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. Because potatoes demand large quantities of nutrients and these soils lack ability to supply nutrients for crop growth, fertilizer use for irrigated potato production is usually high. Over the years, however, continuous fertilizer use has built up the soil test levels of certain nutrients. This is why a sensible fertilizer program for potatoes should be based on soil test recommendations, tissue tests, yield goal projections, and previous crop.
Nutrient removal by the potato crop
The amount of nutrients removed by a potato crop is closely related to yield (table 1). Usually, twice the yield will result in twice the removal of nutrients. The vines take up a portion of the nutrients needed for production. The rest goes to the tuber and is removed with harvest. The primary purpose of table 1 is to provide relative uptake of essential nutrients for potato production, but it should not be used to base fertilizer recommendations.
Table 1. Uptake of soil nutrients by potato vines and tubers as a function of yield.
|Nutrient||Tuber yield, cwt/A|
|Nutrient uptake lb/A|
Soil tests prior to planting provide the best information for predicting crop response to applied fertilizer. It is important to obtain a representative sample from the field in question. Your local county extension office can provide instructions for taking a soil sample and information on obtaining containers for sending a sample. For sandy soils, a soil test can indicate whether there is a need for most essential nutrients including: phosphorus, potassium, magnesium, calcium, sulfur, boron, zinc, manganese, iron and copper. Usually, manganese, iron, and copper will not be limiting (lacking) on acid sandy soils and the need for testing for these nutrients is questionable. The nitrogen soil test for irrigated sandy soils is not reliable because nitrogen can move rapidly and fluctuate widely in these soil types. For crops grown on non-irrigated soils in western Minnesota, soil tests can be used for nitrogen fertilizer recommendations.
Plant analysis and tissue testing
Tissue analysis can be used during the season to monitor nutrient status of the potato plant and diagnose any suspected nutrient disorders. This procedure is particularly useful for monitoring nitrogen. The petiole from the first fully expanded leaf from the top (usually the fourth leaf) is sampled. At least 40 petioles are required for an adequate sample with three samples taken per field. The actual level of petiole nitrogen will vary with time of sampling, cultivar, and region. A midseason (July 1 – July 15) petiole nitrate-N concentration of about 11,000 – 15,000 parts per million is considered an adequate level for most late maturing cultivars. For early tuber set (June 15 – June 30), 17,000 – 22,000 parts per million nitrate-N is adequate. For late season (July 15 – August 15), 6,000 – 8,000 parts per million is adequate; nitrogen applications through the irrigation system can be used if levels fall below these ranges. Keeping yearly records of petiole nutrient levels will help in making fertilizer management decisions. Nutrient concentrations based on whole leaf samples can also be used in diagnosing nutrient disorders. Table 2 shows nutrient sufficiency levels in recently mature leaves (fourth leaf from the top) taken 45 to 55 days after emergence.
Table 2. Nutrient sufficiency levels in recently matured potato leaves (petiole & leaflets) sampled 45-55 days after emergence.
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.
Nitrogen, phosphorus, potassium
Of all the essential plant nutrients, nitrogen is the one most often limiting to potato production on sandy soils. Nitrogen fertilizer recommendations for irrigated soils are based on crop yield goal and previous crop (table 3). Because of lower yield, early maturing varieties generally require less nitrogen than later maturing varieties. Nitrogen management is important both from a crop production and an environmental standpoint. Nitrogen applied early in the season can be easily leached out of the root zone with heavy rainfall or excess irrigation. The result may be increased potential for contamination of the ground water with nitrate and poor yields due to nitrogen deficiency. For sandy soils, nitrogen should be applied in three applications: one quarter to one third banded at planting, one quarter to one third sidedressed at emergence, and the remainder at hilling. Avoid excess nitrogen application, as nitrogen not taken up by the plant can potentially contribute to nitrate ground water problems. Additional nitrogen applications through the irrigation system should be based on petiole nitrogen status. Nitrogen rates can be lowered if the potato crop follows a legume or if manure is used. Although nitrogen content of manure varies, nitrogen fertilizer rates can be lowered by about 5 pounds of nitrogen per acre for each ton of dairy manure applied. Poultry manure has about twice the N content of dairy manure.
Table 3. Nitrogen recommendations for potatoes grown on irrigated mineral soils*
|Yield goal||Previous crop|
|Corn, small grains
|cwt/A||N to apply (lb/A)|
|Less than 200||75||50||25|
|*Rates should be split into at least three applications (see text for explanation).|
Nitrogen deficiency in potatoes results in stunted growth, yellowing of the older leaves, dieback of the vine, and poor yields. Low nitrogen accentuates certain diseases such as early blight and Verticillium wilt. Any nutrient disorder which promotes dieback of the vines will enhance early dieback; however, nitrogen is often implicated with early dieback because it is usually the most limiting for plant growth. Excess nitrogen may delay the onset of tuber growth, increase knobby potatoes, and promote excess vine growth.
Because of the relatively short growing season in Minnesota, excessive nitrogen applications on late season cultivars such as Russet Burbank may delay tuber maturity to a point where yield goals are lower than expected. Yields of early harvested potatoes may also be reduced if too much nitrogen is applied. For potatoes, maintaining the proper balance between tuber and vine growth is critical for attaining high yields.
Phosphorus and potassium fertilizer recommendations should be based on soil tests and yield goal (tables 4 and 5). Use of high rates of phosphorus and potassium fertilizers over the years have resulted in high soil test levels. Phosphorus moves very little in the soil and banded applications at planting are sufficient if soil tests are high. Phosphorus availability decreases with low soil pH. If soil pH is less than 5.2, higher rates than those in table 2 may be required. In contrast to phosphorus, potassium soil tests will drop within 2 to 3 years if broadcast applications are omitted. This is because potassium has a high crop removal rate and a tendency to leach in sands. Usually, a broadcast application of potassium is beneficial when potassium soil tests are in the low to medium range (less than 100 ppm). On very low testing soils, a portion of the fertilizer should be broadcast and incorporated before planting and the remainder banded at planting.
Table 4. Phosphorus recommendations for potatoes grown on irrigated mineral soils.
|Phosphorus (P) soil test (ppm)*|
|P2O5 to Apply (lb/A)|
|*ppm x 2=lb/A.|
Table 5. Potassium recommendations for potatoes grown on irrigated mineral soils.
|Potassium (K) soil test (ppm)*|
|K2O to Apply (lb/A)|
|*ppm x 2=lb/A.|
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.
The secondary plant nutrients (magnesium, sulfur, and calcium) can be limiting on sandy soils. Many sandy soils used for potato production have become low in calcium and magnesium because lime applications are avoided. The reason for not liming is that the potential for scab increases when the soil pH is above 5.2. As a consequence, soil calcium and magnesium may reach low enough levels to limit yield or tuber quality. If soil test calcium is less than 300 ppm (600 lb/A) , then 200 – 300 lb/A calcium as calcium sulfate (gypsum-22% calcium) should be preplant incorporated. High rates of potassium fertilizer coupled with low levels of soil magnesium can lead to magnesium deficiency. If soil test magnesium is less than 50 ppm (100 lb/A) and/or tissue magnesium is less than 0.22%, then 20 lb/A magnesium banded or 100 lb/A broadcast as potassium-magnesium sulfate (11% magnesium) should be applied. An alternative to calcium or magnesium sulfate sources would be to apply finely ground dolomitic limestone (20% calcium, 10% magnesium) at rates of 1 – 2 tons/A in the rotation when potatoes are not grown.
Symptoms of magnesium deficiency include a pale green color between the veins on older leaves. In more severe cases the tissue may appear scorched. Symptoms of calcium deficiency are not as well defined as those for magnesium. In most situations, tubers are small and deformed while the foliage appears normal. High tuber calcium has been associated with improved storage ability.
Sulfur is usually low in sandy soils. Except in extremely deficient soils, potatoes do not usually respond to sulfur applications. If soil test sulfur is less than 7 parts per million and/or tissue sulfur is less than 0.18 percent, then 20 lbs sulfur/A (use a sulfate source) should be banded at planting. Sulfur deficiency symptoms include a general yellowing of the younger leaves.
Micronutrients which include boron, chlorine, copper, iron, manganese, molybdenum, and zinc, are required in lower amounts than the other essential nutrients. Most soils contain sufficient levels of micronutrients to meet crop demands; however, in some areas micronutrient shortages occur and may limit yields. In a 5-year study at the Sand Plains Research Farm at Becker, Minnesota, increases in potato yields were reported with boron and zinc applications but not with manganese or copper applications. If soil test boron is less than 1 part per million, then a broadcast application of 2 lb/A actual boron is recommended. Excess boron applications can be toxic—do not over apply. Monitor boron status of the plants using tissue analysis. For soils with zinc levels below 1 part per million, 1 – 2 lb/A actual zinc should be applied with the banded fertilizer at planting. In acid soils, iron, manganese, and copper should be present and available in adequate amounts to meet crop needs. In extremely acid soils (pH less than 4.8) manganese toxicity may be a problem. Tissue manganese levels greater than 1,000 parts per million are often associated with stem streak necrosis. Potato responses to molybdenum and chlorine applications have not been reported in Minnesota.
Symptoms of zinc deficiency include stunting of the plants and upward rolling of younger chlorotic leaves. Symptoms may be similar to leafroll virus. Boron deficiency symptoms include death of the growing points and shortened internodes. Roots are stunted and the tubers are small, showing surface cracking at the stolon end. Localized brown areas or brown vascular discoloration may appear under the skin near the stolon end.
(Source – http://www.extension.umn.edu/agriculture/crops/potato-fertilization-on-irrigated-soils/)