environmental and economic implications of some slurry management options

J. agric. Engng Res. (1998) 71, 273—283Article No. ag980327

Environmental and Economic Implications of some Slurry Management Options

M. B. McGechan*; L. Wus

Soils Department, SAC, West Mains Road, Edinburgh EH9 3JG, Scotland, UK

(Received 2 December 1997; accepted in revised form 11 June 1998)

Weather-driven simulations which take account ofammonia volatilization, soil water and soil nitrogen dy-namics have been carried out to demonstrate the extentof losses of nitrogenous components which pollute theenvironment, following the land spreading of animalslurry (liquid manure). Alternative slurry managementpolicies concerning selection of the size of store andspreading method were considered in these simulations.Losses were estimated of nitrate leached to field drains,volatilized ammonia and nitrous oxide released by denit-rification. An economic analysis was carried out takingaccount of equipment costs and potential savings in costsof N, P and K mineral fertilizer.

Results showed environmental benefits from a largestore, enabling slurry to be spread at the optimum time forplant growth, compared with a small store requiringextensive winter spreading, and from spreading usingan injection system rather than surface spreading witha splash plate. With the small store, benefits were shownfrom a single annual application on each area of landrather than repeated applications over the same area.Benefits took the form of increased utilisation of slurrynitrogen by growing plants (particularly on grasslandrather than cereal crops), reduced mineral fertilizer re-quirements, and reduced pollution of both water and air.

On economic grounds, results showed that neither anincreased store size, nor injection rather than surfacespreading, could be justified in terms of fertilizer costsavings. Only a small proportion of the additional costsof injection rather than splash plate spreading could berecouped in reduced fertilizer requirements, whereas fora large compared with a small store up to half the addi-tional costs could be recovered in this way. However,repeated spreading of slurry on the same land area was

*Formerly at Scottish Centre of Agricultural Engineering, SAC, BushEstate, Penicuik EH26 0PH, Scotland, UKsNow at: Department of Agrometeorology, China Agricultural Univer-sity, Beijing 100094, China

0021-8634/98/110273#11 $30.00/0 27

shown to be a bad practice from an economic standpoint,as well as being damaging to the environment.

( 1998 Silsoe Research Institute

1. Introduction

Concerns about environmental pollution problems as-sociated with both the use of fertilizer and the disposal offarm wastes have focused attention on opportunities forreducing mineral fertilizer requirements by better utiliz-ation of the nutrient potential of animal slurry and othermanures. Poor utilization and high nutrient losses incurrent farming practice, resulting from inappropriatetechnology and unsuitable timing of spreading opera-tions, represent both a threat to the environment and aneconomic loss of a valuable resource.

Land spread slurry can pollute the environment byvarious routes. Water pollution takes the form of leachednitrate at field drain level and to deep groundwater,leached phosphorus in dissolved or particulate form, andcan also occur due to surface runoff (overland flow)transporting whole slurry with a high BOD and contain-ing nitrogenous and phosphorus components. Air pollu-tion takes the form of volatilized ammonia, which is a ma-jor cause of acid rain and eutrification, and of emissions ofnitrous oxide, a potent greenhouse gas. At SAC, a pro-gramme of research is being undertaken, to assess theextent of pollution in a number of these categories, and thescope for measures to reduce losses, and hence to increaserecycling of nutrients and reduce fertilizer inputs. Leachednitrate resulting from slurry spreading is the main pollu-tion form considered in this paper, although simple esti-mates of emissions of the polluting gases ammonia andnitrous oxide are also included. Overland flow of slurrycomponents occurs only on certain fields or soil typeswhich are prone to surface runoff, and this is the subject ofa separate study.1 Unlike surface runoff pollution whichcan often be avoided by careful management of spread-ing, nitrate leaching is always present to some extent but

3 ( 1998 Silsoe Research Institute

274 M. B. McGECHAN; L. WU

measures can nevertheless be taken to minimize the levelof leaching. In contrast to nitrogenous pollutants, pollu-tion by phosphorus, a large proportion of which is trans-ported in particulate form often attached to small slurryor soil particles, requires a much more complex approachto modelling and will be the subject of future studies.Unlike nitrogen and phosphorus, there has been no con-cern expressed about potassium as a potential pollutant,so losses of potassium can be considered to be of eco-nomic importance but harmless to the environment.

The object of the study described in this paper is to usemodelling to assess the environmental impacts, as well asthe economic benefits in terms of exploiting recycled nutri-ents as fertilizer substitute, of various slurry managementoptions concerned with selection of equipment. Owing tothe current state of development of models, the mainemphasis is on nitrogenous nutrients, but some simpleeconomic calculations for the phosphorus and potassiumcomponents are also presented. The economic analysisalso considers additional costs of procedures to increaseslurry nutrient utilization which offset apparent benefitsand may discourage environmentally friendly practices.

2. Materials and methods

2.1. Selection of slurry equipment and spreading strategy

The slurry management options addressed in thisstudy concern the choice of equipment in which capitalmust be invested, the size of store and the method ofspreading. Two store sizes and two spreading methodswere considered.

A large slurry store enables spreading to be timed tocoincide with a high rate of uptake by growing plants,whereas a small store requires extensive spreading inwinter which inevitably increases losses in various cat-egories. Winter spreading can also aggravate trafficabil-ity and soil damage problems. Spreading during somewinter months is prohibited in a number of Europeancountries including Sweden, Denmark, Germany and theNetherlands, although it is allowed subject to some re-strictions2,3 in the UK. The two store size options con-sidered reflect farmers’ differing attitudes to slurry.A farmer who regards slurry just as an embarrassing wasteto be disposed of will select the small store system, prob-ably spreading slurry whenever the store becomes full onfields nearest to the store. In contrast, a farmer whoregards slurry as a positive resource to be exploited willselect the large store so that he can spread all his slurry inspring when crops can use the nutrients, maximizing thevalue of slurry as a fertilizer substitute. The large sizestore considered was adequate for a whole winter’s pro-duction of slurry, compared with a small store requiring

to be emptied four times per year. A capacity of 1700 m3

was assumed for the large store option, sufficient for1520 m3 annual production (see Section 2.3.2) with 10%spare capacity, compared with a capacity of 500 m3 as-sumed for the small store.

Surface spreading equipment options considered werea vacuum tanker with a splash plate, and a tanker-moun-ted shallow injector. Splash plate spreading leads to highammonia volatilization losses, whereas injection reducesthese losses to a very low level but at a cost of a moreexpensive machine with a higher power requirement andlower workrates. A 60 kW 2WD tractor was assumed tobe adequate for spreading slurry with a splash-platevacuum tanker. For slurry injection with its higher powerrequirement a 90 kW 4WD tractor was selected. Thesame 7 m3 tanker was assumed for both spreading sys-tems, with a four tine winged injector unit mounted onthe back when used for injection. These are the sameequipment combinations as suggested by Warner et al.,4

based on several years of operating slurry spreadingequipment as described by Godwin et al.5

2.2. Modelling tools

It is a long-term aim to complete a ‘‘whole system’’model comprising a series of linked submodels represent-ing the important processes concerned with the fate ofthe nitrogen and phosphorus components of slurry.These processes and submodels include the following:

(1) quantity and composition of slurry produced by aruminant livestock herd;

(2) storage and land spreading of slurry;(3) volatilization of ammonia following spreading;(4) soil nitrogen dynamics, including

(a) additions of slurry and mineral nitrogen fertilizer,(b) nitrification of ammonia in slurry to nitrate,(c) mineralisation of organic soil nitrogen and slurry

components,(d) solute transport of nitrate through the soil profile

and leaching to field drains and deep groundwater,(e) denitrification to nitrous oxide and nitrogen;

(5) soil phosphorus dynamics, similar to soil nitrogendynamics without the volatile or gaseous emissionsbut considering particulate as well as solute trans-port; and

(6) uptake of nitrogen and phosphorus as plant nutrientsby cereal and grass crops.

Since models of soil phosphorus dynamics and of theproduction and composition of slurry have yet to bedeveloped, the present study uses published typical values.

Ammonia volatilization during and after spreadingof slurry is represented using the model described

SLURRY MANAGEMENT OPTIONS 275

by Hutchings et al.6 This model simulates ammonia lossin relation to historic windspeed, temperature, rainfalland potential evaporation data. It also indicates thequantity of nitrogen which infiltrates through the soilsurface, and so forms the link with the soil nitrogendynamics model.

The fate of nitrogen within the soil profile is simulatedusing the Swedish soil nitrogen dynamics modelSOILN,7–9 which includes representation of all the im-portant transport and transformation processes whichnitrogen undergoes in the soil. Parameter values for thetransformation rates in SOILN, particularly for mineral-ization of soil humus and slurry organic components,and for denitrification, have been selected by Wu et al.10

based on a combination of experimental data from theSAC Soils Department and literature sources reviewedby Wu and McGechan.11 The SOILN model has beenvalidated using measured data for leaching of nitrate tofield drains in experimental plots for two arable soils (aclay loam and a sandy loam) and a silty clay loamgrassland soil.10

Uptake of nitrogen by crops in the standard SOILN isrepresented by a crop growth submodel representinga cereal crop.12 As an alternative sub-model for grasslandcrops, a monoculture grass growth model developed byTopp and Doyle13 has been adapted to link with theSOILN model14 in place of the cereal submodel. Valida-tion of the combined soil nitrogen and crop growthmodels by comparing simulated and measured cropyields has been described by Wu et al.10 for cereal cropsand by Wu and McGechan11 for a grass crop with threesilage cuts.

Simulations with the SOILN model must be run inconjunction with simulations using the soil water andheat model SOIL,15 since transport of dissolved nitratedepends on soil water movement, and all the transforma-tion rates are temperature dependent. Selection of para-meter values for the same three soils for the SOIL model,and validation using drainflow data, have been describedby McGechan et al.16 and Cooper et al.17

2.3. Simulation procedure

Simulations over the period 1 April 1986 to 31 March1996 were carried out for each of the six slurry manage-ment options (two spreading methods and two storesizes, with the further option for the small store of spread-ing on different areas of land or repeated spreading onthe same land). Each simulation was repeated for thegrassland soil at Dumfries and for the two arable soils atBush Estate, using the linked SOIL, SOILN and ammo-nia volatilization models. The two weather sites were thesame as those for which the SOIL and SOILN models

had been parameterised and validated. Dumfries is in theWest of Scotland, an area of predominantly dairy farm-ing with mainly permanent or long-term grassland, andBush Estate (near Edinburgh) is in an area of mixedfarming with both arable cropping and grassland sup-porting mainly beef production.

2.3.1. ¼eather dataTen year historic daily meteorological data files were

prepared for the two weather sites in the format requiredby the models. The ammonia volatilization model re-quired conversion to short runs of parameters at inter-vals of 10 min over the periods immediately followingslurry spreading. A program to do this using daily inputdata was provided with the model. Daily weatherparameter values required for simulations with thelinked SOIL and SOILN models were daily mean tem-perature (estimated as the mean of the maximum andminimum temperatures), daily total precipitation,global radiation, net radiation, wind speed and vapourpressure. Values of these data sets for both modeswere prepared from daily weather records for the twosites held in the ‘‘METDATA’’ database.18 Globalradiation was estimated from sunshine hours by theAs ngstrom19 formula and daylength was estimated fromtrigonometric relationships. Vapour pressure was esti-mated from dry bulb and wet bulb temperatures re-corded at 09 00 h and net radiation was estimated fromglobal radiation, temperature and vapour pressure by theBrunt20 formula.

2.3.2. Slurry production and compositionA common set of slurry parameters was selected for

all the management options, based on typical approxim-ate values reported by Dyson.11 A herd of 100 dairycows (mean weight 500 kg) plus 40 young cows (meanweight 250 kg), housed for 26 weeks per year, wasassumed to produce 1520 m3 of slurry, with mean com-position 1)58 kg/m3 available N, 3)23 kg/m3 total N,1)61 kg/m3 total P

2O

5, 3)01 kg/m3 total K

2O, and 7%

dry matter.

2.3.3. Slurry spreadingSlurry was assumed to be spread at a rate of 50 m3/ha,

the maximum application rate for surface spreading per-mitted by environmental protection regulations in theUK.2,3 Although a higher rate (up to 140 m3/ha) ispermitted for liquid wastes applied by injection, the samelower rate was selected for both application methods sothat the results could be compared directly. Also forthe slurry composition assumed here, the higher ratewould have given nutrient applications in excess of croprequirements, a practice discouraged in the codes ofpractice.2,3 A single application over 30)5 ha in spring

Table 1Correction factors for calculation of available N from slurry

between store and soil incorporation (from Dyson12)

Correctionfactor

Step 1 In store Aeration: short (2—3 d) 0)9long (1 month) 0)75

Agitation 0)9

Step 2 Time ofapplication

Autumn 0)1November—December 0)4

January—February 0)7Spring 1)0

Step 3 Method of Injection 0)9application Low level, band 0)8

Vacuum tanker,splash plate 0)7

Step 4 Arable land(ploughed inwithin one dayin cold weather) 1)0

Grassland 0)6


was assumed with the large store, and four applicationsof 380 m3 over 7)6 ha for the small store distributedduring the period for which the animals were housed.There were also two alternative options for thesmall store, one in which slurry was assumed to bespread repeatedly on the same 7.6 ha area of land(the worst case), the other n which slurry was spreadon four occasions throughout the housed animal periodbut on different land each time. The worst case is permit-ted by farm waste disposal regulations in the UK, pro-vided repeated applications are more than six weeksapart, and is common practice where a farmer regardsslurry as a waste product to be disposed of. Target datesfor spreading were 15 October, 15 December, 1 Februaryand 15 March for the small store and 15 March forthe large store. Spreading was simulated as actuallytaking place at a rate of 2)6 ha/d on the first available‘spreading days’ after the target date. A spreadingday was defined as having to meet all the followingconditions:

(1) soil water content in the upper topsoil layer (fieldcapacity #2%;

(2) temperature in the upper topsoil layer '0°C;(3) no snow cover;(4) rainfall on current day (2)5 mm.

Spreading days were determined from weather-drivensimulations with the SOIL model, with the upper topsoillayer defined as zero to 0)1 m depth. This is a similarprocedure to that adopted for estimating number ofworkdays for winter field operations by McGechan andCooper22 and Cooper et al.17

2.3.4. Nitrogen inputs to SOI¸N modelA simulation with the SOILN model requires an

input file with information about dates and applicationrates of urinary N in slurry, faeces N in slurryand mineral fertilizer N, as well as the ploughing datefor an arable crop. The rate of urinary N was estimatedfrom the available N in the slurry depleted by afactor of 10% to represent losses in the store as suggestedby Dyson21 (Table 1), while faeces N was calculatedfrom the non-available N in the slurry withoutadjustment. Simulations with SOILN also requireinitial values for the soil nitrogen pools, whichwere estimated on the basis of soil organic matterlevels of 7% for grassland and 3% for both arablesoils.

2.3.5. Crop managementAssumed fertilizer applications to spring barley

and grass crops were based on rates recommended bySAC,23 listed as ‘‘crop requirements’’ in Table 2, reducedto allow for the contribution from available N and

total P and K in the slurry using guidelines described(in the form of a farmer’s advisory note) by Dyson.21

The actual mineral fertilizer application rates arelisted in Table 2, with those for N differing between eachslurry management option when expected losses of avail-able N from slurry21 (summarized in Table 1) were takeninto account. For barley fertilizer application was as-sumed to take place at sowing time, while for grassthe total quantities were split equally between threeapplications, taking place after the March slurry applica-tion and after the first and second silage cuts.

With four repeated slurry applications on the samearea of arable land, the contribution from slurryexceeded the requirement, so no mineral N, P or Kwas required. Similarly, no P or K was required withrepeated applications on the same area of grassland,or with a single slurry application on arable land.

Ploughing of the arable soils was assumed totake place on the first available ploughing workday(day with soil water content in top layer (field capacity#2%, as described by McGechan and Cooper22

and Cooper et al.17) after the following target dates:clay loam soil and large store, 15 October; clay loamsoil and small store, day following October slurryapplication; sandy loam soil, day following Marchslurry application. In simulations with SOILN, plough-ing distributes plant litter, from both the soil surfaceand the stubble from the previous crop, throughoutthe profile down to the ploughing depth (specified inthis case as 0)27 m).

Table 2Available N, total P and K, kg/ha from 50 m3/ha slurry application, and adjustment to mineral fertiliser application

Available N P2O5 K2O

Arable GrasslandInjected Surface Injected Surface Arable Grassland Arable Grassland

Crop requirement 100 100 300 300 50 115 50 200

Slurry nutrientsSpring slurry application 63)8 49)6 38)3 29)8 80)5 80)5 151 151Autumn slurry application 6)38 4)96 3)83 2)98 80)5 80)5 151 151December slurry

application25)5 19)9 15)3 11)9 80)5 80)5 151 151

February slurry application 44)7 34)7 26)8 20)8 80)5 80)5 151 151Four slurry applications 140 109 84)2 65)5 322 322 602 602

Mineral fertilizer requiredSpring slurry application 36)3 50)4 261 270 0 34)5 0 49)5Autumn slurry application 93)6 95)0 296 297 0 34)5 0 49)5December slurry

application74)5 80)2 284 288 0 34)5 0 49)5

February slurry application 55)3 65)3 273 279 0 34)5 0 49)5Four slurry applications 0 0 215 234 0 0 0 0


2.4. Economic analysis procedure

2.4.1. CostsThe capital costs of the two sizes of slurry store con-

sidered (Table 3) were estimated from a formula fromSAC.23 Capital costs of tractors, tankers and the injector,taken from SAC,23 are also listed in Table 3.

The procedure for calculating the annual cost of itemsof equipment is based on the method of Audsley andWheeler24 as modified by Witney and Saadoun25 and setup for calculation on a spreadsheet as described byWitney.26 For field machines, cash flows are calculatedover the period of ownership. A final sale value must bespecified. The procedure incorporates a treatment of ma-chine repair costs as described by Rotz.27 For the storeand any other fixed structures, calculations are made on

TablCosts of equipme

Capital cost, £

Tractor, 60 kW, 2WD 25000Tractor, 90 kW, 4WD 50000Tanker, 7 m3 with splashplate 5800Tanker, 7 m3 with injector 11 000Store, 1700 m3 27 000Store, 500 m3 10 700

Mineral fertilizer prices, £/kg, NKP

the basis of straight line depreciation over 25 yr with zerovalue at the end of the period. Annual or hourly costscalculated by this method assuming a tax rate of 25%,a borrowing interest rate of 8%, an investment interestrate of 5%, and an inflation rate of 3% (the cost dependsmainly on the difference between the borrowing interestrate and the inflation rate) are listed in Table 3. Costs oftractors, which are shared between slurry spreading andother operations on the farm, must be considered on anhourly basis, using an estimate of total annual use of thetractor. Annual usages of 1000 h for the 60 kW 2WDtractor and 90 kW 4WD tractor were assumed, as sug-gested by Warner et al.14 Hourly tractor costs estimatedin this way are listed in Table 3. Costs of £0)14 /1 fortractor fuel (tractor fuel consumption 0)344 l/kWh), and£4)50 /h for labour were also assumed.

e 3nt and fertiliser

Hourly cost £Annual cost, £ with repairs, £ Hourly fuel cost, £

4330 4)33 2)897350 9)80 4)33841

17401376545

0)420)230)41


2.4.2. ¼orkratesOverall work rates for slurry spreading were estimated

from the time requirements for components of the opera-tion described by Godwin et al.,5 assuming a 500 mdistance from field to store when spreading in springonly (for the large store) or four applications through-out the season on different areas of land (Table 4).For the easy option of four applications on the same areaof land, a 250 m field to store distance was assumed witha reduced time requirement for transport from store tofield.

2.4.3. »alue of recycled nutrientsThese calculations depend on the spreading option

(including timing of spreading), and whether arable land(assumed to be a cereal crop) or grassland is selected.When considering the nitrogen available as a plant nutri-ent, the initial available N component of the slurry mustbe adjusted to allow for losses which occur during stor-age, while being spread, and while lying on or in the soilbefore uptake by plants. These adjustments according toTable 1 are taken from Dyson.21 The same assumption ismade as made by Dyson,21 that there are no losses ofP and K. While these nutrients are not volatile and areless soluble than N, some losses by leaching must takeplace especially when slurry is spread in autumn orwinter, so clearly this assumption is an oversimplifi-cation. Quantities of total P and K plus available N afterallowing for losses are multiplied by their respectivevalues in £/kg, and summed to give the fertilizer substitu-tion value of slurry.

3. Results

3.1. Fate of applied nitrogen

Results are expressed in terms of the mean quantity ofnitrogen recycled into biomass (crop and soil organicmatter) over 10 year simulations, relative to the extent oflosses which pollute the environment. These are presentedin the form of input/output balances in Fig. 1. Thelosses are categorized as nitrate leached to field drains,volatilized ammonia and denitrified N. For the smallstore options involving spreading on different areasof land, results presented are the mean of four simula-tions representing each area of land receiving slurryapplications at different times. Results show sub-stantial variations in the proportion of nitrogenpassing by the different routes between the differentslurry management options, between grass and arablecropped land, and between the two soil types on arableland. Relatively small year to year variations were foundfor each option.

The levels of all the nitrogen pools in the soil profile,including litter, faeces, ammonia and nitrate, can differfrom the beginning to the end of the year, and these area significant part of the nitrogen balance when con-sidered over an individual year. However, when averagedover a 10 yr period, the imbalance in each of these pools,which tends to be positive in some years and negative inothers, is small. The total imbalance from these pools(which are not shown in Fig. 1) accounts for the smallimbalance between each pair of total inputs and totaloutputs in Fig. 1.

The proportion of nitrogen recycled for different timesof slurry spreading (as with spreading on different areasof land for the small store), are plotted in Fig. 2. Thisshows a rise in the proportion recycled with progressivelylater spreading from October to the spring. The propor-tion recycled is lower with repeated spreading on thesame land than with a single application at any time.Separate figures for the proportion recycled to the har-vested crop alone, and to the humus pool plus the crop,show that most of the variation with time of applicationis attributable to that recycled to the crop.

3.1.1. Crop comparisonsResults show a higher proportion of nitrogen recycled

into crop biomass by grass than by spring barley (Fig. 1),despite higher nitrogen fertilizer inputs on grassland.This suggests that grass has a greater potential to absorbexcess nitrogen from the system, partly because it hasa longer growing season, and partly because annual drymatter biomass offtakes are higher for grass than forcereal crops. The higher total slurry application with thesmall store system and repeated applications on the samearea of land, compared with the large store system, pro-duces a response in terms of a higher yield in both crops,but this response is greater with grass than barley.

3.1.2. Effect of store size and spreading methodAbout 10% of total N or 20% of available N is lost by

ammonia volatilisation from slurry applied by a splashplate system on arable land, while on grassland thefigures rise to about 15% of total N or 30% of availableN (Fig. 1). These losses are higher with winter spreadingas required with the small store system, than when slurryis all spread in the spring which can only be done with thelarge store. There is a substantial environmental benefitfrom an injection system which cuts this gaseous emis-sion to almost zero. The adjustment to mineral fertilizersuggested by Dyson21 to allow for higher availableN from injected slurry (compared with surface spreading)is about right for arable land with the large store system.Repeated slurry spreading on the same land area (withthe small store system) results in excess nutrients in thesoil, particularly on bare soil or stubble with spring cereal

Fig. 1. Mean annual nitrogen input/output balances for ten year simulations


land in winter. No mineral fertilizer at all is required withsuch repeated slurry applications, so no adjustment canbe made and injection contributes further to excess nitro-gen in the soil. For grassland, results indicate that the

suggested mineral fertilizer reduction for injection couldbe increased slightly, as the current adjustment onlypartially compensates for nitrogen lost by volatilisationwith surface spreading.

Fig. 2. Proportion of applied nitrogen recycled with different numbers of slurry applications and times; d, j, m, ———, to harvested crop;s, h, n, - - - -, to crop and soil humus; d, s silty clay loam grassland soil; j, h, sandy loam arable soil; m, n, clay loam arable soil


3.1.3. Nitrate leachingNitrate leaching is the main route by which nitrogen is

lost if it is in excess in the system. Leaching losses arealways highest for the small store system with repeatedapplications on the same land (Fig. 1), since four applica-tions of slurry of which some take place at an inappropri-ate time of year contribute to available N well in excess ofrequirements for crop growth. With this system, leachinglosses are particularly high for the arable soils, andhigher for the clay loam than for the sandy loam soil.They are also higher with injection than with splash platespreading, since retaining available N at the spreadingstage contributes further to excess N in the system. Withthe large store system and spreading only in the spring,leaching losses are higher for the sandy loam than for theclay loam soil. With the large store system, leachinglosses are very low from grassland; this is the only casewhere denitrification losses exceed leaching losses.

3.1.4. DenitrificationDenitrification losses are more than twice as high from

grassland compared with the arable soils (Fig. 1). Thismay reflect the larger nitrogen pools, particularly organicnitrogen, in the grassland soil. However, denitrificationlosses differed little between the different slurry manage-

ment options and between the two arable soils. With thecurrent state of development of the SOILN model, it isnot possible to subdivide denitrified N into nitrogen gasand nitrous oxide, but it is reasonable to assume thathigh levels of denitrification will be associated with highnitrous oxide emissions.

3.1.5. Soil humusIn contrast to the other soil nitrogen pools, the one

pool where there is a noticeable change over 10 yr is soilhumus. This is a very large pool, so the change is small asa percentage of the total pool size. However, it appears tobe much larger when expressed as a percentage of thenitrogen added to and lost from the system, and is a sig-nificant part of the ten year balance. For all optionsconsidered, there is an increase in the soil humus nitrogenpool over the years (Fig. 1), due to the addition ofnon-available N from the faeces component of slurry,which subsequently declines only slowly by mineraliz-ation. As would be expected, this increase is greater forfour repeated slurry applications over the same area ofland than for a single slurry application on each area.The smallest such increase occurs with the single surfacespread application of slurry on the sandy loam arablesoil, where the rate of mineralization is almost as great


(greater in some individual years) as the input of faeces inslurry to the humus pool. An increase in soil humus canbe regarded as one of the positive benefits of slurryapplication, since an increase in soil organic matter isgenerally associated with an improvement in soil struc-ture for mineral agricultural soils. While there is littlecontrol of the timing of the release of humus nitrogen bymineralization to nitrate, mineralization is a temper-ature-dependent process so takes place at a faster rate insummer months when opportunities for utilization ofnitrate by crops are highest. Nitrogen from applied slurrywhich ends up in the humus pool can therefore be re-garded as a component of recycled N rather than a loss.

3.2. Environmental problems of phosphorus

Due to availability of appropriate models, the assess-ment of environmental impacts presented in this paperhas concentrated on nitrogenous pollution. Nevertheless,some simple indications about environmental impacts ofphosphorus are given by the calculated reductions infertilizer application rates (according to Dyson,21 seeSection 2.3.5) as shown in Table 1. In particular, thisindicates that repeated spreading of slurry on the sameland area leads to a gross overload of P (and also K)nutrients. This suggests that there will be a serious envir-onmental problem of leaching of dissolved and partic-ulate phosphorus, which can be explored more fullywhen a soil phosphorus dynamics model has been de-veloped. Similarly, for arable land there is some overloadof P and K with all the slurry management optionsconsidered.

TablWork rates, costs and benefits (in terms of value of reduced fertilise

Column no (used in Table 5) 1 2

Store size Large LargeApplications on each area of land 1 1Spreading method Splash pl InjectWorkrate, ha/h 0)31 0)28Time required, h 98)4 109Total hourly cost, £/y 710 1540Annual tanker cost, £ 841 1740Annual store cost, £ 1380 1380Total cost, £/y 2930 4650Fertiliser value, £ N Arable 635 817

P 625 625K 351 351Total 1610 1790N Grassland 381 490P 1010 1010K 1060 1060Total 2440 2550

3.3. Economic analysis

For each of the six slurry management options con-sidered, the overall mechanization cost (including labourand fuel) and value of mineral fertilizer saved are listed inTable 4. Comparisons between relevant pairs of optionsas differences between the figures in each column of Table4 are shown in Table 5. One option is only beneficial overanother in pure economic terms if the difference betweenvalue of fertilizer saved is greater than 100% of theadditional equipment cost.

3.3.1. Spreading methodInjection incurs large additional costs (around £1700

per annum) compared with spreading using a splashplate, and only a small proportion of this (3—10%) can berecouped by reduced fertilizer costs (Table 5). The largestsaving occurs with the large store and arable crops.

3.3.2. Store sizeThe large store, compared to a small one, incurs an

additional cost of £830 per annum, and the benefit inreduced requirement for chemical fertilizer is up to £370or nearly half of the additional cost (Table 5). Thesefigures are calculated assuming that with the small store,slurry is spread on different land each time the store isemptied. There are even larger apparent benefits from thelarge store compared with repeated spreadings on thesame land with the small store. In this case, the benefit isgreater than the additional cost of the large store. How-ever, this is a comparison with a practice which is badfrom both an environmental as well as an economicstandpoint.

e 4r requirement) of different spreading method and store size options

3 4 5 6

Small Small Small Small1 1 4 4

Splash pl Inject Splash pl Inject0)31 0)28 0)38 0)32

98)4 108 80)3 95)3710 1540 580 1350841 1740 841 1740545 545 545 545

2100 3820 1970 3630350 449 320 320625 625 156 156351 351 88 88

1330 1425 564 564210 270 210 270

1010 1010 360 3601060 1060 351 3512270 2330 920 980

Table 5Comparisons of costs and benefits (in terms of reduced fertilizer requirement) of different spreading method and store size options

Columns in Table 4 compared 2-1 4-3 6-5 1-3 2-4 3-5 4-6 1-5 2-6

Total hourly cost, £/y 830 830 770 0 0 130 190 130 190Annual tanker cost, £ 899 899 899 0 0 0 0 0 0Annual store cost, £ 0 0 0 835 835 0 0 835 835Total cost, £/y 1720 1720 1660 830 830 130 190 960 1020Fertiliser value, £ N Arable 182 99 0)0 285 368 30 129 315 497

P 0)0 0)0 0)0 0)0 0)0 469 469 469 469K 0)0 0)0 0)0 0)0 0)0 263 263 263 263Total 180 95 0)0 280 365 766 861 1056 1226N Grassland 109 60 60 171 220 0)0 0)0 171 220P 0)0 0)0 0)0 0)0 0)0 650 650 650 650K 0)0 0)0 0)0 0)0 0)0 709 709 709 709Total 110 60 60 170 220 1350 1350 1520 1570

Benefit, % of Arable 10)4 5)5 0)0 34)7 43)9 589 453 110 120Additonal cost Grassland 6)4 3)5 3)6 20)5 26)5 1040 711 158 154


3.3.3. Spreading policy for small storeThe comparison of the options available with the

small store, i.e. repeated spreading on the same landarea against spreading on different land, shows verylarge economic benefits from spreading on different land(Table 5). This is because the nutrient value of slurryas a fertilizer substitute cannot be exploited properlywith repeated spreading, so even with no chemical ferti-lizer supplement for some nutrients the economic benefits are small. Benefits of spreading on different land canbe up to ten times the additional costs of a longer farmto field transport distance with a slightly lower overallwork rate.

3.3.4. Further potential economic benefitsThe economic analysis presented here is based on the

recommended fertilizer adjustments for slurry presentedby Dyson,21 which make assumptions about the likelylosses of nitrogen with surface spreading and withspreading in autumn and winter, as listed in Table 1.These figures assume losses of nitrogen both by leachingand by ammonia volatilization. Results presented inSection 3.1 suggest that losses of nitrogen are lower thanwere assumed in Table 1, and hence that a larger allow-ance could be made for the nutrients in slurry for someoptions. This is clearly illustrated in the no-slurry option,where the recommended fertilizer applications lead tolower crop yields than where slurry is applied (Fig. 1). Itis likely that a substantial safety margin has been left inthe adjustments. If the fertilizer applications were re-duced to make a true allowance for the nutrients inslurry, the cost benefits of reduced fertilizer requirementwould be greater than calculated here. This might doublethe benefit of injection, although it would still be a longway short of covering the cost of the equipment. It would

also enhance the benefit of the large store which is quiteclose to being economically justified anyway, althoughthe increase in benefit would probably not be quite asgreat as to double it.

4. Conclusions

Weather-driven simulations with ammonia volatiliz-ation, soil water and soil nitrogen dynamics models havedemonstrated the extent of losses of nitrogenous compo-nents which pollute the environment, from land spread-ing of slurry with alternative management policies con-cerning storage and spreading method. Combining thiswith some economic calculations of costs of equipmentand mineral fertilizer substituted, the overall conclusionsare as follows.(1) Losses in the form of leached nitrate are high where

there is a requirement for some slurry to be spreadduring the winter, because the store is too small tohold all the slurry produced during the housing peri-od, compared with spreading all slurry in the springas is possible with a large store.

(2) Losses in the form of leached nitrate areextremely high, due to a gross overload of nutrients,where repeated slurry applications during thewinter period take place on the same area of land,and this often happens where the store is too smallto hold all the slurry produced during the housingperiod.

(2) Slurry application by injection reduces ammoniavolatilization losses to a low level, compared withsurface spreading.

(3) Slurry management measures have little effect onnitrous oxide emissions from denitrification.


(4) There is a greater potential for recycling nutrients inslurry by spreading on grassland than on arable landunder spring barley.

(5) There are economic and environmental benefits fromreducing mineral fertilizer applications to allow fornutrients in slurry.

(6) Taking account of equipment costs and potentialsavings in costs of N, P and K mineral fertilizer, it hasbeen shown that only a small proportion of the addi-tional costs of injection rather than splash-platespreading can be recouped in reduced requirementsfor mineral fertilizer, whereas for a large comparedwith a small store up to half the additional costs canbe recovered in this way.

(7) Repeated spreading of slurry on the same land areacannot be justified on economic grounds, as well asbeing damaging to the environment.

Acknowledgements

The authors would like to thank Professor P.-E. Jan-sson and Dr H. Eckersten of the Department of SoilSciences, The Swedish University of AgriculturalSciences, Uppsala, for assistance in work with theSOILN model, and Dr N. Hutchings of the Departmentof Soil Science, Danish Institute of Plant and SoilScience, Foulum Research Centre, Tjiele (formerly of theMacaulay Land Use Research Institute, Aberdeen) forallowing us to use his ammonia volatilization model.Funds to carry out the work were provided by the Scot-tish Office Agriculture, Environment and Fisheries De-partment, and also by the European Union under theproject ‘Optimal use of animal slurry for input reductionand protection of the environment in sustainable agricul-tural systems’’.

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environmental and economic implications of some slurry management options

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