Essentials of nutritional management of potato crops aiming for high productivity

In tropical and subtropical conditions in Brazil, the potato plant has a relatively short cycle, with high growth rates and tuber production per unit area. However, due to the relatively delicate and superficial root system of the potato plant, adequate and balanced fertilization at the right times is essential to achieve high yields of better quality tubers.

Knowing the quantities of nutrients available in the soil and those extracted by the potato plant in its development stages is extremely important to maximize the cost-benefit ratio with fertilization. The extraction of nutrients by potato crops is quite variable, as it depends on several factors, such as: cultivar, soil, climate, spacing and irrigation. Furthermore, it must be considered that, depending on the level of productivity, the quantities of nutrients extracted and exported may vary, although there is not necessarily a relationship between these factors, as there are differences between potato cultivars in the efficiency of nutrient use. .

Fertilization without considering the cultivar's requirements and the availability of nutrients in the soil are the main factors that lead to inadequate, and sometimes even indiscriminate, use of fertilizers in potato cultivation, generating nutritional imbalance, decline in productivity with unnecessary increase in cost of production. Corrective agents and fertilizers are among the items that most impact the variable production cost of potato crops, and in some cases can represent more than 20% of the total production cost, particularly in crops for frying industries. However, the tuber productivity potential of modern potato cultivars can be significantly leveraged with adequate and balanced management of nutrients at the right stages, also increasing the cost-benefit ratio and consequently the producer's profit.

Accurate knowledge of the availability of nutrients for potato plants depends on adequate soil sampling, a critical and underestimated step by technicians and producers who, no matter how qualified and tech-savvy they may be, will never be able to correct errors in soil sampling. poorly done. Furthermore, knowing precisely the nutrient levels in the soil becomes of particular importance for potato plants, as possibly as important or more important than knowing the availability of nutrient levels in the soil is understanding the importance of interactions between nutrients for high tuber yields of modern potato cultivars. These interactions begin with the correction of soil acidity and the supply of calcium (Ca) and magnesium (Mg) to the plants, and can affect an entire fertilization plan for potato crops.

Need for liming

Although potato crops are less demanding on soil reactions that are closer to neutrality, more modern and productive potato cultivars are more demanding on nutrients and extract more basic cations from the soil (Ca, Mg and K). When fertilized with nitrogen fertilizer with an acidic reaction, such as ammonium sulfate, they result in intense acidification in the soil of the ridges. In soils with the potential to release aluminum (Al) toxic to plants (such as the gibbsite soils of central-southern Paraná, among others), this acidification solubilizes this Al3+ in the soil of the ridges, which, in turn, reduces the absorption of Ca by plants and, therefore, limits the filling of tubers. Therefore, despite being tolerant to soil acidity, liming is essential for modern potato cultivars.

Ca is the third most extracted nutrient by potato plants and must be available at adequate levels in the soil during tuberization and tuber growth, since Ca translocation to tubers is reduced. Low levels of this nutrient in tubers have also been associated with greater susceptibility to rot and physiological disorders, such as brown spots and black hearts (and hollow hearts, especially in the case of the Atlantic cultivar). Maximum tuber productivity is normally obtained when pH (CaCl2) is around 5,5 and 6,0 and base saturation is above 60%, situations that increase the efficiency of subsequent fertilization.

Some producers are concerned about raising the pH of the soil to be cultivated with potato crops, due to the possibility of an increase in the incidence of common scab (Streptomyces scabies) in the tubers. Despite this, in recent years high yields and tubers of excellent quality have been obtained even in fertile soils with relatively high pH, ​​as well as there are reports of the incidence of scab in potato tubers even in soils with pH values ​​of 3,9, suggesting that the causal agent is adapted to acidic conditions. In this case, an alternative that has given interesting results is to plan liming further in advance and apply it to the crop preceding the potato crop. This ensures that the soil is corrected when potatoes are grown and prevents sudden changes in pH during the development of the tubers, thus minimizing the possibility of scab occurrence.

Excessive inputs, especially of Ca, via liming can result in an imbalance between it and the amounts of K absorbed by plants, which are notably harmed due to the antagonistic interactions between them; that is, the presence or absorption of one inhibits the absorption of the other. Magnesium (Mg) deficiencies due to excessive Ca inputs with the use of calcitic limestone and phosphate fertilizers containing Ca in the planting line are common and, even if hidden, have an impact on the absorption of another important element for potatoes, phosphorus (P) . This nutrient, although little absorbed by potato plants, is especially responsible for the initiation of tubers and is actively absorbed with energy expenditure by the plants, requiring Mg as a carrier for its symplastic absorption, or that which causes nutrients to enter the cells vegetables. On the other hand, the possibility of P retrogradation with excess Ca in the soil cannot be ruled out, a phenomenon that makes it insolubilized and unavailable for absorption by the roots of potato plants.

Therefore, liming for potato cultivation should not be overestimated, much less underestimated. As long as it is applied with criteria based on rigorously conducted soil sampling, the increase in root development and the consequent absorption of nutrients with the application of limestone, as well as the insolubilization of toxic elements and the increase in fertilization efficiency, are aspects that make it a highly cost-effective input and a basic prerequisite in Brazilian potato farming, as long as it is in the correct dose.

Primary macronutrients

Traditional applications of high doses of the NPK 04-14-08 formula in potato planting furrows exceed the amounts of N and K required in the vegetative phase of the plant, causing increased costs and reduced tuber yields, especially in older years. dry or in areas with poor irrigation.

NO has a great influence on the growth of the aerial part, tuberization and the quality of potato tubers, however, high doses of N, especially at the beginning of the cycle, can promote excessive vegetative growth, increase the incidence of diseases and even reduce the rate of tuber growth and starch storage in them, resulting in lower productivity and worse tuber quality. However, the N doses that provide maximum tuber productivity in potatoes are very variable, depending on several factors: cultivar, seed tuber size, stem density, planting time, soil type and, mainly, previous crop ( history of the area). Extra doses of N during planting, for example, with high doses NPK 04-14-08, do not consider the potential supply of N through the mineralization of soil organic matter, a process that can intensify with the supply of more N via fertilization of planting. Furthermore, it must be taken into account that N absorption by the crop only increases substantially after 35-40 days after planting, when tuber development begins. Precise management of nitrogen fertilization in potato cultivation is one of the main prerequisites for obtaining high yields of better quality tubers.

To define the need for N application within the period of maximum nutritional requirement of potatoes, that is, during the approximately 45 days of tuberization, foliar diagnosis can be a very useful tool that has been improved for more technical production systems. In these, the possibility of foliar nutritional diagnosis in more advanced phases of the cycle, including at the beginning of tuberization, allows diagnosing whether or not the crop is responsive to N application in a more advanced phase of the process. An alternative to making N available gradually during the crop cycle is the use of controlled or gradual release sources, which do not even require top-dressing nitrogen fertilization.

On the other hand, K is the nutrient most extracted and exported by the potato plant, with its export from the tubers normally being 1,8 times greater than that of N and up to 10 times greater than that of P. The application of high doses of K in the furrow planting, using formulas with high concentrations of this nutrient, can reduce the plant population and tuber production, due to the significant increase in electrical conductivity and the imbalance in the K/Ca+Mg ratio in the soil, which can generate an induced deficiency of Mg by competitive inhibition of excess K. Furthermore, excessive doses of K cause a reduction in tuber dry matter. In view of these deleterious effects of the massive application of K during planting, part of it can be transferred to pre- or post-planting with the possibility of gains in productivity of special class tubers and also in operational yield in fertilization and planting of seed tubers .

With positive effects on tuber filling, the application of K can also be done together with nitrogen fertilization before hilling. Synergistic effects with the application of both N and K occur even in soils of medium and high fertility, however in the latter and in years with less rainfall, higher doses of both nutrients can reduce the percentage of dry matter in the tubers. This is not good for those intended for frying. Therefore, care is needed, very well done soil sampling to diagnose the availability of K and accounting for the amount of K applied at planting are essential to plan potassium top dressing.

Mainly for potatoes intended for industrial processing and when applications of high doses of K are not necessary, the use of other sources of K instead of potassium chloride can be interesting, leading to greater quality of the tubers, especially in terms of the percentage of dry matter . Another aspect that must be considered, both with potassium fertilization for planting and covering, is that the Cl supplied with KCl may require the application of more N for potato plants, since the absorption of this anion is antagonistic to the absorption of nitrate (NO3-) by plants. In amended and well aerated soils this is the predominant form of N.

Despite being one of the macronutrients absorbed in smaller quantities by potato crops (around 0,5 kg of P per ton of tuber produced), P is normally the nutrient applied in greater quantities in this crop, with doses that can exceed 600 kg ha-1 of P2O5. Indeed, in soils with low levels of available P, crop responses to phosphate fertilization have been high. This is due to the low efficiency of the crop in absorbing P from the soil, which is mainly related to its restricted root length. However, the response to phosphate fertilizer is smaller and even occurs at much lower doses when the P content available in the soil is medium or, mainly, high. Therefore, it is extremely important to coherently interpret scientific results (or recommendation tables) and evaluate the extent to which the productivity increases achieved with increased P doses are economically viable for the producer. In short, adequate availability of P is essential to achieve high productivity, however the definition of the dose of phosphate fertilizer must take into account soil analysis, as, in soils well supplied with P, its addition has little effect on production or on the quality of tubers.

Secondary macronutrients and micronutrients

As potato cultivation requires the application of high doses of K and possibly Ca, there may be an imbalance between Mg and these other cations in the soil. Even though we do not absorb high amounts of Mg (normally about 0,32 kg of Mg for each ton of tuber produced), Mg deficiency can greatly affect the productivity, size and quality of tubers (dry matter, starch and reducing sugars). ). Therefore, fertilization with Mg aiming to balance the availability of this nutrient in the soil in relation to K and Ca is very important, especially critical due to its function in the absorption of P by plants. Recent studies have indicated that Magnesian fertilization can have positive effects, even in soils with high Mg levels. This is because high levels in the soil and/or high doses of Ca and/or K applied can cause imbalance and are antagonistic to Mg absorption.

OS is the macronutrient required in smaller quantities by potato crops. However, in soils with levels of this element considered low (<5,0 mg dm-3 of S-SO42-) and also with low levels of soil organic matter (< 20 g dm-3), it must be supplied via plaster or fertilization, with several sources and formulas used in potato cultivation.

B is the micronutrient that has had the greatest effects on crop yields, especially when soils are deficient in this micronutrient, and its application is recommended in soil with a medium or low B content. To define the dose of B to be applied, soil analysis is also a fundamental tool, since the threshold between deficiency and phytotoxicity for plants is very narrow in this particular case.

Potato crop responses to other micronutrients have been less frequent, probably because in Brazilian soils, in general, there is high availability of Fe and Mn and pesticides frequently used in the crop provide Cu, Mn and Zn. Even so, attention to the availability of all micronutrients in the soil, especially in more amended soils, as well as the application of recommended doses via fertilization are important strategies for obtaining high potato productivity. Associated with heavy fertilization with P via NPK formulas, mainly 04-14-08, foliar fertilization with cationic micronutrients has been carried out by producers, which in theory, can have a palliative effect in the face of excess P applied, which can cause precipitation and insolubilization of cationic micronutrients.

Supplements with foliar applications, especially micronutrients, but also some macronutrients, such as Mg, K, N and S, and beneficial elements such as Si, can result in productivity gains. Furthermore, the use of biostimulants, plant bioactivators and beneficial microorganisms, whether in the furrow, aiming for greater rooting, initial growth and tuberization, or via the foliar route, to provide maintenance of the active leaf area and, consequently, greater filling of the tubers, can also contribute to gains in productivity and technological quality of tubers. However, it should not be forgotten that the basis for high potato productivity begins with an accurate and correct soil analysis, adequate acidity correction and balanced supply of nutrients via the soil. Finally, well-made “rice and beans” also guarantees potatoes on the table with greater cost-benefit and profitability for the producer.

By Rogério Peres Soratto (FCA and CERAT/UNESP) and Renato Yagi (IDR-Paraná)

Article published in issue 145 of Revista Cultivar Hortaliças e Frutas