Effects of Different K, Ca, and Mg Ratios in Soil on Nutrient Uptake by Pummelo

Hien Huu Nguyen^1* and Somsak Maneepong²

¹ School of Agriculture and Environment Resources, Vinh University, Nghe An, Vietnam

² School of Agricultural Technology, Walailak University, Nakhon Si Thammarat, Thailand

*Corresponding email: huuhiendhv@gmail.com

ABSTRACT

Potassium (K), Calcium (Ca), and Magnesium (Mg) are important nutrients for pummelo growth and production. However, these nutrients are strongly antagonistic to each others. This study aims to examine the effects of different K, Ca, and Mg ratios in soil on nutrient uptake. Soil samples were collected under pummelo canopies at a depth of 0 cm to 20 cm. Leaf samples of 3 to 5 month olds were obtained from the same trees. The soil chemical properties and nutrient concentrations in the leaves were determined. The results showed that the P uptake was inhibited by a high Ca concentration in soil. High K/Mg ratio in soil (0.52 ± 0. 27) positively affected on K uptake. Low K/Mg ratios in soil (0.42 ± 0.20) negatively affected on K uptake, although exchangeable K in soil was much higher than optimum ranges. The K/Ca mole ratio in the leaves was higher than that in the soil, therefore the uptake of K was better than that of Ca, despite the lower concentration of K in the soil. Moreover, the uptake of K was better than that of Mg, because the K/Mg ratio in the leaves higher than that in the soil. However, the leaf K concentration in Pakapanang orchard was slightly lower than its optimum ranges, because high exchangeable Mg in soil inhibited the uptake of K. Low exchangeable Ca negatively affected on Ca uptake, although Ca/Mg mole ratio in soil was high (2.77 ± 1.18).

Keywords: Nutrient uptake, Plant nutrition, Pummelo, Cation ratios

Introduction

Pummelo (Citrus maxima Merr.) is the biggest fruit of citrus species and high yield like other citrus. It requires larger amounts of K, Ca and Mg for growth and production. According to Maneepong (2008) the quantities of K, Ca and Mg which required for fruit growth were 3005, 1506, and 228 mg/fruit, respectively. However, the nutrient uptake processes of K, Mg, and Ca are strongly antagonistic resulting in a deficiency of the depressed nutrient (Voogt, 1998; Jakobsen, 1993). A deficiency of one element could imply a relative or absolute excess of the others resulting in an imbalance for the plants (Bergmann, 1992). A sufficient Ca concentration in soil or nutrient solution is important; however, major cations frequently interfere with Ca uptake (Barber, 1995). Magnesium may strongly modify the uptake of Ca and K, whereas K and Ca can restrict the uptake and translocation of Mg from the roots to the upper plant parts (Schimanski, 1981). On the other hand, ability of nutrient uptake does not depend only on its concentration in the soil, because mobility of each nutrient into plant root is different.

The optimum K, Ca, and Mg for pummelo growing soil and nutrient concentrations in leaves were established in our previous study (Maneepong, 2008; Zhuang et al., 1991). However, the effects of K, Ca, and Mg ratios in soil on their uptake were ambiguous. Hence, the present study aims to examine the effects of K, Ca and Mg ratios in soil on nutrient uptake.

Materials and Methods

Two representative pummelo orchards in Pakpanang (latitude 8⁰ 31^’ 0749^’’ N longitude 100⁰ 12^’ 05516^’’ E) and Khanom (latitude 9⁰ 21^’ 1369^’’ N longitude 99⁰ 79^’ 0178^’’ E) District, Nakhon Si Thammarat Province, Thailand were selected for this study. The Tuptim Sayam and Thong Dee cultivars were selected for this study. Most of the pummelo trees were planted using air-layering stocks in 1997. Some trees that were replanted later were not included in this study.

Soil samples were collected from 4 positions directly beneath the canopy of each tree between 0 and 20 cm depth by a sampling tube. The samples were mixed, air-dried, ground and gravel and debris were removed by sieving through a 2 mm screen. Eighteen composite samples were selected from 6 pummelo groups. Soil pH and electrical conductivity (EC) were measured using 1:2.5 and 1:5 of soil:water ratios, respectively. EC at the saturation point (ECe) was estimated by multiplying the EC by 6 (Shaw, 1999). Available P was extracted by 0.03 M NH₄F in 0.10 M HCl (Bray II solution), and its concentration was analyzed by the molybdenum blue method. Exchangeable K, Ca and Mg were extracted with 1 M NH₄OAc at pH 7.0. Concentration of K was analyzed by a flame photometer. Concentrations of Ca and Mg were analyzed by an atomic absorption spectrophotometer (AAS) (Jones, 2001; Jones, 2003).

Three- to five-month-old pummelo leaves were sampled from 3^rd or 4^th position of newly flush and non-fruiting twig on the outer canopy. Thirty pummelo trees were selected, and 12 to 16 leaves from each tree were collected. The samples were dried at 65 ˚C, ground, passed through 1 mm sieve. N was analyzed by the Kjeldahl method. The samples were digested with 2:1 mixed of HNO₃: HClO₄ for P, K, Ca and Mg analysis. The concentration of P was analyzed using the vanadomolybdate method. Concentration of K was analyzed by a flame photometer. Concentrations of Ca and Mg were analyzed by AAS (Soil and Plant Analysis Council, 1998).

Results and Discussion

The soil chemical properties and their optimum ranges are listed in Table 1. The soil pH in Pakpanang orchard was neutral and higher than its optimum ranges, whereas the soil pH in Khanom orchard was in optimum ranges. Soil ECe in Pakpanang varied greatly in a range of 1.3 to 5.2 mS/cm; however, most of these values fell in the optimum range. Slightly saline soil is recommended for pummelo growing. Although the soil tends to retard growth rate, but better fruit quality can be obtained. (Maneepong, 2008; Samarankoon et al., 2006). While the ECe in Khanom was very low compared with optimum ranges. Available P was much higher than its optimum ranges in both Pakpanang and Khanom; however, it was higher in Pakpanang than in Khanom. Therefore, pummelo growing in the orchards should not require additional P fertilizers. The exchangeable of K, Ca and Mg in Pakpanang were higher than their optimum ranges, whereas the exchangeable of K and Mg in Khanom were in than optimum ranges, but the exchangeable Ca was lower than its optimum ranges. If nutrient assimilation depends only on the nutrient concentration, the amounts of K, Ca and Mg in Pakpanang should be sufficient for pummelo. However, these nutrients are strongly antagonistic to each other. High Mg concentration either in soil or plant often causes poor K status in plant (Kirkby and Mengel, 1976). Zamaniyan et al. (2012) found that the K uptake by chicory cultured in nutrient solution depends on K/Ca ratio. Increasing the K/Ca ratio also increased K concentrations both in leaves and root. A K/Ca ratio higher than 1.5 decreased the yield and caused morphological damage related to Ca deficiency, such as pith hole and tip burn. The molar concentrations of K in the study soils were lower than those of Ca and Mg. Therefore, the K/Ca and K/Mg ratios oppose to those of nutrient solution for soilless culture.

The nutrient concentrations in pummelo leaves and their optimum ranges are listed in Table 2. The N concentration in Pakpanang was lower than its optimum range according to Maneepong (2008), and fell at lower margin according to Zhuang et al. (1991). N fertilizer may not apply sufficiently, or may cause from a high N loss in NH₃ form, while the N concentration in Khanom was in optimum ranges. The leaf P concentration in Pakpanang was also low, despite its high concentration in the soil, whereas it was in optimum ranges in Khanom. Although, the available P in Pakpanang was higher than in Khanom, but exchangeable Ca in Pakpanang was higher than in Khanom. Restriction in P uptake may be ascribed to a high Ca concentration in the soil together with a neutral pH. Jakobsen (1993) demonstrated that Ca can both support and inhibit P uptake. The inhibition effect results from the precipitation of less soluble calcium phosphate in the rhizosphere. The leaf K concentration was lower in Pakpanang than in Khanom, it was lower than its optimum ranges in Pakpanang, while it was slightly higher than its optimum ranges in Khanom. Although, the exchangeable K in Pakpanang g was much higher than its optimum ranges, it was also much higher than in Khanom, but the soil K/Mg mole ratio was higher in Khanom (0.52 ± 0. 27) than in Pakpanang (0.42 ± 0.20). Moreover, the concentration of Mg was higher in Pakpanang than in Khanom in both soils and leaves. Pummelo trees in Pakpanang cannot uptake K to a sufficient level despite the excessive K concentration in the soil. An antagonism between K and Mg was previously described. (Jones, 1999; Kirkby and Mengel, 1976). On the other hand, low the K/Mg mole ratio in soil negatively affected on K uptake. The leaf Ca concentration was higher in Pakpanang than in Khanom, although the soil Ca/Mg mole ratio was higher in Khanom (2.77 ± 1.18) than in Pakpanang (1.30 ± 0.55), but low exchangeable Ca negatively affected on Ca uptake. The Mg concentration in leaves was over optimum ranges in Pakpanang, whereas it was in optimum ranges in Khanom, it was affected by high exchangeable Mg. The K/Mg mole ratio in the leaves was higher than that in the soil, indicating that pummelo prefers K over Mg. The K/Ca and Ca/Mg mole ratios in the leaves were also higher than those in the soil. These results indicated that the preference order of the pummelo over these nutrients is K > Ca > Mg. However, this preference order does not agree with the concentration order in the leaves (Ca > K > Mg). Generally, farmers apply a large amount of K but ignore Ca and Mg. This practice induces Ca and Mg deficiency (Voogt, 1998; Bartal and Pressman, 1996; Jakobsen, 1993).

Table 1   Chemical properties of pummelo growing soils. Soil samples (0 cm to 20 cm) were collected from 18 subplots in a salt marsh growing area.

Table 2 Nutrient concentrations in pummelo leaves. Three- to five-month-old leaf samples were collected from 30 pummelo trees.

The K/Ca, K/Mg and Ca/Mg mole ratios in pummelo leaves according to the optimum ranges suggested by Maneepong (2008) were 0.5, 2.8 and 5.4, respectively. Similar ratios suggested by Zhuang et al. (1991) were 0.6, 2.8 and 4.3, respectively. The results in Pakpanang showed that K was reached the optimum ratio compared with Ca, but less than the optimum compared with Mg. Ca also exhibited an uptake level less than the optimum compared with Mg. Excessive Mg in the soil inhibited K and Ca uptake. This problem may be solved by applying K fertilizers. The leaf K/Ca and K/Mg ratios in Khanom were higher than their optimum ranges, therefore pummelo trees can uptake K and Mg to sufficient levels. However, the leaf Ca/Mg ratios in Khanom was lower than its optimum ranges, it was affected by a low Ca concentration in soil. This problem may be solved by applying Ca fertilizers.

Conclusion

The exchangeable K, Ca, Mg, and available P in Pakpanang orchard were higher than their optimum ranges. The exchangeable K and Mg in Khanom orchard was in optimum ranges, the exchangeable Ca was lower than its optimum ranges, and available P was much higher than optimum ranges. The P uptake was restricted by a high concentration of Ca in the soil. The antagonistic effect of Mg inhibited the uptake of K, thereby causing an excessive consumption of Mg. High K/Mg ratio in soil positively affected on K uptake. Low K/Mg ratios in soil (0.42 ± 0.20) negatively affected on K uptake, although exchangeable K in soil was much higher than optimum ranges. Low exchangeable Ca negatively affected on Ca uptake, although Ca/Mg ratio in soil was high.

Acknowledgments

We would like to thank the Center of Scientific Equipment, Walailak University, Thailand for analytical support.

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