international fertilizer correspondent
Feed the soil to feed the people,
the IPI jubilee
Session 1: Policy issues related to food supply
and the environment
Session 4: Potash in agriculture
Anatoly Lomakin from IPC, Moscow, Russia provided information on world potash supply and demand. As shown in the graph, global potash use progressed steadily until the late '80s. By that time, developed countries, including the former Soviet Union (FSU), shared about three-quarters of total consumption, with the remainder used in developing countries. The last decade, however, has been characterized by a sharp drop in potash consumption in the FSU as well as in Central/Eastern Europe. Absence of market structures, economic constraints and unclear land titles are some of the reasons. Western Europe and North America have also shown a declining trend in potash use since the early '90s. This has been caused by economic considerations as well as set-aside programmes, environmental considerations and more efficient fertilizer use. In contrast, potash use in developing countries has enjoyed a steadily increasing demand. Nowadays, developing countries take more than half global potash consumption.
Potash, when compared to the other nutrients, is a natural resource that occurs as underground deposits or in salt lakes found in only a few countries. Major deposits are exploited in Canada, Russia, Belarus and in Germany. There are smaller deposits in the USA, England, Spain, Chile and Brazil. The Dead Sea is used for potash production by Israel and Jordan and China also produces potash from a salt lake. Current production represents about 70% of the installed capacity.
Of total potash fertilizers, 91% are in form of muriate of potash, MOP (KCl). North America and the FSU produce almost twothirds of the total MOP. Due to its relatively low price, it is the common source of potash for most field crops. The other potash products, namely sulphate of potash, SOP, and nitrate of potash, NOP, account for 7% and 2%, respectively. SOP is mostly used for chloride sensitive crops like tobacco, vegetables and fruit trees and is the preferred K source on salt affected land. Main producers of SOP are in Western Europe (55% of total), Asia (19%) and North America (17%). NOP, which is the most expensive source of K, finds its use mostly in irrigation systems (fertigation) and is produced mainly in Latin America (43%), the Middle East (26%) and North America (15%).
Asia uses about one-third of the total global potash production. Most of the potash has to be imported because Asia produces only 1% of global production. China and India are the main consumers, followed by Malaysia and Vietnam. High population pressure and export oriented crop production are some of the reasons for increasing consumption. Latin America, which absorbs about 17% of the total K production also shows an increasing use, mainly driven by a large area of soybean, sugar cane, coffee and bananas. North America and Europe (ex FSU) consume about 21% and 19% of total potash production. As indicated earlier, economic constraints as well as changing agricultural policies are reasons for a downward trend in potash use. Potash use in the FSU and Central/Eastern Europe almost collapsed with the economic reforms during the late '80s and have not yet recovered. Potash use in Africa, when compared to other regions, is almost negligible. The same applies for Oceania.
Of major concern is the imbalance in fertilizer use. Although crops take up potassium and nitrogen in almost the same quantity, the common fertilizing practice shows a great imbalance between these two nutrients. This applies in all regions but to Asia in particular. In consequence, because potash use fails to meet the demand of crops, soil K mining is inevitably having a deleterious effect on soil fertility. The concentration of potash production in the few countries which have been endowed by nature with natural resources, and thus the need by most other countries to import potash, is another factor that leads to unbalanced fertilization.
What is the outlook for regional potash supply and demand? A capacity survey for the mid-term shows a modest increase of primary potash production capacity by 2% with an over capacity of around 8.8 Mt by 2006. These capacity reserves allow the suppliers to respond with flexibility to any fluctuations in the world potash market. On the other hand, world potash demand is projected to increase gradually at an annual rate of 2.3% to reach 42.1 Mt of product or some 24 Mt K2O by 2006. Of course, higher growth rates might be achieved if farmers aim at balanced fertilization and try to overcome the current deficit of potash in soils, says Mr. Lomakin.
Why do farmers neglect potash and persist with unbalanced fertilization? Is it a question of cost, agricultural policy or because they fail to understand the function of potassium in the plant and its behaviour in soils. Keith Syers from the Naresuan University in Phitsanulok, Thailand, tried to respond to the first part of this question with his presentation on "Potassium in soils: current concepts". He rightfully mentioned that there is a lot of confusion in the terminology used to describe the status of soil K. Mineral K, labile K, practically unavailable exchangeable K, fast K, postpone effect K, structural K are just few of terms to characterize soil K according to its availability. However, we know there are many dynamic exchange processes in soils which make the assessment of soil K rather difficult. In an attempt to create a more transparent view on these processes and their relation to the K availability to plants, Keith Syers proposed a model which takes into consideration the different exchange processes and the corresponding flux of soil K (see diagram). He differentiates soil K into four main fractions, namely structural K, slowly-exchangeable K, exchangeable K, and soil solution K. It is in this order that readiness of availability improves while at the same time the K content/concentration in the different fractions decreases.
Structural K is present in the lattices of clay minerals and is released as these minerals are weathered. The K release rate is rather slow and thus is unlikely to be of practical significance, especially when considering the very high K demand of high yielding crops at specific times.
The slowly-exchangeable K is a kind of K buffer. It fixes K on wedge sites and expanded interlayers and thereby decreases the risk of losing K through leaching. The K release rate, albeit better than that of structural K, is still, as the name says, too slow to meet the demand of fast growing plants. Nevertheless, this fraction is an important pool of soil K.
The third fraction, namely the exchangeable K, is in rapid equilibrium with the fraction of soil solution K. But it is also in close relationship with the fraction of slowly-exchangeable K. It releases K quite rapidly because K is retained by the negatively-charged exchange sites. This is the fraction that is used to determine soil K status in routine soil tests.
K ions in the soil solution, the fourth fraction, are immediately available to the plant. The concentration varies widely between soils. Only about 5% of the K requirement of the crop is present in the soil solution. As shown in the diagram, the plant takes up its K from the soil solution. When the soil K concentration decreases, K is released by both exchangeable and slowly-exchangeable K fractions. The release intensity is higher in the first and slower in the latter fraction.
In reverse, when K is applied to the soil, either as potash fertilizers or through decomposing organic manure and plant residues, the K concentration of the soil solution increases. Due to the exchange processes, any excess K in the soil solution will be absorbed in both the exchangeable K and slowly-exchangeable K fractions.
Knowledge about these exchange processes is of utmost importance for assessing soil K status. The content of exchangeable K, that is determined by routine soil tests is, without doubt, a good indicator at times but, as Syers says, "the response predictions, using exchangeable K, were wrong about as often as they were right". Accordingly, in some situations, there is a close relationship between the content of exchangeable soil K and yield, as in the case for sugar. Rice in contrast to wheat shows a rather poor response to the content of exchangeable K. To what extent and under which conditions the K released from the K reserve pool (the slowly-exchangeable K), determines the response of the crop is not yet fully understood. There is a need for further research in order to develop more precise fertilizer recommendations.
Another subject which needs further research is the assessment of K stocks in the soil. As will be apparent from the discussion above, this is difficult to do but more information is certainly needed on the impact of K depletion on soil K status.
From the point of view of the plant itself, what is the effect of K on yield and quality? Guohua Xu from the Agricultural University of Nanjing, China, presented some answers.
Potassium, present in concentrations of the order of 100 mM in plant cells, plays a number of central roles in plant growth and the quality of the harvested produce. K is important because it activates biochemical processes in the plant, particularly its ability to make proteins. When plants lack K, they develop mechanisms to make best use of the K that is available. For example, tomato roots will take up K more efficiently if they have been temporarily deprived of it. And potato varieties that can absorb K efficiently can make better use of K from the slowly-exchangeable K fractions in the soil than cultivars with a low K uptake.
The interaction of ammonium and K uptake is also of practical relevance. If ammonium (NH4) concentration in nutrient solutions are high, plant growth slows down and total K uptake is reduced. But the negative effect depends on both the concentration of NH4 and of K. If NH4 is used to partially replace nitrate, growth in sweet peppers is stimulated and this then increases total K uptake. Similar results were found with rice cultivated in dryland soils. If plants lack K they are less efficient at photosynthesis and transpiration, and also at moving sugars and other organic compounds within the plant. This is partly because their leaves work less efficiently, in particular the stomata. Symptoms of K deficiency are usually first seen on older leaves which start to wither and die from the bottom of the plant to the top. K is mobilized preferentially from adult leaves to the actively growing younger parts of the plant but this can be overruled in plants which have specific characteristics that demand K. For example, the developing bolls of high yielding varieties of cotton create such a strong demand for K that even young leaves can turn red and fall earlier than usual as the K within them is transferred to the boll.
The crucial importance of K, especially in relation to crop quality, stems from its role in promoting the production of photo-synthates and their transport to storage organs like fruits, grains, tubers, and to enhance their conversion into starch, protein, oil, vitamins, etc. Numerous results are available which demonstrate the close relationship between K and crop quality. Of course, this relationship, like yield, also follows an optimum curve. The fact that excessive K in sugar beet decreases the sugar extractability is well documented. However, high K concentrations in beet do not necessarily derive from high potash rates; soil type, soil K status, previous cropping and manuring history also have their influence on K content.
Often there is an inverse relationship between yield and quality but this is not so in the case of soybean and its content of isoflavones. The latter are a group of phyto-chemicals that have health benefits for humans and animals. Guohua Xu reported that not only is there a positive correlation of total isoflavones with seed yield but significant isoflavone increases were always accompanied by significant increase of leaf K concentration. A recent mapping study has shown that the isoflavone genistein is closely linked at the genetic level to seed yield.
Understanding the molecular basis of active and passive K transporters in plants has increased enormously in recent years. This has opened up important new avenues for research on plant K nutrition. For example, several K transporters, which have different degrees of affinity to K in the soil solution, have now been identified. Such information may ultimately help to control uptake of Na under saline conditions.
Another important aspect in the relationship between potash and plant growth is tolerance to stress. Ismail Cakmak from the Sabanci University, Istanbul, Turkey, presented a closer look into the relationship between potassium and abiotic stress, that is stress caused by climatic factors such as heat, frost or drought, or soil-borne factors such as salinity.
He explained the central role K plays in controlling plant growth. When K is lacking, net photosynthesis is very much reduced and affects the distribution of carbohydrates within the plant. More carbohydrates are retained in the shoot to the detriment of roots. When plants not only lack K but are also exposed to high light intensity, they develop more pronounced deficiency symptoms than plants kept under low light intensity. Even partial shading reveals this effect. If part of a leaf on a K deficient plant is shaded it remains relatively green whereas the rest of the same leaf shows heavy damage. The question is why should K deficient plants be so susceptible to high light intensity?
The answer lies in the damage caused to plant cells by reactive O2species (ROS). Under normal conditions, the electrons produced by photosynthesis are used to convert CO2 from the air into assimilates such as sucrose. If photosynthesis is reduced - which it will be if K is lacking because leaf stomata function less well - less CO2 is available and there is therefore a surfeit of unabsorbed electrons. These stimulate the production of ROS and leaves are damaged as a result. This occurs not only under high light conditions but also as a result of drought, chilling and heat. However, if plants have an adequate supply of K, photosynthesis is more efficient, leaf cells are protected from ROS damage, and the plant is better able to withstand stress.
The lower yields produced by plants grown in saline conditions are also due to oxidative damage. Such plants contain less chlorophyll and show more cell damage. If extra K is supplied, this alleviates the damage caused by Na and both chlorophyll content and cell structure improve.
In conclusion, Cakmak said that K deficient plants are highly sensitive to light intensity and therefore crops grown under high light require more K than those grown under low light.
This work of Cakmak highlights the fact that in arid zones, such as the WANA region, adequate K supply is of utmost importance. Crops have to be produced at high light intensity, when soil water is lacking and salinity is a problem. All three factors lead to oxidative damage, especially when K supply is inadequate. The reality is, however, that the WANA region in particular is known for its very low potash use and a very wide N:K ratio in fertilizer application.
Potassium and biotic stress was the subject of the presentation by Rolf Haerdter, IPI Coordinator for China and SouthEast Asia. Biotic stress refers to infestations and attacks by insects, fungi, bacteria and viruses. The economic value of the damage caused by biotic stress is considerable; more than 40% of the obtainable yield may be lost. On the other hand, increasingly stringent legislative measures and increasing public concern over the use of crop protection products demand alternatives to the common practice of reliance on pesticides. Integrated pest management, IPM, offers an approach in which cultivation techniques, including plant nutrition, plays a significant role in minimizing damage from biotic stress.
Potassium has a central role in the plant's metabolism and development and is therefore essential in improving the resistance and/or tolerance of the host plant to pathogens. Plants with Isoflavone concentration in relation to seed yield and K content inadequate K supply and thus a relative surplus of N have soft tissue, less silica incrustations in the epidermis, and a much higher content of low molecular weight assimilates in the cell than plants fed in a balanced manner. Soft tissue is a weak mechanical barrier to invading pathogens, and an increased content of low molecular weight assimilates such as sugars, provides easy food for all kinds of parasites. Discoloration and early wilting, both signs of K deficiency, also attracts pests, particularly aphids.
It is well known that adequate fertilization with potassium helps to protect crops from fungal and other diseases. IPI has compiled in its Research Topics No. 3 on "Potassium and Plant Health" more than 2400 indications concerning this relationship between potash nutrition and plant health. In rice for instance, incidences of brown leaf spot (Helminthosporium oryzae), rice blast (Piricularia oryzae) or sheath blight (Thanatephorus cucumeris) are significantly reduced when adequate potash is supplied to the crop. More recent results show that a range of fungal diseases in tea such as anthracnose (Gloeosporium theaesinensis), brown blight (Guignardia camelliae) and grey blight (Pestalotiopsis theae) are less severe in tea plants receiving adequate potash than in plants with K deficiency.
The preventive effect of balanced fertilization on biotic stress has to be seen in context with other plant internal factors such as the genetically controlled resistance/susceptibility mechanism, the anatomy of plants, formation of inhibitory substances, escape mechanisms etc. Also external factors other than nutrition play a role in the host/pathogen relationship, especially temperature and humidity. Nevertheless, nutrition can modify the impact of the different factors either towards higher susceptibility as is the case with unbalanced, N oriented fertilization, or towards strengthening resistance with balanced fertilization and adequate potassium.