agriculture group symposium sodium and potassium in plant nutrition

J Sci Food Agric 1988,43,319-331

Agriculture Group Symposium Sodium and Potassium in Plant Nutrition

The following are summaries of papers presented at a joint meeting of the Agriculture Group of the Society of Chemical Industry and the Fertiliser Society, held on 20 October 1987 at the Society of Chemical Industry, 14-15 Belgrave Square, London SWlX 8PS . The papers published here are entirely the responsibility of the authors and do not reflect the views of the Editorial Board of the Journal of the Science of Food and Agriculture.

The Cellular and Genetic Basis for Cation Discrimination by Plants

Roger A Leigh

AFRC Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Hertfordshire AL5 UQ, UK

John Gorham and R Gareth Wyn Jones

Department of Biochemistry and Soil Science, University College of North Wales, Bangor, Gwynedd LL57 2UW, UK

Potassium is essential for plant growth, and a minimum concentration must be present in plant tissues to obtain maximal growth rates (Leigh and Wyn-Jones 1984). This can easily be achieved by potassium fertilisation, but this decreases the uptake of other cations such as sodium and magnesium (Leigh et al 1986). This is relatively unimportant with cereals but can be serious in grassland where shortage of sodium and magnesium can affect the quality of herbage used for animal production. The problem is howto replace part of the potassium with other cations while still maintaining maximum yields. Some insight into this problem has been obtained from studies of the cellular roles of potassium, which have identified that fraction of cellular potassium which can be replaced by other cations. In addition, experiments on the salt tolerance of wheat have identified the chromosomal location of a set of genes that confer cation discrimination.

In plant cells potassium is located in both the vacuole and cytoplasm but fulfils different functions in each (Leigh and Wyn-Jones 1984). The largest proportion is in the vacuole where it acts as a turgor-generating osmoticum. In plants given adequate potassium the potassium concentration in the vacuole will be about

319

J Sci Food Agric (43) (1988)- 1988 Society of Chemical Industry. Printed in Great Britain

320 Agriculture Group Symposium

200 mM, and once this level is achieved it does not respond to further increases in potassium supply (Leigh and Johnston 1983). However, the ability to generate osmotic pressure is not unique to potassium salts, and therefore other solutes can potentially replace vacuolar potassium. In contrast the cytoplasmic potassium has important biochemical functions, and other cations cannot substitute for it. The cytoplasmic potassium concentration must be maintained at about 100-150 mM. Mechanisms exist to achieve this at the expense of vacuolar potassium and to prevent the accumulation of excessive amounts of sodium in the cytoplasm (Leigh et a1 1986).

Thus only vacuolar potassium is potentially replaceable by other cations without large effects on yield. In practice, however, the nature of the substituting cation appears to be important. In experiments with barley, potassium concentration could be decreased to much lower levels, with a smaller loss of yield, if extra sodium were supplied (Leigh e t a1 1986). This was because without sodium the plants accumulated high concentrations of calcium and magnesium, and it was these cations which decreased yield. This suggests that sodium can substitute for potassium in the vacuole without penalty, but calcium and magnesium cannot. The basis for this is not understood.

The experiments to identify the chromosomal location of genes conferring high K:Na ratios on wheat grown under saline conditions have taken advantage of the fact that of the three genomes (A, B and D) in modern hexaploid breadwheats it is the D genome which carries this trait (Shah et a1 1987). Substitution lines were examined in which each of the A- or B-genome chromosomes in the tetraploid Triticum turgidum cv Langdon was systematically replaced by the equivalent D- genome chromosome. Only lines containing the 4D chromosome were able to maintain high K:Na ratios when grown under saline conditions, indicating that the genes resided on this chromosome. Other substitution lines showed that the genes were located on the long arm of this chromosome, and physiological analyses indicated that the genes controlled the transport of sodium into the xylem and therefore limited the amount of sodium that reached the shoot.

Thus there appears to be scope for manipulating cation discrimination of plants by altering, through breeding, their ability to control the cations that reach the shoot. However, once the cations are in the shoot their effects on yield will depend on the ability of the plants to differentially accumulate them in the vacuole.

References Leigh R A, Johnston A E 1983 Effects of fertilizers and drought on potassium concentrations

in the dry matter and tissue water of field-grown spring barley. J Agric Sci Camb 101 741- 748.

Leigh R A, Wyn Jones R G 1984 A hypothesis relating critical potassium concentrations for growth to the distribution and functions of this ion in the plant'cell. New Phytol97 1-13.

Leigh R A, Chater M, Storey R, Johnston A E 1986 Accumulation and subcellular distribution of cations in relation to the growth of potassium-deficient barley. Plant Cell Environ 9 595-604.

Shah S H, Gorham J , Forster B P, Wyn Jones R G 1987 Salt tolerance in the Triticeae: the contribution of the D genome to cation selectivity in hexaploid wheat. J E x p Bot 38 254- 269.

Sodium and potassium in plant nutrition 321

Potassium Absorption and Distribution in Grasses

Stephen C Jarvis, James H Macduff, Jill Webb and Amelia Mosquera AFRC Institute for Grassland and Animal Production, Hurley, Maidenhead, Berkshire SL6 SLR, UK

In the field, forage plants are subjected to periodic defoliation through grazing or cutting, and to fluctuating N supplies. Defoliation has an immediate impact in reducing the absorption of nutrients by grasses (Clement et a1 1978). The extent and duration of this effect with potassium (K) is not clear; the absorption and distribution of K in Lolium perenne L and Lolium multiflorum L have therefore been examined after defoliation and with different NO; -N supplies.

The plants were grown in flowing solution culture with NO;-N and K maintained at 10 PM. In one treatment, N supplies were withdrawn for 12 days and then re-supplied (=LN); control plants (=HN) had a maintained supply throughout. At the same time as NO; was re-supplied, some plants were defoliated to give, for both species, entire and defoliated plants of HN and LN status. Potassium uptake and absorption rates were calculated from plant growth data and the quantities of K were automatically supplied to the culture solutions to maintain 10 PM. Cellular K distribution in young root material was also examined under scanning electron microscopy by X-ray micro-analysis of frozen hydrated samples.

With L perenne, cumulative uptake of K was reduced after NO; supply was withdrawn; at the end of the experiment overall uptake was reduced by 15 %. In contrast, there was no reduction in cumulative uptake by L multiforum. There was a similar reduction of uptake in the two species after defoliation. When expressed on a unit absorption basis (PM K g-' d- ' fresh root), different patterns were displayed: absorption by control plants of L perenne reached a steady state, whereas that by L rnultijlorum declined throughout. Absorption rate decreased rapidly after NO; was withdrawn and then increased after NO; was re-supplied in L perenne but not in L rnultijlorum. Defoliation had an immediate effect, and absorption was slightly reduced in both HN and LN plants; absorption then increased after a lag of 6-7 days after cutting, in HN plants to twice that of control plants.

K distribution in cortical cells of actively growing roots showed considerable differences at 10 days after NO; removal. In HN plants there was little difference between concentrations in cytoplasm and vacuole: concentrations in LN plants were generally lower than in HN plants and were greater in cytoplasm than in the vacuole. The patterns of distribution after defoliation were not so distinct.

Patterns of K uptake and distribution are significantly influenced by both defoliation and changing NO; supply. Although there were differences between the species, recovery after defoliation was rapid, and unit absorption, even in the absence of a large shoot system, was greater than that of control plants within 7-8 days of cutting.

Rekrence Clement C R , Hopper M J, Jones L H P, Leafe E L 1978 The uptake of nitrate by Lolium

perenne from flowing nutrient solution. 11. Effect of light, defoliation and relationship to

'

CO, flux. J EX^ Bot 29 1173-1183.


Potash Recycling on Pastures

Olivier Jourdan

SCPA Centre de Recherches d'Aspach le bas, Cernay, France

Nitrogen (N), phosphorus (P) and potassium (K) contained in animal manure can be used as fertiliser when manure is spread on grassland; however, what happens when excreta go directly to the grass? Some estimations of recovery are about 50% (Garaudeaux 1975; Witzenmeyer 1963; Lombaert 1983). The coefficient of recovery of K has been measured in intensive grassland in order to understand the role of excreta in meeting the fertiliser needs of grass (Jourdan 1986).

A split-plot design crosses two levels of K fertilisation, K,O = 0 and K 2 0 = 225 kg ha-' yr-' with two systems grazing pasture with 1400m' plots and a mowing system with 12mZ plots. (P fertilisation was 40kg ha-' yr-' and N fertilisation was about 300 kg ha-' yr-I.) Four blocks were sown with tall fescue (Festuca arundinacea) and four blocks with cocksfoot (Dactylis glornerata). Two groups of steers grazed the K,O=O and K20=225 meadows from April to October. Results are given for 9 years (from 1975 to 1983).

Grazing and fertilisation affected DM yield, although grazing effects were small even after 5 years. Effect of grazing is the global effect of the N, P and K in manure and also of trampling by the animals. The average yield of 5 years is 9.46 t ha- yr - ' (mown) and 9.68 (grazed) but the average yield of 9 years is 9.05 (mown) and 10-21 (grazed).

The coefficient of recovery (C %) of K was calculated as follows:

K uptake from grazing - K uptake from mowing K returned with excreta

C % = x 100

We assume 97% of K ingested is returned to the grass.

i 2 3 1 1 1 1 I l

4 5 6 7 8 9 Years Years

(a ) (b)

Fig 1. Yearly (a) and cumulative (b) coefficient of recovery of K. KO: V, cocksfoot, 0, tall fescue. K1: V, cocksfoot, 0 , tall fescue.


The coefficient of recovery of K improved during the experimental period but it took many years to reach equilibrium (Fig 1). Recycling rates differ according to fertilisation level and grass species.

Nine years of experience confirm that the recovery of potassium on pasture is about 50% if no losses occur, but this period is not sufficient to establish a stable and balanced potassium cycle.

References

Garaudeaux J 1975 C R Acad Agric France No 10, 571-580. Jourdan 0 1986 Dossiers Agronomiques d’rlspach. SCPA, Cernay Lombaert V 1983 Thbe, Universite de Bruxelles. Pfitzenmeyer C 1963 Fourrages No 15, 67-81.

Potassium, Sodium and Magnesium Requirements of Grazing Ruminants

Helmut Beringer

Agricultural Research Station Buntehof, Bunteweg 8, D-3000 Hannover 1, West Germany

A pre-requisite for profitable dairy farming is good fodder quality. This not only allows the production of more milk from cheap farm-produced fodder, but also helps to achieve a high level of herd fertility and herd health.

For many enzyme reactions K and Mg ions are essential. Both are present in high concentrations within animal cells. Na ions, on the other hand, and C1- dominate in the blood and extracellular liquids. This different ionic distribution is the basis of electrochemical gradients which are involved in energy-requiring metabolic processes as well as in the transmission of signals in sensory, nerve and muscle cells. The animal has very efficient control and hormonal response mechanisms to maintain its electrolyte balance. Aldosterone, one of the hormones of the adrenal gland, is most important in the homeostasis of Na.

The Mg and Na requirement of grazing cows (30 g day-’ each for a 30 litres day- milking cow) is generally not satisfied by grass. Thus hypomagnesaemia of the animal can occur. Also the K/Na ratio in the ration can exceed 30:l or the Na concentration in the saliva can fall below 130 mM Na. Under such conditions the resorption of Mg is impaired, and decreasing herd fertility (conception rates) may result.

Though Mg and Na as the limiting cations could be offered to grazing animals in the form of salt licks, their acceptance by individual animals is very variable. All measures should therefore be taken to establish pastures with a high proportion of those species (especially Loliurn perenne) having a high uptake rate for Na and responding well to Na- and Mgcontaining fertilisers.


Experiences with ‘Magnesia-Kainit’ on Grassland

Gerhard Horn

Kali und Salz AG, Kassel, FRG

The need to improve farm income against the background of fixed costs has never been greater. As grass is still a relatively cheap source of feed, dairy farmers are forced to get the best from their pastures.

The Mg and Na contents in herbage are generally below the standard levels of 0.2% Mg and 0.2% Na in dry matter required for adequate animal nutrition. Incidence of grass tetany and higher rates of infertility have been reported to increase with deficient Mg and reduced Na contents in grass (Butler 1963; Giihne 1987).

By the application of ‘Magnesia-Kainit’ (11 % K,O, 3% Mg, 22% Na) the contents of sodium and magnesium in the herbage can be increased to the desired minimum levels. Various species of grass respond differently.

Cattle prefer fodder from pastures that have been treated with ‘Magnesia-Kainit’. It is obvious that there is a correlation between the Na contents and increased feed uptake. This is associated with an improvement of palatability. This results in a decrease in the amount of herbage rejection and higher average weight gains of heifers and lactating cows (Poeschel 1987).

High K levels, characteristic of young spring grass, or induced by high rates of slurry, are balanced by ‘Magnesia-Kainit’. The best application time is spring before the vegetative period and after the first cut.

References Butler E J 1963 The mineral element content of spring pasture in relation to the occurrence of

grass tetany and hypomagnesaemia. J Agric Sci 60 329. Guhne W 1987 Gezielter Einsatz organischer und mineralischer Dungung zur Steigerung der

Tierproduktion aus Grundfutter. Kali-Briefe (Buentehof) 18(7) 51 1-522. Poeschel 1987 Untersuchungen zur Beseitigung des Natrium-Mangels einer Mastrindherde

bei Weidegang durch Dungung des Grunlandes mit natrium-haltigen Dungemitteln. Dissertation, Gottingen.

Rational K Manuring

Edward Johnston and Keith Goulding

AFRC Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Henfordshire AL5 UQ, UK

Interest in potassium manuring has decreased in recent years because (i) applying K leads to no environmental problems, (ii) few soils are K deficient, and (iii) K is cheaper than N. However, newly applied K optimises yields, especially with high-


yielding crops and through its interaction with N, and K reserves benefit crops in a way that often cannot be offset by fertiliser K. Thus K manuring should not be disregarded. To achieve a rational basis for K manuring, the various sources and demands must be taken into account.

Sources of K other than fertilisers are the atmosphere (which annually supplies on average about 5 kg K ha- ') and the soil-as water soluble, exchangeable and nonexchangeable K. Water soluble K is a direct measure of K in solution, and it is the activity of this which determines the plant availability of K. This in turn is buffered by exchangeable K and nonexchangeable K. The amount and rate of release of these two fractions and the relationship between them are crucial in determining the ability of a soil to continue to supply K to a crop, and depend largely on soil clay (and sometimes silt) content and mineralogy. Both fractions are increased by K residues, and residues of nonexchangeable K in particular can be released in large amounts. Our ability to predict fertiliser requirements is hampered by our inability to reliably estimate nonexchangeable K and its rate of release to crops, and by the inherent variability-both spatial and temporal-of soil K.

Regarding the demands of thecrop for K, new methods of plant analysis, in which K concentrations are expressed on the basis of tissue water, distinguish K sufficiency and deficiency levels in cereals. The actual crop requirement for K depends on the final crop yield. K removed in wheat grain is approximately 30,40 and 60 kg ha-' for 5,7.5 and 10 t ha-' crops; K removals are at least doubled if straw is removed, showing how important the fate of straw is to determining fertiliser K requirements.

Balance sheets provide the simplest option for rational K manuring. Quantities required can be calculated from expected yields and crop composition tables, and applied annually on light-textured soils (perhaps as an NK compound). On heavier soils, they can be applied at any convenient point in a rotation, probably just before the most K-sensitive crop is grown, as practised at Rothamsted. Applications should match or slightly exceed crop requirements. There is certainly little to gain from omitting K, and every reason, both economic and agronomic, to include it. Changes in exchangeable K levels should be monitored and, if deficiencies are suspected, crop analyses made. Such a rational policy, with careful but not time- consuming or expensive attention to soil K levels, balance sheets and crop analyses, will provide due reward in current crop yields and quality and in future soil productivity.

Phosphate and Potash Removal in Cereal Straw

Paul J A Withers

Soil Science Department, ADAS, Government Buildings, Kenton Bar, Newcastle upon Tyne, UK

ADAS phosphate (P205) and potash (K,O) fertiliser recommendations for cereals are based on crop phosphate and potash removal. Recent data from Norsk-Hydro

3 26 Agriculture Group Symposium

trials 1983-85 (Chaney 1987) suggested that the current ADAS recommendation for the amount of potash removed in cereal straw (8.2 kg K,O tonne-' at 85 %dry matter) is too low. An investigation, partly sponsored by the Potash Development Association, was therefore started in 1986 to monitor phosphate and potash removal in the straw of modern cereal varieties receiving ADAS-recommended rates of nitrogen.

Grab samples of winter wheat, winter barley and spring barley were taken from the combine swath at or near harvest and analysed for dry matter, phosphorus and potassium content. Further straw samples were taken from ADAS nitrogen response trials to investigate the effect of increasing nitrogen rate on straw phosphate and potash removal.

Analysis of 203 samples from the 1986 cereal harvest showed a median offtake of phosphate for both wheat and barley straw of 1.2 kg P,O, tonne-' (85 % dry matter). The offtake of potash in barley straw was 9.8 kg K,O tonne- ' (85 % dry matter), significantly higher (P<O-l) than that in wheat straw which was 7.5 kg K,O tonne-' (85% dry matter). There was no significant difference in phosphate and potash removal between winter and spring barley. Data from 11 ADAS nitrogen response trials showed that increasing nitrogen rates generally increased the potassium concentration in the straw but had no consistent effect on straw phosphorus concentrations.

The results for phosphate and potash removal from the 1986 cereal harvest agree very favourably with current ADAS recommendations which do not distinguish between wheat and barley. The investigation has continued for the 1987 harvest but results are not yet available. With the larger data-set the variation in phosphate and potash removal between sites will be correlated with soil, cultural and meteorological parameters.

Reference Chaney K 1987 Phosphate and potash removals in cereals. Norsk Hydro Agtec Bulletin 1986187 16-17.

The Effect of Different Levels of Potassium and Sodium on Yield and Quality of Herbage

John D Webb

ADAS, Woodthorne, Wolverhampton WV6 8TQ, UK

Natural deposits of potassium chloride occur in admixture with other salts; that with sodium chloride is termed sylvinite and is available from the Cleveland mine. There would be product'ion advantages in the mixed salt if agronomic uses could be demonstrated. Crops such as sugar beet respond to sodium; others substitute


sodium for potassium in metabolic processes. Sodium is an important nutrient for grazing livestock. The relationship between potassium and sodium in their effects on grass growth and nutrient content was investigated, and a summary of the results are presented here.

An experiment was carried out at six centres in England and Wales to investigate the effect of applying potassium at 0,3705 and 75 kg K 2 0 ha-' in all combinations with sodium at 0,20 and 40 kg Na ha- I . The nutrients in the form of chlorides were applied to perennial ryegrass swards for each of several cuts per year for up to 4 years. Additional plots received sylvinite at 120 and 240 kg ha-'.

At three sites, after depletion of soil potassium reserves, significant (P < 0.05) yield increases due to potassium and sylvinite applications were measured (MAFF 1986). These significant yield responses were associated with 'available' soil potassium reserves < 100 mg litre-' and a potassium content in the herbage from the control

At another site, soil potassium was below 100 mg litre-' from the beginning of the experiment, yet no significant yield response occurred until the second year. At this time the potassium content of the herbage DM taken from the control plot had fallen from 1-67 (first cut, year 1) to 079%.

At a further site, no significant (P < 0.05) yield responses were recorded even though at the beginning of the fourth year soil potassium reserves were below l00mg litre-' in the control plot and the herbage taken from this plot contained < 1.0% K.

A sixth site was discontinued after 2 years having given no yield responses. Where yield responses occurred, there was no significant difference (P < 0.05)

between yield from 37.5 kg K,O ha-' and yield from 75 kg K,O ha-'. At two sites in the absence of potassium there were significant (P< 0.05) yield

responses to sodium. Examination of the data does not suggest why this response occurred at these sites and not at the other two that responded to potash.

In all aspects sylvinite behaved similarly to mixtures of the two salts at the appropriate levels of application.

plot of < 1.2%.

Reference MAFF 1986 The Analysis of Agricultural Materials. Reference Book 421. HMSO, London.

Grass Silage Nutrition with Particular Reference to Potassium

Michael Daly and George H Mackenzie

UKF Fertilisers Limited, Ince, Chester, UK

Grass, cut for silage, was evaluated on two sites of contrasting soil texture over a period of 3 years. Different levels of potassium up to 360 kg K 2 0 ha-' per year were


applied. Four cuts were taken each year, and assessments were made of herbage yield and K and Mg levels of the herbage, and also soil K content.

The potash reserves of both sites came within the ADAS Index 1 category at the beginning of the trial. One of the soils was sandy loam (Newport Series) and the other a clay texture (Crewe Series). Both swards were of the long ley type but the clay site was reseeded immediately prior to the start of the trial. A randomised block design was used and there were three replicates. Nitrogen was applied at the rate of 150,125, lOOand 75 kg ha-' forthe first, second, third and fourthcuts, respectively. The total phosphate and potash levels were divided proportionally between the cuts in decreasing amounts from Cut 1 to Cut 4. Herbage samples, taken from each cut, were analysed for K and Mg.

First-cut silage A K response was obtained at both sites. Maximum yield was obtained using 90 kg ha-' K 2 0 on the sandy site, but on the clay site increasing responses were obtained up to the highest level applied (120 kg ha-' K,O).

Seasonal production A linear yield response was obtained on the clay site up to 360 kg ha-' K,O, and a yield increase up to 270 kg ha-' K 2 0 occurred on the sandy site. The clay site was more responsive due to better moisture retention and sward conditions.

K and Mg content of herbage Herbage K levels increased and Mg decreased as K fertiliser inputs were raised. At both sites 270 kg ha-' K 2 0 was the minimum required to raise herbage K levels above 2.0%.

Potassium removal by herbage and remaining available soil potassium A similar level of K was released from both soils in the initial year of the trial, but more rapid depletion of reserves occurred from the sandy soil in subsequent years. At the sandy site 270 kg ha-' K,O was required to maintain soil reserves as at the beginning, but on the clay site the two highest K treatments (270 and 360 kg ha-' K,O) raised the soil K index from 1 to 2. As yield increased, a greater uptake of K was required for maximum yield on the clay site.

Conclusions 1. Maximum yield offirstcut silage was obtained from 90 kg ha-' K,O on a sandy

site and 120 kg ha-' on a clay site. Over the whole season a total of 270 and 360 kg ha- ' K 2 0 respectively was required from the two sites.

2. Increased fertiliser K levels raised the herbage K content but decreased the herbage Mg content. In this trial there was no evidence of luxury uptake of K.

3. Optimum fertiliser inputs maintained soil K levels on the sandy soil, and on the clay site K indices increased from 1 to 2. Soil analysis of exchangeable K does not wholly account for K release on soils of differing texture and similar K index. A test for non-exchangeable reserves of K would be beneficial, particularly for grassland.


The Role of Nitrogen Fertiliser in the Mobilisation of Potassium in the Soil Solutions of Water Barley

Denis J Linehan* and Alex H Sinclaie

Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB9 245, UK

Soil potassium occurs in a number of different forms of differing availability to plants :

111 If I Mineral K e non-exchangeable K e exchangeable K e soil solution K.

Although dynamic equilibria1 reactions exist between these forms, equilibria are not achieved in the field because of the rather slow rates of some reactions compared with the relatively rapid uptake of potassium by plants. Because plants absorb their nutrients from the soil solution, potassium nutrition is dominated by reaction I with a smaller input resulting from reaction 11. The exchangeable nature of the short- and medium-term reserve of potassium has implications for the relationships between potassium availability to growing crops and non-potassium fertiliser practice, eg nitrogen application.

Two sites were chosen on coarse-textured sandy soils (Boyndie Association) of low exchangeable K status and low linear buffer capacity (LBC) but high BCo (soil buffer capacity at the equilibrium K-activity ratio, ARo) resulting from their low ARo values (Sinclair 1979). At both sites, KCl was applied to an autumn-sown barley crop either in late autumn or in early spring. At one site (site 2), NH,NO, was applied in late autumn, and at both sites at the time of spring KCI application and again one month later. Soil samples were taken from the major root zone (100- 150 mm depth) through the growing season. Soil solutions were isolated by a centrifugal method on field moist soil or soil moistened to 80% field capacity (Sinclair and Linehan 1987).

At site 1, soil solution K was high in early winter only where KCl was applied in autumn (Fig 1). It dropped to low values in January and February when NH,NO, plus or minus KCI was applied. Irrespective of the application of KCl a substantial increase in K occurred. The peak in concentration was immediately after the first spring application of NH,NO,. At site 2, where NH,NO, was applied in autumn as well as in spring, a major peak in K concentration occurred subsequent to the autumn NH,NO, application. It was of similar magnitude but shorter duration than where KCI was applied in addition to NH,NO,. As at site 1, spring NH,NO, resulted in increased concentrations, which were higher where K had been applied in the previous autumn.

Nitrate concentrations followed predictable courses resulting from the application of nitrate to the soil as NH,NO,. The changing patterns of potassium concentrations are a result of ion exchange with fertiliser NH: .

Present address: Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK. * Present address: North of Scotland College of Agriculture, 581 King Street, Aberdeen AB9 lUD, UK.


200 - f Y

100 - \\, 0 -

NHqNOjf KCI

0 I 1 Scpt. Oct. Nov. Dec. Jan. Feb. March April May

Fig 1. Potassium in soil solution through the growing season at two sites, showing the effect of addition of ammonium nitrate and potassium chloride. 0 K applied in autumn; 0 K applied in spring.

These results have implications for fertiliser practice since they indicate that autumn-applied potassium is not leached by winter rains but is available for mobilisation by spring NH: fertiliser, thus making spring application of potassium to autumn-sown crops unnecessary even in low exchange soils.

References Sinclair A H 1979 Availability of potassium to ryegrass from Scottish soils I. Effects of

Sinclair A H, Linehan D J 1987 Mobilisation of trace metals in the rooting zone of growing intensive cropping on potassium parameters. J Soil Science 30 757-773.

plants. Aberdeen Lett Ecol 1 3-4.

Nutrient Removals in Cereals

Keith Chaney and Geoffrey A Paulson

Norsk Hydro Fertilizers Ltd, Levington Research Station, Ipswich, Suffolk IPlO OLU, UK

Three years of fully replicated experiments have been carried out at Norsk Hydro Demonstration Centres throughout England from 1983 to 1985. Although the trials

Sodium and potassium in piant nutrition 331

TABLE 1 Nutrient Removal (kg ha-') for Different Levels of Grain Yield

Grain yield Phosphate (P,O,) Potash ( K , O ) at 85% DM

grain Grain removed Grain and Grain removed Grain and straw removed straw removed

6 t ha-' 48 56 30 90 8 t ha-' 64 72 40 120

10 t ha-' 80 90 50 150

were designed to assess the optimum nitrogen rate for winter cereal yield, both grain and straw samples were saved for nutrient analysis. The phosphate and potash contents were assessed in 46 trials with 24 on winter wheat and 22 on winter barley.

Both winter wheat and winter barley were found to have similar phosphate and potash contents. Taking the mean contents over both wheat and barley and for the treatments receiving 120,160 and 200 kg ha-' of nitrogen, the phosphate contents were 0-87 % and 0.19 % for the grain and straw, respectively, whereas the mean potash content of the straw was 1.70% and the grain contained only 061 %.

The phosphate and potash contents of the grain and straw and the potash content of the grain tended to decrease with nitrogen rate. In contrast the potash content increased from 1.49 % with no fertiliser to 1.80% where 200 kg ha- of nitrogen was applied. Similarly the year-to-year variation was only significant for the potash content of the straw. This was especially marked for winter barley with higher straw potash contents from the drier harvests of 1983 and 1984 than from the wetter harvest of 1985. This indicates that heavy rainfall at harvest can leach significant quantities of potash from the ripe straw.

Taking the mean phosphate and potash contents for both grain and straw it is possible to calculate nutrient removals at different levels of grain yield, both where straw is returned and where it is removed (Table 1). Because of the low phosphate content of the straw, removals of phosphate were similar whether the straw was returned or removed. However, the removals of potash were three times greater where the straw was removed than where it was returned.

agriculture group symposium sodium and potassium in plant nutrition

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