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Sodium: A functional element

Sodium (Na) is one of the sixteen elements determined in a routine sap analysis in Omnia Fertilizer’s OmniSap® laboratory. Although sodium may be essential to only a very few plant species, it does not generally conform to any of the criteria necessary for a plant nutrient to be seen as essential. These criteria for essentiality include the following:

  • that an element should be necessary for completion of the life cycle,
  • it should not be replaceable by other elements, and
  • that it should be directly involved in plant metabolism.

Consequently the term functional element have been suggested for sodium as it is not essential for most species according to any of these criteria (Barker and Pilbeam, 2007).
In a plant sap analysis, it would seem that sodium may be an indicator of irrigation water quality and  soil salinity, therefore in essence osmotic potential, which could be linked to water deficit. Hence it is crucial to have a recent water and routine soil analysis at hand when interpreting a sap analysis. Sodium levels in the plant sap is also an indicator of salt tolerance. Tolerant cultivars are able to maintain lower sodium levels in the leaf sap despite exposure to high sodium levels. Metabolic activity is thus maintained despite the presence of high sodium levels (Akthar et al., 2010). 


Phillips and Smythe (2010) reported sap sodium levels in vines over time, as seen in Figure 1. Peaks in sodium levels correspond with the drying of soil profiles and dehydration of plants. During moisture deficit, plant sap Na levels may rise as salts accumulate in the soil profile, but also as the plant endeavours to compensate for the change in osmotic potential to maintain turgor.


In soils with low potassium levels, sodium can substitute potassium and help with osmoregulation amongst other plant functions (Wakeel et al., 2011).

In-season sap sodium levels in Australian vines for soils treated with polyacrylamides.
Figure 1: In-season sap sodium levels in Australian vines for soils treated with polyacrylamides. A6W, A41/42 and A13/14 represent three different soil types, A6W being a heavier soil.

Although the concept of sodium as an osmotic indicator in plant sap has long been advocated and used, it was decided to re-evaluate the hypothesis. During the 2015/2016 summer season, OmniSap® samples were combined with formal water use efficiency (WUE) trials in a joined project between Omnia Fertilizer and the North West University. The trials were conducted on dryland maize at Coligny and Bothaville. The trial at Bothaville specifically experienced severe drought during the mid-season. The results observed are shown in Figure 2.

Influence of soil water content (SWC) at different soil depth levels on sap sodium (ppm) levels for 37, 65 and 93 Days After Emergence (DAE).
Figure 2: Influence of soil water content (SWC) at different soil depth levels on sap sodium (ppm) levels for 37, 65 and 93 Days After Emergence (DAE).

In Figure 2, the dramatic increase in sap sodium (Na) levels as soil water content (SWC) declines is clearly demonstrated. During the early season, SWC content was sufficient and corresponding sodium levels in the sap was low. As moisture decline towards the mid season (65 days after emergence) due to drought, there was a dramatic increase in the sap sodium levels. After 65 DAE, soil moisture again increased after rain and the sodium content returned back to almost normal levels. Severe drought in the late season almost resulted in total crop failure, and no further measurements were done. This example illustrates  the sensitivity of sap Na levels to changes in SWC and highlights its potential use as an early drought stress indicator.

A second very interesting observation was the difference in average sap sodium levels with the use of different nitrogen sources. From the trial results it was observed that in the case of seven nitrogen treatments at the V8 stage (with three replications each), there was an average threefold increase in sodium sap analysis values versus the pre and post growth stages where the soil water content was adequate. The average sap sodium levels varied from 100 to 400 ppm over the treatments during the drought period while it was hardly measurable during the wet periods. The reduced nitrogen sources (such as urea) gave values of higher than 300 ppm, while the nitrate based sources gave values less than 200 ppm. This shows the advantage of nitrate based fertilizers compared to ammonium based sources regarding sodium sensitivity as also reported by Lewis et al., 1988.   The higher sodium levels may be due to the replacement of sodium by ammonium (NH4+) from the reduced nitrogen sources. Lower sodium levels from nitrate based sources mean less osmotic stress and better WUE. Plants in osmotic stress use more energy to maintain an inward flow of water into their roots. As a result, less energy is available for tissue growth and crop yields are reduced.

In conclusion

The observations discussed above clearly show yet another potential advantage of OmniSap®, namely as an early indicator of moisture stress. Field trial results also seem to support the higher WUE of nitrate based fertilizers, compared to urea and ammonium based sources during drought conditions.

References

  • Barker and Pilbeam, 2007. Handbook of Plant Nutrition. CRC Press. Taylor and Francis Group. P 604.
  • Akhtar. J.; Saqib. Z.; Sarfraz. M., Saleem I. and Haq, M. 2010. Evaluating Salt Tolerant Cotton Genotypes at Different Levels of NaCl Stress Insolution and Soil Culture. Pak. J. Bot., 42(4): 2857-2866, 2010
  • Phillips, S. and Smythe, T. 2010. Role of Polyacrylamides (PAM) in Drip Irrigated Vineyards in the Riverland of South Australia. Internal Report. Riverland Wine Industry Development Council (RWIDC)
  • Wakeel, A.; Farooq, M.; Qadir, M. and Schubert, S. 2011. Potassium substitution by sodium in plants. http://www.tandfonline.com/toc/bpts20/current
  • Lewis, O.; Leidi, E. and Lipos, S. 1989. Effect of Nitrogen Source on Growth Response to Salinity Stress in Maize and Wheat. New Physiology. Vol. 111. Pages 155-160.

By: Willem Jonker – Specialist: OmniSap®, Strategic Agricultural Services