Importance of potassium in the quality of strawberry production

The different regions of Brazilian production, in addition to presenting climatic variations, have different types of soil with different levels of fertility. Cultivars, soil types, planting system, density and destination of the fruit (in natura or industry) are factors that contribute to variations in the nutritional needs of plants. In Brazil, fertilizer recommendations for strawberry cultivation are regionalized, with specific interpretation and recommendation manuals for the main producing states.

Among the most important factors for maintaining a constant supply of nutrients to plants is soil pH, which is maintained in the optimal range (5,8 to 6,2) through liming. In Rio Grande do Sul and Santa Catarina, the amount of lime to be applied to strawberry cultivation aims to maintain the pH at around 6. In the states of Minas Gerais and São Paulo, the liming recommendation takes into account saturation. by bases and the magnesium content of the soil. In São Paulo, it is recommended to increase base saturation to 80% and a Mg content⁺² greater than 0,9cmol_c dm^-3, while in Minas Gerais the saturation must be increased to 70% with a Mg content⁺² minimum of 1cmol_c dm^-3. Limestone is generally used to correct acidity, preferably dolomitic, as the strawberry plant is quite demanding on magnesium.

Fertilization is one of the main management practices in strawberry cultivation and aims to overcome a possible lack of nutrients in the soil. Among the nutrients, potassium (K) plays an important role in plant formation and maintaining fruit quality.

Part of Brazilian soils lack K. One of the reasons is that the soluble form, used by plants, is easily leached. Furthermore, the supply of K to plants largely depends on its diffusion into the soil, an aspect that is strongly dependent on soil moisture. In this sense, irrigated cultivation systems favor the use of K present in the soil by the strawberry plant.

K performs several metabolic and structural functions in plants, with an important role in regulating the osmotic potential of cells and acts as an activator of numerous enzymes involved in the metabolic processes of respiration and photosynthesis. Plants adequately nourished with K have greater tolerance to stress (frost, wind, drought, pests and diseases). Potassium favors the size, texture and organoleptic characteristics of strawberries (flavor, aroma, sugars and vitamin C). Low potassium levels are associated with increased incidence of fungi Verticillium, Pythium, Phytophthora e rhizoctonia. Excess potassium in the soil decreases the absorption of magnesium, in addition to calcium, to a lesser extent.

In strawberries, K is the most extracted nutrient, followed by nitrogen, calcium, magnesium, sulfur and phosphorus. On average, approximately 130 kg of K are needed₂For the production of 50 tons/ha of strawberries.

In the predominant production systems in the main producing regions, two forms of fertilization are carried out: pre-planting and production. Pre-planting fertilization is carried out when preparing the beds, where the fertilizer is incorporated into the soil. Now, after planting the seedlings, strawberry fertilization is carried out mainly by fertigation, which begins around 30 days after planting, with weekly applications of macro and micronutrient nutrient solution. This type of fertilization is composed of salts or soluble fertilizers, with potassium nitrate being the most used source of K.

The necessary amount of K to be applied to a crop must be determined by jointly evaluating the results of leaf and soil analysis. Through these results, it is possible to diagnose whether the nutrient levels in the plant are adequate, lacking or excess. As well as identifying whether the nutrient levels present in the soil are sufficient to meet the needs of the crop.

For leaf analysis, it is recommended to collect the third and fourth leaves (without petiole) from the apex, of 50 plants at the beginning of flowering. The K levels considered adequate in the leaves, to obtain good production, are between 2% and 3%.

In addition to checking nutrient levels in the soil, it is extremely important to monitor soil salinity. Salinity can be managed by controlling the electrical conductivity (EC) of the nutrient solution. EC values ​​between 1,4dS/m^-1 and 2,0dS/m^-1 provide good strawberry quality and productivity. On the other hand, in situations of high salinity there may be problems in the absorption of some ions, especially K⁺.

The amounts of fertilizers to be applied vary depending on the production region. In Rio Grande do Sul, the application of 60kg/ha, 80kg/ha, 120kg/ha, 160kg/ha and 200kg/ha of K is recommended.₂O, in pre-planting fertilization, when the levels in the soil are very high, high, medium, low and very low, respectively. In São Paulo the amounts of K₂They vary from 100kg/ha, 200kg/ha, 300kg/ha and 400kg/ha when the concentration of K in the soil is high, medium, low and very low, respectively. In Minas Gerais, for K concentrations in the soil considered very good, good, medium and low, the application of 56kg/ha, 105kg/ha, 175kg/ha and 245kg/ha of K is recommended.₂O, respectively.

The main potassium fertilizers used in agriculture are potassium chloride (KCl – 58% K₂O and 45%-48% Cl), potassium nitrate (KNO₃ – 44% to 46% of K₂O and 13% to 14% of N) and potassium sulfate (K₂SO₄ - 48% K₂O, 15%-17%), for strawberry cultivation, care must be taken when administering fertilizers based on potassium chloride, as strawberry plants are poorly tolerant to chlorine.

Symptoms of potassium deficiency can easily be confused with those of magnesium deficiency, or with leaf scorch caused by salinity, wind, sun or water deficiency. In the plant, it can initially cause a generalized reduction in growth, later the leaves turn a reddish-purple color that progresses to necrosis of the leaflet edge and limb. As potassium is a mobile nutrient within the plant, these symptoms normally appear in older leaves. Furthermore, potassium deficiency affects root development and shoot dry matter production. This is because potassium is involved in the process of photosynthesis. A lack of this nutrient can cause a decrease in the photosynthetic rate and a reduction in the plant's carbohydrate reserves. On the other hand, it also prevents the stomata from opening regularly, which can lead to less CO assimilation.₂ in chloroplasts and consequently cause a decrease in photosynthetic rate.

Another aspect that can be harmed by potassium deficiency concerns the organoleptic characteristics of the fruits (color, texture and flavor), which may result in loss of firmness and reduced shelf life. A problem that occurs because deficient plants have less rigid tissues, as a consequence of the thinner cell wall, in addition to less formation of sclerenchymatous tissues, less lignification and suberization. Research already carried out has verified improvements in the physical-chemical quality of strawberries due to potassium fertilization, with an increase in soluble solids content being observed up to a dose of 600 kg of K₂O/ha for the Camarosa cultivar and up to 464kg of K₂O/ha for the Sweet Charlie cultivar, showing different responses according to the cultivar adopted.

On the other hand, excess K can cause dehydration and rupture of cell membranes, causing the appearance of necrotic spots on the leaves. Plants with excess K may also have impaired absorption of calcium and magnesium, as these nutrients compete for the same absorption sites. Effect that can reduce the quality of the fruits, as well as the storage period. Some researchers also indicate that very high levels of K in the soil can reduce production and the number of fruits. According to studies, the cause of this reduction in production would be due to the combined effect of the low supply of photoassimilates, resulting from the reduction in leaf area growth and changes in the absorption of calcium and/or magnesium induced by K.

Therefore, potassium fertilization must be carried out on strawberry crops to improve the organoleptic quality of the fruits, however, it is recommended to be careful not to apply excess K so as not to harm productivity. Therefore, its application must always take into account the history of the crop, cultural treatments carried out, amount of nutrients available in the soil, cultivar planted, environmental condition, among others.

Fruit production and consumer habits

The eating habits of Brazilian consumers have been changing in recent years, following a global trend, which is the search for a healthier diet. In the recent past, eating habits were more focused on the consumption of industrialized products. However, nowadays, the search for healthier products has increased the consumption of fruits and vegetables.

Brazil is the third largest fruit producer in the world. Among the most cultivated species is the group of small fruits, which includes the strawberry, which is cultivated in regions with a subtropical and temperate climate in Brazil, with the main areas located in the states of Minas Gerais, São Paulo and Rio Grande do Sul. South.

Figure 1 - Pre-planting potassium incorporation

Figure 2 - Strawberry plants with adequate potassium fertilization

Figure 3 - Quality of strawberry fruits with adequate potassium levels

Click here to read the article in issue 81 of Revista Cultivar Hortaliças e Frutas.