IPI International Potash Institute
IPI International Potash Institute

Research Findings: e-ifc No. 15, March 2008

Evaluations of K availability in selected soils from Lebanon

Al-Zubaidi1, S. Yanni2, and I. Bashour3

(1)Professor of Social Science, College of Agriculture, University of Baghdad, visiting professor at FAFS, American University of Beirut

(2)Senior Research Assistant and Professor of Soils at FAFS, American University of Beirut

(3)Senior Research Assistant and Professor of Soils at FAFS, American University of Beirut

Introduction
Different approaches are used to evaluate the availability of potassium in soils. Potassium extracted by 1N ammonium acetate is considered as a good indicator for potassium availability (Mishra and Singh, 1994), while other investigators (Memon et al., 1988, and Al-Zubaidi and El-Semmak, 1995) prefer acid extracted K for evaluation of potassium nutrition and its availability for plant uptake.

Recently, new approaches for evaluating potassium status and its availability have been applied. Thermodynamic parameters including: ionic activity, activity coefficient, ionic strength, activity ratio, buffering capacity and free energy of replacement have been considered by several researchers as suitable for evaluation of potassium availability in soils. Several kinetic models and equations: zero order, first order, parabolic diffusion, Elovich and power function have also been successfully used for mathematical description of potassium adsorption and release in soils (Sparks, 1989; Al-Zubaidi and Al-Obudi, 2001).

This study was initiated to evaluate the availability of potassium in selected Lebanese soils by extracting potassium with ammonium acetate and calculating K-thermodynamics and K-kinetic measurements.

Methods
Nine surface soil samples (0-30 cm) were collected from locations in Lebanon where field experiments are being conducted. The soils are nonsaline, slightly alkaline and calcareous, with CaCO3 ranging from 2.5% to 29.7% and organic matter from 1.4 to 5.1%. Exchangeable Ca and Mg values are described as follows: Ca is high and Mg sufficient with CEC values ranging from 6.5 to 54.5 cmolc kg-1. Textural classes range between sandy loam to clay.

In order to evaluate potassium content and availability in these soils, traditional methods of K measurement were used, as well as K-thermodynamic and K-kinetic measurements. These various methods included: extraction of K with H2O, 1N NH4OAc and 1N HNO3, determination of K-fixation capacity, calculation of K-activity ratio, and free energy of replacement (-ΔF and Q/I relationship) according to Beckett, 1964. Five kinetic equations: zero order, first order, parabolic, Elovich and power function to determine the suitable ones for describing K-release from soils by successive extraction with citric acid.

Results
The results of this study can be summarized as follows: The tested soil samples varied widely in K content and K supplying power to the plant. The concentration of soluble potassium ranged between 1.0 to 11.2 mg kg-1. These low values do not retard the release of exchangeable K, but are not enough to support plant growth. Water soluble K constitutes only 0.7-12.7% of NH4OAc - extractable K. This indicates that a very small part of the available potassium is in the soil solution. The amounts of NH4OAc-K range between 42-575 mg kg-1 which corresponds to 169-2,300 kg ha-1 with an average 151 mg kg-1, or 604 kg ha-1. If we consider the critical value for available potassium 160 mg kg-1, then six of the nine soils investigated are poor in available potassium and are expected to respond positively to potassium fertilization.

The values of HNO3-K show a wide variation, ranging from 136 to 1,460 mg kg-1, which is attributable to different types and mineralogy of the studied soils. If we consider the critical values of HNO3-K to be 400 mg kg-1, then six of the soils are poor in supplying power of K.

The values of potassium saturation percentage ranged between 1.73-7.75% with low values in most of the soils studied. Values of fixed potassium expressed as total amount and as a percentage of applied potassium are shown in Fig. 1. The variation in the amounts of fixed potassium in these soils could be attributed to the amount and type of clay minerals. Statistical analysis showed that there are significant correlations (P<0.05) between the amount of fixed potassium and clay related parameters (CEC, clay per cent and clay + silt content).

The values of free energy of replacement (-ΔF) varied from -4,021 to -2,376 cal.mol.-1. According to Woodruff's classification of -ΔF, only two soils were shown to have medium supplying power of K, while the rest of the samples were (1) Professor of Soil Science, College of Agriculture, University of Baghdad, visiting professor at FAFS, American University of Beirut. (2) Senior Research Assistant and Professor of Soils at FAFS, American University of Beirut. Fig. 1. Total amounts of fixed potassium and percentage of applied potassium are shown for the studied soils. 0.00 1.00 poor in K-supplying power. Many workers have used the Q/I ratio linear curves to evaluate the ability of soils to maintain K level in soil solution (Beckett, 1964; Rasnake and Thomas, 1976; AL-Zubaidi, 2003). The values of the slope represent the K-buffering capacity and those of the intercept, the labile potassium in soil. This data show that the values of K-buffering capacity vary from 22.00 to 297.79 cmolc kg-1mol.-1L-1 with an average value of 133.91 cmolc kg-1mol.-1L-1. Statistical analysis indicated that there is a significant positive correlation (r=0.975) at P<0.05 between buffering capacity and CEC values of the studied soil.

Fig 1. Total amounts of fixed potassium and percentage of applied potassium are shown for the studied soils.
Fig 1. Total amounts of fixed potassium and percentage of applied potassium are shown for the studied soils.

The obtained values of labile potassium are very low in the studied soils ranging between 0.044-0.114 cmolc kg-1 with an average 0.071 cmolc kg-1. The values are lower than those of NH4OAc-K which means that part of the exchangeable potassium in the analyzed soils is held on exchange sites with high bonding energy.

The cumulative amount of released potassium versus time of extraction with 0.5 mM citric acid is shown in Fig. 2. The variation in the amount of released potassium from different soil samples could be attributed to many factors such as particle size of K-bearing minerals and soil environment.

Fig 2. Cumulative released potassium vs. time of the different soils tested.
Fig 2. Cumulative released potassium vs. time of the different soils tested.

The least square regression analysis for five kinetics equations used in this study to describe K-release from soil indicated that significant linear correlations were obtained. Based on the highest value of correlation coefficient (R) and the lowest value of standard error (SE), all equations except zero order satisfactorily describe the reaction rate of potassium release in the studied soils. The first order equation was the most suitable and was selected to determine the release rate coefficient value of potassium. These values ranged between 0.025h-1 to 0.0347h-1 with an average value 0.031h-1. It seems that the studied Lebanese soils have relatively high release rate of coefficient values, indicating that the Lebanese soils, compared to Iraqi soils, are younger and their degree of weathering is relatively higher.

In conclusion, the results of this study showed that more than one laboratory method should be applied for the evaluation of K-availability in soils and the results of laboratory analysis should be compared with field data.

Acknowledgements
Special thanks go to the University Research Board at American University of Beirut, the Institute of International Education's Scholar Rescue Fund (SRF) and the International Potash Institute (IPI) for their support to this research.

References

  • Al-Zubaidi, A. 2003. Potassium status in Iraqi soils. Proceedings of the regional workshop: "Potassium and water management in West Asia and North Africa." Edited by A.E. Johnston. International Potash Institute, 2003: 129-142.
  • Al-Zubaidi, A., and M. Obudi. 2001. Mathematical Description of potassium release Kinetics in some Iraqi soils. Iraqi Journal of Soil Science, Vol. 1: 282-290.
  • Al-Zubaidi, A., and K. El-Semmak. 1995. Effect of soil salinity on potassium equilibrium as related to cropping. Mesopotamia Journal of Agriculture, 3: 99-105.
  • Beckett, P.H.T. 1964. Studies on soil potassium. I. Confirmation of the ratio law: measurement of potassium potential. J. Soil Sci. 15: 1-8.
  • Memon, Y.M., Fergus, I.I., and J.D. Page. 1988. Utilization of non-exchangeable soil potassium in relation to soil type, plant species, stage of growth. Aust. J. Soil Research. 489-496.
  • Mishra, M.K., and M. Singh. 1994. Soil test/ crop response to potassium with respect to yield and quality. Potash Review, subject II, 9th suite, No. 3.
  • Rasnake, M., and G.W. Thomas. 1976. Potassium status of some alluvial soils in Kentucky. Soil Sci. Soc. Am. J., 40:883-886.
  • Sparks, D.L. 1989. Kinetics of Soil Chemical Processes. Academic Press Inc., England.

Further reading

  • Darwish, T., Nasri, T., el Moujabber, M., and T. Atallah. 2003. Impact of soil nature and mineral composition on the management and availability of potassium in Lebanese soils. Proceedings of the Regional workshop: "Potassium and water management in West Asia and North Africa." Edited by A.E. Johnston. IPI, 2003: 152-160.
  • Sayegh, A., Khazzakah, K.H., el-Khatib, A., Sfeir, S., and M. Khawlie. 1990. Soil mineralogy of Lebanon. Soil resources, Land and Water Development Division, FAO.