View of Effect of soil compactness on the growth and quality of carrot

Effect of soil compactness ^on the growth and quality of carrot

Liisa Pietola

AgriculturalResearch Centreof^{Finland,Institute}of^Cropand Soil Science, Jokioinen,Finland DepartmentofApplied ChemistryandMicrobiology, Universityof^{Helsinki,Finland}

ACADEMIC DISSERTATION

To be presented, with the permissionofthe Faculty

ofAgricultureandForestry of^{the University}of^Helsinki, for^{public criticisminAuditoriumXII,}Aleksanterinkatu5, Helsinki,

on^June14th, 1995,_at12noon.

Preface

The experimental part of this study was mainly carried out at the Agricultural Research Centre of Finland in 1989-1992, theroot analyses being completed in 1993 atMichigan State University, USA. The work was finalized in 1994^atthe University ofHelsinki,Sec- tion of Agricultural Chemistry and Physics of the Department of Applied Chemistry and Microbiology.Iwishtothank Dr. EskoPoutiainen, Director General of the Research Cen- treand Dr. PaavoElonen, Professor of Agricultural Chemistry and Physicsatthe Research Centre, for providing me the main financing and facilities for the experiments. I amvery gratefultoDr. Alvin J. M.Smucker,Professor of Soil Biophysics atMichigan State Univer- sity, East Lansing, for allowing meto join his laboratory forroot analyses, and Dr. Antti Jaakkola, Professor of Agricultural Chemistry and Physics atthe University ofHelsinki, for his constructive criticism and ^support atthe various stages of the work.

I wishtothank Docent Irma Voipio and Docent Erkki Aura for the valuable suggestions tothe manuscript. Iamgrateful tothe staff of the Agricultural Research Centre, especially

toMr. Tapio Salo, M.Sc., Mr. Risto Tanni, Mrs.Erja Äijäläand Mrs. Ritva Niemiat the Institute of Crop and Soil Science,for the skilful technical assistance in the experiments. I greatly appreciate the guidance and technicalsupport of Mr. John. C. Ferguson, B.Sc. at Michigan State University. I also wish to thank the staff of theDepartment of Applied Chemistry and Microbiologyat the University of Helsinki for the analysis of soil air com- position and Dr. IngeHåkansson,Professor of the Department of Soil Sciences atthe Swedish University of AgriculturalSciences, for allowing the determinations of soil reference bulk densitiestobe made in his department. The Figures (6-35)weredrawn by Mr. AriTörmä, M.Sc. and the English manuscript was revised by Mrs. SevastianaRuusamo, M.A., and edited by Mrs. SariTorkko, M.Sc.,which work I greatly appreciate. This investigationwas financially supported by the Academy of Finland and the TiuraFoundation, which isgrate- fully acknowledged. I would also like to thank the board of the Agricultural Science in Finland for including this study in their journal. Finally, mywarmestthanksare dueto my family whosegreat support made it possible forme tocomplete this work.

East Lansing, February 1995 ^Liisa Pietola

Contents

Abstract

1 Introduction 145

1.1 Importance^ofsoil compactness toplant growth 145

1.1.1 Consequencesofexcess soilcompactionandloosening 145

1.1.2 Optimumsoil compactness 146

1.2 Significanceof soilproperties tocarrot rooting and yield quality 148

1.3 Efficiencyof root system 148

1.4 Background and aim of the present study 149

2 Materials and methods 150

2.1 Experimental fields ¹⁵⁰

2.2 Treatments 152

2.2.1 ^Irrigation 152

2.2.2 ^Soilmechanical treatments 152

Soilloosening 152

Soilcompaction 152

2.3 Establishmentand managementof field experiments 153

2.3.1 Fertilization and spring tillage 153

2.3.2 Sowingand managementduring growth 153

2.3.3 Sampling 154

2.4 Weather conditions 154

2.5 Soil measurements 154

2.5.1 Soil water content 154

Electrical resistance 154

Samplingand drying 155

2.5.2 Soildrybulkdensity 155

Gravimetricanalyses 155

Gamma raytransmission 155

2.5.3 Degreeof soilcompactness 156

2.5.4 Soilpenetrometer resistance 156

2.5.5 Soilpore size distribution ¹⁵⁶

2.5.6 Soil aircomposition 157

2.6 Plant measurements 157

2.6.1 Sampling 157

2.6.2 Yield 158

2.6.3 ^{Tap root}quality 158

Externalquality 158

Internalquality 159

2.7 Studies on^carrotfibrousroot system 159

2.7.1 Destructivesampling 159

2.7.2 Dry weight 160

2.7.3 Root morphology 161

Videorecording 161

Image analysis 162

2.8 Statistical analysis 163

3 Results 164

3.1 Effects of^treatmentsonsoilphysical properties 164

3.1.1 Soil moisture 164

3.1.2 Soil drybulkdensity 167

Gravimetricanalysis 167

Gamma ray transmission 170

Gravimetricanalysis vs.^{gamma ray} transmission 171

3.1.3 Degree of surface soil compactness 171

3.1.4 Penetrometerresistance 172 Relationto soil moisture anddrybulkdensity 175

3.1.5 Soilporosityand waterretention capacity 177

Pore volume 177

Water retention 179

3.1.6 Soil aircomposition 182

3.2 Response of^carrotgrowth andyield qualitytotreatments 183

3.2.1 Yieldand externalqualityoftap roots 183

Biomass accumulation ¹⁸³

Final yield 186

Root size 187

Root splittingandbranching 190

Taprootlength and diameter 193

Rootshape 193

3.2.2 Internalqualityof tap roots 196

Dry matter content 196

Crude fibre^{content 197}

Juice content 197

Juicedry matter content 197

Sugarcontent 206

Carotene content 206

Internalqualityvs.root size 206

3.3 Response ^ofwhole-root system and shootgrowthto differentlytreated soil

profilesin PVCcylinders 209

3.3.1 Distribution ofweights 209

Taprootand shootweight 209

Fibrous^rootweight 209

Fibrous^rootweight to tap rootweightratio 210

3.3.2 Distribution of^fibrousroot length 213

Rootlength perplant 213

Root lengthto tap rootweightratio 215

3.3.3 Distribution of fibrous root surfacearea 217

Root surfaceareaperplant 217

Root surfaceareato tap rootweightratio 218

3.3.4 Fibrous root width 219

4 Discussion 221

4.1 Effect of soilloosening, compactionandirrigation onsoilphysical

growth factors 221

4.2 Effect of soil physical growthfactorsoncarrotgrowth andyield quality 223 4.3 Role of fibrous root systemincarrot response tosoilcompactness 226

5 Conclusions 228

References 230

Selostus 236

Appendixes 1-5

Effect of soil compactness ^on the growth

and quality ^{of carrot}

Liisa Pietola

AgriculturalResearch Centreof^{Finland,Institute}of^Cropand Soil Science, Jokioinen,Finland Present address:DepartmentofApplied ChemistryandMicrobiology, ^P.O. Box27,FIN-00014

Universityof^{Helsinki,Finland}

Field experimentswereperformed in^Southern Finlandonthree soil types: fine sand (1989-1991), clay (1989)and mull(1990-1991).Thefollowingsoil mechanical treatmentswereappliedto autumn ploughed land: soilloosening by ridge preparation (ridge distance 45cm), rotary harrowing ^(toa depthof20cm,clay 15^cm), and soilcompactiontrackby trackbyatractorweighing 3 Mg (1 or 3 passes, wheel width 33 cm)before ^{seed bed}preparation.^Oneplot wasuntreated. These treatments were ^{set upinApril(on}clay in May)undermoist soilconditions. Sprinkler irrigation(one applica- tion of 30mm) was applied to clayand fine sand when soil moisture intop soil had decreased to around50%ofplant-available watercapacity.PVCcylinders^(r= 15cm,h⁼60cm)werefixedinthe experimentalareasduringthegrowing periods.Atharvest,thesecylinderswereremoved forspecific analysisof tap and fibrousroots of carrot.Length and width of fibrous roots were quantified by image analysis intheUSA.

Theimpactsof soilloosening andpartial compactionweredeterminedby measuring soilphysical parameters to a depth of 25 cm inmineral soils, and to greater depths in organic soil. Dry bulk densities of theplough layersincreased withincreasing tractor passesby 8%, 10%and 13%for fine sand, mull and clay soils, respectively. The lowest dry soil bulk density in the plough layer was obtainedbyrotary harrowingtoadepth^of20cm. Comparisonof gamma ray transmission and gravi- metricanalysisindicated thatdrysoil bulkdensity ^wasslightly lower when determinedby gravimet- ricanalysis.Increased soil bulk densities werereflectedbyincreased^waterretentioncapacity(mat- ric suction <lOkPa) and greater penetrometer resistance.Relatively similar increasesinbulkdensity increased the penetrometer resistance much less in mull thaninfine sand. In contrast,greater bulk densitiesinthe mull soil affected soil aircomposition adversely by decreasingthe0

2content to10%

when the subsoil hadhighwetness.Inothersoils,the lowest soil oxygen contents of 16-18%were recorded inearlysummer(compacted clay)and during periods ofvigorous plant growth (fine^sand) when soil water contentswerehigh. Eventhough the highest degree ofsoilcompactness (D) in a plough layer approached 93 (gravimetric) inallsoils,only claysoil wascompactedtoasoilmacro- porositybelow 10% (pore diameter^>30 pm).

Soilcompaction promotedcrop establishment andearly growthascomparedwith loose soil beds.

Optimumsoil compactness for carrotyield^(D=82) wasobservedonly in clay field where excess looseningorcompactionaffectedyield quantity adverselyatdifferentstages ofgrowth. During bio- massaccumulation, excessive penetrometer resistances limited tap root growth in compacted fine sand withoutirrigation.Waterapplications promotedshootgrowth,but did not affect final shoot and tap rootyield. Amongthe three soil types testedinthis study, compactionof mull soil had the least effectoncarrotgrowth and external quality.

©Agricultural ^ScienceinFinland

Pietola,L.:

Effect of

^soilcompactness on the growth and quality

of

^carrot

This paper presents evidence that theinternalqualityofcarrots isonly slightlyaffected by chang- esⁱⁿsoilphysical properties, while the adverse effects of soilcompactiononcarrotexternalquality (short,deformed and conicaltap rootswithgreatermaximumdiameters)areclear. Even thoughcom- pacted clay soilgreatly limited the biomass accumulations inthe tap root, which had ^ahigh crude fibre content, the carotene(10 mg/100gcarrots)and sugar contents(5%)reached acceptablelevels.

The lowestcarotene contents(4 mg/100 gcarrots)wereobservedinloosemull, followingacoollate

summerin 1990.Theeffectofirrigationoncarotene contentvaried fromoneyear to another.

Highsugar and carotene contentsappeared torespond tothehigh below-ground absorptionsur- face.Thefibrous root system of carrots,consistingofmostlyvery fine roots(diameter 0.15mm), had totallengthsof150min loose fine sand at^asoildepth^of0-50cm(rotary harrowed), 200m and300 min fine sand and mull soils subjectedto3passesby atractorwheel.Themaximumdry weight⁽⁶⁰ pg), length (1.2 cm)and surfacearea(0.05cm²)of the fibrous root system per soil volume (cm³⁾were observed in compactedor irrigated soil to adepth of 30cm, and also inrelation to tap root dry weight.This suggests^acapacity of carrotplantforhigh below-ground absorption potential andopti- mal biochemial maturation of tap root tissueevenwhen surface soilsarecompacted.Thisis support- edby higherleaf area,astheearly shootgrowthwaspromoted by partial soilcompaction.

Soil compaction affected the soil physical properties and carrot external quality in agreement withprevious studies. Carotene and sugar contents appeared tobe unaffectedor were slightly in- creasedinriper and firmer carrots ofcompactedsoils. This is consistent with the earlier information about the internal quality of carrot which is shown to be highly dependenton genetic factors and developmental stage of carrot. The presentstudy emphasizes the surfaceareaof carrot fibrous root systemas abeneficial factor formaintaining highlevels of carotene and sugar contentsintap roots

afterpartial soilcompaction.

Keywords: tillage, traffic, soil physical properties,^carotene, sugar, root length, root surface area, image analysis

Introduction 1.1 Importance of soil

compactness ^to plant growth

1.1.1 Consequences of excess soil compaction and loosening

Poor aeration and high mechanical impedance arethe majorstressfactors affecting the growth ofmostplants in compacted soil. Tractor wheel traffic, with axle loads ofno morethan3 Mgon wet clay soils,destroys the total macroporosity (pore diameter >3O pm) toaportion below 10%

of soil volume(Aura 1983) which is considered tobe the critical limit for soil aeration maintain- ing plant growth(Glinskiand Stepniewski 1985).

This detrimental effect onsoil porosity reflects

in altered soil air composition (Eavis 1972, Si- mojoki etal. 1991)and in an oxygen diffusion ratebelow30 pg nr²s'

1

^{which istoo}low for plant growth (Erickson 1982, Asady etal. 1985). In highly compacted soils theroot to soil contact may besointense that aeration of the_roottissue is completely dependentonthe internal air chan- nels in theroot (Veen etal. 1992).Further, as soil compaction has an adverse effecton satu-

rated _water retention capacity and infiltration (Blake etal. 1976, Reicosky etal. 1981, Ankeny etal. 1990),poor aeration of_compactand flood- ed soil inhibits_root growth and nutrient uptake (Grath and Håkansson 1992). Under localized anoxia, compensatory root growth causesinef- ficient utilization of carbon and additional up- take of water and nutrients (Schumacher and Smucker 1984, 1987).

Vol. 4: 139-237.

Mechanical impedance is anothercomponent of soil compaction often implicated in poor growth. Pohjanheimo and Heinonen(1960) re- ported that the hardening and dryingoutof clay loam soil may entirely inhibit the penetration of barley roots into the subsoil. Soil strength on some roots is reflected also asreduced growth rate of other plant parts ^(Massle and Passioura 1987).In the absence of continuous large pores, soil resists the local deformation caused by roots, and there is a definite upper limitto apressure near 1 MPa in the axial direction and 0.5- 0.9 MPa in the radial direction which can be exerted byroots of_agiven species (Pfeffer 1893, ref. Gill and Bolt 1955, Misra etal. 1986).Ra- dial expansion ofaroot behind the tip causes a lowering of external mechanical impedance ahead of the elongatingroot(Abdalla etal. 1969, Richards and Greacen 1986).

Mechanical impedance measured by pene- trometersshould be regarded ascomparative,not absolute values. The static and additive frictional components ofpenetrometer probe results in greater soil resistance values than foraroot (Far- rell and Greacen 1966, Whiteley etal. 1981).

Accordingto Materecheraetal. (1991), the soil penetrometerresistance of4.2 MPa corresponds toanexternal mechanical resistanceontheroots of approximately 1.14 MPa.

The pressure at the^root apex cannot, how- ever,be the only determining factor regulating the root elongation rate, as aclear relationship has been observed between the penetrometer resistance in the ^toplayer and root growthrate in the subsequent loose layer (Bengough and Young 1993).The restrictedroot growth with morphological deformities, such as diameter growth and compensatorygrowth oflaterals, is ageneral _symptom of_toohigh soil strength_un- der field conditions(Wiklert 1960, Voorhees ^et al. 1975).Mechanically impededroots, inturn, exhibitanincreased respirationrateper unitroot length (Schumacher and Smucker 1981, Atwell

1990

a,

b).

Poorwatersupply and smallroot tosoilcon- tactareaof loose soilcanalso contributetopoor growth. Soil needs some compaction, i.e. soil

compression (Smucker and Erickson 1989), to be themostproductive (Håkansson 1966, John- son etal. 1990). This has been proved particu- larly during drought periods, dueto lower volu- metric water content (Boone etal. 1978, Dom- zal and Hodara 1992) and, thus, a weaker un- saturated hydraulic conductivity in loosely packed fine textured soils than under slightly compacted conditions (Kemperet

al.

1971, Voor- heesetal. 1979, Mehtaetal. 1994). On the oth- er hand,soil compaction reduceswaterconduc- tivity ofcoarse textured soils atmatric _poten- tials between 0 and -60 kPa (Lipiec and Tarkie- wicz 1984).According to Agrawal (1991), this improves the productivity of sandy soils withtoo high water infiltration by reducing losses of water and nutrients. Compaction of sand has been reported to increase yields by as much _as 30-50%.

The poor contact area between root and loosely packed soils has been discussedrecent- ly: Kooistra et al. (1992) measured by a thin- section tehnique averageroot tosoilcontactsof 60,72 and 87% for sandy loam ofaporosity of 60, 51 and 44% (v/v), respectively. They also proved that water absorption and nitrate uptake per unit maizeroot length decreased with loos- ening soil and with decreasing^{root to} soil con- tact (Veen etal. 1992). Similarly, Huang (1990) demonstrated that the_{greater root to} soil con- tact at the high bulk density of water deficit plants increasedwater absorption per unit of rootsgrowing in clay soil. These results arewell in agreement with the earlier measurements of Lipiecetal. (1988, 1992) which indicatedafaster and higher waterabsorption by _aplant growing in compacted soil than in loose media.

1.1.2 Optimum soil compactness

Measurements for soil compactness include analysis of soil dry bulk density and soil poros- ity. Different sizes ofpenetrometers have been used _to indicate soil strength of dense layers (Bengough and Mullins 1990)orprofile charac- teristics after various tillage operations and com- Pietola, L^:

Effect of

^soilcompactnesson the growth and quality

of

^carrot

paction(Carter 1988, Akkeretal. 1994).Ameas- urementfor soilproperty which is relatedtothe variation in continuous pore size distribution, like saturatedwaterconductivity, should be in- cluded in the analysis of soilcompactness aswell (Hartge 1992).To be abletocompare the results of soilcompactnessbetween different soil_types, soilcompactness has been determined also_{as a} relative value by means of the reference state.

Håkansson(1990)specifies therelative soilcom- pactness as a “degree of soil compactness” (D)

which is given by the^formula;

D= 100 (p_d/p_dp⁾ (1)

where p_dis the dry bulk density ofasoil (gcm³⁾ and p_{d p}the dry bulk density of the same soil (gcm³⁾ after excessively compacted tothe ref- erence stateby an unaxial pressure of 200 kPa

until drainage and compaction ceases (after 1 week). Initial soilwater contents are nearfield capacity (matric suction^=° 10^kPa).

Optimum soil compactness is reached by avoiding excessive soil loosening and compac- tion. According^tofield experiments in Sweden during the last threedecades,barley yield is high- est atD around 87 in all mineral soiltypes.This compactnessis achieved withonepass ofa^trac- torwheelon moist ploughed land during spring tillage. The finding is well in agreement with results from Norway (Riley ¹⁹⁸⁸⁾ and Poland (Lipiec ^etal. 1991).

The variation in seasonal precipitation ex- plains well the differences between yields at- tainedatthe optimum and non-optimum states of soilcompactness(Lipiec^etal. 1992). The dis- advantages of soil compaction aremost promi- nent after excessive rains which cause oxygen stress.Similarly, loweroptimum soil densities areneeded during rainy seasonsbecause there is asmaller need toretain all of thewater(McKyes

etal. 1979, Medvedev 1992).Thus, according toHåkansson (1992), the relation between plant growth and soil compactness is understandable only if soil moisture status has been measured along the growing period. He outlined the criti- cal values of soil air content, penetrometer re-

Matricwatertension (kPa)

sistance and unsaturated hydraulic conductivity for plant growth in relation ^toboth soil moisture and degree ofcompactness(Fig. 1).The data in Figure 1 is based onthe analysis oftwo Polish loam soils by Lipiec etal.(1991)but, according

tothe authors, the information is generally ap- plicable toall mineral soils. In organicsoils, or- ganic particles may be irreversibly deformed during prolonged loading, resulting in an over- estimation of the reference bulk density (Håkans- son 1990).

The optimumcompactness varies according toplant species, and precipitation atthe time of themostintensive yield productionis^adecisive factor (Soane 1992). In the Nordic countries, mostof the precipitation during growingseasons falls at the end of thesummer. Consequently,_a high state of soil compactness may affect most adversely crop production of late species, such as potatoes androot crops. For these crops, the disadvantages of soil compactionare common- ly prevented by preparing very loosely packed raised beds or narrow ridges (Millette et al.

1981). The bed cultivation_system is often_car- ried out with a controlled traffic system using

permanent traffic lanes (Monroe and Taylor 1989). Water deficit may, however, occur soon- Fig.I.^{Criticallimits for}plant growth inrelation todegree of soil compactness and matric waterpotentialof theplough layer (Håkansson 1992). Shadedareaindicates adverse soil physical propertiesforplant growth.

Vol.4: 139-237.

erin raised beds thanon flat land(Mahrer and Avissar 1985). This is well in agreement with the above mentioned low hydraulic conductivi- tyandwaterretention capacity of loosely packed soils.

1.2 Significance ^of soil properties

to carrot rooting and yield quality

Carrot growth and yield qualityareexcellent in- dicators of soil compactness, as the external quality ofcarrot taprootis sensitivetosoilcom- paction, even on organic soils (Strandberg and White 1979). Mechanical impedance and poor aeration of compacted soil restricts the carrot rootsize and modifies theroot,making it coni- cal. Followed by youngcarrot root buckling, branching and thickening (Strandberg and White 1979), the number ofshort,thick and branched

tap rootsis high under compacted soil conditions (White 1978,Taksdal 1984, Kesik 1990).On the other _{hand, tap} root yield (Olymbios and Schwabe ^1977,Agung and Blair ¹⁹⁸⁹⁾and seed- ling emergence (Strzalka 1990)arereduced by excess soil loosening.

The role of soil chemical and physical prop- erties in the internal quality ofcarrots is minor ormaynot be well understood. Although place- ment fertilization (Evers 1989^a) and drought (Dragland 1978)have been foundtobe favoura- ble to carotene production, the developmental stage of^carrothas amajor influence on the in- ternal quality factors ofcarrottap root, such as dry matter, sugar (Platenius 1934, Hole and McKee 1988) and carotene contents (Barnes 1936, Evers 1989^{a).These and}tap^rootfirmness increase along the growing season in develop- ing tap root, indicatingroot biochemicalmatu- rity, i.e. ripeness (Fritz and Habben 1974).Also genotype and weather conditions have a clear impact _on carrot internal quality (Bradley and Smittle 1965, Simonetal. 1982, Miedzobrodz- kaetal. 1993, Evers ^1994).

Agung and Blair(1989)reported of poorex- ternal quality of ^carrot tap ^root and decreased fibrous root length density caused by highly compacted soilbut, unfortunately,nodataonthe internal qualitywere collected. The total length ofcarrotfibrousroot system increased signifi- cantly during the last 50 days before harvest⁽¹⁵³ days after sowing) in all pots. The pots were filled by packing homogeneous soil to different dry bulk densities to^create artificial soil pro- files of verygreat variation in soil compactness which hardly exist under field conditions witha normal field traffic. Moreover, this study was carriedoutwithout any biopores orcracks which contribute^{to root} penetration, but havea limit- ed effect on average soil physical properties, such as meanbulk density or penetrometer re- sistance. As the thickness of thecompactlayer representsusually less than 25% of the finalroot- ing depth of cultivated plants(Tardieu 1994),the effects of localizedcompact zones on theroot- ing characteristics of whole-rootsystemsshould be emphasized.

1.3 Efficiency of ^root system

The response of growth to soil compactness is determined also byroot characteristics of_agiv- en species. These areroot dimensions (length, diameter,surfacearea)which affect the require- ments of carbon (Eissenstat 1992), nutrient and wateruptake (Barber and Silberbush 1984), abil- ity topenetrate into soil(Richardsand Greacen

1986, Materechera^etal. 1991)and capacity to maximize_{root to} soil _{contact area} (Veen etal.

1992).

Root length is themostfrequently measured plant property influencing water and nutrient uptake (Nye and Tinker 1977, Molz 1981, Noor- wijk and Willigen 1991).Root diameter is also important, as species which haveroot systems withnumerousfineroots containing onlyafew layers of cells produce more root surface area with less photosynthesized carbon (Eissenstat Pietola^,L:

Effect of

^soilcompactnessonthe growth and quality

of

^carrot

1992).Moreover, rootswithasmaller diameter will haveabetter nutrient uptakeasdemonstrat- ed by Itoh and Barber (1983a) for therootdiam- eterrange of 0.03-0.3 mm.Inaddition,fineroots remove more water (Eissenstat and Caldwell 1988^a) stored in the soil profile than do larger roots of similarroot biomass and distribution with depth. AccordingtoRichards and Greacen (1986), fine_roots arealso less affected by high mechanical impedance, even though in very compacted homogeneous soil (penetrometerre- sistance 4.2 MPa)there is apositive correlation between root diameter and elongation (Mater- echera et al. 1991). Noordwijk et al. (1993) found, however, no relation between root-soil contactandroot diameterorroundness.

The absorptive capacity of theroot systemis aproduct ofroot surface areaand ^root permea- bility. AccordingtoFiscus and Markhart(1979), the system size seems to be the dominant and root permeability the minor factor. Also age of the root seems relatively unimportant for the nutrient uptake (Clarkson and Hanson 1980).

Root hair lengths less than 0.4 mm which are common for field crops havea minor effect on ion uptake andwaterenteringasshortroothairs donot increase significantly the absorbing sur- face ofroots in the soil(Jones etal. 1983, Bar- ber and Silberbush 1984, McCully and Canny

1989).

Fibrousrootsareseldom distributed uniform- ly within a soil layer (Logsdon and Allmaras

1991, Aiken 1992).Roots tendtopreferentially colonize loose zones, between soil aggregates, biopores or other planes of weakness by buck- ling _{as a}root grows across a macropore and meets asolid surface (Dexter and Hewitt 1978, Pietola 1991).Roots congregatealso in the fer- tile(Eissenstat and Caldwell 1988^{b)and wetter}

zones (Smucker 1993).Thus, root characteris- tics should be quantified from numerous hori- zontal and vertical soil planes or layers. While non-destructiverootsampling by rhizotrons and minirhizotrons (Tayloretal. 1990)offers possi- bilitiesto do research on spatial and temporal root dynamics, i.e. root growth and turnover rates, destructive samples are useful for quanti-

fyingrootbiomass, length,diameter,surfacearea and volume ateach sampling time (Smucker

1993).

Earlyroot length analyses rely upon New- man’s line-intersect method (Newman 1966).

Alsoacore-break method has been used forroot length, but it provides only_alow precision esti-

mateofroot length density (Bland 1989).Auto- mated_root intersect counting by videocamera imaging (Voorheesetal. 1980) has been extend- ed_{to root} image analyses (length,width,branch- ing) by using a scanner and a microcomputer (Zoonand Tienderen ^1990,Pan and Bolton 1991, Ewing and Kaspar 1993). For recording project- edroot area orroot surfacearea,avideo_camera and a computer system for digitizing and ana- lysing video images have been developed for clean washed^roots(Harrisand Campbell 1991, Kokkoetal. 1993)and also for washedroot sam- ples with residues (Smucker 1993). The image analysis permits examination of^root length, di- ameterand surfaceareawhich all areneeded for the evaluation of the efficiency ofroot systems on amorphological basis.

1.4 Background and aim of the present study

Growth of above-ground biomass in compacted soil has been the subjecttomuch research over many years in the Nordiccountries, but mainly onsmall grainson clay soils. Information about the optimum state of soilcompactnessfor other plant speciesondifferent soiltypesis still need- ed. For understanding the relations between soil compactnessand plant growth, responses of dif- ferent physical growth factors of soiltoboth soil loosening and compaction should be quantified on different soil_typesunder varying soil mois- ture conditions, including impacts on below- ground growth.

The disadvantages of soil compaction are better known than the negative effects of inten- Vol. 4: 139-237.

sive soil loosening, especially in vegetable pro- duction where the bed cultivation system has been applied extensively. This cultivation tech- nique aims ateven and high-quality products.

However, evenif the external quality of_carrot benefits from loose soilconditions,the question sofar unanswered is: “To whatextentand how is the carrot internal quality affected”. Since waterand nutrientsareabsorbed through^roots, an extensive functioning root system is of pri- mary importance toplant growth. If the distri- bution of fibrousroot surfacearea were notad- versely affected by moderate soil compaction, comparison of loose and slightly compacted soil conditions would suggest moreintensive ripen- ing (i.e. caroteneand sugar synthesis) in slight- ly compacted soil where the soilto root contact and the soil moisture retention capacityarehigh- er. Therefore, quantitative information about the interaction of soil compactnessand moistureon carrotrooting and yield quality under fieldcon-

ditions isdesirable,with special reference toin- ternal quality factors.

The objective of this study was to evaluate the response ofcarrotgrowth and qualitytothe soil compactness of different soil _types under different soil moisture conditions. Field experi- ments wereconducted:

1) To determine the alterations of soilcompact- nessand physical growth factors (soilwater, mechanical impedance,^aeration) induced by soil loosening, partial compaction and irri- gation.

2) To identify the changes in carrotgrowth and quality caused by soil compactness,empha- sizing_taprootsize, shape and ripeness (dry mattercontent, firmness, sugar andcarotene content).

3) To evaluate the role ofcarrotfibrousrootsys- tem in the response ofcarrotbiochemicalma- turityto the soil compactness.

2 Materials and methods 2.1 Experimental fields

The empiric datawascollected from three field experiments established^at the Agricultural Re- search Centre of Finland in Jokioinen(60°49’N;

23°28’E) on undifferentiated Spodosols (Foth 1990)(Appendixes 1-3). The experimentonfine sand soil was set up in all experimental years (1989-1991),but the experiment ofclay soil rich in organic matter was conducted only in 1989.

These^twofields were situated in the samefield area.The thirdfield, amull soil(mixture ofpeat and clay),wasstudied in 1990-1991, and itwas situated around 10 km away from the other^two fields. The sown area was 1040

m

2 for theex- periments of sandy and clay soils and 520 m

2

for organic soil.

Topsoil(i.e.plough layer) and subsoil samples

weretaken from all four blocks of the experimen- tal fields. In finesand,the thickness of topsoil was approximately 25cm,that ofclay soil 26cm,and that of mull soil 27 cm.The samples were ana- lysed for particle size distribution by the method ofElonen (1971) and for organic carbon^content by a Leco analyzer at 1370°C (Sippola 1982) (Table 1).Accordingtothe soil classification used in Finland(Juuselaand Ware 1956),the topsoil of the sandy fieldwasloamy fine sand. Even though the topsoil ofclay field hadahigh clayfraction,it had 9.4% organic matter, as the organic carbon content wasmultiplied by 1.724(Allison 1969).

Hence, this soil _typecould be used for_carrotpro- duction and for the purposes ofthis study. The mull fieldwasrich in clay fraction which made itsus- ceptible_tocompaction.

The experimental soils were analysed for potassium, phosphorus, calcium and magnesium Pietola,L:

Effect of

^soilcompactnesson the growth and quality

of

^carrot

Table 1.Particle size distribution andorganic^Coffinesand,clayand mull atexperimentalsite^(%).

Soil Particle size fractions(|im)

depth,^cm

<2 2^_ 20- 60- 200- 600- Org.C

6 20 60 200 600 2000

Fine sand

0-25 13 3 4 9 33 36 2 2.4

25- 31 6 7 16 23 16 1 0.9

Clay

0-26 75 7 5 7 3 2 1 5.8

26- 78 6 6 7 2 I 0 0.7

Mull

0-27 81 7 4 4 1 2 2 18.1

27- 85 7 4 3 1 0 0 6.9

extractable in acid ammoniumacetate(pH4.65) (Vuorinenand Mäkitie 1955, Kurki etal. 1965), and pH and electrical conductivity ^(EC) in _wa- ter suspension, atthe Institute of Soils and En- vironment at the Agricultural Research Centre of Finland. The boron content was determined

by the azomethine-H method (Sippola and Erviö 1977).These characteristics of experimentaltop- soils and subsoils before (April 1989, for mull

1990)and after(November 1991)the study are presented in Table2. At the beginning of the field experiments the nutrient contentsfor fine sand

Table2.Chemicalcharacteristics ofexperimentalsoils before (a) and after (b) field establishment.

Soil pH ^{EC Ca} K Mg P B

depth,^cm _lOmScnr

1

^{mg dnr3}air-dried soil

Fine sand

0-25 a 6.8 0.65 2510 210 112 81.8 0.94

b 6.7 0.68 2240 125 129 70.5 1.42

25- a 6.9 0.55 2000 151 147 21.8 0.73

b 6.9 0.74 2350 145 651 6.2 0.60

Clay

0-26 a 6.3 0.83 4200 452 742 18.9 1.35

b 6.3 0.71 4230 436 765 19.5 1.43

26- a 6.4 0.64 3250 272 1146 2.8 0,70

b 6.4 0.70 3390 276 1265 4.4 0.71

Mull

0-27 a 5.5 0.46 2470 290 232 5.8 0.50

b 5.3 1.33 2450 235 221 7.6 0.94

27- a 5.4 0.59 2530 266 266 4.1 n.d.'>

b frö E2O 2650 189 818 3.7 0.40

11n.d. ⁼notdetermined

Vol. 4: 139-237.

and clay soilsweregoodonaverage,eventhough the magnesium content in sandy soil was only fairly good, according to the common classifi- cation used in Finland (Soil Testing Laboratory of Finland 1990). In mull,both phosphorus and boroncontentswere only fairly good.

2.2 Treatments

sector 90°). Thus, onesprinkler was needed to irrigate each main plot. Rate of irrigation was 5 mm per hour. Theamountof_watergiven_toever

rysprinklingsectorwascontrolled by eight plas- tic flasks eguipped with funnels (Elonen etal.

1967). The dates of irrigation and quantities of water appliedarepresented in Appendixes 1-2.

The water applications _were rather uniform in- side the main plots withavariation less than20%

(1989), 15%(1990) or25% (1991).

The field experimentswereestablishedas asplit- plot design with four replicates (Appendixes 1—

3). The effects of irrigation were studied in the main plots whichweredivided into five subplots accordingtodifferent soil mechanicaltreatments before sowing asfollows:

A. Sprinkler irrigation A 0 ⁼0 mm

A 1⁼2 x 30 mm

B. Mechanicaltreatment Loosening

B ⁼ narrow^ridges

B 2

^{=rotary harrowing}

B 3^{= notraffic}(untreated) Compaction

B 4

^{= one}pass of thetractorwheel

B 5

⁼ three passes of the_tractorwheel

2.2.1 Irrigation

Irrigationtreatments were studied insandy and

clay soils and consisted ofnoirrigation(A₀⁾and sprinkler irrigation(A(). Irrigation wasnot performed on mull. One irrigation of around 30 mm was given when the water content at a depth of 15cmhad been depleted^toaround 50%

of the plant-available watercapacity, _{as meas-} ured by the gypsum block method^(see2.5.1).

Irrigation was performed ^atnight by rotary sprinklers (radius 14 m and angle of irrigated

2.1.2 Soil mechanical treatments

Spring tillage treatments in the sub-plots were setup on autumn ploughed land, by loosening (B[-B 2) orcompacting (B₄-B₅⁾ the experimen- tal field_areabefore fertilization and sowing. All tractor operations were performed by thecon- trolled trafficconcept.The distance of both the rearand front wheelswasadjustedto2 m which

wasthe width of the sub plots. Thetractorwheel was neverallowed ^tocompactthe experimental areain treatmentsBj-B3^.

Soilloosening

After harrowingtothe depth of 5 cm, fournar- row ridges were prepared for treatment

B,

^by

means of coulters used for_potatoplanting. The ridge tops were slightly compacted by rolling (10kPa) to thestatewhich could carry the sow- ing units. The final height of the loose ridges wasaround 10cmand the ridge distance 45^cm, which_wasthe_carrotrowdistance in all subplots.

In 1991, the ridges were 1-2cm higher than in 1989-1990 because the design and the compac- tion of ridges wererepeated. In theB,^{plots, the} soil _was loosened by arotary harrow_toadepth of20cm(clay 15cm). The topsurface was af- terwards slightly compacted by aroller.

Soilcompaction

Fortreatments 84-B₄-B₅^,wheel trafficwasimposed on ploughed land by a ^tractorwith a harrow which _waskept up. In treatment 84,B₄^, the subplot Pietola, L.:

Effect of

^soilcompactnessonthe growth and quality

of

^carrot

wasestablished by consecutive passes across^the plot with the_rear wheels compacting the entire subplot. Because the track distancewas kept at 2 m, and the width ofone rear wheel_was33cm, six passes compacteda4-m wide lane. A buffer zone of2 m was leftat the outer sides of the main plots (Appendixes 1-3). In treatment 85,B₅,

the subplots wererecompacted twice(18drives).

Therearaxle load of thetractorwith the harrow was 3030 kg. Because the diameter of therear wheelwas 145cm, the contact area of therear wheelwas 1290cm²according to the equation of Inns and Kilgour (1978, ref. Soane ^et al.

1980):

S ^{= 0.87b,}x 0.31d (2)

where S is thecontact ^area,b the section width_C and d_cthe diameter ofawheel. This equation is designed for a hard surface. Because the soil surface of clay and mull sank during the first pass, thisequation could^notbe appliedtothese soil typesin treatment 84,B₄^,but it was applicable toall soils after the first pass. According toequa- tion (2),the ground pressure of therear wheel was 120kPa, that of the front wheel60 kPa. The pressures used in thepresent studywere low as compared to those applied, e.g. in the United States where the ground pressures of agricultur- al vehiclescanbe_ashigh_as490 kPa (Gupta and Larsson 1985).

2.3 Establishment and manage-

ment of field experiments

Prior ^to establishing the experiments in early spring when soil was moist and susceptible to compaction, the entire studyarea was ploughed to adepth of 25-26 cm each^autumn. The first operation was 1-3 passes witha tractor wheel, whichwas carried out under thesame moisture conditions each experimental year. Soil compac- tion and sowing dates arepresented in Appen- dixes 1-3.

2.3.1 Fertilization and spring tillage

After soil compaction, fertilizersweredrilledat therates of N 80, P 50, K 160 kg per hectare ^to fine sand and clay, and N 30. P 70, K 170 kg per hectare tomull,in accordance with general rec- ommendations (Soil Testing Laborotory of Fin-

land1990). Infine sand and clayfields,thecom- pound fertilizerwasapplied withafertilizer drill tothe soil surface. Inmull,nitrogenwas broad- casted. The fertilizers were harrowed in treat- mentsB(and 83-B₃-B_stothe depth of 5 cm, being

1-2cm shallower in the mostcompacted treat- ments.After harrowing, the looseningtreatments

B,

^and

B 2 were

^performed (see 2.2.2).

2.3.2 Sowing and management during growth

For one subplot, seedswere sown in four row beds 10 m long with 45 cmbetweenrows.The carrot (Daucus carotaL.) cultivar grown in these experimentswasNantes Duke Notabene370 Sv which has been used also in othercarrotinvesti- gations in the Nordic countries concerning land preparation (Taksdal 1984) and fertilization (Evers 1988).Seeds(in 1989-1990coated)were sown 1cm deep and 6 cm wide with a Nibex sowing machine with four units. The seeding depth ranged 0.5-1.5 cm, with a greaterdepth on loose organic soil and smaller depth on com- pacted clay soil. In 1991,a pneumatic Gaspor machinewasused^toavoid gaps in the loose sub- plots. Later, thinning wasdoneatthe sametime

for all plots ofonefield to 40 plants per I m.

Flowever, in 1989, emergenceon clay was une- venand slow due tothe lack of rains following planting although the whole field_was irrigated (5 mm). Plant populations were thinned to 20 plants permetre.

Weeds were sprayed with linuron (only in 1989)and methoxuron(1990-1991). Inaddition, setoxidimwasused for protection against couch grass(1989-1990). Couch grasswas aproblem especially in the clay soil field. Also hand weed- Voi4: 139-237.

Table3.Weather conditionsinJokioinenin 1989-1991and 30-year averages.

Month Meanairtemperature(°C) Precipitation^(mm)

1989 1990 1991 (1961-91) 1989 1990 1991 (1961-91)

April 5.3 5.6 3.4 ⁽ 2.4) 40 35 14 ⁽³¹⁾

May 10.4 9.3 7.2 ⁽ 9.4) 41 22 29 (35)

June 15.4 14.4 12.1 ^(14.3) 30 20 69 (47)

July 16.3 15.2 16.6 (15.8) 85 85 55 (80)

August 13.7 15.0 16.2 (14.2) 92 90 92 (83)

September 11.0 8.0 9.1 ( 9.4) 51 62 80 (65)

October 4.7 4.9 5.4 ⁽ 4.7) 49 48 49 (58)

Mean: Sum:

11,1 10.3 10.0 (10,0) 388 362 388 (399)

ing was carried out during the growing season as needed. Dimethoate was applied 3-4 times againstcarrotpsyHit, Iriosa apicalis, atthe be- ginning of the growingseason.Carrot fly, Psila

rosae,wascontrolled by ^traps.

2.3.3 Sampling

Both ends of each subplotwereused for soil and plant sampling (Appendix 4). The final carrot yield was measured froman area in the middle ofeach sub-plot (length 6^m) wherenosampling had been carriedout.Because the samplingareas used for the cylinders in 1989-1990 were no moreuseful due^tomixedsoil, anexceptionwas made in 1991.The plastic cylinders forroot sam- pling (see 2.7) _were fixed inside the yield area of subplots fortreatmentsB, ^{and 85.B}5^. From four

carrotrows,only thetwo inner_{rows were} used for plant sampling and harvest.

2.4 Weather conditions

After the dry earlyseason in 1989, the growing season wasfavourable for growth. Precipitation

wasslightly higher than normal in July and Au- gust, and Septemberwas verywarm (Table 3).

In 1990, like in 1989, the soils weretoodry for good emergence. Dry June was followed by a moderately rainy season,and Augustwas warm.

Septemberwas, however, cooler than in 1989, witha difference of 3°C in mean temperature.

In 1991, the soils were moist enough during emergence which was, however, much slower than in 1989-1990because of lowtemperatures in May and June. Only late Julywasdry. During thewarm and rainy^autumn, carrot growthwas very intensive and recoveredsoonafter the cool early_seasonof 1991.

2.5 Soil measurements

2.5.1 Soil water content

Electrical resistance

The moisture condition of the soil during the growingseason wasstudied by the gypsum block method (Bouyoucos ^1954). Immediately after sowing in 1989, blocks were dug to a depth of 15 cm in subplots B_p B, ^and

B 5

^{(Appendix 4).}

Pietola, L:

Effect of

^soilcompactnessonthe growth and quality

of

^carrot