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A collection of papers based on a talk at the IPI-ISSAS 12th International Symposium on “Management of Potassium in Plant and Soil Systems in China”, Chengdu, Sichuan, China, 24-27 July 2012 and published in the June issue of the “Journal of Plant Nutrition and Soil Science” (J. Plant Nutr. Soil Sci. 2013, Volume 176, Issue 3)
 

Editorial

by Ernest Kirkby and Sven Schubert

China is a major player in world agriculture. The country accounts for 20% of the global population but has only 8.4% of global arable land at its disposal to support its huge population numbering about 1.34 billion people. Food security is therefore always of prime concern and great efforts have been made to maintain self-sufficiency. During the past 15 years food production has increased per capita by more than 54% which means that currently China is about 95% self-sufficient. To a considerable extent this great achievement has resulted from the application of mineral fertilizers. China, now being the largest global consumer, accounts for one third of total use with an average rate of fertilizer application approximately four times higher than the world average.

Nevertheless in China, potassium (K)-fertilizer consumption relative to that of nitrogen is 50% lower than the average global ratio, indicating excessive use of nitrogen (N), or inadequate use of potassium, or both. While China produces large amounts of cereals (20% of global), its vegetable production is much higher (52% of global). Since K is required to a considerably greater extent in the cultivation of vegetables, fruit trees, and other cash crops, these figures suggest an overall suboptimal K application even though the N:K ratio in fertilizer use has increased in recent years in favor of K.

Of the three major crop plant nutrients, N, P, and K, supplied as mineral fertilizers, K is certainly the one which receives least attention. There is a minority view in China that mineral K fertilization in some staple crops and regions is not required because K balance in crop cultivation can be sustained by recycling K from crop residues. For various reasons this concept may be questioned. There is certainly good evidence from a number of long-term crop experiments carried out in China that when K supply is inadequate, a decrease occurs in exchangeable or plant-available soil K, indicative of a loss of K from the soil–plant system which is in accord with findings of accompanying negative K-balance data (Xie and Zhou, 2012). Similar observations have been made in other countries (Römheld and Kirkby, 2010). Amounts of soil K reserves differ markedly throughout China and high rates of K fertilization appear to be essential especially on soils in the various agro-ecological regions where K reserves are low, as for example in the south of the country.

Low levels of plant-available K in the soil constrain crop growth by restricting uptake and utilization of N and P. This imbalanced fertilization is detrimental to the farmer not only because of waste of expenditure of N and P fertilizers and the resulting low-yielding poor-quality crops but also for the reason that unused N and P fertilizers in the soil present an environmental hazard. More and more attention is being given to considering the widely varying and intricate functions of K in plants from studies in molecular biology, biochemistry, and plant physiology and their relevance to agronomy. In addition to response to K in yield and quality of harvest, the stress-mitigating role of K under conditions of drought, salinity, and pathogenic infection is of particular practical importance (Cakmak, 2005), and requires additional studies and agronomic work.

With this background, the International Potash Institute and the Institute of Soil Science, Chinese Academy of Sciences (ISSCAS), Nanjing, in co-operation with China Agricultural University (CAU), Soil and Fertilizer Institute, Sichuan Academy of Agricultural Sciences, and Sinofert Holding LTD, organized an international symposium entitled Management of Potassium in Plant and Soil Systems in China (July 25-27, 2012) in Chengdu, Sichuan. The meeting brought together dozens of scientists from China and more than 25 from abroad making up more than 150 participants. Thirty-three papers were presented, and more than 40 posters displayed.

In reviewing the presentations, those on K management provide an overview of studies investigating optimal K fertilization of some of the main crops grown in China including maize, lowland rice, rice–maize systems, cotton, oilseed rape, and tea. In China, like elsewhere in the world, the assessment of available K in soils is still mainly made by chemical extraction of dried topsoil. This method lacks precision in concept in relation to uptake of K by crop-plant roots and there is a need to improve it particularly where measurements show low correlation with K fertilization and crop growth. In this respect the thought-provoking presentation of Simonsson (Andrist-Rangel et al.) discussed the various pools and fluxes of K in soil in field balances in the context of mineralogical studies of long-term agricultural experiments, which was of much interest to the meeting.

A novel conceptual approach was presented by White based on the physiological need to maintain concentrations of K in the plant within defined limits to prevent loss of growth. White argues that on the one hand this is determined by roots facilitating K uptake and on the other by efficient translocation of K within the plant after uptake and he suggests potential targets to maximize these two features of roots. Further investigations on roots and root–shoot interactions are needed to study these targets. Both features vary among plant cultivars and genotypes indicating the close interdependency between mineral nutrition and plant breeding. Results reported from a field experiment carried out by Hao et al., comparing two contrasting cotton cultivars in K efficiency, show that not only was the more K-efficient cultivar better able to take up K, but was also more efficient in taking up and assimilating N and P. This approach in comparing cultivars seems to be a promising line of study. Interestingly in relation to the concepts of White, is the report by Raveh of the need to maintain orchards with trees at optimal leaf mineral concentrations as a key issue for maximizing fruit yield.

A most valuable review of K in relation to stress alleviation and physiological functions in cotton management is presented by Oosterhuis et al. The physiological processes described include enzymes and organic compound synthesis regulation, water relations and stomates, photosynthesis, transport, cell signaling, and plant response to drought stress, cold stress, salt stress, as well as biotic stresses. The beneficial effect of K in alleviating salinity reporting up to date literature is well reviewed by Wakeel as is also drought by Grzebisz et al., which is supported by experimental field work from east Europe. As mentioned by Ooesterhuis et al. there is still inadequate data relating soil K availability and water stress. There remains therefore a need for precise greenhouse studies as well as for more field experimentation in this area.

Finally, mention should be made of two valuable presentations relating to the environment. Bar-Yosef and Ben Asher show through mathematical modeling the potential effect of K fertilization on CO2 binding in cropland soils, opening the door for discussion on the role of proper fertilization as a whole on carbon sequestration, and in relation to the carbon footprint of fertilizer manufacturing. This tool can also be used for assessment of N-fertilizer application in China and the benefits from balanced fertilization with K on the energy or carbon footprint of achieved yield. The paper of Woodson et al. presented by Brouder examines the fluxes of P and K in the bioenergy crop switchgrass and the findings are discussed in relation to nutrient management of the crop growing on marginal lands with both nutrient and environmental limitations. Of particular interest was the close relationship between optimized dry-matter yield and bioethanol yield. With not much research yet done on this topic, the new findings presented are of high importance to understanding fertilization needs of bioenergy crops, and pathways of nutrients and residues after energy production which is all part of the system.

The editors thank the authors for their papers and also express their gratitude to all the reviewers for their dedicated work.

References

  • Cakmak, I. 2005. The Role of Potassium in Alleviating Detrimental Effects of Abiotic Stresses in Plants. J. Plant Nutr. Soil Sci. 168:521-530.
  • Römheld, V., and E.A. Kirkby. 2010. Research on Potassium in Agriculture: Needs and Prospects. Plant Soil 335:155-180.
  • Xie, J., and J. Zhou. 2012. History and Prospects of Potash Application in China. Electronic International Fertilizer Correspondent (e-ifc) 32:19-27.

Papers

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June 2013

English