1.0+ high 0

Table 11. Zinc recommendations for irrigated potato production.

Zinc soil test Relative level Zinc to apply
Broadcast Row
ppm lb/A
0.0-0.5 low 10 2
0.6-1.0 medium 5 1
1.1+ high 0 0

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