Effects of Potassium Fertilizer Application on Festuca arundinacea I: Plant Growth and Potassium Requirement

This study sought to clarify the effects of potassium (K) fertilizer application on the growth and K requirement of Festuca arundinacea and determine the appropriate K fertilizer application rate for its normal growth. A pot experiment was conducted to test the plant growth and K uptake of Festuca arundinacea response to different K fertilizer rates (0, 100, 200, 300, and 400 mg K2O kg−1) in two K levels (medium K and low K) of soils. Potassium fertilizer application and soil significantly affected plant growth, K uptake and K concentration in the shoot, and K uptake in the root. Low-K soil and without K fertilizer application increased the root-shoot ratio. Increasing K fertilizer application enhanced K distribution in the shoot. The recommended optimum K fertilizer rate to obtain 80% of the maximum shoot biomass in medium-K and low-K soils was 88.9 mg K2O kg−1 and 71.1 mg K2O kg−1, and the corresponding critical K concentration of shoot was 17.9 g kg−1 and 14.4 g kg−1. Appropriate K fertilizer application could promote plant growth and K uptake. An optimized K fertilizer management strategy and K diagnostic concentration should be carried out according to soil K levels during the establishment phase of Festuca arundinacea.


Introduction
Potassium ion (K + ) is the most abundant inorganic cation in plants and plays an important role in many fundamental processes, such as enzyme activation, protein synthesis, material transportation, and osmotic regulation (Römheld and Kirkby 2010). Plants absorb K + from the environment through cytoplasmic membrane K + transporters and K + channels and then efficiently transport it to the shoot (Duan et al. 2019). Many studies have proven that sufficient potassium (K) supply can increase K + absorption (Wu et al. 2019;Mardanluo et al. 2018), improve plant resistance to adversity (Nascimento et al. 2021;Li and Si 2019;Shahid et al. 2019), and promote plant growth (Huang et al. 2019;Shahid et al. 2020). Deficient or excessive K supply can affect various physiological or metabolic activities in the organisms and, as a result, limit plant growth and quality (Tewari et al. 2021).
Plants absorb K from the environment in two sources: soil and fertilizer (Zörb et al. 2014). The K level of soil significantly affects the growth and development of plants (Komosa and Szewczuk 2002). With the current increase in yield and multiple cropping index, as well as the unbalance nutrients input in traditional agriculture management practice, soil K has decreased significantly and the overall K balance has shown a deficit (Ji et al. 2017;Philp et al. 2021;Fan et al. 2013). China is a country with scarce inorganic K fertilizer resources, nearly 50% of K annually used in agriculture has been dependent on imports in the past ten years (FAOSTAT 2021). Soil K deficiency and the shortage of K fertilizer have become the important limiting factors for the development of agricultural production in China and even in the world (Niu et al. 2019;Ӧborn et al. 2006). Limited resources are now forcing us to investigate how the plants' Shen Zhang has the same contribution as the first author K + demand can be satisfied with the minimum of fertilizer application and to realize green and sustainable agricultural production (Dreyer 2014).
Scientific and rational application of K fertilizer is necessary in plant cultivation system. The abundance-deficiency index method based on soil available K test or plant tissue K concentration test is usually used for plant nutrition diagnosis and K fertilizer recommendation (Zhang et al. 2010), and it has achieved good results in optimizing K fertilizer application rate and improving K utilization efficiency in recent years. However, the related research and this technology extension are mainly concentrated in grain crop and economic crop production area (Lu et al. 2017), and rarely on the turf.
Potassium, as one of technically macronutrients, is often deficient in soils on which turf is grown, and has received the greatest attention in the development of a fertility program for turf (Christians et al. 2016). Previous studies have shown that the application of K fertilizer can promote the growth of turfgrass and enhance its adaptability to adversity (Wu et al. 2019;Schmid et al. 2018), which is beneficial to promote the turf cover and turf quality, prolong the lawn green period, and improve the ecological environment (Ihtisham et al. 2018). Recent studies have shown that the effect of applying K fertilizer on the growth of turfgrass is closely related to the K level of the soil (Ihtisham et al. 2020;Dokbua et al. 2021). Thus, it is necessary to understand the response of turfgrass to K fertilizer and establish an optimized K fertilizer application strategy and K nutrition diagnosis technology under different soil conditions. Festuca arundinacea, a poaceae, provides a high-quality turf. It is most widely used cold-season turfgrass on athletic fields, lawns, parks, and urban landscapes in cold temperate, temperate, and tropical alpine regions (Surhone et al. 2010). Thus, it is also wildly cultivated in northern China . Till now, little is known about effects of soil initial K and applying K fertilizer on the plant growth and K demand of Festuca arundinacea and the optimized K application rate based on soil K supply level. However, it is very important to promote the growth of turfgrass and maintain a good lawn quality, meanwhile, saving K fertilizer resources and improving the utilization efficiency of K fertilizer. The aim of this study was to investigate the effects of K application on shoot growth, root development, and K requirement, especially to determine the recommended rate of K fertilizer and critical K concentration for K diagnosis and management of Festuca arundinacea under medium and low soil K conditions.

Experimental Site and Materials
The experiment was conducted from April to July 2017 at the farm of Liaocheng University (36°45′N, 115°97′E, altitude 34 m), which located in Dongchangfu District, Liaocheng City, Shandong Province, China. The experimental site belongs to a typical warm-temperate, subhumid, continental, and monsoonal climate. During the test period, the average temperature was 23.5 °C (range: 9.1-32.4 °C), and the total precipitation was 226 mm, which close to the average over the same period in the experimental site.
The cultivar of Festuca arundinacea was "Fire Dragon," provided by Jiangsu Manqing Seed Industry Co., Ltd. (Suyang, Jiangsu Province, China). The fertilizers used in this experiment were potassium sulfate (K 2 SO 4 , 52%), urea (CO(NH 2 ) 2 , 46%), and diammonium phosphate ((NH 4 ) 2 HPO 4 , 18-46-0), purchased from Shandong Zhongnong Hui Defeng Seed Industry Technology Co., Ltd. (Liaocheng, Shandong Province, China). The basic physical and chemical properties of the two tested soil showed in Table 1. According to the content of available K in the two soils, they were divided into two levels: medium K and low K.

Experimental Design
The experiment was a pot experiment with a two-factor randomized block design, in a total of 10 treatments with 4 replicates. The main factor was K fertilizer application rate, including 5 K application levels, which were 0, 100, 200, 300, and 400 mg K 2 O kg −1 . The secondary factor was soil types including medium-K and low-K levels, recorded as S1 and S2. In order to study the effects of K fertilizer application under the condition of sufficient nitrogen (N) and phosphorus (P) supply, 200 mg N kg −1 and 90 mg P 2 O 5 kg −1 were added in each treatment. Polyethylene pots were used in the experiment (33 cm × 26 cm × 15 cm), and each pot contained 4.3 kg of soil (passed through a 3-mm sieve). On April 15, 2017, the fertilizer and soil were mixed and installed in the pots, and the healthy seeds almost in the same shape and size were selected for uniform sowing. The seeding rate was 38 g m −2 , and the sowing depth was 1 cm. After sowing, each pot was irrigated with 1000 mL of water; then, the pot was covered with plastic film to reduce water evaporation, and it was removed at emergence, on April 24. After emergence, watered 1000 mL per pot every 3-4 days, and weeds were removed manually when necessary. On July 25, plants were harvested and sampled to assess various shoot and root growth parameters.

Data Collection
One week after emergence, the number of Festuca arundinacea plants from each treatment was counted to calculate the emergence density. At harvest, the plants were separated into shoot and root. The tillers number of the shoot was counted and the harvest density was calculated. The average number of tillers per plant was calculated according to the harvest density and emergence density. Twenty plants were randomly selected from the shoot of each treatment, flattened, and straightened, and the height of the plants was measured with a steel tape. Taking 1/2 of the root according to the diagonal sampling method and washed the root out with clean water. The roots were scanned using an EPSON V700 Scanner (EPSON (China) Co., Ltd.), and the length, surface area, and average diameter of root were analyzed with WinRHIZO PRO 2012 Root Analysis System (Regent Instruments Inc., Quebec, Canada) (Zhu et al. 2020). The shoot and root samples were dried to a constant weight at 75 °C for 48 h to measure the biomass and calculate the root-shoot ratio.
All dried shoot and root samples were ground and passed through a 1-mm screen. The ground material was digested in a mixture of concentrated H 2 SO 4 and H 2 O 2 to determine K concentration using a flame photometer (FP6450, Shanghai Xinyi Precision Instrument Co., Ltd., Shanghai, People's Republic of China). Potassium uptake was calculated according to the biomass per unit area and the K concentration of the shoot or root. Potassium distribution ratio was calculated by the shoot (or root) K and the total K uptake.
The correlation between the amount of K fertilizer rate and shoot biomass was analyzed, and then, the maximum biomass of shoot and the corresponding K fertilizer rate were calculated in the two soils according to the mathematical function equations. Based on the nutrient abundancedeficiency index method, 80% and 50% of the relative biomass of shoot were used as the critical value to judge the supply level of K fertilizer. When the relative biomass of shoot was ≥ 80%, the amount of K fertilizer was sufficient, 80-50% was slight deficient or slight toxic, and < 50% was serious deficient or serious toxic. The relative biomass of shoot was calculated by the following formula: Relative biomass of shoot (%) = biomass of shoot/maximum biomass of shoot × 100%

Statistical Analysis
Data were analyzed using analysis of variance (ANOVA) and quadratic regression and linear regression in SPSS software version 20.0 (SPSS Inc., Chicago, IL, USA). Significant differences were determined using Duncan's multiple range test at P ≤ 0.05. Figures were drawn by GraphPad Prism 8.0.2. (GraphPad Software, Inc., San Diego, CA, USA).

Shoot Growth
The plant height, shoot biomass, harvest density, and the number of tillers per plant were significant different among treatments, but the difference in emergence density did not reach a significant level (Table 2). In S1 and S2 soils, plant height, shoot biomass, and tillers number per plant all increased with the increase of K fertilizer application rate and reached the maximum value when K fertilizer rate was 300 mg K 2 O kg −1 and then reduced. The harvest density also showed a trend of first increasing and then decreasing with the increase of K fertilizer rate. The K fertilizer rates for soil S1 and S2 with the highest harvest density were 300 mg K 2 O kg −1 and 200 mg K 2 O kg −1 , respectively. Soil type significantly affected harvest density and tillers number per plant and had extremely significant effects on plant height and shoot biomass (P ≤ 0.001). The plant height, shoot biomass, harvest density, and tillers number per plant of soil S1 were all higher than those of S2, which were 1.20, 1.42, 1.19, and 1.15 times of S2. The amount of K fertilizer had significant effects on the growth indicators of shoot except the emergence density (0.001 < P ≤ 0.01). There was a significant interaction between soil and K fertilizer on the shoot biomass.

Root Growth
There were significant differences in the root biomass and root surface area of Festuca arundinacea among different treatments, and the differences in root length density and average root diameter did not reach a significant level (P = 0.056) ( Table 3). The root biomass of plants in both soils increased first and then decreased with the increase of the K fertilizer application rate and reached the maximum value when K fertilizer application rate was 300 mg K 2 O kg −1 . The root length density, root surface area, and average root diameter of S2 had the same variation trend, and the root length density and root surface area of S1 were the largest when the K fertilizer application was 200 mg K 2 O kg −1 . Soil type had Table 2 Effects of potassium application rate (0, 100, 200, 300, 400 mg K 2 O kg −1 ) on shoot growth status of Festuca arundinacea in medium-K (S1, 117 mg kg −1 ) and low-K (S2, 57 mg kg −1 ) soils Values represent the mean ± SE, n = 4. P, S, K, S × K represent ANOVA results between all treatments (P), variation sources from soil (S), K fertilizer (K), and soil × K fertilizer (S × K) interaction. Different letters in the same column indicate significant differences between treatments (P ≤ 0.05). ns, *, **, *** indicate significant difference at P > 0.05, 0.01 < P ≤ 0.05, 0.001 < P ≤ 0.01, or P ≤ 0.001 level, respectively  Table 3 Effects of potassium application rate (0, 100, 200, 300, 400 mg K 2 O kg −1 ) on root growth of Festuca arundinacea in medium-K (S1, 117 mg kg −1 ) and low-K (S2, 57 mg kg −1 ) soils Values represent the mean ± SE, n = 4. P, S, K, S × K represent ANOVA results between all treatments (P), variation sources from soil (S), K fertilizer (K), and soil × K fertilizer (S × K) interaction. Different letters in the same column indicate significant differences between treatments (P ≤ 0.05). ns, *, **, *** indicate significant difference at P > 0.05, 0.01 < P ≤ 0.05, 0.001 < P ≤ 0.01, or P ≤ 0.001, respectively. Root biomass refers to the amount of root dry matter accumulation per unit area of soil. Root length density and root surface area refer to the root length and root surface area per unit volume of soil significant effects on various morphological indexes of plant roots, but had no significant effect on root biomass (P = 0.090). The root length density, root surface area, and root diameter of soil S1 were 65.1%, 98.7%, and 23.5% higher than those of S2. The amount of K fertilizer significantly affected the root biomass (P ≤ 0.001), but had no significant effect on the morphological indexes of roots. When the amount of K fertilizer was 300 mg K 2 O kg −1 , the root biomass was higher than that of the other treatments, and the root biomass significantly decreased when excessive K fertilizer applied. There were significant interactions between soil and K fertilizer on root length density and average root diameter.

Root-Shoot Ratio
There were significant differences in the root-shoot ratio of Festuca arundinacea among different treatments (Fig. 1A). The root-shoot ratio of K0 (without K fertilizer) in soil S1 was the largest, which was significantly higher than that of 400 mg K 2 O kg −1 K fertilizer treatment in S1, and had no significant difference with other treatments. There was no significant difference in rootshoot ratio among all treatments in S2. Both soil type and K fertilizer rate significantly affected plant rootshoot ratio. The average root-shoot ratios of soil S1 and S2 were 0.524 and 0.634, respectively, and soil S2 was 1.21 times that of S1 (Fig. 1B). The root-shoot ratio of plants without K fertilizer application was the highest, and the difference in root-shoot ratio was not significant when the K fertilizer application rate ranged from 100to 400 mg K 2 O kg −1 (Fig. 1C). There was no significant interaction between soil and K fertilizer on root-shoot ratio (P = 0.128).

Potassium Concentration, Accumulation, and Distribution
Potassium concentration and K uptake of shoot and K uptake of root were significant different among all treatments (P ≤ 0.001) and showed a trend of first increasing and then decreasing with the increase of K fertilizer rate both in S1 and S2, but the difference in root K concentration did not reach a significant level (Table 4). Soil type significantly affected K concentration and K uptake of plant. Potassium concentration and K uptake of shoot, K concentration, and K uptake of root in S1 were all higher than these of S2, which were 1.09, 1.59, and 1.18, 1.31 times of S2. The amount of K fertilizer had extremely significant effects on K concentration and K uptake of shoot, and K uptake of root (P ≤ 0.001), all in a trend of increased first, and reached the maximum value when K fertilizer rate was 300 mg K 2 O kg −1 , and then reduced. Potassium fertilizer application had no significant effect on root K concentration. There were significant interactions between soil and K fertilizer on K concentration and K uptake of shoot. No significant differences were found in the K distribution ratios of shoot (P = 0.108) and root (P = 0.116) between all treatments, although there was an increasing trend of K distribution in shoot and a decreasing trend in root with the increase of K fertilizer application rate in the two soils (Table 4). Soil type had no significant effects on K distributed in shoot and root. However, K fertilizer application significantly affected K distribution. With the amount of K fertilizer application increase, the percentage of K distribution increased in shoot and decreased in root and the difference between the K fertilizer rate of 200 mg K 2 O kg −1 and no K application treatment (K0) reach a significant level. No significant interactions were shown between soil and K fertilizer on K distribution. Fig. 1 Root-shoot ratio of Festuca arundinacea plants with different potassium application rate (0, 100, 200, 300, 400 mg K 2 O kg −1 ) in medium-K (S1, 117 mg kg −1 ) and low-K (S2, 57 mg kg − . 1 ) soils. A, B, and C represent root-shoot ratio of plants with different treatments, different soils, and different potassium application rates, respectively. Error bars are standard error of means. Different letters above the bars indicate significant differences between individuals in each subfigure (P ≤ 0.05)

Correlation Between Shoot Biomass and K Application and Accumulation
Both in S1 and S2 soils, the mathematical relationship between the shoot biomass and the K fertilizer application rate could be simulated by a quadratic function equation, and the correlation between the two was extremely significant ( Fig. 2A, B). According to the functional equation between shoot biomass and K fertilizer application rate, the maximum shoot biomass of Festuca arundinacea in S1 and S2 Table 4 Potassium concentration, potassium uptake and potassium distribution of Festuca arundinacea with potassium application rate (0, 100, 200, 300, 400 mg K 2 O kg −1 ) in medium-K (S1, 117 mg kg −1 ) and low-K (S2, 57 mg kg −1 ) soils K con. represents K concentration. Values represent the mean ± SE, n = 4. P, S, K, S × K represent ANOVA results between all treatments (P), variation sources from soil (S), K fertilizer (K), and soil × K fertilizer (S × K) interaction. Different letters in the same column indicate significant differences between treatments (P ≤ 0.05). ns, *, **, *** indicate significant difference at P > 0.05, 0.01 < P ≤ 0.05, 0.001 < P ≤ 0.01, or P ≤ 0.001 level, respectively  . 2 Response curve of shoot biomass of Festuca arundinacea to potassium fertilizer application rate in medium-K (S1, 117 mg kg −1 ) and low-K (S2, 57 mg kg −1 ) soils. A and B represent shoot biomass of plant with different potassium application rates in S1 and S2, respectively. *** indicate extremely significant correlation between the shoot biomass and potassium fertilizer application rate at P ≤ 0.001 level were 289.2 g m −2 and 200.9 g m −2 , and the corresponding K fertilizer application rates for obtaining the maximum shoot biomass were 254.9 mg K 2 O kg −1 and 234.8 mg K 2 O kg −1 , respectively. According to the relative shoot biomass of 80% and 50%, the suitable K application rate of medium-K soil S1 for Festuca arundinacea normal growth was 88.9-420.8 mg K 2 O kg −1 , and K fertilizer rate < 88.9 mg K 2 O kg −1 was deficient, in the range of 420.8-517.3 mg K 2 O kg −1 the plant appeared slight toxic, > 517.3 mg K 2 O kg −1 produced serious toxic (Fig. 3A). The suitable K fertilizer rate ranged from 71.1 to 398.5 mg K 2 O kg −1 for low-K soil S2, and K fertilizer rate < 71.1 mg K 2 O kg −1 was deficient, slight toxic phenomenon occurred in the range of 398.5-493.6 mg K 2 O kg −1 , > 493.6 mg K 2 O kg −1 the plant showed serious toxic (Fig. 3B). Without K fertilizer application, S1 and S2 soils could obtain 52.8% and 58.8% relative biomass of shoot, respectively (Fig. 3A, B).
The shoot K uptake and K concentration were significantly correlated with the shoot biomass (Fig. 4A, B). The mathematical relationship both between K uptake and biomass, and between K concentration and biomass of shoot could be simulated by linear equations, and the correlation between each two parameters all reached an extremely significant level (P ≤ 0.001). From these two linear equations, the variance of shoot biomass could be explained by the K uptake at the level of 95.8% (Fig. 4A) and by the K concentration at the level of 59.1% (Fig. 4B). According to the linear equation between K concentration and shoot biomass (Fig. 4B), the K concentrations corresponding to 80% shoot biomass in S1 and S2 were 17.9 g kg −1 and 14.4 g kg −1 , respectively. These two K concentrations could be used as critical values for K nutrition diagnosis in medium-K and Low-K soils.

Discussion
In higher plants, potassium affects photosynthesis at different levels. An increase of K content in the leaves can increase the rate of photosynthesis and the formation of photosynthetic products by promoting the activity of RUBP carboxylase, regulating stomatal opening, stimulating the activity of membrane-bound proton-pumping ATPases, etc. (Marschner 1995). In the current study, the K concentration and K uptake in the shoot had an extremely significant linear positive correlation with the shoot biomass. It was proved that increasing the K concentration and K uptake in the shoot of Festuca arundinacea was beneficial to the promotion of photosynthesis and the accumulation of photosynthetic products. Under medium-and low-K soil conditions, K deficiency or insufficient K supply results in reducing the dry matter accumulation due to an impaired phloem transport of sucrose, the increase of oxidase content, and the decrease of photosynthesis (Tewari et al. 2021). Excessive K supply inhibit plant growth may be due to the potential Fig. 3 The deficient, sufficient, and toxic potassium application range corresponding to the relative biomass of shoot of Festuca arundinacea in medium-K (S1, 117 mg kg −1 ) and low-K (S2, 57 mg kg −1 ) soils. A and B represent relative shoot biomass of plant with different potassium application rates in S1 and S2, respectively. The amount of potassium fertilizer with relative biomass of ≥ 80% is sufficient. The amount of potassium fertilizer with relative biomass of 80-50% and < 50% is slight deficient or slight toxic and serious deficient or serious toxic reduction of starch synthase activity (Preusser et al. 1981), K exocytosis (Peng et al. 2012), or even ionic toxicity (Zörb et al. 2014). Therefore, appropriate K supply in mediumand low-K soils is an effective measure to promote the growth of turfgrass, improve turf quality, and maintain a good landscape (Campbell et al. 2020), for Festuca arundinacea or other turfgrass, such as Paspalum vaginatum Sw. (Du et al. 2018) and bermudagrass (Cynodon dactylon (L.) Pers) (Ihtisham et al. 2020). In high-K soils, studies have shown that some turfgrasses, Lolium perenne (Bian et al. 2000), a creeping bentgrass (Agrostis stolonifera L.) (Bier et al. 2018), and hybrid napier grass (Dokbua et al. 2021) for example, do not produce a positive fertilization effect to K fertilizer application, and K fertilizer is not required.
Potassium supply level significantly affects the root growth in terms of root dry matter accumulation, root morphology, and root-shoot ratio during plant cultivation (Pourranjbari Saghaiesh and Souri 2018). The growth of plant root is determined by the distribution of photosynthetic products from shoot to root and the resulting changes in root morphology (Gao et al. 2019). The photosynthate of aboveground transport to the root in the form of sucrose through the phloem, which is regulated by the K supply level. With the increase of sucrose concentration in leaves of K-deficient plants, the distribution of sucrose from aboveground to root decrease, and the root growth is inhibited (Zörb et al. 2014). Otherwise, the content of soluble carbohydrates in roots of K fertilized plants increases (Zhu et al. 2008), and the carbohydrates distributed from aboveground parts to roots enhance, which promote root growth . This study demonstrated that the appropriate amount of K fertilizer in medium-K and low-K soils could obtain a larger plant root system, while K deficiency limited the growth of shoot and increased the proportion of dry matter distributed to root. Other studies have shown that there are no significant changes in biomass allocation and root structure establishment after the plant roots feel the K deficiency signal (Römheld and Kirkby 2010) or in the case of applying K fertilizer (Bai et al. 2009). This different response to K supply may be related to the plant genotypes and the initial K level of the soil. In addition, different responses of soil K and K fertilizer application to root dry matter accumulation and root morphology in this study indicated that K sources might have different effects on root development.
In low-input systems, such as turfgrass cultivation, 80% of the maximum dry matter yield is usually used as a critical reference indicator for determining mineral nutrient deficiency (Li 2008). Here, in this study, based on the quadratic function equations between shoot biomass and the amount of K fertilizer in the two soils, the nutrient abundancedeficiency index method was used for nutritional diagnosis of plant, and 80% relative shoot biomass was used as the critical value for judging whether the nutrient supply was sufficient, the appropriate K fertilizer application ranges for normal growth of Festuca arundinacea were 88.9-420.8 mg K 2 O kg −1 and 71.1-398.5 mg K 2 O kg −1 in medium-K and low-K soils, respectively. It showed that the suitable K fertilizer ranges were wide, and this result is consistent with Fig. 4 Correlation analysis between shoot biomass and potassium uptake or potassium concentration of Festuca arundinacea. A represent correlation between shoot biomass and potassium uptake and B represent correlation between shoot biomass and potassium concen-tration. *** indicate extremely significant correlation between the shoot biomass and potassium uptake or potassium concentration at P ≤ 0.001 level the discussion on K supply thresholds by Zörb et al. (2014). Considering to save K resources and to reduce the risk of K losses together, the recommended K fertilizer amount for the two soils were 88.9 mg K 2 O kg −1 and 71.1 mg K 2 O kg −1 , respectively. The recommended amount of K fertilizer for medium-K soil was higher than that of for low-K soil, which may be related to the different nutrient preserving capability of soil itself.
Nutrient concentration in plant tissue is usually used to judge the degree of nutrient abundance and to guide fertilization (Marschner 1995). The critical concentration of nutrients varies with plant growth rate, development stage, leaf age, plant species, and nutrient interactions (Li 2008). Here, the critical concentrations of K for Festuca arundinacea to obtain 80% of maximum shoot biomass in medium-K and low-K soils were 17.9 g kg −1 and 14.4 g kg −1 respectively; these critical values are within the critical K concentration range of 5.0-20 g kg −1 reported by Leigh and Wyn Jones (1984) for most crops. The difference in critical K concentration indicates that soil K supply level is also a factor affecting the critical concentration of nutrients, which may be related to the dependence of plant growth on the soil fertility.
Plants absorb K + from soil solution through roots. Generally, K + concentration in soil solution increases with the increase of K fertilizer application and is also concentrated or diluted by evaporation and precipitation under field conditions (Dokbua et al. 2021). Within a certain range, K absorption by plants increases with the increase of K + concentration in soil, while excessive K supply reduces K absorption, as shown in this test. Therefore, appropriate K fertilizer application and medium-K soil improved K absorption, resulting in changes in plant features, such as the increase of plant height, harvest density, tillers number per plant, dry matter accumulation of shoot and root, and the decrease of root-shoot ratio. There was no regular response of root morphology to K absorption in this study. The K + concentration in soil solution and plant tissue for optimum plant growth varies greatly (Havlin et al. 2006). It is not only controlled by plant genetics (Philp et al. 2021;Dbara et al. 2019) and also affected by soil properties and K fertilizer application. Under this experimental condition, K absorption, K recommendation, and critical K concentrations of "Fire Dragon" cultivar were different in medium-K and low-K soils, indicating that K management should be performed according to soil K levels for Festuca arundinacea cultivation, and the differences among varieties should also be considered.
It has been reported that plant growth is limited by rooting volume (pot size) when the water, nutrients, and other resources provided by a small quantity of soil or other substrate in small containers cannot meet the needs of plant. In this case, the results of the pot experiment are biased and cannot be used to guide field production. But in the early stage of plant growth in a pot experiment, the expansion space of plant roots and the demand for water and nutrients have not yet become obstacles to plant growth, the shoot biomass will not decline significantly, and this limiting effect is not very pronounced (Poorter et al. 2012). The average shoot biomass of all treatments observed in this study is very close to the shoot biomass of Festuca arundinacea with similar phase in a field experiment reported by Norton et al. (2006); thus, the similar shoot biomass confirms that the results of this pot experiment can be used in the early stage of building a tall fescue lawn in the field.

Conclusions
This study demonstrated that appropriate amount of potassium fertilizer and medium-K soil could promote plant growth and increase the potassium absorption of Festuca arundinacea. Potassium fertilizer mainly affected the dry matter accumulation of root, and soil potassium mainly affected the root morphology. Low potassium soil and without potassium fertilizer application could increase the rootshoot ratio. The optimum potassium fertilizer application rate and critical potassium concentration of shoot for normal growth of Festuca arundinacea in medium-K and low-K soils were different; then, the potassium fertilizer recommendation and potassium nutrition diagnosis for Festuca arundinacea management should be carried out according to the soil potassium supply levels.