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.

Soil testing

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.

Tissue analysis

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.

Soil pH

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.

 | 

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.

 | 

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. 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.

Carl J. Rosen

Reviewed in 2018

Share this page:

© 2018 Regents of the University of Minnesota. All rights reserved. The University of Minnesota is an equal opportunity educator and employer.