FERTILIZERS AND FERTILIZATION 2005(VII) Nr 3(24)
Spis treœci Wprowadzenie
1.Syers J. Keith - Potas w glebie i roœlinach uprawnych ( Artyku³ przegladowy) .... 9 Sesja I. Bilans potasu w systemach rolniczych
2. Alar Astover, Hugo Roostalu, Indrek Tamm, Vahur Vingisaar – Bilans potasu w glebach gruntów ornych w Estonii ... 37 3. Jerzy Barszczewski, Barbara Sapek – Bilans potasu na ³¹ce trwa³ej deszczowa-
nej ... 46 4. Pavel Èermak , Michaela Budnakova – Zawartoœæ potasu w glebach i bilans
tego sk³adnika w rolnictwie Republiki Czeskiej ... 57 5. Tadeusz Filipek, Mieczys³aw Bojarczyk – Zawartoœæ sk³adników mineralnych w
kainicie z Ukrainy, stosowanym jako nawóz w Po³udniowo- Wschodniej Polsce ... 69 6. Stephan Gorbanov, Toni Tomov – Bilans potasu w 40 letnich, p³odozmiennych
doœwiadczeniach polowych w Bu³garii ... 76 7. Aldis Karklins, Inara Lipenite – Bilans potasu w wybranych gospodarstwach na
Litwie ... 81 8. Margarita T. Nikolova – Bilans potasu w gospodarstwach i na poziomie kraju w Bu³garii ... 89 9. Stanislav Torma – Bilans potasu w rolnictwie Republiki S³owackiej w latach
1989 – 2003 ... 98
Sesja II. Formy potasu w glebie
10. Jacek D³ugosz, Ewa Spychaj-Fabisiak, Piotr Malczyk – Zmiennoœæ przestrzenna zawartoœci potasu przyswajalnego w poziomie powierzchniowym gleb
wybranego obszaru Równiny Sêpopolskiej ... 105 11. Mariusz Fotyma, Stanis³aw Gosek, Damian Str¹czyk – Nowe podejœcie do kali- bracji testu potasu przyswajalnego Egner-Riehm DL ... 113 12. Mariusz Fotyma, Stanis³aw Gosek, Damian Str¹czyk – Zawartoœæ ró¿nych form potasu w glebach województwa lubelskiego ... 124 13. György Füleky – Wyczerpanie rezerw potasu przyswajalnego z gleby ... 133 14. Ireneusz Grzywnowicz - Zmiany wielkoœci sorpcji niewymiennej potasu w
ró¿nych glebach wyczerpanych z tego sk³adnika ... 141 15. Ireneusz Grzywnowicz - Mo¿liwoœci pobierania potasu przez roœliny z
g³êbszych poziomów profilu glebowego ... 149
16. Janusz Igras, Vida Danyte – Bilans potasu w gospodarstwach rolnych na terenie lubelszczyzny ... 156 17. Barbara Wiœniowska-Kielian, Marcin Niemiec – Zawartoœæ potasu w glebach
³¹kowych po³o¿onych w gminie górskiej Ujso³y ... 162 18. Miros³aw Kobierski, Halina D¹bkowska-Naskrêt – Potas w zró¿nicowanych
typologicznie glebach Równiny Inowroc³awskiej ... 172 19. Wojciech Lipiñski, Ma³gorzata Walendziuk – Potas przyswajalny w glebach
Polski ... 182 20. Barbara Murawska, Ewa Spychaj-Fabisiak – Wp³yw ró¿nych systemów
nawo¿enia na mobilnoœæ potasu w glebie i jego bilans... 189 21. Andrzej Sapek – Stê¿enie potasu w wodzie glebowej z ró¿nie u¿ytkowanych
gleb torfowych ... 206 22. Barbara Sapek – Stê¿enie potasu w wodzie gruntowej na terenie gospodarstwa i w jego pobli¿u ... 219 23. Waldemar Spychalski, Andrzej Mocek, Miros³awa Gilewska – Formy potasu
w glebach antropogenicznych na zwa³owiskach pokopalnianych ... 230 24. Micha³ Stêpieñ, Stanis³aw Mercik – Regeneracyjne dzia³anie obornika na
glebach bardzo ubogich w potas i silnie kwaœnych ... 242 25. Ewa Szara, Wojciech Stêpieñ, Stanis³aw Mercik – Zawartoœæ ró¿nych form K
oraz wymiennych form Ca, Mg i Al w zale¿noœci od wieloletniego nawo¿enia tymi sk³adnikami ... 253 26. Magdalena Szymañska, Jan £abêtowicz, Marian Korc – Zawartoœæ form potasu w glebie w warunkach trwa³ego doœwiadczenia nawozowego ... 262
Sesja III. Nawo¿enie potasem roœlin uprawy polowej i u¿ytków zielonych 27. Jerzy Barszczewski – Nawo¿enie potasem u¿ytków zielonych jako element
zrównowa¿onego systemu gospodarowania ... 272 28. Zygmunt Brogowski, Alicja Gawroñska-Kulesza – Zawartoœæ potasu w fazach
rozwojowych wybranych roœlin uprawnych ... 283 29. Gra¿yna Anna Ciepiela, Kazimierz Jankowski, Joanna Jode³ka, Roman
Kolczarek – Zawartoœæ potasu w 3 gatunkach traw zale¿nie od dawki i formy nawozów azotowych ... 294 30. Daniela Dana , Emilia Dorneanu , Dorneanu A. , Timbota I. ,Povarna Fl.,
Clotan Gh., Serdinescu A., Calinoiu I. – Efektywnoœæ nawo¿enia potasem wybranych roœlin uprawy polowej w Rumunii ... 301 31. Vida Danytë, Mariusz Fotyma – Stê¿enie potasu w soku roœlin zbo¿owych jako
wskaŸnik stanu od¿ywienia tym sk³adnikiem ... 310 32. Ewa Fotyma – Interakcja potasu i azotu w nawo¿eniu roœlin uprawy polowej319 33. Witold Grzebisz – Nawo¿enie potasem roœlin uprawy polowej – koncepcja na-
wo¿enia uwzglêdniaj¹ca zmianowanie roœlin ... 328
34. Kazimierz Jankowski, Roman Kolczarek, Bo¿ena Kisieliñska, Joanna Jode³ka, Gra¿yna Ciepiela – Wp³yw nawo¿enia wermikompostem na zawartoœæ
potasu w runi ³¹kowej ... 342 35. Barbara Borawska-Jarmu³owicz – Zawartoœæ potasu, wapnia i magnezu w
odrostach runi mieszanek pastwiskowych ... 349 36. Dorota Kalembasa, El¿bieta Malinowska, Dawid Jaremko, Stanis³aw Je¿owski – Zawartoœæ potasu w ró¿nych klonach trawy Miscanthus w zale¿noœci od nawo-
¿enia mineralnego ... 359 37. Rudolf Kastori, Maja Cuvardic – Wieloletnie doœwiadczenia polowe z nawo¿e-
niem potasem w Serbii ... 365 38. Kovacevic V., Seput Miranda, Simic B. – Reakcja kukurydzy na nawo¿enie KCl
– artyku³ przegl¹dowy ... 372 39. Tomáš Lošák, Rostislav Richter, Jaroslav Hlušek, Thomas Popp, Jacek
Antonkiewicz, Ladislav Ducsay – Nawo¿enie potasem maku siewnego (Papaver somniferum L.) ... 379 40. Gra¿yna Mastalerczuk, Piotr Stypiñski – Wp³yw intensywnoœci u¿ytkowania i
uwilgotnienia gleby na zawartoϾ, pobieranie i rozmieszczenie potasu w
roœlinnoœci ³¹kowej ... 384 41. Péter Ragályi, Imre Kádár – Wieloletni wp³yw nawo¿enia mineralnego na plon i
sklad chemiczny runi ³¹kowej ... 395 42. Wojciech Stêpieñ, Stanis³aw Mercik, Tomasz Sosulski – Wp³yw formy nawozu
potasowego i sposobu nawo¿enia na plon i jakoœæ roœlin ... 401 43. Barbara Symanowicz, Stanis³aw Kalembasa – Dynamika pobierania potasu
przez rutwicê wschodni¹ (Galega orientalis Lam.) w trzecim i siódmym roku uprawy. ... 409 44. Wies³aw Szulc, Beata Rutkowska, Jan £abêtowicz – Dzia³anie nawozowe
potasu zale¿nie od wapnowania i nawo¿enia obornikiem ... 415 45. Ivan Vasilev, Margarita Nikolova – Wp³yw ró¿nych form nawozów potasowych na wielkoœæ i jakoœæ plonu ziemniaka ... 423
Sesja IV. Nawo¿enie potasem roœlin ogrodniczych i sadowniczych 46. Maciej G¹sto³ – Zawartoœæ potasu w ró¿nych organach jab³oni
odmiany Jonica ... 427 47. Ewa Jadczuk – Od¿ywianie mineralne roœlin sadowniczych potasem w œwietle
badañ naukowych ... 432 48. Józef Nurzyñski – Wp³yw nawo¿enia ró¿nymi formami nawozów potasowych
na plon oraz sk³ad chemiczny pod³o¿a i liœci warzyw ... 448 49. Jan Skrzyñski, Maciej G¹sto³ – Wp³yw podk³adek na zawartoœæ potasu
u jab³oni ... 457 50. Iwona Domaga³a-Œwi¹tkiewicz – Wp³yw stanowiska glebowego na stan
od¿ywienia jab³oni potasem ... 467
Contents Introduction
1. Syers J. Keith - Soil and plant potassium in agriculture (A review) ... 9
Session I. Potassium balance in agricultural systems
2. Alar Astover, Hugo Roostalu, Indrek Tamm, Vahur Vingisaar – Potassium balan- ce of arable soils in Estonia ... 37 3. Jerzy Barszczewski, Barbara Sapek – Potassium balance of a sprinkled irrigated permanent meadow ... 46 4. Pavel Èermak , Michaela Budnakova – Potassium content in the soils and
potassium balance in the Czech agriculture ... 57 5. Tadeusz Filipek, Mieczys³aw Bojarczyk – The content of nutrients and trace
elements in Ukrainian kainite distributed in South-East Poland ... 69 6. Stephan Gorbanov, Toni Tomov – Potassium balance in 40 years field
experiment in crop-rotation system ... 76 7. Aldis Karklins, Inara Lipenite – Potassium balance in selected farms of Latvia81 8. Margarita T. Nikolova – Potassium balance on field, farm and country level in
Bulgaria ... 89 9. Stanislav Torma – Potassium balance in agriculture of Slovak Republic in the
years 1989 – 2003. ... 98
Session II. Potassium forms in soils
10. Jacek D³ugosz, Ewa Spychaj-Fabisiak, Piotr Malczyk – Spatial differentiation of available potassium content in the soil surface horizon from selected area Sêpopolska Plain ... 105 11. Mariusz Fotyma, Stanis³aw Gosek, Damian Str¹czyk – New approach to cali-
bration of the Egner-Riehm(DL) soil test for available potassium ... 113 12. Mariusz Fotyma, Stanis³aw Gosek, Damian Str¹czyk – The content and rela-
tions of different forms of potassium in the soils of Lublin Region ... 124 13. György Füleky – Exhaustion of available soils potassium ... 133 14. Ireneusz Grzywnowicz - Possibilities of potassium uptake by plants from
deeper lagers of soil profile ... 141 15. Ireneusz Grzywnowicz - Changes in amount of non exchangeable sorption of
potassium in various soils depleted of this element ... 149 16. Janusz Igras, Vida Danyte – Potassium balance on farms in the Lublin Region ..
... 156 17. Barbara Wiœniowska-Kielian, Marcin Niemiec – Potassium content in grassland
soils on an example of mountain commune Ujso³y ... 162 18. Miros³aw Kobierski, Halina D¹bkowska-Naskrêt – Potassium in soils of differ-
ent type from Inowroc³aw Plain ... 172
19. Wojciech Lipiñski, Ma³gorzata Walendziuk – Available potassium content in Pol- ish soils ... 182 20. Barbara Murawska, Ewa Spychaj-Fabisiak – Modeling the contents of available forms of potassium in soil in relation to the use of fertilization and crop rotation
... 189 21. Andrzej Sapek – Potassium concentration in groundwater from the different
used peat soils ... 206 22. Barbara Sapek – Potassium concentration in ground water from under the farm- stead and its vicinity ... 219 23. Waldemar Spychalski, Andrzej Mocek, Miros³awa Gilewska – Potassium form
in soils formed from postmining lands ... 230 24. Micha³ Stêpieñ, Stanis³aw Mercik – The regenerative effect of FYM on soils
depleted from potassium and strongly acid ... 242 25. Ewa Szara, Wojciech Stêpieñ, Stanis³aw Mercik – Content of different forms of K and exchangeable forms of Ca, Mg i Al in dependence on long-term fertiliza- tion with these elements. ... 253 26. Magdalena Szymañska, Jan £abêtowicz, Marian Korc – Estimation of the form
of potassium affected by fertilization factors in long-term fertilization
experiment. ... 262
Session III Potassium fertilization of arable crops and grasslands
27. Jerzy Barszczewski – Potassium fertilization of grasslands in the process of achieving sustainable management ... 272 28. Zygmunt Brogowski, Alicja Gawroñska-Kulesza – Potassium content in growing stages of selected crops ... 283 29. Gra¿yna Anna Ciepiela, Kazimierz Jankowski, Joanna Jode³ka, Roman
Kolczarek – Content of potassium in three grass species subject to a dose and nitrogen form of nitrogenous fertilizers ... 294 30. Daniela Dana, Emilia Dorneanu, Dorneanu A., Timbota I., Povarna Fl., Clotan
Gh., Serdinescu A., Calinoiu I. – Efficiency of different potash fertilizers on some field crops in Romania ... 301 31. Vida Danytë, Mariusz Fotyma – Concentration of potassium in plant sap as the
indicator of potassium requirements ... 310 32. Ewa Fotyma – Interaction of nitrogen and potassium in fertilization of arable
crops ... 319 33. Witold Grzebisz – Potassium fertilization of arable crops- the crop rotation ori-
ented concept ... 328 34. Kazimierz Jankowski, Roman Kolczarek, Bozena Kisieliñska, Joanna Jode³ka,
Gra¿yna Ciepiela – Influence of meadow fertilization with vermicompost on the content of potassium in fodder ... 342
35. Barbara Borawska-Jarmu³owicz – The content of potassium, calcium and mag- nesium in regrowth of pasture mixture sward ... 349 36. Dorota Kalembasa, El¿bieta Malinowska, Dawid Jaremko, Stanis³aw Je¿owski –
The content of potassium in different clones of Miscanthus upon the mineral fertilization ... 359 37. Rudolf Kastori, Maja Cuvardic – Long-term field trials with potassium fertilizers in Serbia ... 365 38. Kovacevic V., Seput Miranda, Simic B- Maize response to fertilization with KCl
under conditions of Eastern Croatia – a review ... 372 39. Tomáš Lošák, Rostislav Richter, Jaroslav Hlušek, Thomas Popp, Jacek
Antonkiewicz, Ladislav Ducsay – Potassium and its forms in the nutrition of poppy(Papaver somniferum L.) ... 379 40. Grazyna Mastalerczuk, Piotr Stypiñski – The influence of the management in-
tensity and soil water conditions on the content, uptake and distribution of potas- sium in grassland sward ... 384 41. Péter Ragályi, Imre Kádár – Long term effects of mineral nutrition on the yield
and element content of grass ... 395 42. Wojciech Stêpieñ, Stanis³aw Mercik, Tomasz Sosulski – The influence of potas- sium form and methods of application on the yield and quality of selected crops
... 401 43. Barbara Symanowicz, Stanis³aw Kalembasa – The dynamic of potassium uptake
by goat’s rue (Galega orientalis Lam.) in the third and seventh years of cultiva- tion. ... 409 44. Wies³aw Szulc, Beata Rutkowska, Jan £abêtowicz – The effect of potassium on plant yields in dependence on liming and farmyard manure application ... 415 45. Ivan Vasilev, Margarita Nikolova – Influence of different potassium forms on the
yield and quality of potatoes ... 423 Session IV. Potassium fertilization of horticultural crops
46. Maciej G¹sto³ – The content of potassium in different organs of apple tree var.
Jonica ... 427 47. Ewa Jadczuk – Potassium nutrition of fruit trees in the light of scientific research
... 432 48. Józef Nurzyñski – Effect of potassium fertilization (KCl, K2SO4, KNO3) on the
yield and chemical composition of substrate and leaves of greenhouse vegetables ... 448 49. Jan Skrzyñski, Maciej G¹sto³ – The influence of rootstocks on the content of
potassium in apple trees ... 457 50. Iwona Domaga³a-Œwi¹tkiewicz – Effect of the soils properties on the potassium
nutrient status of the apple tree ... 467
J.Keith Syers
University of Newcastle, Newcastle upon Thyne, UK
Abstract
Potassium (K) is an essential nutrient for plants and animals, including humans; its role in agricultural production is firmly established. Also, K constitutes no hazards to human health and has no deleterious effects on water quality. It is highly unlikely that the world’s supply of high quality K sources will be exhausted within the next few centuries. The needs and opportunities for further work on K in agriculture relate to economic issues, particularly improved recommendations for K fertiliser use. To achieve this a better understanding and integration of the relevant soil and plant factors is required.
The chemistry of K in soils is relatively simple, compared to that of nitrogen and phosphorus. Exchangeable K is a good indicator of soil K status and the likelihood of obtaining a response to K fertiliser in many soils. However, for soils containing partially weathered micaceous minerals, fixed K (which is slowly available) is a potentially important source of K for crops. Work in the United Kingdom in the 1950s and 1960s emphasised the likely significance of fixed K (often referred to as non-exchangeable K) as a source of K for crop growth. As exchangeable K levels have been built up and the likely contribution of fixed K has diminished, this interest has waned but it would be worth reconsidering the relationship between non-exchangeable K and initially exchange- able K in soils where mineralogical composition is known. This is because it may be possible to moderate exchangeable K values, based on an estimate of the supply of fixed K, in developing more precise fertiliser recommendations for K.
Expressing plant K concentrations for cereals on a tissue water basis provides es- sentially constant values for K concentration which are little affected by fertiliser N and P and water supply. This method offers promise for diagnosing K deficiency for these crops but commercialisation of the possibility has been slow. Foliar application of K has produced very positive results in some experiments but further work is required for this method of application in specific situations, such as high magnesium soils. Crop offtake of K has increased substantially in recent years as yields have increased and more straw has been removed. Fertiliser recommendations for K are being modified accordingly.
Reprinted from Proceedings No. 411. By the kind permission of The Fertilizers Society
Modelling offers the best prospects for improving our understanding of the dy- namic interactions between the soil supply of K and crop demand for K. There have been advances in modelling the uptake of K by crops but there is a shortage of data to validate such models. To find widespread acceptance at the field level, such mechanis- tic models require a better integration between the soil and crop components, simplifi- cation, and validation.
Introduction
Potassium (K) is found in all living cells and is an essential nutrient for plants and animals, including humans. Of the three major plant nutrients, nitrogen (N), phospho- rus (P), and potassium, the requirement for K is intermediate between that of N and P.
In both animals and plants, not only is K the major intracellular mineral ion, but the K concentrations in cells are essentially the same,100-160 mM in animals, [Preston and Linsner 1985], and 100-200 mM in plants Leigh and Wyn Jones [1984]. Potassium has both a biophysical and a biochemical function in cells. A larger amount of K is required to maintain the osmotic potential of cells than is needed for biochemical processes. The biochemical function of K is primarily concerned with the activation of enzymes which play a key role in metabolic processes [Sueller, 1985].
At the practical level, K can increase the efficiency of use of other nutrients by plants, particularly of N,have a beneficial effect on the quality of a wide range of crops [Usherwood 1985], especially in terms of improved protein quantity and quality,; de- crease the incidence of plant diseases [Huber and Amy, 1985]) and reduce abiotic stresses, particularly cold stress. Thus the role of K in crop production is clearly established.
In 1997, world potash capacity was estimated to be 36.3 million tonnes K2O [Prud’Homme, 1997] with 96% of this capacity being accounted for by muriate of potash (MOP). Based on 1996 production figures published by IF A, the potash indus- try operated at only 65% of its MOP capacity in that year, with the world’s surplus largely localised in Canada and the Commonwealth of Independent States (Russia and Belarus). In the medium to long term, it is likely that the current global surplus will decline as demand increases. Known estimates of high quality reserves (defined as ore which can be recovered at or near current market prices) range from 9 to 20 billion tonnes of K2O . Total resources (defined as potentially minable ores which because of cost or other constraints cannot necessarily be recovered at current prices but might at a later date) amount to 150 billion tonnes of K2O [Sheldrick, 1985]. Thus there is a sufficient supply of high-quality K sources in the world to last for many centuries. This contrasts with the situation for P where high quality phosphate rock reserves are being depleted fairly rapidly, as shown by the decrease in the average P2O5 content of mineral rock from 32.7% in 1980 to 29.5% in 1986 [Notholt et al, 1989].
Although K is the mineral nutrient for which humans require the greatest quan- tity [Serfass and Manatt, 1985], the K requirement of humans is not known precisely [Anke et all, 1992 ]. However, it is almost impossible to induce K excess with a normal diet, provided renal function is normal. Equally, K deficiency resulting from inad- equate dietary intake is highly unlikely, given that K is ubiquitous in foodstuffs [Grossklaus, 1992]. Far from a high K intake having any harmful effect, it may have a beneficial effect on human health. In fact there is a reciprocal relationship between K and Na, whereby K can reduce hypertension in humans induced by Na [Serfass and Manatt, 1985]. Thus, there have been no concerns about possible adverse effects on human health from excess dietary intake of K, as there have been for nitrate [House of Lords, 1989].
Unlike N and P, which have been implicated as causative factors in the acceler- ated eutrophication of surface waters [OECD, 1982], K has no known deleterious effects on the quality of natural waters. The EEC Directive on the quality of water for human consumption [Directive 80/778/EEC] establishes a guide level of l0 mg dm-3 and a maximum admissible concentration (MAC) value of 12 mg dm-3 for K. Grossklaus (1992) indicates that there is neither a physiological nor a toxicological justification for such a low MAC for K given the very much higher K content of cow’s milk and fruit juices (both approximately 1600 mg dm-3) than of drinking water. Significantly, the average K content of human breast milk is 13 mM ,approximately 500 mg dm-3 [Wissenschaftliche Tabellen Dokumenta, Geigy, 1975)] When it is considered that the intake of K in drinking water is only about 0.1% of total daily K intake, the MAC cannot be justified.
There has been a steady stream of reviews on different aspects of soil and fertiliser K over the last forty years [e.g., Munson, 1985; Sparks, 1987] Significant reviews have been published by the Fertiliser Society [e.g., Warren and Johnston, 1962; Arnold, 1962; Arnold, 1970; Beringer, 1992] and by HGCA [Arnold and Shepherd, 1989].
What is clear from the early work is that although the terminology for the forms of soil K often varied, the scientists involved had a clear understanding of their meaning and the implications for plant availability. More recently, and with the increasing involve- ment of non-specialists, the terminology has become less precise and often quite con- fused. Grimme [1985] has commented on the liberal use of vague terms, such as available K, K status, and K availability, and on the fact that they are often used interchangeably, even though they have different meanings. It may be timely to reconsider the terminology for the forms of soil K and to integrate this with an assess- ment of plant availability.
If, as suggested, the role of K in agricultural production is well established; if there is no limitation to the world’s supply of K for agriculture, both in terms of the quantity and quality of K source materials; and if there are no hazards to human health and deleterious effects on water quality from K, what are the outstanding issues and what information is required? In essence, the issues are largely economic and relate prima-
rily to the development of improved recommendations for K in crop and grassland production. A better understanding and integration of the soil and plant factors which are relevant to making improved recommendations are necessary. This review places particular emphasis on the information needs for those aspects of soil and plant K which are required to increase the efficiency of K fertiliser use.
Forms and availability of soil potassium
In comparison to the chemistry of N and P in soils, that of K is relatively simple.
This is because K exists:
(i) in the soil solution as K ions (K+),
(ii) on the solid phase as an exchangeable cation on the surfaces of clay minerals and organic matter,
(iii) tightly held or «fixed» by weathered micaceous minerals, (iv) present in the lattice of certain K-containing primary minerals.
Fixed and lattice K can be grouped together in that they make up the non-exchange- able pool of soil K. In each case, K exists in the inorganic form and does not enter into organic combination, per se; unlike N and P, K is not a biogenic element. In this review it is intended to develop further the concept of fixed K. But where previous workers have used the term non-exchangeable K, it will continue to be used here in referring to their work.These four forms of soil K vary in their availability to the plant and it is useful to try to put this on a scale of relative availability. In Fig. 1 the mode of occur- rence of the forms of soil K (solution, exchangeable, fixed and lattice K) has been integrated with an assessment of plant availability (immediately, readily, slowly, and very slowly available, respectively).
The distinction between fixed and lattice K is that the release of the former is reversible whereas that of lattice K is not [Kirkman et all, 1994]. The representation in Fig. 1 provides a useful conceptual basis for discussing the plant availability of soil K and any moves towards improving the assessment of soil K status and fertiliser rec- ommendations.
Soil solution potassium
Plants remove K+ from the soil solution and this form of K can be regarded as being immediately available to plants (Fig. 1). The concentration of K+ in the soil solution varies appreciably with soil weathering, cropping history, and fertiliser use [Bar- ber, 1984], but the amounts present are far too small to meet the requirements of the crop. McLean and Watson [1985] have estimated that approximately 5% of the K requirement of the crop is in solution at any one time. Based on the water requirement
Fig. 1 - The potassium cycle in the soil-plant-animal system. Fixed K alone or fixed K plus lattice K has often been referred to as non-exchangeable K. For feldspars, lattice K is expected to release K to the soil solution
of a given crop, Barber et al. [1962] reported that soil solution K would only supply up to 5% of the K requirement on leached soils containing between 2 and 5mg dm-3 of K in the soil solution. Even in soils containing up to 60mg dm-3 of soil solution K, uptake by transpiration-induced mass flow is insufficient to meet the Krequirements of the plant and additional K must be supplied by diffusion [Barber, 1985].
Warren and Johnston [1962] obtained a good relationship between water-soluble K and exchangeable K, with approximately 15% of the exchangeable K above 170 mg K kg1 being soluble in water. As emphasised by Johnston and Goulding [1990], however, a simple estimate of water-soluble K, gives no indication of the rate of replenishment.
The addition of K in fertiliser and manure will increase soil solution K concentrations and this K can potentially be leached from surface soils. Uptake by the plant and the rapid equilibrium with exchangeable K (Fig. 1) reduces the extent to which this occurs.
The K-retention characteristics of the topsoil and subsoil, arising from both inorganic (particularly the amount and type of clay) and organie components, in addition to water movement through the soil, will influence K loss. Unlike K in drainage water, K retained in the subsoil is not lost from the system when deep-rooted crops are grown. For drainage waters, Johnston and Goulding [1990] reported annual mean K concentrations from a sandy clay loam of 3.4mg K dm-3 under arable cropping and 0.5mg K dm-3 under herbage crops, and from a sandy loam of 3.4mg K dm-3. Depend- ing on rainfall and evapotranspiration, and on the cropping system, the annual leaching losses ranged from 0.2 to 5.0kg K ha4 from the clay loam and from 3 to 20kg K ha-1 from the sandy loam. Using data from Europe and North America, Johnston and Goulding [1992] suggested that for each l00 mm of through drainage, approximately l kg K ha-1 would be lost.
Exchangeable potassium
Exchangeable K is retained by negatively-charged exchange sites on organic matter and clay minerals in soils. This form of K is in the most rapid equilibrium with K in the soil solution (Fig. 1) and for this reason is considered to be readily available. There is also abundant evidence in the literature which indicates that exchangeable K is the predominant source of plant-available K in soils which contain small amounts of fixed K. Neutral ammonium acetate has been used for more than fifty years [Schollenberger and Dreibelbis, 1930] to estimate exchangeable K in soils; currently this reagent (1M concentration) or 1 M ammonium nitrate (as used in the UK) is widely used. Values in soils usually range from 40 to 500mg K kg-1 with a value of 150mg K kg-1 usually considered to be adequate for most crops in the USA [Barber, 1984]. Results from soil-testing laboratories are usually expressed as mg K dm-3, in contrast to those coming from research experiments which are usually expressed as mg K kg-1 soil.
For most mineral soils, sieved to pass 2mm, the density is approximately 1 so that the values are essentially the same, regardless of the units used. The frequently observed close relationship between crop response and soil exchangeable K in greenhouse ex- periments, but more particularly in the field, has resulted in the adoption in the UK of exchangeable K as a useful indicator of soil K status and the likelihood of obtaining a response to fertiliser K. A close linear relationship was obtained for the grain yield of winter wheat and exchangeable K on the Exhaustion Land experiment at Rothamsted (Fig. 2).
A yield increase of approximately 1.8 t ha-1 of wheat grain for an exchangeable K increase from 55 to l00mg K kg-1 was obtained. In contrast, there was little response of spring barley grain to increasing exchangeable K values at Rothamsted (Fig. 3);
for a grain yield of 6t ha’ no more than 80mg kg-1 of exchangeable K was required [Johnston and Goulding, 1990]. Winter wheat, with its longer growing season and deeper rooting, might have been expected to be less responsive to K than spring barley.
The larger response by winter wheat could have been due to the larger yield, requiring more K, or to differences between the two soils in the amount of fixed K they contain.
Fig. 2 - Relationship between grain yield of winter wheat and exchangeable K in soil on the Exhaustion Land experiment at Rothamsted. Unpublished data
from P.R. Poulton and A.E. Johnston
In terms of crop quality, the yield of sugar from sugar beet grown at Rothamsted was still increasing up to 200mg kg-1 of exchangeable K (Fig. 4). The scatter in the values for sugar beet may reflect the fact that subsoil K contributed to K supply, given that the roots of sugar beet explore the subsoil more extensively than do the roots of barley [Johnston and Goulding, 1990].
Field beans at Rothamsted showed a pronounced response to increasing soil ex- changeable K levels up to approximately 200mg K kg-1 (Fig. 5). The high K demand and poor root system of this crop are likely to be responsible for this result.
The above results from work at Rothamsted suggest that exchangeable K is a useful indicator of the soil K which is readily available to the crop. Cropping will decrease the amounts of exchangeable K but it is generally not possible to remove exchangeable K in a soil below a certain level. This is because when the «minimal» level is reached, the release of fixed K, if present, is triggered by the low concentration of soil solution K, exchangeable K, or both [McLean and Watson, 1985)].
There are many soils in temperate regions, where fixed K makes an important contribution to plant-available K [Bertsch and Thomas, 1985]. Such soils often contain small amounts of exchangeable K and maintaining the pool of available K is dependent on the transformation of fixed K to the exchangeable and solution phases following Fig. 3 - Relationship between grain yield of spring barley and exchangeable K in soil at Rothamsted. Redrawn from Johnston and Goulding (1990). Different symbols relate to different cropping and fertiliser histories
Fig. 4 - Relationship between sugar yield from sugar beet and exchangeable K in soil at Rothamsted. Redrawn from Johnston and Goulding (1990). Different symbols relate to different cropping and fertiliser histories
depletion by crop removal [Doll and Lucas, 1973] and by leaching [Sparks,1987].
Results reported by Eagle [1967)]for the relationship between the yield response of potatoes and exchangeable K (30-500mg K kg-1) in K fertilizer trials on widely-differ- ing soils in England are particularly interesting. A pronounced separation of data points for shale soils and sandy soils was obtained but the inclusion of an estimate of the release of non-exchangeable K, using a cation-exchange resin, with that for ex- changeable K greatly improved the relationship (from r = -0.23* to r = -0.53***) and substantially reduced the separation between data points for the different soils. Eagle (1967) stated: „The fact that the release of non-exchangeable potassium is more highly correlated with yield response to potash fertiliser than exchangeable potassium suggests that a considerable part of the potassium requirements of the potato crop are obtained from non-exchangeable forms. If this is so any reliable method of soil analysis must take this into account and no determination on the exchangeable fraction only will be entirely satisfactory”.
It is now necessary to briefly consider the origin and dynamics of fixed K before assessing its likely significance for crop growth.
Fig. 5. - Relationship between grain yield of field beans and exchangeable K in soil at Rothamsted. Redrawn from Johnston (1997). Different symbols are data from
different experiments
Fixed potassium
There are several excellent reviews which cover K fixation and the release of fixed K in soils [Beckett, 1970; Sparks and Huang, 1985; Sparks, 1986; Goulding, 1987]. It is well established that micaceous clay minerals contain adsorption sites which show a high specificity for K+; these are the so-called wedge sites, which are created as micas weather and partially expanded (illites) or expanded interlayers (vermiculites) are created [Rich, 1968] These sites are primarily responsible for the fixation of K. Thus clay mineralogy (type, amount, and particle size distribution of clay minerals) essen- tially determines K fixation and release but many factors have an influence on the rate and extent, including K inputs from fertiliser and manures, depletion by cropping, and soil pH, to name just three. As emphasised by Goulding [1987], K fixation in the long term should be viewed as a beneficial process in that it can decrease the loss of K by leaching and can retain K in a slowly-available form which can be important in meeting the needs of the crop.
The definition, conceptualisation, and measurement of fixed K is more prob- lematic than for exchangeable K. Some 16 procedures for assessing non-exchangeable K in soils were discussed by Martin and Sparks [1985]; the most common being extraction with strong acids (usually boiling nitric acid), the use of other extraction procedures, and exhaustive cropping in pot experiments conducted in the green- house. There is a substantial body of literature on the effectiveness and usefulness of the boiling nitric acid procedure for measuring non exchangeable K [e.g., Doll and Lucas, 1973; McLean and Watson, 1985]. In some studies, e.g., Hsu et al. [1979], boiling nitric acid gave a good prediction of K availability to plants in the latter stages of growth in a pot experiment when it is likely that non-exchangeable K is being released.
However, as pointed out by Johnston and Goulding [1990] the amounts of K extracted by strong acids often relate poorly to the response of annual field crops to fertiliser K.
Other laboratory techniques for assessing fixed K include electro-ultra filtration [Beringer, 1985] and the use of cation-exchange resins [Goulding, 1987], and sodium tetraphenylboron [Jackson, 1985]. Whereas these procedures may be useful for re- search purposes, it seems unlikely that they will find any significant, regular use for advisory purposes in the UK. This should not rule out their use in providing back- ground data on soil K reserves. In fact Goulding and Loveland [1986)] suggested that ion-exchange resin data could usefully be combined with that for strong acid extraction to give a complete picture of soil K content and reserves. This could be used to classify and map K reserves in soils, but it is not readily applicable to routine soil analysis. In New Zealand, an estimate of the long-term release of K from the non-exchangeable pool (Kc), is obtained by boiling with M nitric acid for 10 min and used to moderate exchangeable K values in the Computerised Fertiliser Advisory Service model for fertiliser K recommendations [Cornforth and Sinclair, 1984]. Because of the time and cost constraints imposed by using boiling nitric acid extractions on all soil samples
[Kirkman et al, 1994], three categories of Kc, low, medium, and high, have been established and all New Zealand soils are grouped into one of these according to their clay mineralogy. Campkin [1985] showed that categorising long-term K release based on soil genetic groups results, in some instances, in either overestimation or under- estimation of fertiliser K requirements. Further rigorous testing of this model is re- quired and it may be worthy of evaluation elsewhere.
Levington Agriculture, a UK-based laboratory, provides a „Reserve Potash Analy- sis Service” based on the use of three extractants, including strong nitric acid, mainly for heavy-textured soils growing arable crops. For soils with a reserve K index greater than 2, reduced K recommendations may be made for the crops grown on heavy soils It is informative to review some of the earlier work conducted at Rothamsted on exhaustive cropping and the depletion of soil K. Johnston and Goulding [1990] have pointed to the attractions of this technique for measuring the K-releasing capacity of soils and relating the data obtained to laboratory estimates of exchangeable K, fixed K, etc. However, such experiments are time consuming and costly, and as indicated by Ogunkunle and Beckett [1988] relationships between K release data obtained in the glasshouse and in the field are sometimes poor; potential limitations which were well recognised by Johnston and Goulding [1990)]. Nevertheless, intensive cropping in glasshouse experiments can provide useful information on the release of K which could be useful in enhancing predictive ability for K recommendations. Johnston and Mitchell [1974] conducted a detailed study of the behaviour of K remaining in 48 soils from the Agdell experiment at Rothamsted where the manuring and cropping histories since 1848 were known. They obtained a strong correlation (r = 0.94) between K uptake by ryegrass in the first cropping period (7 harvests) in the glasshouse and the initial amount of exchangeable K. Also, the amount of K taken up at the first harvest was larger than the decrease in exchangeable K during the first cropping period, suggesting that K taken up by subsequent harvests came from initially non-exchange- able sources. The release of non-exchangeable K was calculated from total K uptake minus the decrease in exchangeable K, the latter being obtained by subtracting the value for exchangeable K in the cropped soil from that for initial exchangeable K.
When the release of non-exchangeable K was compared with the decrease in ex- changeable K during the first cropping period (Fig. 6), a close relationship was obtained (r = 0.90); that for K uptake by ryegrass and the decrease in exchangeable K was r = 0.95. Over the range of values tested, the release of non-exchangeable K was approxi- mately twice that of the decrease in exchangeable K.
When the release of non-exchangeable K in the same experiment was plotted against initial exchangeable K, a close relationship (r = 0.87) was again obtained (Fig. 7). This suggests that for soils developed from the same parent material and containing non- exchangeable K, accumulated from past applications of K fertiliser, then the release of non-exchangeable K is related to initial exchangeable K, i.e., a reversible equilibrium exists between the two, as shown in Fig. 1. It is of interest to consider whether this relationship holds for a wider range of soil types. Arnold and Close [1961] found that the release of non-exchangeable K during exhaustive cropping with ryegrass in the glasshouse tended to increase as the initial exchangeable K content of 20 different soils increased (Fig. 7).
When a soil developed on glauconitic sand and with a very high exchangeable K value (720mg K kg-1) was excluded from the comparison, a correlation of r = 0.84 was obtained. Arnold and Close [1961] considered that the correlation was too poor for the trend to be a useful guide. However, it may be worth reconsidering the relationship between the release of non-exchangeable K and initial exchangeable K in soils which have a known mineralogical composition. It may be possible to develop modifying Fig. 6 .- Relationship between release of initially non-exchangeable K and the de- crease in exchangeable K during seven harvests ofryegrass. Redrawn from Johnston and Mitchell (1974). Open circles - soils in arable cropping. Closed circles - soils under grassland
factors for different soil groups based on the ratio of non-exchangeable to exchange- able K based on soil mineralogy, and to use these to moderate exchangeable K values in developing K fertiliser recommendations. The work of Arnold and Close [1961] and Johnston and Mitchell [1974)] forms a very good basis for this. However, several studies have shown that equilibrium levels of exchangeable K in different soil types are not necessarily related to the ability of soils to release fixed K [Arnold and Close, 1961], and it may be necessary to temper any undue enthusiasm for thinking that major advances in this area are possible. These can only come from a better understanding of the dynamics of fixed K, particularly that which is derived from accumulating K fertiliser resides and that from the release of lattice K.
Two further points should be made with regard to the forms and availability of soil K. Firstly, Figure 1 indicates that there is a reversible reaction between exchange- able K and soil solution K, and between exchangeable K and fixed K [Reitemeier, 1951]. This emphasizes the dynamic nature of the pools of K and the need to inte- grate kinetic and equilibrium measurements in assessing the availability of K to crops [Bertsch and Thomas, 1985; Grimme, 1985]. This is where modelling offers good potential, initially as a research tool but perhaps eventually, in advisory work.
Lastly, one might ask why interest in the likely importance of fixed K for crop growth has waned, at least in the UK. Exchangeable K levels can usually be topped up readily by using relatively inexpensive fertiliser K and consequently reserves of fixed K, if present, are less likely to be drawn upon. Added to this, exchangeable K provides a reasonably reliable assessment of the K status on many soils, relieving pressure on the need to improve the assessment of K availability. Furthermore, if the fixation process is reversible, as suggested, it may not be necessary to determine the amount and release of fixed K, provided exchangeable K levels are adequate, but this requires further evaluation.
Lattice potassium
It is difficult to obtain reliable estimates of the significance of very-slowly available lattice K in soils (Fig. 1). The results for the NP treatments in long-term experiments, where K has not been added, indicate an average annual crop removal of approxi- mately 18kg K ha-1 on Broadbalk and 27kg K ha-1 on Park Grass on silty clay loam soils; 8kg K ha-1 on a sandy loam soil at Woburn; and 47kg K ha-1 on a sandy clay loam at Saxmundham [Johnston, 1986]. These results clearly indicate that lattice K is present in these soils and is a source of K to the crop in this treatment. However, because most cultivated soils have received manures and fertilisers containing K, the exchangeable and to a lesser extent the fixed K pools are expected to dominate K supply to crops. If the lattice K occurs in micaceous minerals then this is likely to contribute to fixed K
during the weathering process; if present in K-feldspar minerals, then the weathering process will release K, albeit slowly, to the soil solution (Fig. 1).
Plant potassium Plant uptake of potassium
Plants remove K+ from the soil solution (Fig. 1) by root uptake; a process which is affected by both absorption and translocation of K within plants [Barber, 1984]. The supply of K+ to the root surface has been studied extensively and it is now well estab- lished that diffusion, along the concentration gradient created by root absorption, is the main mechanism [Mengel, 1985]. Calculations which integrate information on crop water use and soil solution K concentrations [Barber, 1984; Barraclough, 1989] and also field data [Renger et al., 1981, cited by Grimme 1985] indicate that mass flow, induced by transpiration, makes a very small contribution to K uptake. Data for spring wheat reported by Grimme [1985] indicate that only 4kg K ha-1 had been contributed by mass flow at anthesis when the maximum uptake of 205kg K ha] had occurred. The plant factors which affect K influx include root morphology, particularly root length, plant K status, plant age, and environmental parameters, such as light and tem- perature. The complex interactions between root growth, the dynamics of nutrient uptake, and soil solution concentrations are well illustrated by the work reported by Barraclough [1989]for a high-yielding winter oilseed rape crop. Soil solution concen- trations for K+ during the growing season were very much higher than the predicted concentration differences required to sustain K influx by diffusive supply and it is unlikely that K uptake (and also that of N, P, and Ca) was limited by potential supply rate.
Concentration of potassium in plants
Crop plants differ in the rate of K uptake and also in total K uptake. It is well established that K concentrations, expressed on a dry-matter basis, decrease with in- creasing age of the plant, and are influenced by the supply of other nutrients, particu- larly N. Thus in establishing critical levels for K it is necessary to know the stage of growth of the plant and to be sure that only one nutrient is limiting, in this case K.
However, as emphasised by Johnston and Goulding [1990], it is often difficult to relate
%K in the dry matter of arable crops to soil or fertiliser K. Expressing K concentration on a dry matter basis is essential to calculate K uptake and offtake and there is the advantage that K uptake and offtake can be calculated, if yield is known.
Leigh and Johnston [1983] questioned the usefulness of expressing K concen- tration on a % dry matter basis and suggested that tissue water may provide a better basis for expressing K concentrations. When expressed in this way, the K concentra-
tion values were essentially constant during the season (for spring barley) until the onset of ripening, when they increased significantly as the crop lost water. Concentra- tions of K were little affected by fertiliser N, P, and water supply throughout the season. When soil K supply was adequate (an exchangeable K value of 325mg K kg-1), tissue water K levels were approximately 200 mM (Fig. 9), in contrast to those (50 mM) in crops grown on a K-deficient soil (an exchangeable K value of 55mg K kg1).
An average concentration of 200 mM of K in tissue water was reported for permanent grass well supplied with K, at a soil exchangeable K value of 114mg K kg-1 [Barraclough and Leigh, 1993)] and an average critical leaf value of 150 mM for winter wheat be- tween growth stage 31 and 61 [Barraclough et al, 1997)] For a winter oil-seed rape crop, tissue water K concentrations were maintained at around 100 mM, even during a drought, whereas % K dry matter values decreased from 3.4 to 1.9% over the same period. Barraclough et al.[1997] have proposed critical values of 130, 150, and 170 mM for the minimum, mean, and maximum K concentrations in leaf tissue water of winter wheat during stem elongation.These results hold promise for diagnosing K defi-
ciency in cereals at any time during spring and summer. This is particularly the case where plants are not growing well on soils where soil K levels appear to be adequate.
The potential for in-situ testing, using test strip technology or a miniaturised specific ion electrode system, is currently being evaluated. However, it must be said that commercialisation of the possibility has been slow.
The situation for vegetables appears to be different and responses to freshly-ap- plied K fertiliser are often small, even when soil available K is in short supply [Costigan and McBurney, 1983; Costigan et al., 1983]. Thus petiole sap analysis should offer the prospect of diagnosing any K shortage during the seedling stage which cause an initial growth reduction not recovered before harvest [Burns, 1986]. However, for reasons which are not fully understood, sap K concentrations in lettuce are not constant during the growing season, changing with water content, and vary with the age of the leaf sampled [Burns and Hutsby, 1986)] The petioles of young, expanding leaves gave more precise results and were more sensitive to changes in external K supply. Values were also substantially lower than for cereals. Despite initial promise from early research, there has been little interest in commercialising sap testing for K in veg- etables. Diagnosis of K deficiency, using sap analysis, is arguably its most useful appli- cation but K deficiency in vegetables is not common. Also, the technique is not suffi- ciently sensitive for scheduling fertiliser application with vegetables [Burns, personal communication].
Foliar application of potassium
Foliar sprays are used with a view to enhancing nutrient uptake, particularly during critical periods such as grain filling, and when soil conditions do not favour root ab- sorption, e.g., during drought [Kannan, 1990]. Positive results have been obtained with wheat and rice in India [Das and Sarkar, 1981], with cotton in the USA [Hake, 1991], and with maize in Thailand [Suwanarit and Sestapukdee, 1986]. With the ex- ception of a study recently reported by Barraclough and Haynes [1996)] no similar work appears to have been published in the UK.
In the study of Suwanarit and Sestapukdee [1986)] suitably-timed applications of 2.5% KNO3 to all aerial parts of maize plants grown in large pots containing a soil with adequate exchangeable K (190mg K kg1) increased grain yield by up to 74%. It was concluded that the K foliar spray enhanced chlorophyll synthesis by overcoming a K limitation at full tassel emergence. In contrast, Barraclough and Haynes [1996]
found no effect of foliar sprays of KNO3 solution on grain yield, yield components, or grain N of winter wheat grown on a soil with adequate supplies of soil nutrients, including K. This was the case even in a season with a dry spring when a foliar spray may have been expected to show some benefit. Given the positive results obtained in
some experiments with foliar K applications to some crops, it may be worth conduct- ing further work in specific situations where K deficiency is anticipated. One such example is soils high in magnesium (Mg) where K deficiency can occur in mid-season under conditions of moisture stress.
Crop offtake of potassium
Nutrient offtake is a function of yield and nutrient concentration in the harvested crop. As such it is strongly influenced by management practices, particularly the use of fertiliser N. The yield of arable crops, and increasingly yield variation within fields, can be measured with reasonable precision. However, if the concentration of K in har- vested crop products varies considerably with yield or soil factors, reliable estimates of offtake are more difficult to obtain. Nevertheless, it is possible to make reasonable estimates of K offtake and these are useful for preparing K balances and assisting with fertiliser recommendations. Straw and tops invariably contain more K than grain or roots [Johnston and Goulding, 1988] and the K they contain then contributes to soil reserves when they are returned. In the case of cereal straw, the extent of removal must be considered in calculating K offtake.
Increases in the yield of several crops during the last decade or so have resulted in larger offtakes of K. This can be seen in Table 1 where the crop yield, K concentra- tion, and estimated K offtake for wheat, barley, oil-seed rape, and potatoes in 1982 and 1997 are given.
Table 1. Estimates for the offtake of K by wheat, barley, oilseed rape and potatoes calculated from the mean yield and K concentration (both on a fresh weight basis). Values in parentheses are those for 1982 recalculated from Church and Skinner [1986]
Wheat Barley Oilseed rape potatoes
Grain Straw Grain Straw Seed Tubers
Average yield 7,8(6,2) 5,1(4,8 5,8(4,9) 3,8(3,2 3,2(3,3 40,7(37,3) kg K t-1 4,6(4,6 7,9(6,8) 4,6(4,6) 8,8(6,8) 9,1(9,1) 4,8(4,8) kg K ha-1 36(29) 28(13) 27(22) 32(9) 29(30) 195(180) Total offtake kg
K ha-1 64(42) 59(31) 29(30) 195(180)
Average yield for 1995,1996 and 1997 (UK MAFF Statistics).
Assuming straw yield for baling and removal is 65% of grain yield.
% K in straw based on recent Potash Development Association data. For barley, national average yield and % K based on a ratio of 60% winter barley and 40% spring barley.
Based on a 70 and 96% removal of wheat and barley straw, respectively.
Church and Skinner assumed that 40% of cereal straw was removed; much straw at that time was burned in situ.
Also, the fact that more cereal straw has been removed in recent years contributes substantially to the larger value for K offtake. Thus the estimated total K offtake has increased from 42 to 64kg ha-1 for wheat and from 31 to 59kg ha for barley, between 1982 and 1997, respectively. This increase in K offtake is now being recognised and K fertiliser recommendations adjusted accordingly. Calculations of K offtake are impor- tant for balancing K removal from the system and should be made at regular intervals if management practices are changing.
Needs and opportunities
In the introduction to this brief review it was emphasised that the needs and oppor- tunities for further work on K in agriculture were related to economic issues, particu- larly the development of improved recommendations for K fertiliser use. This arises because K is an environmentally benign element, posing no threat to human health or the quality of natural waters.
The consumption of potash fertiliser in the UK has changed remarkably little during the last 36 years, unlike the situation in France which has tracked world consumption fairly closely (Fig. 10). During this time, crop yields and crop offtake of K have in- creased, the latter often substantially so during the last 15 years based on the data presented in Table 1. It is of interest that, during recent years, soil K levels appear to have changed little, at least using the distribution of soil K within each of soil indices 0- 5 (Table 2). A more detailed analysis of the change in soil K levels in relation to fertiliser use and increasing K offtake may be warranted.
A case for obtaining more information on the amounts and release of fixed or slowly-available K in soils has already been made. The objective would be to deter- mine whether and the extent to which exchangeable K values should be moderated to take account of fixed K, where present, in making K fertiliser recommendations.
Also, further work on the effectiveness of foliar K applications for correcting K limita- tions in particular situations, such as high Mg soils, has been suggested.
It is likely that there will continue to be a requirement for further research and development work on specific issues relating to K. For example, there is a need for an improved understanding of within-field variability of K and other nutrient levels in the context of the developing interest in precision farming. The suggestion [ADAS, 1995]
that, considering all sources of error, the minimum difference in soil analysis values for practical decision making for K might be 25mg K l-1 is somewhat disturbing and one which offers the prospect for further work. Also, the extent to which K fertiliser appli- cation rates should be varied as soil pH varies requires further evaluation.
Table 2. Distribution (%) of exchangeable K values across six Indices established for available K assessment for arable, ley-arable, and grassland soils in the United Kingdom for the periods 1984-1988 and 1989-1993. Data from the Representative Soil Sampling Scheme RSSS for the respective years. [Personal communication]
Time period Distribution (%) of values within Index
0* 1 2 3 4 5
Arable
1984-1988 2 26 52 18 3 1
1989-1993 2 24 53 19 4 1
Ley-arable
1984-1988 6 37 42 11 3 0
1989-1993 6 35 45 12 3 0
Grassland
1984-1988 7 41 41 9 1 0
1989-1993 6 42 42 9 2 0
Index : 0 = <60 mg K l-1, 1 = 61-120 mg K l-1, 2= 121-240 mg K l-1, 3 = 241-400 mg K l-1, 4 = 401-600 mg K l-1, 5 = >600 mg K l-1
Modelling arguably offers the best prospects for improving our understanding of the dynamic interactions between the supply of soil K and the demand for K by the crop. The growth and activity of the root system links the two processes of supply and demand for K [Barraclough, 1990]. Although good progress has been made in model- ling the uptake of K by crops [Classen et ah, 1986; Seward et ah, 1990] there is a shortage of the detailed measurements needed to validate and drive such models [Barraclough, 1990]. Very recently, details of a dynamic model for describing the effects of K fertiliser on crop growth, K uptake, and soil K have been published by Greenwood and Karpinets [1997a], which interestingly includes a term for the gain and loss of fixed K.
The validity of the model was tested against the results of multi level K fertiliser experiments [Greenwood and Karpinets, 1997b], with encouraging results. Simula- tions with the model indicated that, in Central England, no response of 10 arable crops (principally field-grown vegetables) to fertiliser K would be likely on soils containing more than 170mg K kg1 of exchangeable K and with clay contents of be- tween 15 and 45%. Such a mechanistic model is potentially very useful for under- standing the relative contribution of the various soil and plant factors which contribute to crop growth and K uptake. Further validation and simplification of this and related models is required if they are to find widespread acceptance at the field level.
Acknowledgements
The author is particularly indebted to «Johnny» Johnston (IACR-Rothamsted) for his encouragement and valuable interaction during the preparation of this paper.
Chris Dawson (The Fertiliser Society), John Hollies (The Potash Development Asso- ciation), Rolf Hardter and Thomas Berg (Kali und Salz), Adolf Krauss (Interna- tional Potash Institute), Keith Isherwood (International Fertilizer Industry Associa- tion), Peter Arnold (Emeritus Professor, University of Newcastle), and Ian Burns (Hor- ticultural Research International) provided much useful information; their contribu- tions are gratefully acknowledged.
REFERENCES
ADAS (1995) A Review of Spatial Variation of Nutrients in the Soil Report of Project CEO139 to MAFF CSD, London.
Anke, M., Losch, E., Muller, M., Kramer, K., Glei, M. and Bugdol, K. (1992)Potassium in human nutrition. In: Potassium in Ecosystems: Biogeochemical Fluxes of Cations in Agro- and Forest- Systems. International Potash Institute, Basel, pp. 187-204.
Arnold, P.W. (1962) The potassium status of some English soils considered as a prob- lem of energy relationships. No. 72 The Fertiliser Society, York, UK, 25-43.
Arnold, P.W. (1970) The behaviour of potassium in soils. No. 115, The Fertiliser Society, York, UK, 3-15.
Arnold, P.W. and Close, B.M. (1961) Release of non-exchangeable potassium from some British soils cropped in the glasshouse. Journal of Agricultural Science, Cambridge 57, 295-304.
Arnold, P.W. and Shepherd, M.A. (1989) Phosphorus and potassium requirements of cereals. Home Grown Cereals Authority Research Review No. 16.
Barber, S.A. (1984) Soil Nutrient Bioavailability. John Wiley, New York, 398p. Barber, S.A. (1985) Potassium availability at the soil-root interface and factors influ- encing potassium uptake . In: Potassium in Agriculture (Ed R.D. Munson) American Society of Agronomy, Madison, pp. 309-324.
Barber, S.A., Walker, J.M. and Vasey, E.H. (1962) Principles of ion movement through the soil to the plant. Proceedings International Soil Science Society Confer- ence, Palmerston North, New Zealand, pp. 121-124.
Barraclough, P.B. (1989) Root growth, macro-nutrient uptake dynamics and soil fertil- ity requirements of a high-yielding winter oilseed rape crop. Plant and Soil 119, 59-70.
Barraclough, P.B. (1990) Modelling K uptake by plants from soil. In: Development of K- Fertilizer Recommendations. International Potash Institute, Basel, pp. 217-230.
Barraclough, P.B. and Haynes, J. (1996) The effect of foliar supplements of potassium nitrate and urea on the yield of winter wheat. Fertilizer Research 44, 217-223.
Barraclough, P.B. and Leigh, R. A. (1993) Grass yield in relation to potassium supply and the concentration of cations in tissue water. Journal of Agricultural Science, Cambridge 121,157-168.
Barraclough, P.B. Bollans, H.M., Chambers, B.J., Hatley, D.J. and Moss D.P. (1997) Plant testing to determine the P and K status of wheat. Home Grown Cereals Authority Project Report No. 37, HGCA, London.
Beckett, P.H.T. (1970) Fixed potassium and the residual effects of potassium fertilisers. Potash Revue Subject 16, 52nd suite.
Beringer, H. (1985) Adequacy of soil testing for predicting fertiliser requirements. Plant and Soil 83, 21-37
Beringer, H. (1992) Environmental aspects of potash needs, use and production.No.
328, Proceedings of The Fertiliser Society, York, UK. Bertsch, P.M. and Tho- mas, G.W. (1985) Potassium status of temperate region soils. In: Potassium in Agriculture (Ed R.D. Munson), American Society of Agronomy, Madison, pp.
131-162.
Burns, I.G. (1986) Determination of critical K concentrations in sap from individual leaves or from whole plants using K-interruption experiments. Plant and Soil 94, 301-312.
Burns, I.G. and Hutsby, W. (1986) Choice of leaf for estimation of K status by analy- sis of petiole sap. Journal of the Science of Food and Agriculture 37,115-128.
Campkin, R. (1985) Model for calculating potassium requirements for grazed pasture.
New Zealand Journal of Experimental Agriculture 13, 27-37.
Church, B.M. and Skinner, RJ. (1986) The pH and nutrient status of agricultural soils in England and Wales 1969-1983. Journal of Agricultural Science, Cambridge 107 21-28.
Claassen, N., Syring, K.M. and Jungk, A. (1968) Verification of a mathematical model by simulating potassium intake from soil. Plant and Soil 95, 209-220.
Cornforth, I.S. and Sinclair, A.G. (1984) Fertiliser and Lime Recommendations for Pastures and Crops in New Zealand. Second Edition, New Zealand Ministry of Agriculture and Fisheries, 76p.
Costigan, P.A. and McBurney, T. (1983) Variation in yield between two similar sandy loam soils. II. Causes of yield variation in cabbage and lettuce crops. Journal of Soil Science 34, 639-647.
Costigan, P.A., Greenwood, D.J. and McBurney, T. (1983) Variation in yield between two similar sandy loam soils. I. Description of soils and measurement of yield differences. Journal of Soil Science 34, 621-637.
Das, S. and Sarkar, A.K. (1981) Effect of post-flowering foliar spray of potassium nitrate solution on grain filling and yield of rice and wheat. Indian Agriculture 25, 267-273.
Doll, E.C. and Lucas, R.E. (1973) Testing soils for potassium, calcium and mag- nesium. In: Soil Testing and Plant Analysis (Eds L.M. Walsh and J.D. Beaton), Soil Science Society of America, Madison, pp. 133-151.
Eagle, D.J. (1967) Release of non-exchangeable potassium from certain soils. In: Soil Potassium and Magnesium, Ministry of Agriculture, Fisheries and Food, HMSO, London, pp. 49-54.
Goulding, K.W.T. (1987) Potassium fixation and release. In: Methodology in Soil-K Research International Potash Institute, Basel, pp, 137-154..
Goulding, K.W.T. and Loveland, P.J. (1986) The classification and mapping o potas- sium reserves in soils of England and Wales. Journal of Soil Science 37,555-565.
Greenwood, D.), and Karpinets, T.V. (1997a) Dynamic model for the effects of K fertiliser on crop growth, K-uptake and soil-K in arable cropping. l. Descrip- tion of the model. Soil Use and Management 13, 178-163.
Greenwood, D.J. and Karpinets, T.V. (1997b) Dynamic model for the effects of K fertiliser on crop growth, K-uptake and soil K in arable cropping. 2. Field test of the model. Soil Use and Management 13,184-189.
Grimme, H. (1985) The dynamics of potassium in the soil-plant system. In: Potassium in the Agricultural Systems of the Humid Tropics. International Potash Insti- tute, Basel, pp. 127-154
Grossklaus, R. (1992) Potassium concentrations in beverages as a guidance for potas- sium threshold levels in drinking water. In: Potassium in Ecosystems: Bio- geochemical Fluxes of Cations in Agro- and Forest-Systems. International Potash Institute, Basel, pp. 127-154.
Hake, K. (1991) Overcoming K deficiency in cotton. Solutions, November / Decem- ber,1991, 38-40.
House of Lords Select Committee on the European Communities (1989) Nitrate in Water,HMSO, London.
Hsu, W.W., Liebhardt, W.C., Cotnoir, L. and Spolgaric, W. (1979) Potassium release by some Delaware soils. Communications in Soil Science and Plant Analysis 10,1037-1056.
Huber, D.M. and Arny, D.C. (1985) Interactions of potassium with plant disease. In:
Potassium in Agriculture (Ed R.D. Munson), American Society of Agronomy, Madison, pp. 468-488.
Jackson, B.L.J. (1985) A modified sodium tetraphenylboron method for the routine determination of reserve potassium status of soils. New Zealand Journal of Experimental Agriculture 13, 253-226.
Johnston, A.E. (1986) Potassium fertilisation to maintain K balance under various farming systems. In: Nutrient Balances and the Need for Potassium. Interna- tional Potash Institute, Bern pp. 199-226.
Johnston, A.E. (1997) Fertilisers and agriculture: Fifty years of development and chal- lenges. No 396, The Fertiliser Society, York UK, 19-48
Johnston, A.E. and Goulding, K.W.T. (1988) Rational potassium manuring for arable crop systems. Journal of the Science of Food and Agriculture 46,1-11.
Johnston, A.E. and Goulding, K.W.T. (1990) The use of plant and soil analyses to predict the potassium suppying capacity of soil. In: Development of K- Fertil- izer Recommendations. International Potash Institute, Bern, pp. 177-204.
Johnston, A.E. and Goulding, K.W.T. (1992) Potassium concentrations in surface and groundwaters and the loss of potassium in relation to land use. In: Potassium in Ecosystems: Biogeochemical Fluxes of Cations in Agro- and Forest-Sys- tems. International Potash Institute, Basel, pp. 135-158.
Johnston, A.E. and Mitchell, J.D.D. (1974) The behaviour of K remaining in soils from the Agdell experiment at Rothainsted, the results of intensive cropping in pot experiments and their relation to soil anałysis and the results of fie1d experi- ments. Rothamsted Experirnental Station Report for 1973, Part 2, 74-97 Kannan, S. (1990) Role of foliar ferlilization in plant nutrition. In: Crops as Enhancers
of Nutrient Use (Eds. V.C. Baligar and R.R. Duncan), Academic Press, San Diego, pp. 313-348.
Kirkman, J.H., Basker, A., Surapaneni, A. and MacGregor, A.N. (1994). Potassium in the soils of New Zealand — a review. New Zealand Journal of Agricultural Research 37, 207-227
Leigh, R.A. and Johnston, A.E. (1983) Concentrations of potassium in the dry matter and tissue water of field-grown spring barley and their relationships to grain yield. Journal of Agricultural Science, Cambridge 101, 675-685.
Leigh, R.A. and Wyn Jones, R.G. (1984) A hypothesis relating critical potassium con- centrations for growth to the distribution and functions of this ion in the plant cell. New Phytologist 97, 1-13.
McLean, E.O. and Watson, M.E. (1985) Soil measurements of plant-available potas- sium. In: Potassium m Agriculture (Ed R.D. Munson), American Society of Agronomy, Madison, pp. 277-308.
Martin, H.W. and Sparks, D.L. (1985) On the behaviour of non exchangeable potas- sium in soils. Communications in Soil Science and Plant Analysis 16,133-162.
Mengel, K. (1985) Potassium movement within plants and its importance in assimilate transport. In: Potassium in Agriculture (Ed. R.D. Munson), American Society of Agronomy, Madison, pp. 397-412.
Munson, R.D. (1985) Potassium in Agriculture, American Society of Agronomy, Madi- son, 1223p
Notholt, A.J.G., Sheldon, R.P. and Davidson, D.F. (eds.) (1989) Pliosphate Deposits of World. Volume 2, p. 24. Cambridge University Press.
OECD (1982) Eutrophication of Waters: Monitoring, Assessment and Control. OECD, Paris.
Ogunkunie, A.O. and Beckett, P.H.T (1988) The efficiency of pot trials on undis- turbed soil cores, as predictors of crop behaviour in the fie1d. Plant and Soil 107, 85-93.
