This article was downloaded by: [University of Hong Kong Libraries]On: 04 October 2013, At: 15:55Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number:1072954 Registered office: Mortimer House, 37-41 Mortimer Street,London W1T 3JH, UK

Communications in SoilScience and Plant AnalysisPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/lcss20

Efficient use of nutrientsin agricultural productionsystemsA. E. Johnston a , Lawes Trust a & SeniorFellow aa IACR‐Rothamsted, Harpenden, Herts, AL52JQ, UKPublished online: 11 Nov 2008.

To cite this article: A. E. Johnston , Lawes Trust & Senior Fellow(2000) Efficient use of nutrients in agricultural production systems,Communications in Soil Science and Plant Analysis, 31:11-14, 1599-1620, DOI:10.1080/00103620009370527

To link to this article: http://dx.doi.org/10.1080/00103620009370527

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of allthe information (the “Content”) contained in the publications on ourplatform. However, Taylor & Francis, our agents, and our licensorsmake no representations or warranties whatsoever as to the accuracy,completeness, or suitability for any purpose of the Content. Anyopinions and views expressed in this publication are the opinions andviews of the authors, and are not the views of or endorsed by Taylor& Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information.Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities

whatsoever or howsoever caused arising directly or indirectly inconnection with, in relation to or arising out of the use of the Content.

This article may be used for research, teaching, and private studypurposes. Any substantial or systematic reproduction, redistribution,reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of accessand use can be found at http://www.tandfonline.com/page/terms-and-conditions

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

COMMUN. SOE. SCI. PLANT ANAL., 31(11-14), 1599-1620 (2000)

Efficient Use of Nutrients in Agricultural Production Systems

A. E. Johnston, Lawes Trust Senior Fellow

IACR-Rothamsted, Harpenden, Herts., AL5 2JQ, UK

ABSTRACT

In the 1950s and 1960s, the near universal aim was to increase food production at almost anycost. The introduction of improved cultivars and chemicals to control weeds, pests and diseasesjustified the use of more fertilizers. Worldwide, the largest increase was in the use of nitrogen(N), partly because it had the largest effect on yield and partly because large quantities wereavailable once fixed N was no longer required for use in the weapons of war. Many developedcountries, however, were comparatively quick to satisfy their food needs by the intensification oflocal production and imports. Interest then turned to issues related to the impact of thisintensified agriculture on the environment. Among the more important issues were theincreasing concentrations of nitrate in potable water. Although these increases were,simplistically, related to the increased use of fertilizers, they led to a greater interest in fertilizeruse efficiency. Increasing fertilizer use efficiency must be seen as a component of integratedplant nutrient management. Integrated because farmers should consider all nutrient sourcesavailable to them when deciding fertilizer rates; management because all decisions will have tobe taken at the field level by each farmer. The latter requires guidelines. For N, annual field-specific recommendations will be required because after harvest there is little or no mineral Nresidue from the fertilizer in the soil to benefit subsequent crops and any nitrate can be readilylost from the soil by leaching and denitrification. For many crops, N use efficiency, determinedby the difference method, has improved greatly but there is concern that a significant proportion,perhaps 20% for rainfed cereals and more for paddy rice, of the applied fertilizer N still cannot beaccounted for. The difference method for assessing efficiency may not be appropriate fornutrients like phosphorus (P) and potassium (K). Calculating the P or K balance and determiningits effect on readily soluble P and K in soil may be more useful. This is because only very smallamounts of P and K are usually lost from soil and on many soils, a part, at least, of the residuefrom each application accumulates as a plant available reserve. Thus, a critical level of readilysoluble P and K can be determined for each crop on each soil. The guideline to fanners would beto maintain their soils just above the appropriate critical value and assess the efficiency ofdifferent nutrient sources on their ability to maintain readily soluble soil reserves.

© Marcel Dekker, Inc.

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

1600 JOHNSTON

INTRODUCTION

In the past, much effort has been devoted to eliminating plant nutrient deficiencies inagriculture. Even today, this must still be the priority in many countries as they seek to providesufficient wholesome food at affordable cost for their increasing populations. In more fortunatecountries, however, farmers, aided by excellent research and advice, have succeeded in achievinglarge yields but with much increased inputs. The costs of such inputs and concerns that some oftheir residues could impact adversely on the environment, have led comparatively recently todiscussions on whether nutrients can be used more efficiently in food production systems.Improving nutrient use efficiency is a part of precision farming which can be defined as spatiallyvariable management, often down to the within-field scale, to allow more efficient use of inputs,greater profitability and more environmentally benign husbandry.

This paper briefly reviews the historical background to nutrient use and considers someways to improve nutrient use efficiency. Many of the examples will be based on small graincereals because both historically and presently these are the staple food in many parts of thetemperate world.

Historical Background

By the early years of the 19th century, farming in Britain and most of Europe was firmlyrooted on mixed animal and crop husbandry. Arable cropping was based on the Norfolk four-course rotation: turnips, spring cereal, legume (herbage or grain), winter wheat (occasionallyfollowed by another cereal). As the century passed, fertilizers became generally available;initially, nitrogen (N) as ammonium sulphate and sodium nitrate, phosphorus (P) as guano andpotassium (K) from plant ash. J.B. Lawes at Rothamsted took out his patent for the manufactureof superphosphate in 1842 and was producing it commercially at a factory in London in 1843.The German potash mines were in production by the 1860s. Concerns about the availability ofcombined N at the end of the 19th century were dispelled by the industrial fixation ofatmospheric N, first by combination with oxygen in 1902 (in Norway) and then with hydrogenon a commercially viable scale in 1913 (in Germany).

Even in 1936, however, the amount of fertilizer used in most European countries wassmall and was negligible in the USA (Table 1). The greater proportional use of P and Kcompared to N was, in part, because most soils were deficient in these two nutrients and alsocrop cultivars then available had only a small yield potential. This second, very important,limitation to yield can be shown for the UK. At the start of the 20th century, the national averageyield of winter wheat, grown in rotation and following a legume, was just over 2.0 t ha"1 grainand yield increased slowly to a little under 2.51 ha"1 by the 1940s. These yields can be comparedwith those on the Broadbalk experiment at Rothamsted where wheat had been grown each yearsince 1843. Average annual grain yields between 1843 and 1944 were 1.55 t ha"1 with 48 kg ha"1

N and 2.45 t ha"1 with 144 kg ha"1 N. The response to the extra 96 kg ha"1 N was much smallerthan the 2.9 t ha"1 grain obtained in 1985-90 with a much improved cultivar. In 1950-52, the UKaverage yield of winter wheat had only increased to 2.711 ha"1 and of spring barley to 2.511 ha"1;the average N application was 32 and 25 kg ha'1 respectively.

Worldwide, fertilizer use increased dramatically after the Second World War. Countrieswhich had been devastated by war sought to achieve food security at almost any cost andcountries with the possibility to export food sought the opportunity to do so. In England andWales, the average annual N application in the 3-year periods 1950/52,1977/79 and 1994/96 was32, 125 and 190 kg ha"1 for winter wheat and 25,84 and 97 kg ha"1 for spring barley.

Environmental issues began to be discussed in Britain in the early 1970s. Increasingconcentrations of nitrate in potable waters, especially some of those abstracted from aquifers,

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

EFFICIENT USE OF NUTRIENTS . 1601

Table 1. Fertilizer use in six European countries in 1936 and the United States in1939.

Country

HollandBelgiumGermanyDenmarkGreat BritainFranceUnited States

N685325141062

kg ha1

P452214111282

of arable land

PA10351322528195

K83364212882

K2O10044501410102

N1111111

Nutrient ratio

PA1.51.01.31.92.82.81.8

K2O1.50.82.01.01.11.11.0

was attributed to the rapidly increasing use of N fertilizers. Much subsequent research showedthat for many arable crops, if N fertilizers were applied in the correct amount at the mostappropriate time, then little nitrate remained in the soil after harvest. When "N labelled fertilizerwas applied to a cereal crop, often less than 2% of the N was found as mineral N, mainly nitrate,in the soil to about 100 cm at harvest. Thus, only a very small amount of the applied fertilizer Nwas directly at risk to loss by leaching. Most of the nitrate in soil in autumn came from themineralisation of organic matter. Some of the N mineralised from organic matter can come fromrecently added organic manures. Because organic manures tend to be applied in large quantitiesin intensive husbandry systems, restrictions are now in place on the amount and time at whichsuch manures can be applied.

More recently, it has become apparent that an increasing proportion of the P in surfacewater is coming from agricultural land, mainly where there is intensive animal husbandry, andthe adverse effects of eutrophication are a cause for concern.

In many parts of Europe, the post 1950s increase in N use on many crops was on soilswhich had accumulated reserves of P and K. Farmers followed the advice to maintain suchreserves by applying P and K fertilizers. In many developing countries, however, although moreand more N was used, with obvious immediate benefit to yields, there was no commensurateincrease in the use of P and K. Thus, these two nutrients were being mined from soils withpotentially disastrous results for future productivity.

Thus, because of issues related both to present and future productivity and protection ofthe environment, it is essential to develop guidelines for the efficient use of plant nutrients inagriculture. But, because it will be farmers who will have to implement such guidelines it will benecessary to get them to consider integrated plant nutrient management and think not just aboutfertilizers but about individual plant nutrients and the role they play in crop production and soilproductivity.

Integrated Plant Nutrient Management

Current understanding of the soil-crop-animal system recognises that there are inevitablelosses of plant nutrients from the system. Thus, even when nutrients are recycled efficientlythrough agricultural wastes and organic manures, the total stock of nutrients, especially N, willdecline through inevitable losses. But even the most efficient recycling does not allow for anylong-term expansion in production because the amount of plant available nutrients in most soils

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

1602 JOHNSTON

is invariably too small to consistently support large yields. An example of this at Rothamsted iswhere spring barley given N is grown on a silty clay loam soil which has received no P or K forthe past 150 years. The annual offtakes of P and K in grain plus straw are 4 and 15 kg ha"1

respectively. A lighter textured sandy loam at Wobum releases only about 10 kg ha'1 K eachyear. Thus, the availability of plant nutrients must be increased by using fertilizers. But the useof fertilizers must be related to the availability of nutrients from all other sources on each farm.Hence, the concept of integrated plant nutrient management; integrated because it includes theneed to consider all sources of nutrients and management because it implies the need for anactive on-farm management approach by individual fanners.

The need to consider all sources of nutrients, especially those from organic manures isimportant in developing nutrient balances. The intercontinental transfer of nutrients in food andfeedingstuffs shipped around the world has been recognised for some years (see for exampleCooke, 1986). Steen (1997) gave an overview of nutrient balances in various Europeancountries. In more detail, Tunney (1990) showed that in the Republic of Ireland, annual Pfertilizer use increased steadily from 1950 to the early 1970s, it then fluctuated somewhat beforedeclining slightly and remaining reasonably constant from 1980 to 1993. However, soil P statuscontinued to increase steadily throughout the whole period due to the import of P in animalfeedingstuffs.

At a first level, integrated plant nutrient management can apply to each nutrientseparately. At a higher level, it can apply to interactions between nutrients. The decliningresponse to N by a range of crops in a number of agricultural systems worldwide, and reports ofdecreasing responses to the same amount of N over time in some situations may well be due todecreasing amounts of another nutrient. Thus, ensuring efficient use of nutrients requires that allare at optimum levels. An example from an experiment on spring barley at Rothamsted is in Fig.1. At the lowest levels of Olsen P and exchangeable K (Kexch) it was not justified to apply morethan 48 kg ha"1 N. With adequate K and not quite enough P, 96 kg ha"1 N was used effectively.With adequate P and K, the largest amount of N gave an economically viable yield.

Soil Productivity and Crop Production

The term soil productivity or productive capacity can be used to describe a soil's ability tosupport crop biomass production. The productive capacity of many soils in developed countrieshas been increased gradually by inputs which, if not used by the crop, have left residues whichhave persisted in plant available or soil ameliorative forms. Thus, on many soils, residues ofsoluble P and K fertilizers have built up plant available reserves of these two nutrients; on acidsoils, rock phosphates and limestone have similar beneficial effects.

If a soil is maintained at its optimum productive capacity, crop production can, within thelimits of climatic variables, be managed by annual inputs specific for each crop. Such inputs willinclude N, pest control and water. Because reserves of inorganic N rarely accumulate in soil,different criteria should be used to ensure the efficient use of N fertilizers than those criteriaapplied to say P and K fertilizers which can build up plant available reserves in many soils.

The productive capacity or productivity of a soil is related to its biological, chemical andphysical properties which tend to change comparatively slowly over time. The various factorswhich contribute to each of these three groups of properties must be managed in such a way as toprovide the best biological, chemical and physical environment within the soil. This enablesroots to grow quickly and explore the whole soil mass to find both nutrients and water.

Nutrient Use Efficiency

There are two important aspects to this term. First, it can apply to the fertilizer productand the way in which its components react with the soil constituents; this aspect is not discussed

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

EFFICIENT USE OF NUTRIENTS 1603

cd

C 3

P Kmg kg1

140 329

13 104

68

48 96 144

Fertilizer N, kg ha"1

Figure 1. Response of spring barley, cv. Triumph, to nitrogen on soils with different

concentrations of Olsen P and exchangeable K, Hoosfield, Rothamsted.

here. Second, it can apply to the recovery of applied nutrients, i.e. the efficiency of their use byplants.

To achieve improvements in the second aspect of nutrient use efficiency will depend, inpart, on the acceptance by farmers and their advisors that the productive capacity of a soil canvary at a range of scales, within field, within farm, and within region. Some causes of yieldvariation can be controlled once identified by soil and crop analysis, others cannot. Those in thesecond category often exert the greater influence on yield. An excellent example was the datafrom the ICI Ten-Tonne Club Competition in 1979 and 1980 (Weir et al. 1984). Fanners whoentered the competition presumably considered that they could achieve a yield of 10 t ha ' winterwheat grain, but in both years the mean and range of yields varied widely between soil series.

In 1980, for example, yields of 10 t ha'1 or more were achieved on only 36 sites out of atotal of 633. The latter were the sites where the grid reference was sufficiently accurate toidentify the soil series from the Soil Survey map. Further analysis of the data, however, wasrestricted to those soil series which were represented by at least 3 sites. This decreased the

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

1604 JOHNSTON

number of sites yielding 101 ha"1 or more grain to 22 out of a total of 410 sites on 19 soil series.The average grain yield for these 410 sites was 7.2 t ha"1. The largest mean yield, 9.1 (± 0.40) tha"1, was achieved by 15 farmers on Park Gate series soil; 25 farmers on Ragdale series soil hada mean yield of 7.2 (± 0.76) t ha"1 equal to the mean yield of all the sites, while the smallestaverage yield 5.3 (±0.32) t ha"1 was obtained by 28 fanners on Curtisden series soil. (The errorsshown are twice the standard error of the series mean and give an indication of the variability).This data suggests that it is essential to identify the yield potential of soils and manage inputs toachieve the yield appropriate to each soil type.

Soil Analysis

Some of the components contributing to the biological, chemical and physical propertiesof soil can be measured by soil analysis. It would be ideal to define critical values for each ofthese components based on the assumption that, if all other factors controlling yield are optimal,then crop yield will respond exponentially as the factor under test increases until it reaches anasymptote (Fig. 2). For a soil factor, like the quantity of readily soluble P or K, the value atwhich the yield reaches say 90 to 97% of the asymptote may be considered as the critical soilvalue.

There have been many improvements in soil analytical methods in recent years. However,it is essential to give more consideration to taking the samples and to the interpretation of the data.Protocols for sampling soil to provide a representative bulked sample have been discussed byOlivere/ al (1997).

Often, attention is drawn to apparent limitations of soil analysis without adequateconsideration being given to possible reasons. Frequently, there are strong correlations betweensoil analytical data and yield and nutrient uptake in pot experiments in the glasshouse (see, forexample, Johnston and Mitchell, 1974) but poorer correlations under field conditions. This suggeststhat it is not the analytical method which is wrong. The weaker correlations often found in fieldexperiments are probably due to other factors. These can include the volume of soil explored byroots and the effects of weather on yield. Where soil analytical data are compared with the cropresponse to freshly applied fertilizer then the thoroughness with which the fertilizer was mixed withthe soil is crucially important.

Soil analytical data must be combined, therefore, with a better understanding of how thedata relate to yield. Usually this will depend on defining a critical value for each parameter asdiscussed previously (Fig. 2). Below the critical value, there is frequently an appreciable loss ofyield, a financial penalty to the farmer. It is also an unnecessary expense to maintain soils muchabove the critical value. Also for P there may be an increased risk of loss of P to water fromexcessively P-rich soils.

It is generally accepted that soils growing mainly arable crops in England and Wales shouldbe maintained at not less than pH 6.5 (water). It is much more difficult to generalise about soilorganic matter (SOM). This is because the amount is very dependent on soil type, climate andfarming system. Johnston (1986, 1991) has given examples where increased yields were obtainedwhen crops were grown on soils with more organic matter in experiments on the same soil type.This was probably because of the effect of SOM on soil structure. The large potential yield ofmany cultivars available today is only achieved when plant roots have the greatest opportunity toacquire nutrients and water from the soil.

Critical Values for Olsen P

Experiments on the silty clay loam at Rothamsted, the sandy loam at Woburn and the sandyclay loam at Saxmundham have sought to determine critical Olsen P values for a range of arable

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

EFFICIENT USE OF NUTRIENTS 1605

Î

Critical value

Readily soluble soil P or K

Figure 2. Schematic representation of the relationship between yield and readily

soluble phosphorus and potassium in soil.

crops. Figure 3 shows yields of potatoes (Fig. 3a) and sugar from sugar beet (Fig. 3b) related toOlsen P on a sandy clay loam soil. Yields were averaged for groups of years when the asymptoteyield was similar. Although yields differed by a factor of two, due to differences in rainfall, theOlsen P at which the asymptote was approached varied little. It averaged 25 mg P kg'1 for potatoesand 20 mg P kg"1 for sugar beet. Olsen P accounted for much of the within year variance in yield,viz 84% for potatoes and 73% for sugar yields (Johnston et al, 1986).

Subsequent experiments on winter wheat on the same soil showed that applying more N toget larger yields did not require more Olsen P; in fact the Olsen P was slightly less with the largeryields (Table 2). Olsen P accounted for 63% of the within year variance in yield. This result wassupported by that from another experiment where wheat was given 192 kg ha'1 N in four successiveyears. Average yields for pairs of years with similar asymptotes differed by more than 1.5 t ha"1

grain but the critical Olsen P was 14 and 19 mg kg"1 in the larger and smaller yielding yearsrespectively (Johnston and Poulton, unpublished data). In an experiment on a silty clay loam,yields of spring barley differed three-fold between groups of years (Figure 4), but the critical OlsenP values were closely similar (Table 3).

Each of these experiments was on one soil type. Where yield was related only to Olsen Pand not to the response to freshly applied P, Olsen P accounted for by far the largest variance inyield. On a wider range of soils, Boyd (1965) showed that Olsen P either alone or when allowancewas made for soil type, accounted for between 37 and 50% of the variance in response to freshlyapplied P fertilizer. These experiments were on a wide range of soil types and individual farmerswere left to make most of the management decisions. Olsen P was, nevertheless, by far the largestsingle factor responsible for the variance in yield. Other examples were given by Williams andCooke (1965), Webber et al. (1976) and Draycott et al. (1971).

Various soil and management factors may affect the relationship between yield and OlsenP. For example, in one experiment at Rothamsted, the relationship between potato yields and OlsenP was very different on soils with 2.4 and 1.5% SOM (Figure 5). On the soil with 2.4% SOM, theplateau yield was approached at about 35 mg P kg"1, but on the soil with only 1.5% SOM, the same

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

1606 JOHNSTON

Olsen P, mg kg'1

Figure 3. Relationship between yields of potatoes (Fig. 3a) and sugar from sugar beet

(Fig. 3b) and Olsen P in a sandy clay loam soil, Saxmundham. Where the curves are

means of years, the asymptotic yield was very similar in those years.

Figure 3a. Upper, 1971,1974; Middle, 1969, 1972, 1973; Lower, 1970.

Figure 3b. Upper, 1969, 1971, 1973; Middle, 1972; Lower, 1970, 1974.

Table 2. Average yield of winter wheat grown on a sandy clay loam related to OlsenP, Saxmundham.

Grain, t ha', at 99% ofthe asymptote'Olsen P, mg kg"1,giving the above yield

80

9.15

29

N rate,120

9.71

25

kg ha"'160

10.13

19

200

9.96

18

Yields of grain in this and all subsequent tables are at 85% dry matter

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

EFFICIENT USE OF NUTRIENTS 1607

S

I3-

2-

1

?£*„ a

0 10 20 30 40

Olsen P, mg kg'1

Figure 4. Relationship between the yields of spring barley and Olsen P in a silty clay

loam, Rothamsted. The curves were Upper, 1988; Middle, 1986, 1987, 1990

(averaged because the asymptotic yields were very similar) and Lower, 1989.

Table 3. Asymptotic spring barley grain yields and associated Olsen P values,Rothamsted, 1986-1990.

Yield grain, t ha', at 99% ofthe asymptoteCorresponding Olsen P, mgkg1

% variance accounted for

1989

2.31

14.677

1986,87,90

4.44

14.486

1988

7.05

14.997

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

1608 JOHNSTON

so

. , 4 0 -

X> 302oOS 2 0

510

0 » / - o - . - - - so

40

30

20

10

?' «•

0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80

Olsen P, mg kg"1

Figure 5. Relatíonship between yields of potatoes and Olsen P in two soils with either

2.4% (a) or 1.5% (b) soil organic matter, Agdell, Rothamsted.

yield was only attained with 60 mg P kg"1. This experiment was on a poorly structured, difficult tocultivate soil and the extra SOM appears to have improved soil structure and thereby the ability ofroots to acquire nutrients.

Critical Values for Exchangeable K

The readily available K status of soils is adequately estimated by determining theexchangeable K (Kexch) (Johnston and Goulding, 1990). Usually only the surface soil is sampledand analysed and there is often scatter in the relationship between yield and Kexch, even on onesoil type (Johnston and Goulding, 1990). This is because K can be leached below the surface soiland still remain available to deep rooted crops; also clay in the subsoil can release K. Kuhlmannand Barraclough (1987) showed that winter wheat could acquire 50% of its K from the subsoil.Thus, variable levels of K in subsoils and variable root penetration and nutrient acquisition fromsubsoils can affect the yield/Kexch relationship.

Johnston and Goulding (1990) gave data for the relationship between barley grain and sugarfrom sugar beet on a silty clay loam soil. Spring barley yielding about 6 t ha ' grain did not needmore than 80 mg kg"1 Kexch but sugar yield was still increasing at up to 200 mg kg"1 Kexch. Therewas some scatter about the mean for the reasons given above but the general relationship was clear.More recent results for winter wheat again showed scatter about the general relationship but it wasevident that grain yields increased linearly from about 6.2 to 8.01 ha"1 as Kexch increased from 55to 100 mg kg"' (Figure 6). When the yields of field beans (Vicia faba) were averaged over a numberof years and related to Kexch, there was a good relationship (Figure 7). The much larger criticalvalue of Kexch for field beans than for winter wheat highlights the need to calibrate soil tests forboth crop and soil type.

The Value of Soil Analysis

At Rothamsted both Olsen P and Kexch have been very reliable tools for assessing the Pand K status of three soil types and critical values for both nutrients have been determined. Critical

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

EFFICIENT USE OF NUTRIENTS

9T

8-

1609

6 ••

50 60 70 80 90 100 110

-1Exchangeable K, mg kg*1

Figure 6. Relationship between grain yield of winter wheat and exchangeable K in

soil on a silty clay loam soil, Exhaustion Land, Rothamsted.

•a

0 100 200 300 400 500 600 700

Exchangeable K, mg kg"1

Figure 7. Relationship between the yield of field beans (Vicia faba) and exchangeable

K in a silty clay loam soil, Rothamsted.

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

1610 JOHNSTON

values may vary for different crops and soils but they need to be determined. However, for OlsenP, many of the critical values in the international literature do not vary greatly. For Kexch, it islikely that there will be variation, partly because of the role played by fixed K (Syers, 1998) andpartly because of the dependence of fixed and exchangeable K on the type and quantity of clay andthe amount of soil organic matter. Thus, efficiency in the use of both P and K can be improved ifmanuring seeks to maintain soils at or about an appropriate critical value.

The approach to P and K manuring outlined above can be criticised as manuring the soilrather than the crop. However, when 32P experiments frequently show that less than 25% of a crop'sP requirement is taken up from freshly applied fertilizer, the rest must come from soil reserves, andthese must be maintained. The value of plant-available soil P and K reserves has been welldemonstrated (see, for example Johnston et a!. 1970a; 1970b). Also for P and K fertilizers, theefficiency of use must be related to crop yield and nutrient offtake. For example, on Broadbalk, 33kg P ha"1 was applied annually from 1843 to 1973 and 35 kg ha"1 since then. Table 4 shows annualP offtake in grain plus straw relative to grain yield in four periods with cultivars of increasing yieldpotential. In 1852-71, P offtake was 11 kg ha"1, i.e. about one-third ofthat applied. Now, with ayield of about 8.5 t ha"1 grain, the P offtake in 1991-92 was about 32 kg ha"1 which is almost equalto the annual application, although not all of this P came from that applied to that crop. Similarly,the increased yield of winter wheat now removes in grain plus straw most of the K applied eachyear (Table 4).

If the percentage recovery of the added P in 1852-71 was calculated by the differencemethod, i.e. as the difference in P offtake between plots with and without added P expressed as apercentage of the added P, then the recovery of the 33 kg P ha'1 added each year was, on average,only 14%. Calculated in the same way the recovery of K in 1862-91 was only 29%. Because onlyvery small amounts of P and K applied as fertilizer are usually lost from soil, determining %recoveries as an estimator of efficiency may not be the most appropriate method. It may be betterto determine the P and K balance, i.e. the amount applied minus that removed, and relate anynegative or positive balance to changes in soil analytical data, e.g. Olsen P and Kexch.

The advantage of defining critical soil values for nutrients like P and K is that it does notmatter in what form they have been added to soil. The crucial point is whether the various formsin which P and K can be added to soil will maintain the soil at or just above the critical value andwhat is the financial cost of doing this. Experiments at Rothamsted suggest that the critical valuefor Olsen P and exchangeable K can be maintained either by fertilizers or organic manures or acombination of both. Experimental results also show that the critical value is not affected byvariations in yield caused by variations in weather. Critical values will probably be affected,however, by the volume of soil available for root growth; thus, very shallow or stony soils mayhave higher critical values than deeper soils.

Soil Physical Conditions

It has long been realised that soil physical conditions can be improved to benefit plantgrowth but it has proved very difficult to develop simple laboratory tests which either measure, orcan be related to, soil physical conditions in the field (Rothamsted Soil Structure Group, 1979).Equally well recognised is the fact that plants may benefit by increasing the depth of soil throughwhich their roots may range. In-field soil examination has often included exposing a soil profile toascertain the depth of rooting and the existence of a plough pan. The latter can impede root growthinto the subsoil which can be an important source of water and sometimes nutrients which havebeen leached below the surface layer of soil. Within field variations in the depth of soil and theexistence and thickness of plough pans can all cause yield variation, as will soil physical conditionsthat can lead to droughtiness and waterlogging. Johnston et al. (1998) recently discussed some

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

EFFICIENT USE OF NUTRIENTS 1611

Table 4. Annual yields of winter wheat on Broadbalk, Rothamsted, and P and Kofftake in each of four periods where P has been applied at 33 kg ha"' and K at 90 kgha"1 annually since 1843.

Period

1852-711966

1970-75

1991-92

Cultivar

Red RostockSquarehead's MasterContinuous"1st after fallow'Cappelle DesprezContinuousRotation'AppolloContinuousRotation

Yieldgrain t ha"'

2.70

3.023.07

5.415.48

8.358.69

Offtake :, kg ha"1

in grain plus strawP11

1215

1922

3231

K46

3944

7788

10197

' Continuous: grown year after year; 1st after fallow: first wheat following a one yearfallow; Rotation: first wheat following a two-year break from winter wheat.

interactions between soil physical conditions and nutrient acquisition. When consideringimprovements in nutrient use efficiency this is an area of likely fruitful research.

Nitrogen

Many extradants and methods have been suggested for the analysis of soils to estimate thelikelihood of a crop responding to N but few have been adopted for routine use. Sampling soil to90 to 100 cm in spring and analysing for mineral N (nitrate plus ammonium) has been proposedand used successfully occasionally. However, the large number of cores required to minimise soilheterogeneity makes the analysis expensive. In England and Wales it is usually only recommendedwhen it is thought that there may be large residues of mineral N from previous cropping or manureapplication and considerable savings could be made by decreasing fertilizer N applications.

The basis for assessing the efficiency of N fertilizer use must be different from that for Pand K (outlined previously) because only very small amounts of any excess accumulate in soil, andthen often only in soil organic matter, to benefit subsequent crops and there are adverseenvironmental impacts from excess nitrate or ammonia lost from soil.

The most accurate way of assessing the efficient use of fertilizer N is to use 15N labelledfertilizer but this is costly. The much more widely used difference method (see the section on TheValue of Soil Analysis) is probably acceptable in many cases where the experiment is done on asoil with a uniform history and there is a true control. When there is a true control, N recoveriescalculated by the difference method are often similar to those using I5N labelled fertilizer(Macdonald et al. 1997; Glendining et al. 1997; Powlson et al. 1986). However, when there is notrue control, as when using data from soils with a long continued history of different N treatments,then N recoveries by the crop using "N are very different from those using the difference method.For example, Powlson et al. (1986) showed for Broadbalk data, averaged for 1980 and 1981, that

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

1612 JOHNSTON

the recovery by winter wheat grain plus straw of 144 and 192 kg N ha'1 was 82 and 76%respectively by the difference method but only 56 and 56% respectively when using I5N.

When estimated by the difference method, % recovery is very dependent on the amount ofN taken up by the crop to which none was applied. Throughout the long history of the winterwheat experiment on Broadbalk, some plots have always received the same amount of N and %recovery has improved markedly since the 1960s (Table 5). This is partly due to the increasedyields of current cultivars and their improved grain:straw ratios with grain %N being much largerthan straw %N, It is also partly due to the fact that the crop given no N has removed similaramounts of N each year so that comparisons over time are valid. One of the issues related to thegreater use of N is well illustrated by the data in Table 5. Applying 96 kg N ha"1 to CappelleDesprez in 1970-78 left 36 kg N ha"1 unaccounted for. In 1985-87, applying 192 kg N ha"1 toBrimstone left 83 kg N ha"1 unaccounted for, i.e. more than twice as much.

When N is applied to a soil, the crop and soil microbial population are in competition forthe N. The effect of the long-continued use of inorganic N fertilizers on soil organic N reserves hasbeen reviewed by Glendining and Powlson (1995). For example, in the Broadbalk experiment, theresidue of some 20,000 kg ha'1 fertilizer N, applied annually at 144 kg ha"1 since 1843, hasincreased total organic N by only about 700 kg ha"1 in the top 23 cm soil compared to a total of2900 kg ha'1 organic N in the soil to which no fertilizer N has been applied. The mineralisation ofthis organic N increased the N content of winter wheat by 41 kg ha'1 (Shen et al. 1989). Glendiningand Powlson (1995) also noted that the increases in mineralised N from the extra organic mattertend to be modest although they should be taken into account when recommending N fertilizerapplications. The Broadbalk data also show that the extra soil organic matter on plots givenfertilizer N had reached a new stable equilibrium level within a few decades (Jenkinson, 1977;Johnston, 1969); the consequence of this is that eventually any input is matched by an equal output.

The use of labelled N allows some additional information to be obtained. Powlson et al.(1986) reported results using 15N labelled fertilizer on the winter wheat on Broadbalk (Table 6).Nitrogen unaccounted for (total applied minus that in grain, straw, chaff, stubble and soil) averaged19% (range 8-27%). Little of the N was probably lost by leaching in spring because the N wasapplied to an actively growing crop and in three of the four years there was insufficient rainfall tocause through drainage. Using data from a number of Rothamsted experiments, Addiscott andPowlson (1992) showed that in many cases the major pathway of N loss in spring wasdenitrification. Pilbeam (1996) reviewed "N experiments on rain-fed winter wheat across a widerange of soil and climatic conditions and found that, on average, 20% (range 10-30%) of theapplied 15N labelled fertilizer could not be accounted for. Whatever other considerations might-apply, this is a financial loss to the farmer. Determining the loss processes and considering ways ofpreventing the loss would greatly improve N use efficiency.

Table 6 shows that about 20% of the 144 kg N ha"1 applied to winter wheat remained in thesoil at harvest. This fertilizer-derived N was nearly all in three fractions, stubble and crowns, soilmicrobial biomass and soil organic matter. Less than 2% was present as nitrate at risk to loss byleaching. Many other experiments on cereals have shown that less than 5% of the fertilizer N ispresent in the soil as nitrate at harvest when the nitrogen was applied at the recommended amountand time (Macdonald et al. 1989). On a sandy loam soil, an experiment had plots with a range oforganic matter contents and using 15N showed that all soils had very little labelled nitrate in them atharvest But there was much more unlabelled mineral N in the soils with more organic matterfollowing leys than in the soils after arable crops (Fig. 8) (Macdonald et al. 1989). Again, when144 kg ha'115N labelled fertilizer was applied to spring barley on soils with 0.100 and 0.298% totalN, the total inorganic N in the soil at harvest was 34 and 69 kg ha"1 respectively. However, thelabelled fraction of this mineral N was only 1.6 and 3.2% of the 144 kg N ha"1 applied as fertilizerto the soil with least and most organic matter respectively (Glendining et al. 1997). Thus, much ofthe nitrate in the soil in autumn came from the mineralisation of soil organic matter. The 15N

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

EFFICIENT USE OF NUTRIENTS 1613

Table 5. Percentage recovery of fertilizer nitrogen applied to winter wheat growncontinuously on Broadbalk, Rothamsted.

Period

1852-711966-671970-781979-841985-87

Cultivar

Red RostockSquarehead's MasterCappelle DesprezFlandersBrimstone

48

3232566967

N applied, kg ha'1

96 144% recovery"

3339638377

3236597667

192

29-

526957

Nitrogen applications1852-71, all in autumn as equal weights of ammonium sulfate and chloride1966-67,24 kg ha'1 in autumn remainder in spring as ammonium sulfate1970 and since, all in spring as calcium ammonium nitrate* Determined by the difference method

Table 6. Percentage distribution at harvest of fertilizer-derived nitrogen applied at144 kg N ha ' labelled with 1!N, Broadbalk, Rothamsted

Year

1980198119821983

Mean

Grain55374544

45

% fertilizer nitrogen inStraw

13162313

16

Soil17202416

19

Unaccounted for15278

27

19

experiments showed that for cereals it was not the organic matter formed during that year whichwas breaking down quickly. When the soils were sampled 12 months later, following a secondcereal crop, more than 90% of the labelled organic N measured the previous autumn was still in thesoil. Much of the nitrate must have come from the mineralisation of much older reserves of organicmatter.

In summary, for mainly arable crops, if N fertilizers are applied at the appropriate time andin the correct amount they are used efficiently as estimated by the amount of mineral N remainingin soil at harvest. Using N fertilizer gives small increases in soil organic matter from which modestamounts of N can be mineralised; allowing for this in N recommendations will be discussed later.Improvements in N fertilizer use will come from quantifying the amount of N lost by the variouspathways in different farming systems and attempting to minimise these losses.

Fertilizer Nitrogen Recommendations

In recent years, much effort has been expended in attempts to improve fertilizer Nrecommendations. They have included allowing for the yield and protein potential of the site,

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

1614 JOHNSTON

60 r

50

40

30o

g> 20o

S 10

'ZZL

AB/AF Ln 3/8 Le 3/8

Inorganic N: total l_ J 15N-labelled Y/////k

Figure 8. Inorganic nitrogen in soil (0-50 cm) following the harvest of winter wheat

to which 140 kg ha"1 "N-labelled N had been applied as fertilizer in April. The wheat

was grown in contrasted rotations: AB and AF all arable cropping; Ln grass leys given

fertilizer N and Lc grass-clover leys without fertilizer N. The leys were ploughed

after 3 (Ln3, Lc3) or 8 (Ln8, Lc8) years.

estimating or measuring soil mineral N prior to the fertilizer application, and using chemicalextractants to estimate the organic N which might be mineralised. The calculations are oftencomputer-aided to allow the inclusion of as many variables as possible. Today, for winter sowncereals it is frequently suggested that the total quantity of N recommended is applied in 2 or 3applications.

Recently Poulton and Johnston (personal communication) have summarised data for Nresponses by winter wheat (10 years) and spring barley following the winter wheat (9 years). Thewinter wheat was grown in a rotation of all arable crops or following 3 or 8 years of either grass-clover leys or an all-grass ley given fertilizer N, i.e. potentially there was a wide range ofmineralisable organic N in the soils of this experiment. Four N rates were tested on each crop andfrom the fitted response curves for both cereals each year, the optimum economic yield (Ymax) andits associated N application (Nmax) were calculated. These values, determined by experiment, werecompared with the N fertilizer recommendations using Bulletin 209 (MAFF 1994), a standardsource of recommendations in the UK. For both wheat and barley, an N index system (NO NI N2)is used based on previous cropping and recommendations vary with soil type. For winter wheat theyield potential of the site is an additional factor. The known mean yield of winter wheat in eachrotation was used as the yield potential for each soil. The average values for Nmax for the 10 yearsof wheat data and 9 years of barley data were very close to the N recommendations derived fromBulletin 209 (Table 7). Thus, the average recommendation given by Bulletin 209 was excellent.

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

EFFICIENT USE OF NUTRIENTS 1615

There were discrepancies, however, in individual years. The number of years when the determinedN ^ j , was larger than, sufficient or less than, the recommended N application (+ 10 kg ha"1) isshown in Table 7. Only in about one-third of the observations was N„,„ within ± 10 kg N ha"1 ofthe N recommendation. The recommendation was too large in about 45% of the observations forwheat and 24% of those for barley. It is not yet possible to offer a good explanation for thisvariability although it was clearly due to variability in Y^,^ between years. This has variouspractical implications. If the yield was larger than the assumed potential yield, then the penaltycould be a lower than expected grain %N. If the risk of this happening can be realised soon enoughthen foliar applications of N can be used to increase grain %N. If the yield was smaller than theassumed potential yield, then it is likely that a greater than usual amount of N would beunaccounted for. Lower than expected yields could be due to adverse weather or increased diseaseincidence. Neither can be predicted with precision.

If the variability in Nera„ is only due to the N supply, then it must be related to theavailability of soil N by mineralisation of organic N. In this experiment, annual N uptake when noN was applied varied from 18 to 118 kg ha"1 for winter wheat and from 9 to 83 kg ha"1 for springbarley. When averaged over the 10 and 9 years for wheat and barley respectively, N uptake for thedifferent treatments ranged from 48 to 91 kg ha"1 for wheat and 32 to 65 kg ha"1 for spring barley.The best way to estimate such release would be by a model which could predict the release ofmineral N from soil organic matter up to say anthesis. Because mineralisation is partly dependenton temperature and moisture, some reliable long term weather forecast would be required. A modelfor N release by mineralisation would be applicable to all crops.

For cereals, there is another complicating factor, namely the ability of the crop tocompensate i.e. fewer tillers are frequently compensated for by heavier grain mass but withoutnecessarily requiring more N.

In a 16-year study of the growth and yield after anthesis of winter wheat grown onBroadbalk, Thorne et al. (1988) observed that grain yield was closely related to the number ofgrains m"2. But this property depends on the number of ears m"2 and grains per ear, both of whichdepend on the survival of shoots and florets. This survival is influenced in the few weeks beforeanthesis, after the main N application is given, by a variety of factors of which N supply is onlyone. If this is generally true, then predicting N need for cereals will be liable to errors unless otherfactors like water, temperature and radiation which influence shoot and floret survival can also bepredicted. This does not mean that the search for a reliable N recommendation system should beabandoned. Rather it implies that when comparing or validating systems there must be sufficientobservations on crop growth throughout the season to explain any variations in response to thosewhich were predicted.

Expanding the Horizons for Soil and Plant Analysis

Both the need to continue to improve nutrient use efficiency and develop the concept ofintegrated plant nutrient management have been discussed in this paper. To maximise their benefitswill require the greater use of soil and plant analysis. It will also be necessary to persuade farmersto think more carefully about the role of N in crop production and that of P and K in maintainingthe productive capacity of the soil.

Soil analysis is less relevant to N recommendations than to those for P and K. This isbecause annual site specific recommendations are all important. Because of the numbers involvedsuch N recommendations will have to be computer generated and based on reliable models.Besides factors like the site yield potential, any reliable computer system will have to successfullysimulate the quantity of N mineralised from soil organic matter after the time that a decision has tobe made about the size of the final N application. The Rothamsted SUNDIAL-FRS (FertilizersRecommendation System) aims to do this (Bradbury et al. 1993, Smith et al. 1996). The required

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

1616 JOHNSTON

Table 7. Comparison of the amount of fertilizer N recommended by RB209(" with theamount needed to achieve economic maximum yield as estimated by curve fitting.Ley-Arable experiment, Woburn, 1981-1991.

N Index141

Expected yield'", t ha"1

Recommendation, kg N ha '

Mean value for NOT„(6)

Number of years whenNm„ (7 ) was:smaller than

the same as

larger thanthe recommendedapplication ± 10 kg ha"1

Winter wheat*2', first crop afterAB & Ln3 & Lc3 &

AF Ln8 Leg0.5

7.5

168

166

9

7

4

1.0

7.5

150

144

10

4

6

1.5

8.0

130

124

g

7

5

Spring barleyAB&

AF0

-

125

132

4

6

g

(1>, second crop afterLn3 & 1x3 &

Lng Lc81

-

90

94

4

g

6

1

-

90

92

5

9

4

(l) Fertilizer Recommendations for Agricultural and Horticultural Crops. Reference Book 209 (MAFF,1994)

( a Winter wheat, 1981-90, 10 years data each for Ln3 and Ln8; Lc3 and Lc8(3) Spring barley, 1982-91 (excluding 1983). Nine years data each for Ln3 and Ln8; Lc3 and Lc8'"' Based on previous cropping as defined in RB209(!) Based on the average estimated economic maximum yield (to the nearest 0.5 tha') from the fitted

response curve(6) Mean N O T „ ; winter wheat , 1981-90; spring barley 1982-91 (excluding 1983)(7) Based on the amount o f N associated with the estimated economic max imum yield each year

amount of N can be divided between a number of applications depending on the appearance of thecrop or non-destructive measurements made on it. Alternatively, nitrification inhibitors are againbeing actively researched to find lower cost products. If effective over appropriate timescalessingle N applications would be possible. Another alternative would be a slow release fertilizer.

As a backup to N fertilizer recommendations, non destructive plant analysis methods arebeing developed to monitor the N status of the crop throughout the growing season. This will allowmore accurate decisions about late N top dressings. A hand-held device is used to determine thechlorophyll content of the crop (Schepers et al. 1992). A large number of estimates can be madequickly on leaves at the same stage of development giving a reliable mean value. Such monitorsare being used commercially in Europe by, for example, Hydro Agri. Hydro Agri are developingthe opportunity for the further use of this concept by linking chlorophyll sensors mounted at thefront of a tractor to a rear mounted, variable rate fertilizer spreader through a computer installed inthe tractor cab. As the tractor passes through the crop, the N rate is adjusted according to the sensorreadings via the computer software. Other reflectance measurements are being investigated forother nutrients. The main concern is to ensure that there is no interference from factors like waterstress, nutrient imbalance or foliar disease which may affect the reflectance measurement.

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

EFFICIENT USE OF NUTRIENTS 1617

The more efficient use of P and K depends on defining critical values for both nutrients fordifferent crops and farming systems. This has been discussed fully here and many examples given.For potassium, plant analysis can be used to help distinguish between soils if there is doubt aboutthe Kexch values. For cereals and grasses, Leigh and Johnston (1983) showed that when Kconcentrations were expressed on a tissue water basis they were reasonably constant throughout thegrowing season and the values were very different between K sufficient and K deficient plants.

Where irrigation is required to grow acceptable yields, applying all the required nutrientswith the irrigation, fertigation, offers great promise for improving nutrient use efficiency especiallyif the quantity of water is carefully controlled so that the nutrients are not leached below the zone ofactive root growth.

The introduction of fully automatic and integrated yield mapping systems can nowgenerate information showing variation in crop performance and yield within individual fieldsand this now adds a new dimension in the management opportunities for arable agriculture(Johnston et al., 1998). However, yield maps for each field are required for a succession of cropsto identify areas which yield consistently above or below average. Once these areas areidentified, variations in yield can be related to differences in data from soil analysis and soilphysical examination. However, there is need for care in interpretation. For example,occasionally soils from low yielding areas contain large amounts of readily soluble P (Olsen P)and K (Kexch). This is because in the past the field has been uniformly fertilized with P and Kand consistently small yields have removed little P and K allowing the accumulation of largerplant available reserves in the soil.

The urgent need is to persuade farmers and their advisors that soil and crop analysis has amajor role in an overall farm management strategy. In England and Wales probably less than 5%of fields are sampled and analysed each year and those that are are mainly growing arable crops.The reasons why so few farmers use soil analysis are obscure. Perhaps scientists argue toovigorously about the merits of different methods. Perhaps farmer expectancy is too great; toofrequently other factors have a larger effect on yield than the fertilizer application. Provided asuitable extradant is used the greatest benefit of soil analysis comes from (i) periodic analysis toensure that on a field basis plant available nutrients are not being severely depleted or enriched, (ii)to compare soil nutrient levels in areas of large and small yields within fields so that remedialaction can be taken where appropriate.

REFERENCES

Addiscott, T.M. and D.S. Powlson. 1992. Partitioning losses of nitrogen fertilizer between leachingand denitrification. J. Agric. Sci. (Camb) 118:101-107.

Boyd, D.A. 1965. The relationship between crop response and the determination of soil phosphorusby chemical methods, pp. 94-102. In: Soil Phosphorus. Technical Bulletin 13. Ministry ofAgriculture, Fisheries and Food, HMSO, London.

Bradbury, N.J., A.P. Whitmore, P.B.S. Hart and D.S. Jenkinson. 1993. Modelling the fate ofnitrogen in crop and soil in the years following application of I5N labelled fertilizer to winterwheat. J. Agric. Sci. (Camb) 121:363-379.

Cooke, G.W. 1986. Nutrient balances and the need for potassium in humid tropical regions, pp. 13-32. In: Nutrient Balances and the Need for Potassium. International Potash Institute, Basel.

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

1618 JOHNSTON

Draycott, A.P., M.J. Durrant and D.A. Boyd. 1971. The relationship between soil phosphorus andresponse by sugar beet to phosphate fertilizer on mineral soils. J. Agric. Sci. (Camb) 77:117-121.

Glendining, M.J. and D.S. Powlson. 1995. The effects of long continued applications of inorganicnitrogen fertilizer on soil organic nitrogen - a review, pp. 385-445. In: R. Lal and B.A.Stewart (eds.) Soil Management: Experimental Basis for Sustainability and EnvironmentalQuality. CRC Press Inc., Boca Raton USA.

Glendining, M.J., P.R. Poulton, D.S. Powlson and D.S. Jenkinson. 1997. Fate of l5N-labelledfertilizer applied to spring barley grown on soils of contrasting nutrient status. Plant Soil.195:83-98.

Jenkinson, D.S. 1977. The nitrogen economy of the Broadbalk experiments. I. Nitrogen balance inthe experiment. Rothamsted Experimental Station Report for 1972, Part 2, pp. 103-110.

Johnston, A.E. 1969. Plant nutrients in Broadbalk soils. Rothamsted Experimental Station Reportfor 1968, Part 2, pp. 93-112.

Johnston, A.E. 1986. Soil organic matter, effects on soils and crops. Soil Use Manag. 2:97-105.

Johnston, A.E. 1991. Soil fertility and soil organic matter, pp. 299-313. In: W.S. Wilson (ed.)Advances in soil organic matter research: the impact on agriculture and the environment.Royal Society of Chemistry, Cambridge.

Johnston, A.E., P.B. Barraclough, P.R. Poulton and C.J. Dawson. 1998. Assessment of somespatially variable soil factors limiting yields. 48 pp. Proceedings No. 419, The InternationalFertilizer Society, York, UK.

Johnston, A.E. and K.W.T. Goulding. 1990. The use of plant and soil analyses to predict thepotassium supplying capacity of soil, pp. 177-204. In: The development of K-fertilizerrecommendations. International Potash Institute, Basel.

Johnston, A.E., P.W. Lane, G.E.G. Mattingly, P.R. Poulton and M.V. Hewitt. 1986. Effects of soilfertilizer P on yields of potatoes, sugar beet, barley and winter wheat on a sandy clay loamsoil at Saxmundham, Suffolk. J. Agric. Sci. (Camb). 106:155-167.

Johnston, A.E. and J.D.D. Mitchell. 1974. The behaviour of K remaining in soils from the Agdellexperiment at Rothamsted, the results of intensive cropping in pot experiments and theirrelation to soil analysis and the results of field experiments. Rothamsted ExperimentalStation Report for 1973, Part 2, pp. 74-97.

Johnston, A.E., R.G. Warren and A. Penny. 1970. The value of residues from long-period manuringat Rothamsted and Woburn. IV. The value to arable crops of residues accumulated fromsuperphosphate. Rothamsted Experimental Station Report for 1969, Part 2, pp. 39-68.

Johnston, A.E., R.G. Warren and A. Penny. 1970. The value of residues from long-period manuringat Rothamsted and Woburn. V. The value to arable crops of residues accumulated from Kfertilizers. Rothamsted Experimental Station Report for 1969, Part 2, pp. 69-90.

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

EFFICIENT USE OF NUTRIENTS 1619

Kuhlmann, H. and P.B. Barraclough. 1987. Comparison between the seminal and nodal rootsystems of winter wheat in their activity for N and K uptake. Z. Pflanzenernahr. Bodenkd.150:24-30.

Leigh, R.A. and A.E. Johnston. 1983. Concentrations of potassium in the dry matter and tissuewater of field grown spring barley and their relationships to grain yield. J. Agric. Sci. (Camb).101:675-685.

Macdonald, A.J., P.R. Poulton, D.S. Powlson and D.S. Jenkinson. 1997. The effects of season, soiltype and cropping on recoveries, residues and losses of 15N-labelled fertilizer applied toarable crops in spring. J. Agric. Sci. (Camb). 129:125-154.

Macdonald, A.J., D.S. Powlson, P.R. Poulton and D.S. Jenkinson. 1989. Unused nitrogen fertilizerin arable soils - its contribution to nitrate leaching. J. Sci. Food Agric. 46:407-419.

MAFF 1994. Fertiliser Recommendations for Agriculture and Horticultural Crops. RB 209.HMSO, London, 112 pp.

Oliver, M.A., Z. Frogbrook, R. Webster and C.J. Dawson. 1997. How many soil cores are neededfor bulking? In: J.V. Stafford (ed.) First European Conference on Precision Agriculture,Society of Chemical Industry, London.

Pilbeam, C.J. 1996. Effect of climate on the recovery in crop and soil of 15N labelled fertilizerapplied to wheat. Fertilizer Research 45:209-215.

Powlson, D.S., G. Pruden, A.E. Johnston and D.S. Jenkinson. 1986. The nitrogen cycle in theBroadbalk Wheat Experiment - recovery and losses of 15N-labelled fertilizer applied in springand inputs of nitrogen from the atmosphere. J. Agric. Sci. (Camb). 107:591-609.

Rothamsted Soil Structure Group 1979. A series of papers. J. Soil Sci. 30:377-482.

Schepers, J.S., D.D. Francis, M. Vigil and F.E. Below. 1992. Comparison of corn leaf nitrogen andchlorophyll meter readings. Commun. Soil Sci. Plant Anal. 23:2173-2187.

Shen, S.M., P.B.S. Hart, D.S. Powlson and D.S. Jenkinson. 1989. The nitrogen cycle in theBroadbalk wheat experiment: 15N labelled fertilizer residues in the soil and in the soilmicrobial biomass. Soil Biol. Biochem. 21:529-553.

Smith, J.U., N.J. Bradbury and T.M. Addiscott. 1996. SUNDIAL: a PC useable system forsimulating nitrogen dynamics in arable land. Agron. J. 88:38-43.

Steén, I. 1997. A European fertilizer industry view on phosphorus retention and loss fromagricultural soils, pp. 311-328. In: H. Tunney, O.T. Carton, P.C. Brookes and A.E. Johnston(eds.) Phosphorus Loss from Soil to Water. CAB International, Wallingford.

Syers, J.K. 1998. Soil and plant potassium in agriculture. Proceedings No. 411, The FertilizerSociety, York, UK 33 pp.

Thome, G.N., R.J. Darby, W. Day, P.W. Lane, P.J. Welbank and F.V. Widdowson. 1988.Variation between years in growth and nutrient uptake after anthesis of winter wheat onBroadbalk field at Rothamsted, 1969-84. J. Agric. Sci. (Camb). 110:543-559.

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013

1620 JOHNSTON

Tunney, H. 1990. A note on the balance sheet approach to estimating the phosphorus fertilizerneeds of agriculture. I. J. Agric. Res. 29:149-154.

Webber, J., D.A. Boyd and A. Victor. 1976. Fertilizers for maincrop potatoes on MagnesianLimestone soils: experiments in Yorkshire 1966-71. Exp. Husb. 31:80-90.

Weir, A.H., J.H. Rayner, J.A. Cart, D.G. Shipley and J.D. Hollies. 1984. Soil factors affecting theyield of winter wheat: analysis of results from ICI surveys 1979-80. J. Agric. Sci. (Camb).103:639-649.

Williams, R.B.J. and G.W. Cooke. 1965. Measuring soluble phosphorus in soils, comparisons ofmethods and interpretation of results, pp. 84-93. In: Soil Phosphorus, Technical Bulletin 13,Ministry of Agriculture, Fisheries and Food, HMSO, London.

Dow

nloa

ded

by [

Uni

vers

ity o

f H

ong

Kon

g L

ibra

ries

] at

15:

55 0

4 O

ctob

er 2

013