In many regions of the world, increasing rates of mineral fertilizer application over the past 50 years have been accompanied by growing crop yields. Consequently, production is usually more than adequate to supply food for both domestic purposes and exports. By marked contrast, however, in most parts of Africa fertilizer application rates have increased at a much lower rate, resulting in relatively poor yields that nowhere near satisfy the demands of a rapidly rising population. On a global scale, demand for agricultural produce is currently at unprecedented levels as a result of increasing populations and consumption per capita. In sub-Saharan Africa (SSA), the world’s poorest region, the already high population is set to more than double by 2050 to 2.2 billion. Feeding this huge population increase presents a major challenge to mankind; today, over 200 million Africans are chronically undernourished and 5 million die of hunger every year. To add to these difficulties, are the challenges presented by the impacts of global climate change and the effects of human conflict.
Ethiopia is a mountainous landlocked country in northeastern Africa. It is surrounded by Eritrea to the north and northeast, Djibouti and Somalia to the east, Kenya to the south and Sudan to the west, which all include land situated within SSA. Agriculture is the backbone of the Ethiopian economy. A remarkable feature of this economy is the dominance of small-scale farmers, who cultivate about 95% of the land under agricultural use and are responsible for 90% of total agricultural output. On average, crop production makes up about 60% of agricultural output and provides employment for around 85% of the country’s population. Coffee is Ethiopia’s leading export, followed by oil seeds, pulses, vegetables and fruits.
The climate in Ethiopia results in a wide range of biodiverse ecosystems, which allows a broad variety of crops to be grown, with about 65% of land potentially fertile for agricultural use. Crops used for home consumption include eragrostis tef (teff), sorghum, wheat, lentils, chickpeas and maize. All these crops are rainfed, but there is great potential for improvements in production with the introduction of irrigation. Ethiopia has been referred to as ‘the water tower of East Africa’ with ten rivers that are all capable of being used for irrigation, as well as for providing a clean source of electricity. The population is now over 100 million, with a high proportion of young people and a large potential workforce living mainly in the countryside. The human capital and fertile environment in Ethiopia are conducive to improving soil fertility – through measures such as mineral nutrient application – which could greatly improve crop production and food security in the country to meet the future needs of the growing population.
Fertilizers often account for more than 50% of yields produced and, in soils with low nutrient reserves (as in SSA) fertilizer attributable yields can be much higher. An important approach to enhancing food production and security in SSA is therefore related to improving crop fertilization. In Ethiopia, only two types of mineral fertilizers that supply plant nutrients are currently in common use. These are urea, as a source of nitrogen (N), and diammonium phosphate (DAP), which is a source of N and phosphate (P). The application of farmyard manure (FYM) provides a source of potassium (K) for crops, in addition to whatever K may be taken up from the soil. There has been a long-standing perception among agronomists, researchers, extension officers, and advisors.that Ethiopian soils are rich in K. However, in recent years evidence has suggested a possible depletion of K in these soils, which has limited crop growth.
In cooperation with various Ethiopian agronomists, for the last 10 years, the International Potash Institute (IPI) has organized field experiments involving a variety of crops grown on different soils in various regions of Ethiopia. Reports on the findings of these experiments, together with other complementary and relevant review papers, were key to discussions at the 1st IPI-Ministry of Agriculture-Ethiopian ATA joint symposium on “The Role of Potassium in Cropping Systems of sub-Saharan Africa: Current Status and Potential for Increasing Productivity,” which was held in Addis Ababa, Ethiopia, 4-5 September 2014.
The eleven chapters produced as a result of these proceedings provide a valuable addition to our knowledge of the role of K in cropping systems in subaqueous soil and its potential for increasing productivity. The experimental findings presented at the symposium provided a clear message about the highly beneficial influence of K fertilization in Ethiopian soils. For example, research findings from field experiments in southern Ethiopia highland (chapter 6) soils, which compared P and N fertilization, both with and without K, showed the dramatic influence of introducing K. Potato tuber yields and wheat grain yields also increased with the addition of K. Interestingly, the experiment found that relatively high applications of potash (150 kg/ha) were required to meet the economic optimum yields for both potato and wheat.
Chemical analysis of Ethiopian soils has determined low values of available K, which also highlights the necessity for K supplements through fertilization. Even in vertisols, which are often high in K, the benefits of small applications of K and P were found for growing wheat (chapter 7) due to the soil’s characteristically high fixing capacity for both nutrients. Chapter 9, which specifically deals with soils in Burundi, Mozambique, Rwanda, Uganda and western Kenya, also highlights positive crop responses to K, although in some highly extreme sites, excess soil acidity and a deficiency of other nutrients, including micronutrients, limited the crop response.
Providing balanced crop nutrition is key to increasing yields. Crop nutrient requirements may differ, but numerous observations of many different agricultural crops indicate that they often remove very similar amounts of N and K from the soil. It is only in fruit and vegetable crops that K uptake exceeds N. The role of K in soil-crop relationships can never be overstated, it is an essential plant nutrient involved in physiology, metabolism, growth, development, and thus, in the yield and quality of all crops. K plays a vital role in carbon dioxide fixation by plants, functioning directly or indirectly at various stages during photosynthesis, including light interception and chlorophyll synthesis. K is also closely and essentially linked to nitrate assimilation in plants, as well as protein metabolism. In the vacuole, K plays a key role in the maintenance of turgor and control of stomatal movement. As the predominant cation in plants, K also transports nitrate from roots to shoots and loads assimilates (sucrose and amino acids) into the phloem, which it subsequently transports to the plant’s fruits and storage organs. Furthermore, crops well supplied with K are more resistant to biotic (e.g. pest attack) and abiotic (e.g. drought, cold and salt) stresses.
In the processes described above, field observations have found that relatively similar amounts of N and K are always present. In order to optimize yields, soil fertilization involving these two nutrients must therefore ensure they are provided in similar quantities, with the level of supply depending largely on the crop species and K status of the soil. It should be remembered that, while over fertilization of soil with K is undesirable and a waste of nutrient resources, the K ion itself is harmless to the environment. On the other hand, excess application of mineral and organic N fertilizer is highly detrimental to the environment. Nitrifying bacteria in the soil release excess nitrate, which may be leached into deeper aquifers or acted upon by many species of bacteria capable of reducing nitrates into nitrogenous gases (NO N2O N2) that may then be released into the atmosphere.
Editing of the symposium proceedings was carried out with great dedication by Professor Takalign Mamo of the Ministry of Agriculture, Addis Abba, Ethiopia and the final draft of the manuscript was ready for publication in late August of this year. Very sadly, and most unexpectedly, however, Professor Mamo passed away on September 4th. This volume is therefore dedicated to his memory as a token of respect.
Ernest A Kirkby
Faculty of Biology, University of Leeds, UK
Edited by: Prof. Tekalign Mamo, Ethiopia. 186 pages.
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