The global phosphorus- and potassium resources: A review and its impact for the future (1998)
Esther Clauß1, Andreas Jenet, Börries Oeynhausen1 and Nina Oswald1
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Keywords: potassium, deposits, phosphorus, nutrient resources, phosphate, fertilisersIntroduction
The need to mine of potassium and phosphorous in the world is bigger than the existence resources, which means that the circelasation of use will not be closed. Therefore, in the wealth countries (e.g. USA) the deposits are about to be emptied. The result will be that the developing countries will end up delivering to the industrial countries, in exchange of financial support. The question will be, whether it will be possible to close the circulation of nearly or non renewable resources?Availability of potassium and phosphorus
Availability of Potassium for plants
Potassium is an activator of many of the plants enzymes. It is responsible for processes like energy metabolism, starch synthesis and nitrate reduction. Potassium plays an important role in regulating the opening and closing of the stomata in the leaves and helps regulating the uptake of water by root cells.
Potassium removal by crops is high; about 3 times that of Phosphorous and the same as Nitrogen. For example the removal of 140-180 kg/ha of K by a 60 Mg/ha-1 silage corn crop is not unusual.
These high numbers are even more critical, considering that plants have the tendency to take up K far in excess of their needs. This process is called "luxury consumption". The excess in K uptake however, does not increase crop yields.
The K cycle could be described by the biological cycling of soil solution K to plants, animals and men and its return through the digestion of organic wastes by the soil organisms. The original source of Potassium are the primary minerals, such as the micas and potassium feldspar, where 90%-98% of the existing potassium is held. In this original form it is considered as relatively unavailable. Here it is rather resistant to weathering and released mainly through the action of carbonic acid and of stronger organic and inorganic acids.
Approximately 10% of the soil K is slowly available which means that K released from the primary minerals into the soil solution is usually fixed very readily by 2:1 type clays. As the K ions are just the right size to fit between the layers of the negatively charged crystal they cannot be easily replaced. For a release from the expanding clays temperature and water availability play an important role.
As these clay minerals also take up much of K released into the soil by fertilisers they act as an extremely important reservoir of slowly available nutrients.
Also the organic matter holds and releases some K. Only 1-2% of the total soil K is readily available in the soil solution or as exchangeable K adsorbed on the soil colloidal surfaces. Unfortunately, K unlike P in the soil solution is subject to considerable leaching losses.
In humid region receiving moderate rates of fertiliser the annual loss is about 35 kg/ha. The K in the soil solution is always in a dynamic equilibrium. As soon as plants absorb K from the soil solution, exchangeable K immediately moves into the soil solution until the equilibrium is again established. When water-soluble fertilisers are added to the soil, the equilibrium reverses ? soil solution K moves onto the exchange complex. (Brady, 1990)
In very acidic soils the tightly held hydrogen and hydroxyl aluminium ions keep the K ions from close association with the colloidal surfaces. Therefore K is less susceptible to fixation. The reverse effect occurs in a soil with a low pH value.
Extraction of Potassium
Potassium deposits emerged 200 million years ago. Shallow lagoons filled with sea water were drying out, thus a concentration of crystallised sea salt accumulated. The dissolved salts precipitated in the inverse order of their solubility. Through geological movements they were covered and moved around. (Fink, 1985)
Today we find potassium in its mineral form in 3-8m wide deposits in a depth of 500 ? 1000m. There it is mined in pits that have a length of several km. From there it is hauled by a pit-head frame to the earth?s surface. (K+S GmbH, 1996)
The principal potash evaporate minerals are sylvite, langbeinite kainite and carnallite (Malavolta, Usherwood, N.R., 1978).
World Resources
World-wide potash trade achieved its most rapid growth during the 1960s and early 1970s. The countries which initiated major production expansions, the former USSR and Canada emerged as major producers during this period (1987) and they still are today.
1997 the total yearly potassium production is of the order of 25 million t K2O (1977 it was 22 Mio t; Global 2000, 1980).
Canada overtook the GUS states in its production, with an output of 9 Mio t K2O in 1997 (1976 it was only 5.5 Mio t). Canada?s production increased especially after having discovered the large reserve of New Brunswick additionally to Saskatchewan in 1983 (Pratt, 1984). The GUS states produced 7 Mio t K2O in the same year (1976: 9.1 Mio t). The major production areas lie in Byelorussia, Urals and the Ukraine.
Germany once was one of the most important producers (Fink, 1985), but the production capacity is decreasing (1997: 3.4 Mio t; 1976: 5.7 Mio t). (K+S, 1998; UN, 1980)
The prognosis for the production capacity in the future varies greatly in the different literature. In 1980 Global 2000 estimated a 5% K2O production increase per year. Furthermore they proposed that in the year 2000 the double amount of the K2O produced in 1977 was going to be needed. The numbers of today, however, prove that the production as well as demand is increasing much more slowly. Furthermore it was calculated in Global 2000 that the reserves would last only for 84 years from 1980. Different numbers are given by the UN "Fertiliser Manual" (1980) where Potassium is stated as the 7th most abundant element in the earthly crust. Malavolta,E. and Usherwood, N.R. (1978) calculated that the world?s resources will last another 3.856 years. So it can be seen that the ideas about the future vary greatly.
At this point it is probably sensible to define the difference between reserves and resources (U.S. Bureau of Mines, in: UN,Fertiliser Manual, 1980): Reserves are defined as ores that can be recovered at or near the current market prices.
Resources on the other hand are ores that are potentially mineable, but because of the cost or other constraints, could not necessarily be recovered at current prices.
So all of them are right in their sense. Global 2000 was considering the Potassium reserves discovered and mined at that time. The reason why they have estimated the production increase rate too high might be the fact that the demand could not rise in this rate, first because potash prices increase with increased difficulty in exploitation and also because many countries that, although they would need K fertilisers, cannot afford them as it is the case with most African and other poor countries (Hunstock, 1998). Malavolta and Usherwood might be right as they tried to calculate world?s extractable resources. The question remains to be asked as how far they left a tolerance gap for high exploitation costs. The UN "Fertiliser Manual" talks of K more as an element than an exploitable resource for fertiliser production.
In the same manual it is further stated: "World potash reserves are enormous and sufficient to meet requirements for many centuries to come. The fact that potash can be recovered from sea water means an additional, almost unlimited, but more costly resource of this material."
The fact is that the production depends on the demand and when the latter one increases new resources are found ? this is with varying costs of exploitation, of course. As the demand for potash does rise constantly, new sources are found. (Sheldrick, 1985)
Strong new emerging markets, for example are being developed in China, Israel and Jordan, Brazil and Thailand. The biggest of all being in Thailand (reserves: 9.940 Mio t) followed by the production of Jordan and Israel (Dead Sea). Also Brazil has large deposits, especially in the Amazon basin and in the north-east. (Freire, 1988; Japakasetr et al., 1980)
The largest resources of all remain however in Canada and Russia. These two countries together represent more than half of the world?s reserves and about 80% of the world?s resources. (U.N.,1980)
These number make it look like the potash production is limitless. However there are countries like the United States that are reaching their limits.
The production of Carlsbad, New Mexico, where 80% of the total United States production is mined, has dropped from a peak of more than 3 Mio. t K2O in 1966/67 to a record low of less than 1.6 Mio t in 1981/82. Apart from depleting reserves, poor quality ore, high production costs, low prices and the high competition of imported materials are cited as the main reason for the decline. (Nye, 1984)
Remains to say where the potash fertilisers go to. The main producers are commonly the same as the main consumers. Western Europe and North America seem to be prevalent on the K import market (U.N.,1980). Moreover China, India, Southeast Asia and South America (especially Brazil) are markets which became of great importance in the last few years (Sanders, 1987). The market in Central- and Eastern-Europe almost stagnated since the beginning of the 90´ies (K+S, 1997).
Especially India and China, but also Brazil have caused a rise in demand for potash on the world market from 22.5 Mio t to 26 Mio t in 1997. This demand is mainly covered by Russia but also by Canada. To protect their own market from the low Russian prices the European Community introduced a Anti-Dumping-Regulation since 1994. (K+S, 1997) This again reflects the fact that most potash producing agencies are from the public sector and therefore, realistically, price regulation also depends on political preferences and not only on the demand/supply ratio (Sheldrick, 1985).
This has an important effect on less developed economies, which often cannot afford the expensive fertilisers, nor the sophisticated infrastructure to mine their own. However more developed countries like Brazil and China, who have been major K importers in the last years, are starting to exploit their own reserves. (Sander, 1987)
The Availability of phosphorus
The productivity of soil is largely determined by its fertility, which in turn is depended on rootable soil depth and the nutrients stored in its mineral and organic constituents. The rootable soil depth may be restricted by physical constraints such as water table, bedrock or other impenetrable layers, as well as by chemical problems such as soil acidity, sodicity, salinity or toxic substances. Large reserves of stored nutrients are in themselves no guarantee of high soil fertility as plant availability of nutrients requires the release of these nutrients from their mineral or organic matrix or surface, processes that can be physical, chemical, as well as biological nature. Finally, a fertile soil can be of benefit to the plant only in the presence of sufficient soil water to allow the transport of these nutrients to plant?s roots.
Phosphorus is essential for plant growth and metabolic processes. It is a component of the energy carrier ADP and ATP and it plays also a significant role in the life cycle of plants as a essential component of DNA and RNA which are needed for protein synthesis. 98 to 99% of the phosphorus is associated with primary or secondary minerals and soil organic matter. Some 1 to 2% is microbial tissue, and only 0.01% exist as soluble phosphorus. Therefor is no significant loss of phosphorus by leaching or in gaseous forms. The plant roots absorb soluble inorganic and, to a lesser extent, organic phosphorus compounds and translocate them to aboveground plant parts. The phosphorus in the plants is returned to the soil either in crop residues and temporarily tie up at least part of the phosphorus in their body tissue. Some of it becomes associated with the soil organic matter where it is subject to future release. Some is very slowly converted into soluble forms that plant roots can absorb, thereby start, a repetition of the cycle.
In most soils, the amount of phosphorus in the available form at any one time is very low, seldom exceeding about 0.01% of the total phosphorus in the soil. Thus available phosphorus levels must be supplemented on most soils by adding chemical fertilisers. Unfortunately, much of such added phosphorus is converted to less available secondary mineral forms, from which it, too, is released very slowly and becomes useful to plants only over a period of years. The phosphorus problem is threefold. First, the total P-level of most soils is low, commonly between one tenth to one fourth of the nitrogen (200-2000 kg P2O5 ha-1) (Brady 1990). Second, the native phosphorus compounds are mostly unavailable for plant uptake, some being highly insoluble. Third, when soluble sources of phosphorus such as those in fertilisers and manure?s are added to soils, they are fixed or are changed to unavailable forms and in time react further to become highly insoluble forms.
Sources and methods for Utilisation
Epochs of Phosphorite Formations
Phosphate rocks are the natural source of the element phosphorus. Phosphate rock is the starting raw material for production of all phosphate products. It can be of sedimentary, volcanic or biological origin. However, they have a limited use as fertiliser due to the relatively slow availability for plant growth. Natural phosphates can be found in three main types of deposits: guanos ? organic material with little economical importance; aluminium phosphates ? mineral form with certain economical importance; calcium phosphates ? the most important type of natural phosphate rock. Calcium phosphates can be of sedimentary origin. In igneous deposits is present as apatites. This is a mineral family of which chemical formula can be mainly represented by: Ca10(PO4)6F2 (fluor apatite). Apatites are found in very well formed crystals and with small degree of substitution. In sedimentary deposits, phosphorus is present as phosphorites with the same apatites chemical formula but whith a much higher degree of substitution degree, turn easier the phosphate processing.
About 70% of the estimated world phosphate rock reserves and more than 80% of the world production are from sedimentary origin (Da Silva & Scheid-Lopez, 1991).
It has been established that two global epochs of phosphorite formation can be clearly distinguished in the Vendian-Cambrian and the Cretaceous-Paleogene sections. They can be traced in almost all continents and serve as a specific reference point in the prospecting for large phosphate lodes(see figure below text). Many accumulations of subaquatic phosphorite deposits are usually concentrated within these periods of geological history. They almost simultaneously appear in several shallow-marine basins and are associated with different types of marine sediments (Soubies et al 1990; Hein et al 1993; Compton & Mallinson, 1990 Filippelli & Delaney, 1992).
The Vendian-Cambrian stage of earth?s evolution (650-500 Ma) was marked by the formation of the Kara Tau, Khubsugul, Altai-Sayan, Uda-Shantar, South Chinese, and Pakistanian phosphorite-bearing basins in Eurasia, the Georgian Basin in Australia, the Volta Basin in Africa, as well as the Minais Gerais-Baia Basin in South America (Kholodov, 1997). Stratabound high-quality phosphorites become abundant in the Vendian-Cambrian time. They are usually intercalated with carbonate, as well as with siliceous-clay rocks. These are found in Asia, Africa, Australia and South America. Phosphoric bearing sections which are often found in Eurasia and Africa are usually associated with ferromanganese ores, carbon-bearing black shale?s, containing increasing quantities of metals (Anike et al, 1993; Kholodov, 1997).
During the Vendian-Cambrian epoch, phosphorite formation extensively manifested itself in Asia, Africa, Australia, and South America, but not in North America, where evidently there were no paleogeographic conditions favourable for photosynthesis.
During the Cretaceous and Paleogene time, the continental block was again involved in extensive phosphoric formation. It was expressed in the formation of granular and nodular phosphorites belonging to the two somewhat different facial types. The phosphat nodules occur among glauconitic sandstones and clays and are clearly assigned to washouts in the base of the phosphoric bearing sequence (Kholodov, 1997).
Figure 1: The epochs of phosphoric formations and their resources of P2O5 is shown in the figure.
In North America, the Cretaceous-Paleogene phosphorites are found within the Santa Domingo Basin. In South America, they are found in the area of the Columbian and Venezuelan basins, as well as in the Pernambuko-Paraiba region (Brasil). In Africa, phosphorites of this age are developed in the Egyptian, Algerian-Tunisian, Moroccan, Western Sahara, Senegalian, and Gabonese basins. Their resources are assessed in astronomical figures. In Eurasia, the zone of intensive phosphoric formation of the Cretaceous-Paleogene age is traced in the Eastern Mediterranean, Central Asian, and Aktyubinsk basins. The only continent where nodular and granular phosphorites of the Cretaceous-Paleogene age have not been detected is Australia (Filippellt & Delaney, 1992; Kholo)dov, 1997; Compton & Mallinson, 1991; Hein et al, 1993).
The largest phosphorous resources of the world are based in: Moroccan with an amount of 21t; United States 13,5; Western Sahara 10,0; Former Soviet Union 7,6; Tunisia 2,0 (in 109 tonnes P2O5)
Fertiliser supplies are assured in the coming century. Phosphate reserve estimates vary from 3.4 to 51 Gt. of P, dependent on what is considered economically feasible, like it is discussed in chapter four. Fertiliser consumption in 1990 amounted to 16 Mt yr-1 of P (FAO, 1994). Even with the lowest estimate of 3.4 Gt. of P by the Global 2000 model (BARNEY, 1980), P reserves would represent a supply for 90-130 years. However, considering the estimated 23 Mt yr-1 of P that are irretrievably lost to the oceans (Howarth et al, 1995), P use may have to be increased to compensate for this waste.
Figure 2 Global P balance on a per hectare of arable land basis (total P in crop and livestock imports + fertiliser consumption - total P in crop and livestock exports) avaraged over three periods (Beaton et al, 1995)
Trade in fertilisers and crop and livestock commodities is leading to a net transfer of nutrients from producing continents to the consuming regions. The net movement of P amounts to a transfer from North and Central America and North Africa to Asia and Europe (Beaton et al, 1995). The resulting accumulations in P, depicted in figure 1, are translated into accrual rates of around 30 kg P ha-1, causing serious problems in some regions with highly intensified agriculture. The P accrual rates from Africa and Latin America are likely to be insufficient to compensate for losses due to erosion and accumulations in city waste, and are consistent with the warnings of serious nutrient mining in Africa (Stoorvogel & Smaling, 1996). With current urban expansion rates, nutrients in harvested products are increasingly lost from the rural environment as a whole (Vlek, 1997 et al; Umdp 1996; Bregstenin 1996).
Closing the nutrient cycle at a community level in rural areas may be tedious: on a inter-regional level it is associated with considerable costs of collection, detoxification and transportation to the farms (Vlek et al, 1997). Yet, at a rate at which some of the non-renewable resources such as P and K are being exploited, recycling of these nutrients will soon be required. Here indeed, political foresight is needed to translate our awareness into action programmes.Discussion and Impacts
Consequences on Agricultural practices
Phosphorus and Potassium are important raw materials for the production of fertiliser. Deposits of raw materials are mined and transported over long distances to countries with a high demand in potassium and phosphorus to provide their intensive crop production.
With regard to the decreasing world reserves of mineral potassium and phosphorus it will be more imperative to fall back to the already existing soil K and P reserves.
In agrosystem parts of the potassium taken up from soil is removed from the fields in form of crop yields and is transported to towns (Frissel, 1978; Mengel and Kirkby, 1980). Usually this potassium does not return to the soil unlike in natural ecosystems where plant nutrients enter the soil at or near the place where K is taken up by the plants. The K uptake by plants is generally the main exit for potassium. In agrosystems the K cycle must therefore be considered as open .In case K does not return to the soil, the K soil fertility declines .The philosophy of Liebig explained the "manure by deprivation" (Liebig, 1865), which means that the same quantity of nutrients have to be applied in form of organic input or fertiliser which was taken off by yield.
He creates the idea of "sustainability depending upon its return" (van der Pol, 1990; Poss, 1991) -
This postulate excludes the constant deprivation of nutrients from the soil. (Scheller, 1997)
The roots are the main points for the uptake of potassium for plants. However, if no fertiliser is applied, 100% of the potassium is taken up by the roots. When fertiliser is applied, potassium can be taken of two nutritive sources: the available soil potassium and the K from fertiliser. The experiments from (Slutskaya et al.1980 and Fardeau 1991) et al proved that 94% to about 70% of the K applied
Within a given year, the remain in soil after the first crop. As a consequence, available soil potassium , with or without K fertilisers, is the main source of potassium for plants. (Ellis and Mellor 1995) mentioned that about 5 -15% of the applied fertiliser is taken up by the plants.
The question remains: Are in soil sufficient minerals K and P reserves, for a long-term crop production? What does the mobilisation of the fixed K and P depend upon? .
The soil K reserves, in comparison with the soil P reserves, are much bigger.(Scheller 1993) proceeded that the available K in most of the primary minerals and sediments are sufficient for high yield quantity.
He was able to demonstrate, that on Haplic Luvisols one can find an average K delivery of about 120-200 kg K2O/ha and nearly 100 kg K2O/ha on sandsoil (Classer et al. 1996). The experiments prove, that plants do have the potential capacity to cover their requirements from the available potassium in the soil over a long-term period.(Kerschberger and Richter; 1985)
However the question of how long the nutrient capacity still remains at the moment the prognosis of the available K supply for the crop over a long-term period is very inaccurate. The process of weathering and the quantity of the annual nutrient flux, depends on the type of primary soil and the content of the minerals, the climate conditions, the type of crops and the management of the cultivation. (Scheller 1993) states that intensive crop cultivation releases more nutrients from the minerals, because of the acid secretion by the roots. As a result, more nutrient flux in the rhizoshpere can be expected. Again a general but not quantified statement.
In comparison to Potassium the Phosphorus pool is very low. (Pierou 1976) calculates, that the global Phosphorus balance in the upper part of the agriculturally used soils is 20 to 50 times higher than that in the deposits. Therefore (Scheller 1997) explains, that an additional supply of P and K should be limited to soils, where the deficiency of phosphorus and potassium is high.
For cultivation?s on all other soils, a deficit in potassium in the long-term production could be avoided by a change of cultivation and a change of the frame conditions to improve the mobility of the nutrients. According to the statement of Scheller;
Baier (1974) confirmed, that nutrients of soil origin make a larger contribution to crop requirements than fertiliser. Nevertheless, in case of intensive cultivation, e.g. Czechoslovakia, there is a need of an additional potassium input in order to express the full yield potential. The quantity of fertiliser will result in the difference between availability of soil nutrients and the uptake by the crops. The prime purpose of fertilisation is to replace nutrients removed from the soil in harvested crops.
Also Hinsinger (1990) explained the necessity to compensate the nutrient export from the system, in form of plant produce, animal production, leaching and surface run-off by additional K entry - like rain, fertilisers and crop residues. The result of this experience is the difference between the application of crop residues which has a greater effect on the crop yields than the effect of K fertilisation.
Can organic residues/manure replace the nutrient fertiliser?
Spiess, and Besson, (1992) investigated in efficiency of organic K input, in form of manure, crop residues and of mineral fertilisers. ( K in crop residues are mainly applied in the ionic form, included in the plant cells and K fertiliser as soluble salts).
The results demonstrate, that in the long-term, potassium in animal manure is as effective as mineral potassium. The same applies to phosphorous (Buchner and Sturm, 1980). The short-time efficiency of manure is often lower in comparison with mineral fertiliser, but the residual effect is better. A problem of excessive potassium supply in crops can cause antagonistic effects between K, Mg and Na. (Muller; 1992 and Baier, 1997) In an additional experiment with K fertilisers, it was proven, that it delays or even blocks mineral degradation processes (Martin and Sparks, 1985).
Therefore in all agrosystems, especially in the tropical and arid regions the importance of crop residues must be emphasized because their return to the soil decreases the K output significantly.
(Sanchez et al.,1982)Conclusion
The estimations about the K and P resources give a lot of space for calculations. When considering K is in various agro-ecosystems, the availability and low prices of potash fertilisers in Europe and North America can easily lead to the erroneous conclusion that potassium nutrition of crops will never be a major problem. The reverse is often the case in tropical countries. Many agro-ecosystems have become heavily depleted of K. Although there are more resources of K, it will be more expensive and ineffective to explore. It is a matter of fact, that the duration of the world soil K supply, will not just depend on the intensity of the mineral pool or the availability of potassium in the soil, but on the kind of management methods, the genetic varieties of plants and the way of application of nutrients (environmental effects). Taking in mind the environmental and climatic factors in each place.
Farming must find new directions. This can be achieved mainly by considering the closed K and P cycles in the agricultural systems, using natural K and P input in balance with the output by crop yields. This is in order to maintain the soil fertility over a long-term period.
However, there are not enough data about the various reactions and fluxes within the mineral components, in order to adapt and improve analytical procedures and methods of prediction (Sanchez et.al,1981). Equally knowledge is missing on determination of potential rates of K transfer from soils or soil-manure/fertiliser systems to the soil solution, and from the soil solution to the plant roots. Therefore further investigations need to be focused on nutrient availability and mobilisation in natural soil conditions and the possibility of increasing the efficiency of the applicated K and P input, taking into account the biotic factors of crops under different abiotic conditions.
Sustainable agricultural systems will decrease the economic input and conserve the worlds K pools, limiting the use of mineral K in K poor soil. Mining the K and P reserves, the effects on the environment through destruction of important nature areas - also have to be considered, as it is the case of the Amazons significantly Basin, for example. Furthermore, the high energy and water consumption of fertiliser production should be kept in mind. Applying fertilisers to soil can have effects on the environment by changing the soil conditions and leaching or erosion or in case of P application in high amounts is causing eutrophication in the water.
Last, but not least, there should be given importance to the critical question in how far it is necessary to increase the yield by additional fertiliser application in countries, which already have an agricultural overproduction, while other countries, e.g. the Congo, cannot produce enough to cover their own food requirements yet, due to a lack of financial means, which could either buy their fertilisers abroad or could pay for the infrastructure even to mine their own reserves.
Thesedays many middle-countries like Brazil and Thailand are investigating more and more in the possibilities of exploitation of their own resources in order to become more independent from the expensive imports of fertilisers from the industrialised countries.
In the long run, however, the problem of the depletion of the K and P resources at one point will also urge them in direction of changing their agricultural method in order to rechose the nutrient circleSummary
Phosphorous and potassium are limited resources on earth. In the last 30 years they have been, however, increasingly mined for the production of fertilisers, which became of principal importance to the high output agriculture practised in the Western World.
Both P and K are, although largely present in intensively used soils, relatively unavailable to the plants as both are fixed relatively fast on the soil colloids. This problems is tried to be solved by the use of fertilisers.
The world resources for the mining and processing of these fertilisers are still relatively large. The extraction costs, however, will soon increase as many of the easily minable reserves are already depleted.
It is therefore proposed to change the approach towards this problem and to concentrate on finding more effective methods of soil cultivation which make the P and K that are already accumulated in the soil more easily available. Bibliography
Anike, O.L., Umeji, A.C. and Orajaka, I.P., 1993, Geology of Precambrian Banded Iron-Formation form Muro Hill, N. Economic Geology 88 (1993). 1237-1241
Baier, J. (1974): Principles of basic relations of mineral plant nutrition. DrSc Thesis, 283.pp., Prague
Barney, G.O., 1980, The global 2000 report to the president entering the twenty-first century. Washington, DC: US Government Printing Office
Beaton, J.D., Roberts, T.L., Halstead, E.D. and Cowell, L.E., 1985, Global transfers of P in fertilizer material and agriculture commodities. Phosphorus in the Global Environment (ed Tiessen) New York, John Wiley & Sons, 7-26
Brady, N.C., 1990, The Nature and Properties of Soils/Nyle C. Brady, 10th ed.. Macmillian Publ. Company, New York. 621 pages
Bucher, A. and Sturm,H. (1980) : Gezielter düngen. DLG-Verlag, Frankfurt am Main
Classen,N.,Wulff,F. and Junk,A. (1996): Erträge, K-Gehalte des Bodens sowie K-Auswaschungen infolge von K-Düngung auf Sandböden bei Wasrode. Mitt.dt.Bodenk. Ges.,79 279-282
Compton, J.S. and Mallinson, D.J., 1991, Phosphorit Deposits as Proxies for Organic Carbon Burial: Evidence From the Florida Platform. EOS, Transactions, American
Ellis,S. and Mellor,A. (1995): Soils and Environment, Routledge
FAO, 1994, FAO fertilizer yearbook 1993, Vol. 43
Fardeau,J.C. Morel,C et Boniface,R.(1991) : Cinetiques de transfert des ions phosphate du so vers la solution du sol: parametres caracteristiques Agronomie. 11,787-797
Filippelli, G.M. and Delaney, M.L., 1992, Similar phosphorus fluxes in anciant phosphorite deposits and a modern phosphogenic enviroment. Geology 20 (8:1992). 709-712
Fink, A., Fertilizantes y Fertilización, Verlag Chemie, Weinheim, 1985
Freire, E.S., Petromisa´s plas for expansion, Phosphorus-and Potassium, 154, 1988, 13-16
Frissel, M.J (1978). : Cycling of mineral nutrients in agricultural ecosystems Elsevier, Amsterdam
Geophysical Union 72(51). AGU 1992 Ocean Science Meeting (Suppl). 53
Heathwaite, A.L., 1994, Chemical fractionation of lake sediments to determine the effects of land-use change on nutrient loading. Journal of Hydrology 159 (1994). 395-421
Hein, J.R., Yeh, H.W., Gunn, S.H., Sliter, W.V., Benninger, L.M. and Wang, C.H., 1993, Two Major Cenozoic Episodes of Phosphogenesis Recorded in Equatorial Pacific Seamount Deposits. Paleoceanography 8 (2). 293-311
Hinsinger,P. (1990): -Action des racines sur la liberation du potassium l'alteration des mineraux silicates. these Universite de Montpellier
Howarth, R.W., Jensen, H.S., Marino, R. and Postma, H., 1995, Transport to and processing of P in near-shore and ocean waters. Phosphorus in the Global Environment (ed Tiessen) New York, John Wiley & Sons, 323-345
Hunstock, F., written communication: 1998-06-05, Kalium + Salze ? Bergbau Allgemeines, Kassel, 1998
Institute, Basel Switzerland
Japakasetr, T.; Sheldon, R.P.(ed); Burnett, W.C., Potash Deposits of Northeast Thailand, Fertilizer Mineral Potential in Asia and the Pacific, Bankok, Thailand, 1980
Kalium + Salze, Geschäftsbericht 1997, Kalium + Salze GesmbH, Kassel, 1997
Kalium + Salze, Mineraldünger = Naturprodukte, Kalium + Salze GesmbH, Kassel, 1996
Kalium + Salze, Umweltschutz bei der K+S GesmbH, Kalium + Salze GesmbH, Kassel, 1997
Kaliverein e. V., The Potash Industry in the Federal Republic of Germany, Kaliverein, Hannover, 1982
Kerschberger, M. and Richter,D. (1985): Ergebnisse eines 6-jährigen K-Steigerungsversuchs auf einer
Lößschwarzerde im Bezirk Halle. Archiv Acker-Pflanzenbau. Bodenk. 29,135-141
Kholodov, V.N., 1997, The Epochs of Phosphorite Formations and Biogeochemistry of Phosphorus. Lithology and Mineral Ressources 32(6), 491-504
Liebig, J.V. 1865_ Die Anwendung der Chemie auf Agrikultur und Physiologie. Vorrede XV, S.11, Braunschweig
Malavolta, E., Usherwood, N.R., Adubos e Adubação Potassica, Instituto da Potassa ? Fosfato, Boletim Technico, 03, 1978
Martin,H.W. and Sparks,D.L.(1985) : On the behaviour of non-exchangeable potassium in soils. Commun. Soil Sci.Plant Anal. 16, 133-162
Mengel, K. and Kirkby, E.A.(1980) Potassium in crop production. Adv. in Agron.,33, 59-110
Muller,J.C. (1992) : Potassium Concentrations in Municipal and Agroindustrial Effluents: Data Needed for Good Fertilization Practice , Potassium in Ecosystems International Potash
Nye, P.H., Carlsbad - What Future for Potash?, Phosphorus-and-Potassium, 129, 1984, 17-20
Pierou,U.(1976) : The global phosphorus cycle. In: Svenson, B.H. and Söderlund,R.: Nitrogen, phosphorus and sulphur - global cycles. Scope report No.7, Ecol.Bull. 22,55 - 88, Stockholm
Poss,R (1991).: Transferts de l'eau et des elements mineraux dans les terres de Barre du Togo. Consequences agronomiques. These Universite Paris VI,
Pratt, C.J., Potash Resources of the Maritime Provinces of Canada, Phosphorus-and-Potassium, 133, 1984, 26-29
Sanchez, P.A. and Salinas, J.G (1981): Low-input technology for managing oxisols and ultisols in tropical America. Adv. in Agron., 279-407
Sander, I., 25 Years of Potash, Phosphorus-and-Potassium, 15, 1987, 36-38
Scheller,E. (1993): Wissenschaftliche Grundlagen zum Verständnis der Düngungspraxis im ökologischen Landbau - Aktive Nährstoffmobilisierung und ihre Rahmenbedingungen. Manuskript, Selbstverlag Dipperz
Scheller,E. (1997): Betreibt der organische Landbau Raubbau an den Nährstoffvorräten des Bodens? Ökologie und Landbau 3/97, S.33-37
Sheldrick, W.F., World Fertilizer Review and the Changing Structure of the International Fertilizer Market, Agro-Chemical News in Brief, 8:2, 1985, 12-18
Slutskaya,L.D., Medvedeva,O.P. and Shirsmova, R.A (1980) : Study of potassium nutrition of plants using 40K. Agrokhimiya, 7,30-38
Soubies, F., Melfi, A.J. and Autefage, F., 1990, Geochemical Behaviour of Rare Earth Elements in Alterites of Phosphate and Titanium Ore Deposits in Tapira (Minas Gerais, Brasil): The Importance of the Phosphates. Geochemistry of the Earth?s Surface and of Mineral Formation, 2nd International Symposium July 2-8 (1990) France, 377
Spiess,E. and Besson, J.-M. (1992): Hofdüngereinsatz in der Schweiz: Erhebungen und Umfrage auf Landwirtschaftsbetrieben. Schriftenreihe der FAC Liebefeld Nr.12, Liebefeld-Bern
Stoorvogel, J.J. and Smaling, E.M.E., 1990, Assesment of soil nutrient depletion in sub-Saharan Africa. Wageningen. The Winhard Staring Centre Report 28(I-IV)
UN, Fertilizer Manual, Develoment and Transfer of Technological Series No.13, UN, N.Y., 1980
UNDP, 1996, Urban agriculture: Food, jobs, and sustainable cities. New York
Van der Pol, F. (1990): Soil mining as a source of farmer's income in Southern Mali. Royal Tropical Institute. Amsterdam
Vlek, P.L.G., Kühne, R.F. and Denich, M., 1997, Nutrient resources for crop production in the tropics. Phil.
Trans. R. Soc. Lond. B 352 (1997). 975-985