1. Introduction
Sour cherry trees uptake considerable amounts of nutrients from the soil, and the uptake level of potassium is the second highest, after nitrogen. Annual potash intake by 8–10-year-old sour cherry trees is 27.9 kg/ha [
1].
This element is an essential nutrient for sour cherry productivity and fruit quality because of its important role in many physiological processes. Potassium is not part of the organic macromolecules but is present in plant cells in its ionic form (K
+), which serves as the most important osmotic compound and has an effect on turgor-driven processes like stomatal movement [
2,
3,
4]. Another essential function of potash is connected to its participation in photosynthetic processes. The element’s deficiency is associated with a reduction in the photosynthetic capacity of CO
2 assimilation and anatomical alterations in leaf structure [
5]. Also, potassium contributes to the long-distance phloem transport of photoassimilates. This process is especially important for the fruit trees, which require the translocation of sucrose from photosynthetically active organs (leaves) into sink ones (fruits) [
2]. A positive impact of fertilization with potassium on fruit tree productivity has been repeatedly reported [
6,
7,
8,
9].
On the other hand, excessive rates of potash fertilizers decrease Ca, Mg and Mn uptake by cherry trees and lead to degradation in fruit quality [
10,
11]. Both a deficit and abundance of potassium can result in the imbalance of physiological processes and decrease plant productivity [
2,
12,
13].
Potassium is removed from stone fruit orchards with yield and often with pruned shoots, especially in intensive horticulture. Annual removal impoverishes the soil supply of plant-available potassium compounds. The long-term growing of fruit trees without fertilization led to a significant decrease in exchangeable potassium reserves in orchard soil over 6–8 years [
14,
15]. However, sometimes the rates of potash-containing fertilizers applied in orchards significantly exceed the plant needs for this nutrient [
16]. Such excessive fertilization results in the decrease in fertilizers’ agronomic efficiency [
17]. Thus, for sustainable stone fruit orchard production it is necessary to apply potassium fertilization with rates according to tree needs and to ensure profits for fruit growers. These optimal rates may vary significantly depending on soil–climatic conditions and demands of fruit crops and cultivars.
To determine the optimal potassium rates for orchards planted in different soil types, it is necessary to combine various methods of soil nutrient investigation with the chemical analysis of plant tissues. The availability of soil potassium for plants is linked to the inter-relation of different potassium compounds being in dynamic equilibrium (exchangeable, non-exchangeable and water-soluble) [
18]. The study of these inter-relations enables a complex view of the potassium issue in the orchard agroecosystem.
In the Russian Federation, the assessment of the potassium supply in agricultural soils is based on the determination of exchangeable potassium. These compounds are extracted from the soils with the help of 0.2 mol/dm
3 HCl (Kirsanov method), 0.5 mol/dm
3 CH
3COOH (Chirikov method) or a 1% solution of (NH
4)
2CO
3 (Machigin method), which are recommended for acidic, neutral and calcareous soils, respectively [
19].
Water-soluble potassium is the most mobile part of exchangeable potassium [
18]. It is most accessible to roots and moves easily in the soil profile. The amount of the element extracted by water characterizes the current level of plant potassium nutrition. The content of water-soluble potassium reduced by plant uptake usually replenishes quickly from the reserves of exchangeable compounds with sufficient soil humidity. Thus, this indicator reveals the ability of the soil to desorb potassium ions into the soil solution.
The non-exchangeable potassium, in turn, is a reserve for the replenishment of more mobile bioavailable compounds. Plant roots release H
+ into the rhizosphere [
20], which results in the release of non-exchangeable potassium from clay minerals into the soil-adsorbing complex [
21]. The application of NH
4+-containing fertilizers also leads to alterations in the ratio of potassium compounds with unequal availability for plants [
22]. In long-term field experiments, the non-exchangeable potassium content in topsoil and in subsoil is a sensitive indicator revealing the early stages of soil degradation [
23].
Stone fruit orchards grow on the same plot for a long period, and potassium uptake by trees is continuous, so the potash status of the orchard soil changes gradually. The floor management system and irrigation and nutritional management also affect the inter-relation of potassium compounds in orchard soil from year to year.
Fertilization with potassium is often ineffective in the first years after planting an orchard if the soil originally had heavy texture and favorable agrochemical properties [
24,
25,
26]. However, with increasing tree age, the efficiency of potash fertilizers usually increases, and 10–20-year-old trees are more responsive [
8,
27,
28,
29]. Therefore, the need for potassium in fruit trees increases as they grow and enter the fruiting stage, and the rates of potash fertilizers applied in the orchard should be increased according to the needs of the plants. To determine the optimal period to begin fertilizer application, long-term field experiments should be carried out in orchards growing in various soil–climatic conditions.
Currently, the potassium regime of orchards is poorly studied, and long-term data are collected mostly for pome crops. Sergeeva et al. [
30] ascertained that without fertilization the content of exchangeable potassium in chernozem gradually decreased by half over 10 years after planting an apple orchard. In the next 10 years, this indicator remained consistently low, which measured up to a new equilibrium ratio of different potassium compounds in the soil-absorbing complex. Kuzin et al. [
31], within two vegetation periods, investigated the seasonal dynamics of exchangeable potassium in the meadow-chernozem soil of a high-density apple orchard and recorded similar dynamics in two experimental plots planted with different cultivars. In this experiment, the potassium content varied depending on the soil moisture and fertilizer treatment, but every year the lowest potassium level was in August and September (intensive growth and ripening of apples).
The interannual dynamics of plant-available potassium in the 0–20 cm layer of Humic Cambisol was studied in a pear orchard during 2010–2017 [
24,
27]. In the topsoil (0–10 cm) of unfertilized plots, the potassium content decreased sharply (about 40%) in the second year of the experiment, and in subsequent years the index was relatively stable. In the subsoil (10–20 cm), the decreasing of available potassium was more gradual: it decreased by three times in five years.
Currently, the potassium regime of stone fruit orchards is much less studied than that of pome ones. Nutritional management for stone fruit crops is often based on the assessment of fertilizers’ effect on fruit yield and quality with no consideration of soil properties. The complex view of the potassium issue in stone fruit orchards regarding both the plant diagnosis of potash nutrition and the study of soil potassium ‘behavior’ in specific soil and climatic conditions may be useful for the elaboration of precise nutritional management for specific crops.
The purpose of this research was to study the interannual and seasonal dynamics of different potassium compounds in orchard soil and the potassium status of sour cherry trees affected by the application of nitrogen and potash fertilizers. The results of this research might be useful for adjusting the rates and timing of fertilization with potassium in sour cherry orchards growing in conditions of the East European Plain.
3. Materials and Methods
3.1. The Soil and Meteorological Conditions
The investigation of the relationship between soil potassium conditions and potassium content in fruits and leaves of sour cherry were carried out at the experimental site of the Russian Research Institute of Fruit Crop Breeding, located in the forest-steppe zone of the Central Russian upland (Orel region), Russia. The Orel region is located in a temperate continental climate zone at an altitude of 203 m above sea level. The absolute maximum in summer is +40 °C, the sum of temperatures above +10 °C is 2250 °C and the growing season lasts 175–185 days. The climate in the region is moderately continental with an average annual temperature of 5.5 °C and an average annual precipitation of 450–550 mm. Soil of experimental site is classified as loamy Haplic Luvisol (IUSS Working Group WRB, 2015) with favorable agrochemical characteristics which are presented in
Table 13.
The temperature regime during the 2018–2020 vegetation period was close to the long-term average, and in some months, the temperature exceeded the average monthly level by 0.9–3.5 °C. However, May 2020 was cold, and was unfavorable for fruit setting (
Table 14).
The total amount of precipitation and its regularity varied significantly in different years. Contrasting moisture conditions were a feature of the growing season in 2018: two drought periods were observed from late May to mid-July and in August, while 119 mm of precipitation fell from July 13 to 25. In 2019, the dry period lasted from late May to the third decade of June; in the following months, the periods of precipitation were more regular. In 2020, the greatest amount of precipitation (217 mm) fell from May to July (the period of fruit growth and ripening) (
Table 15). In 2020, due to heavy rainfall, the fruits ripened 10 days later than usual.
3.2. Experimental Design and Treatments
We studied the interannual and seasonal dynamics of different potassium compounds in the soil and potassium status of sour cherry trees in the rainfed orchard planted in 2015 with an allocation scheme of 5 m × 3 m. In this orchard, the field experiment in studying mineral fertilizers’ efficiency started in 2017.
The soil of the experimental orchard initially had a high content of available phosphorus. Because of this feature, nitrogen and potassium fertilizers were chosen for the experiment to optimize the nutritional management of the studied crop. Organic and mineral fertilizers were not applied in the orchard before the start of the research.
The experimental treatments were as follows: 1. without fertilizers (control); 2. N30K40; 3. N60K80; 4. N90K120; 5. N120K160 kg/ha. The applied treatments based on the results of infrequent studies of sour cherry nutrition were performed in similar soil and climatic conditions. The rate of nitrogen and potassium in these investigations varied from60 to 180 kg/ha [
76,
77]. Bearing in mind the modern tendency to minimize the fertilizer rates in stone fruit orchards [
78], we also applied the minimum rate of N30K40.
Fertilizers in the form of urea (46% N) and potassium sulfate (52% K2O) were applied annually in early spring (April) to a depth of 10–15 cm in a 2.2 m wide strip with the center in a row of trees. The experiment was carried out in three repetitions with a randomized arrangement of plots with 4 measurement trees in each plot. The soil management in the rows of trees consisted of herbicide treatments, but in the inter-rows, ploughing was conducted from 2015 to 2019 and the grass in the inter-rows was mowed from 2020 onwards.
The experiment was performed with sour cherry tree cv. ‘Turgenevka’ on the Prunus mahaleb rootstock. The ‘Turgenevka’ cultivar was chosen due to its frost and disease resistance. The fruits of this cultivar are popular with consumers for their large size (5 g) and dark red, juicy dense flesh.
3.3. Soil Sampling and Analysis
Soil samples were taken between trees in the subcronal zone at a distance of 1.0–1.2 m from the tree trunk at depths of 0–20, 20–40 and 40–60 cm. Sampling was carried out with the soil auger five times during the growing season from May to September 2018–2020. Mixed samples (about 500 g) were made of three point samples from each experimental plot of each repetition. Thus, every month we took 45 soil samples. The collected samples were dried at room temperature for a week and grounded and sieved through a 2 mm sieve.
The content of exchangeable and water-soluble potassium compounds was determined in these samples using the flame photometric method. Exchangeable potassium was extracted with 0.2 mol/dm
3 HCl solution at a 1:5 soil:solution ratio. The suspension was shaken for 1 min on a horizontal shaker and was filtered for15 min after shaking [
79]. The 1:5 soil:water ratio was used to extract water-soluble compounds. The suspension was shaken for 3 min on a horizontal shaker and was filtered for 5 min after shaking [
79].
Soil sampling to determine non-exchangeable potassium was carried out twice, at the end of September 2017 (1st year of fertilization) and in 2020 (4th year of fertilization). Non-exchangeable potassium was extracted from the soil with10% HCl at a1:5 soil:solution ratio. The suspension was kept in the thermostat at 90 °C for1 h and then was filtered [
79]. The non-exchangeable potassium of soil colloids and potassium fixed by soil from fertilizers were extracted by this method. The potassium concentration in extract was also measured with a flame photometer. The level of non-exchangeable potassium in soil was calculated as the difference between the value of potash in the extract with 10% HCl and the content of exchange compounds extracted with 0.2 mol/dm
3 HCl.
3.4. Plant Sampling and Analysis
The leaf samples were taken 3 times during the growing season in June (30–40 Days After Full Bloom (DAFB), July (60–70 DAFB) and August (90–105 DAFB). From each plot of each repetition, 40 fully developed leaves were collected around the trees from the middle part of the annual shoots. Leaves were dried at room temperature for a week and then in a drying chamber for 6 h at 40 °C. Then, all plant material was homogenized. The dry leaf material was a shed in a muffle furnace at a temperature of 450 °C for 32 h. The ash was dissolved in 20% HCl. The resulting solution was diluted 10 times and then the potassium content was determined.
Mixed fruit samples (~1000 g) were taken from each plot of each repetition during harvest. The small portions of whole fruits (22–26 g each) were randomly picked from mixed samples and dried at a temperature of 70 °C in a drying chamber for about 96 h. Then, the analysis was performed similarly to that of the leaf samples.
The determination of potassium in ash solution and in soil extracts was performed with a Sherwood 410 flame photometer (Cambridge, Great Britain)
The mixed fruit samples were also subject to the evaluation of fruit quality traits. Total soluble solids (TSS%) were measured with a PAL-3 Digital Refractometer for Refractive Index and Brix (ATAGO, Tokyo, Japan), and titratable acidity (TA%; malic acid) was measured by titration with 0.1N NaOH [
80]. The total sugars (TS) were determined by Bertrand’s method [
80], which is based on the reducing action of sugar on an alkaline solution of tartarate complex with cupric ion. The cuprous oxide formed was dissolved in a warm acid solution of ferric alum. The ferric alum was reduced to FeSO
4, which was titrated against standardized KMnO4; Cu equivalence was correlated with the table to obtain the amount of reducing sugar. This is based on the alkaline solution of the tartarate complex of cupric ion.
3.5. Fruit Weight and Yield
Harvesting took place on 7–8 July, 6–8 July and 17–20 July in 2018, 2019 and 2020, respectively. Fruit harvest was measured by the weight method considering fruit weights from each measurement tree. The weight of a single fruit was measured from a sample of 200 randomly selected fruits.
3.6. Statistical Analysis
The data were subject to dispersion analysis with Microsoft Office Excel 2007. The means were compared with the LSD test. To determine the relationship between the content of potassium in soil and plants, a Pearson correlation analysis was performed, the statistical significance of which was evaluated by Student’s t-criterion. The results are expressed as mean ± standard deviation for three repetitions. The significance level for all statistical analyses was 95% (p < 0.05).
4. Conclusions
The present study clarified some characteristics of the potassium regime in the loamy Haplic Luvisol under a rainfed sour cherry orchard. While the supplies of mobile and fixed potassium compounds in unfertilized soil were linked to soil mineralogy and texture, the interannual and seasonal dynamics of plant-available potash depended on the weather patterns and the uptake of potassium by trees. In 2020 (6th year after tree planting), the first signs of potassium nutrition insufficiency appeared, such as low leaf and fruit potassium status and a decrease in non-exchangeable potassium reserves in the 20–40 cm soil layer in unfertilized plots.
The annual ground fertilization with potash and nitrogen in rates from N30K40 to N120K160 led to the gradual accumulation of exchangeable potassium at depths of 0–20, 20–40 and 40–60 cm. The accumulation rate increased with the fertilizer rate increase.
When the exchangeable potassium level in the topsoil reached 200 mg/kg, the intensification of both the seasonal fluctuations in potash content and the potash leaching into the soil depths occurred in all experimental treatments. Thus, the content of exchangeable potassium exceeding the above-mentioned level was excessive, and the K+ cations were more weakly fixed by the soil-adsorbing complex.
The presented results confirm the expediency of regular fertilizer application with the rates appropriate for the conservation of potassium balance in ‘soil–tree’ systems. In the conditions of our experiment, one-time treatments with superfluous potassium rates (over 80 kg/ha) did not provide an increased stock of plant-available potash in soil but caused unreasonable losses of it due to leaching.
The evaluation of leaf and fruit potassium status and yield and fruit quality parameters correlates with the results of soil diagnostics. An increase in fertilizer rates was not essential for normal metabolic processes and did not manifest itself as an increase in potassium content in leaves and fruits or as an increase in yield.
Since our experiment was carried out in a relatively young sour cherry orchard, the augmentation of tree needs for potassium in the course of their growth and productivity increase should be considered. Also, the potassium removal rates may increase in the future, and for keeping its balance in the ‘soil–tree’ system, higher fertilization rates will be required.