Introduction:

Potato (Solanum tuberosum L.) is the third largest food crop in world, with an annual global tuber production of around 370 Mt [1]. In the Mediterranean basin, the million hectares dedicated to its production yields ∼25 Mt of tubers [1], with most of the crop targeted at the ‘‘early’’ crop market [2,3].

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 (Table 1).

Early” potato tubers are a proven source of vitamin C, vitamin B6 and essential minerals including mainly potassium, but also magnesium, but also magnesium, phosphorus, manganese, zinc and iron to a lesser extent [4]. Essential mineral elements are a class of nutritionally important nutrients which play crucial roles in various biological processes for both plants [5] and human beings [6]. For humans, deficiencies in these elements can cause metabolic disorders and organ damage, leading to acute and chronic diseases and even death [7]; so an adequate dietary intake of mineral elements is necessary for human health and wellness. Unfortunately, mineral malnutrition is still a common problem worldwide and considered one of the most important global challenges for human nutrition [8, 9]. Notably, the micronutrient contents of several crops, including vegetables [9], have declined due to a number of factors, including the use of high yielding crop varieties [10, 7],

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.

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

Soil testing

Fundamental to any effective nutrient management program is a reliable soil analysis and soil test interpretation. Take samples in the top six to 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, although the soil test for S on sandy soils is usually low. 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, but may be deficient in alkaline soils.

Tissue analysis

You can use plant tissue analysis or tissue testing 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. Optimum nutrient ranges provide the basis behind tissue analysis (Table 2). 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 sufficiency levels are much higher for 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 much less nitrate-N in leaflets and total N provides a more accurate measurement of N status for whole leaves.

Figure 1: Top 10 Potato Producing Countries Map

Figure 2:  Micronutrients and Soil Microorganisms in the Suppression of Potato

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)

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 provides 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

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. Irrigation water can be quite alkaline in Minnesota and may also help to slow down soil acidification processes.

Nutrient management suggestions

Potatoes have a relatively shallow root system with most roots located in the top 1.5 to 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 to supply all or a portion of immobile nutrients, such as phosphorus and potassium. 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. For potatoes, yield goal is associated with market class, growth habit (determinate or indeterminate) and the time of the season the vines are killed.

Nitrogen

Of all the essential elements, N is the one most often limiting for potato growth, particularly on soils with low organic matter. 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.

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. 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).

Table 4: P sufficiency levels for Russet Burbank potatoes

Conclusions

The observations about the mineral profiles of the studied samples suggests that the application of foliar micronutrient fertilizers was able to fortify raw potatoes by improving the content of iron and zinc which are important for a healthy diet. The better concentration of minerals in micro-fertilized raw potatoes also led to a better concentration of micro-fertilized minimally processed potatoes, even if some minerals were lost in processing, presumably due to the elimination of the peel. The agronomic approach via foliar microelements-containing solutions proved to be a sustainable, economical and fast strategy to increase micronutrients concentration in early potato tubers. Fertilizer Timing and Placement

Band all phosphorus. Apply 50% to 70% of N and 50% of K2O at emergence and the remaining N and K at 35 to 40 days after planting. Potatoes planted in cool soils might respond to up to 25 lb/ac P2O5applied as starter fertilizer in the furrow with the seed pieces. Fertilizer Sources Supply 25% to 50% of the N in the nitrate form if soils were treated with multipurpose fumigants or if the soil temperature will stay below 60°F for up to one week following transplanting or germination. Water Management Fertilizer and water management are linked. Maximum fertilizer efficiency is achieved only with close attention to water management. Supply only enough irrigation water to satisfy crop requirements. Excess irrigation may result in leaching of N and K, creating possible plant deficiencies. For subsurface irrigation, maintain a constant water table between 18 (at planting) and 24 inches (near harvest) below the top of the bed. Monitor water table depth and do not fluctuate to avoid fertilizer loss below the root zone.

Table 5: Variance analysis of the effects of soil texture and water stress on the growth of potato aerial parts

Nutrient requirement of potato crop is quit high and the application of fertilizers and organic manures is considered essential to obtain economic and high yields. In light soils and places where organic manures are not easily available, green manuring is beneficial. The optimum dose of fertilizer application varies greatly depending upon the soil type, soil fertility, climate, crop rotation, variety, length of growing season and moisture supply. A fertilizer dose of 180-240 kg N, 60-90 kg P O and 85- 2 5 130 K O per hectare is recommended for alluvial soils of Indo Gangetic plains. In the hill zone, the 2 application of 100-150 kg N, 100-150 kg P O and 50-100 kg K O per hectare is recommended. In black 2 5 2 soils of plateau areas about 120-150 kg N, 50 kg each of P O & K O is recommended. In the acidic 2 5 2 soils of southern plateau 120kg N, 115 kg P O , and 120 K O kg per hectare are recommended for 2 5 2 potato production.

Funding

This work was financially supported by  Semnan university.

Acknowledgments

The authors are grateful   Semnan university in Iran in technical assistance.