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A study of the effects of soil bulkdensity on root and shoot growth ofdifferent ryegrass linesD. J. Houlbrooke a , E. R. Thom b , R. Chapman a & C. D. A. McLay aa Department of Earth Science , The University of Waikato , PrivateBag 3105, Hamilton, New Zealandb Dairying Research Corporation , Private Bag 3123, Hamilton, NewZealandPublished online: 17 Mar 2010.

To cite this article: D. J. Houlbrooke , E. R. Thom , R. Chapman & C. D. A. McLay (1997) A study ofthe effects of soil bulk density on root and shoot growth of different ryegrass lines, New ZealandJournal of Agricultural Research, 40:4, 429-435, DOI: 10.1080/00288233.1997.9513265

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New Zealand Journal of Agricultural Research, 1997, Vol. 40:429-4150028-8233/97/4004-0429 $7.00/0 © The Royal Society of New Zealand 1997

429

A study of the effects of soil bulk density on root and shoot growthof different ryegrass lines

D. J. HOULBROOKEDepartment of Earth ScienceThe University of WaikatoPrivate Bag 3105Hamilton, New Zealand

E. R. THOM*Dairying Research CorporationPrivate Bag 3123Hamilton, New Zealand

R. CHAPMAN

C. D. A. McLAYDepartment of Earth ScienceThe University of WaikatoPrivate Bag 3105Hamilton, New Zealand

Abstract A glasshouse pot trial was designed toinvestigate the effects of bulk density on the rootand shoot growth of two pipeline perennial ryegrass(Lolium perenne) lines, NZA1 and NZA3, and acommercial cultivar Yatsyn 1, in a soil derivedfrom volcanic tephra in New Zealand. Bulkdensities of 0.9, 1.0, 1.1, and 1.2 Mg/m3 wereachieved in the subsoil growing medium as asubsurface layer in pots that were arranged in acomplete factorial design, and caused penetrationresistances of 2.2, 2.9, 3.6, and 4.4 MPa,respectively. Five harvests of herbage were madeover 24 weeks from planting. At the final harvest,roots in the surface, subsurface, and lower potstrata were separated from the soil. Estimates ofroot length and dry weight were made. Increasingsubsurface soil bulk density reduced root lengthfrom 20.4 to 8.4 m per pot stratum (P < 0.05). Thisdecrease in root development was associated with

*Author for correspondenceA96058Received 29 July 1996; accepted 4 August 1997

decreased (P < 0.05) herbage yield (4.72 to 2.38 g/pot) and tillering, showing that the increasedmechanical impedance encountered by rootsaffected overall plant growth. The three ryegrassesproduced similar total root length, herbage yield,and tiller numbers. However, the root length,herbage yield, and tillering of Yatsyn 1 were notsignificantly reduced as bulk density increased from0.9 to 1.2 Mg/m3, although few significantinteractions were detected. This was in contrast tothe responses of NZA1 and NZA3 and shows thatYatsyn 1 was the least sensitive to the effects ofsoil compaction. Adverse affects on ryegrass rootand shoot growth were shown at lower soil bulkdensities (<1.3 Mg/m3) than are commonlyreported. It is suggested that reduced plant growthin response to compaction may be occurring atlower bulk density in soils derived from volcanicparent material than those usually reported.

Keywords soil bulk density; ryegrass cultivar;Lolium perenne; mechanical impedance; soilcompaction; root length; tillering; herbage yield

INTRODUCTION

Soil compaction is defined as a change in soilvolume leading to increased soil bulk density (Hillel1980; Marshall & Holmes 1988). Soil compactionreduces air volume, and causes re-arrangement ofsoil particles and closer packing of the soil weight(Harris 1971; Hillel 1980). Soil compaction, byincreasing mechanical impedance, createsunfavourable growing conditions for roots assupplies of oxygen, water, and nutrients are reduced(Dexter 1986; Bengough & Mullins 1990; Bennie1991; Cook et al. 1996).

The point at which increasing soil compactionhas substantial effects on plant growth anddevelopment is poorly defined although it appearsto be related to changes in the physical propertiesof the soil (Campbell & Henshall 1991). It hasbeen suggested by many researchers that soil bulk

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430 New Zealand Journal of Agricultural Research, 1997, Vol. 40

densities from 1.3 to 1.7 Mg/m3, or a penetrationresistance range from 3.0 to 5.0 MPa, may limitroot growth and decrease plant yield (Cannell 1977;Vepraskas 1988; Asady & Smucker 1989;Bengough & Mullins 1990; Kuznetsova 1990).Cook et al. (1996) recently suggested that pene-tration resistances as small as 0.5-1.0 MPa mayadversely affect root elongation rates.

Soils developed on parent material of volcanicorigin often have low bulk densities and are notgenerally considered to cause plants to suffermechanical impedance problems. However,Chapman & Allbrook (1987) reported that bulkdensities as low as 1.1-1.2 Mg/m3 in the subsoil ofWaikato soils, derived from volcanogenic alluvium,may affect ryegrass root development and herbageyield. Furthermore, many Waikato farm advisorsare suggesting an association between subsoilcompaction and 'pulling', whereby dairy cowsinadvertently uproot ryegrass clumps duringgrazing.

The objective of this study was to test theeffect of subsoil bulk density on ryegrass rootgrowth, in a soil derived from parent material ofvolcanic origin, under controlled climate andnutrient supply conditions. The shoot and rootgrowth responses of different ryegrasses weremeasured in a range of soil bulk densities in aglasshouse pot trial. In particular, observeddifferences in their ability to tolerate intensivegrazing regimes (Thorn et al. 1996) may reflectdifferent growth responses to changing soil physicalconditions. This experiment was complementaryto a field trial investigating the effects of sub-soiling and soil compaction on pasture response(Houlbrooke 1996).

MATERIALS AND METHODS

Treatments and experimental designThree perennial ryegrasses (Lolium perenne) weregrown in pots containing soil packed to fourdifferent bulk densities. The ryegrasses were thelines NZA 1 and NZA3 and the cultivar Yatsyn 1.Yatsyn 1 is a popular commercial cultivar derivedfrom an ecotype originating on a Mangere dairyfarm in northern New Zealand (Cumberland &Honore 1970). NZA 1 and NZA3 are breeding lineswhose parents were also selected from the sameecotype. They were chosen for this study becauseof field observations of differences in their abilityto tolerate 'pulling' (complete removal of clumps

of plants or tillers from the sward during grazingby cattle). The pots representing various treatmentcombinations were arranged in a complete factorialdesign and were replicated four times.

Pots containing repacked soil as the growingmedium were chosen for this experiment so thatnutrients could be thoroughly mixed with the soilto minimise possible differences in soil fertilityand the natural variability of soil physical propertiesaffecting plant growth (Cook et al. 1996). Subsoilfrom an Okete clay loam (Typic Orthic Granularsoil, Hewitt 1992) derived from volcanic parentmaterial on rolling hills in the Waikato region, wasused. This soil was packed to a maximum bulkdensity of 1.2 Mg/m3, with intermediate and lowbulk density treatments of 1.1, 1.0, and 0.9 Mg/m3,respectively. These bulk densities are considerablylower than those used in many studies involvingsubsoil compaction, but are the maximum to whichthe soils can be readily compacted using standardprocedures, and are also the maximum bulk densitythat would be encountered in the field (Chapman& Allbrook 1987).

Particle size distribution was measured asdescribed by Lewis & McConchie (1993). The soilwas classified as a loamy clay with 12.7% sand,28.7% silt, and 58.6% clay. It was passed througha 2 mm sieve before packing and adjusted to a pHof 6.2 by adding calcium hydroxide. A balancednutrient solution (Roberts & Edmeades 1993) wasuniformly mixed throughout the soil prior topacking into columns.

The pots were constructed of PVC pipe (10 cmdiam., 24 cm lengths, with a gauze mesh bottom toprevent loss of soil from the base) and weresubdivided into three sections: Stratum A (0-4 cm)contained 4 cm of soil packed uniformly to a lowbulk density of 0.7 Mg/m3, providing thegermination zone for seed and ensuring littlephysical resistance to seedling root penetration;Stratum B (4-14 cm) was compacted to thetreatment bulk densities of 0.9, 1.0, 1.1, and1.2 Mg/m3; Stratum C (14-24 cm) was compactedto a uniform bulk density of 1.0 Mg/m3, to minimisehindrance to root growth that penetrated beyondStratum B. These packing treatments were designedto test the ability of plant roots to penetrate therange of bulk densities created in Stratum B. It wasassumed that the depth and bulk density ofStratum C remained the same after the packing ofStratum B.

Pots were placed in a glasshouse and each wasplanted with approximately 12 ryegrass seeds at 2

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Houlbrooke et al.—Effects of soil bulk density on ryegrass 431

cm depth, on 16 December 1994. Watering tookplace on a 2 day cycle, with pots maintained atapproximately 80% field capacity by weighing andallowing for changes in plant growth. Germinationof seeds occurred from 23 to 29 December 1994.Plants were thinned to two per plot on 16 January1995 to represent field ryegrass plant densities ofabout 250/m2. Plant shoots were cut to a 20 mmstubble at 50, 83, 106, 142, and 171 days fromplanting. A complete nutrient solution (Seager1984) with the addition of 0.22 g of K2HPO4, wasapplied to the surface of each pot after each harvest.

Plant deaths occurred in four pots followingthe second harvest. In each case only one of thetwo plants per pot died. Three deaths occurred inpots of the 1.2 Mg/m3 bulk density treatmentgrowing NZA1, and another in a 1.1 Mg/m3

treatment, also growing NZA1. As all deathsinvolved the same ryegrass, they were consideredpart of the treatment variation for the purposes ofthe statistical analysis.

Soil measurementsThe penetration resistance for each soil stratumwas measured from the surface using a hand-heldpenetrometer (Anderson et al. 1980). Measurementswere replicated five times within each pot stratum.The pots were dismantled after the final harvestand these measurements were repeated. Air-filledporosity was calculated using the water contentsthat the pots were watered to bi-daily.

Plant measurementsClippings obtained at each harvest were oven-driedat 85°C for 24 h, before weighing and thecalculation of dry matter (DM) yield per pot. Tillerproduction was measured before each harvest bycounting ryegrass tillers per pot. After the finalharvest, roots were removed from each pot stratumby wet sieving, using a bank of sieves ranging inmesh size from 2 to 1 mm diameter. The length ofwashed roots was measured using a Comair® rootlength scanner (Hawker De Havilland Ltd,Melbourne). Root samples were oven-dried at 85°Cfor 24 h, before weighing to determine their dryweight (RDW). Mean root diameter was determinedfrom RDW and length data (Matthew 1992).

Statistical analysisThe statistical package Genstat 5.3 provided themodel for the analysis of the factorial design with4 replicates, and normally distributed data. This

analysis of variance provided the SED forcalculation of the appropriate LSD (least significantdifference) for the comparison of treatment meansat each harvest. Root weight and length dataobtained from different pot strata at the final harvestwere analysed separately using a similar model.Total root length per pot was not statisticallyanalysed since it was computed from stratummeans.

RESULTS

Penetration resistance and air-filled porosityThe effect of increasing bulk density of the subsoilin Stratum B was to markedly increase penetrationresistance by up to two-fold (Table 1). Largedecreases in air-filled porosity were also observed,although it remained above critical levels (10-15%) usually recorded for plant growth (McLaren& Cameron 1996), even at the highest bulk density.

Plant herbage yieldHerbage yields increased from harvest 1 to 4 for allbulk density treatments. However, herbage yieldsdecreased (P < 0.05) at harvests 2, 3, 4, and 5 assoil bulk density increased from 0.9 to 1.2 Mg/m3.This decrease was reflected in a 50% decline intotal yield between the lowest and highest bulkdensity treatments over the trial (Table 2).

There was no significant difference in totalcumulative herbage yield between ryegrasses,although Yatsyn 1 tended to yield slightly higherthan NZA1 and NZA3. Soil bulk densities of 1.1and 1.2 Mg/m3 reduced (P < 0.05) herbage yieldsfor NZA1, compared with the lower bulk densities.However, total yield of Yatsyn 1 was notsignificantly affected by increasing soil bulk density(Fig. 1), and the interaction between soil bulkdensity and ryegrass line at each harvest was notsignificant.

Table 1 Selected physical properties of soil in pots.

Stratum

AB

C

Bulkdensity

(Mg/m3)

0.70.91.01.11.21.0

Penetrationresistance

(MPa)

0.52.22.93.64.43.1

Air-filledporosity(m3/m3)

0.510.380.310.240.170.31

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432 New Zealand Journal of Agricultural Research, 1997, Vol. 40

NZA1 NZA3

Ryegrass

Yatsyn I

Bulkdensity(Mg/rr,3)

a 0.9

•1 .0

• 1.1

• 1.2

«-•

oa.

per

to

jü

50

40 -J

30

20 -

10

0

I-1

—

1NZA1 NZA3 Yatsyn I

Ryegrass

Bulkdensity

(Mg/m3)

Fig. 1 Effect of soil bulk density on total cumulativeherbage yield (g/pot) of different ryegrass lines. Barrepresents LSD5%

Fig. 2 The effect of soil bulk density on tillerproduction of each ryegrass at harvest 5, 171 days fromplanting. Bar represents LSD50/

Tiller number

Tillering was continuous over the trial and mostrapid between harvests 3 and 4. Tiller counts forplants growing in the highest soil bulk densitywere lower (P < 0.05) than in all other treatmentsfrom harvests 3 to 5 (Table 3). Yatsyn 1 showed noeffect of soil bulk density on tiller numbers atharvest 5 (Fig. 2). At harvest 4, Yatsyn 1 maintainedtiller numbers despite increasing soil bulk density,whereas that of NZA1 declined as soil bulk densityincreased above 1.0 Mg/m3, as did tillering ofNZA3 at soil bulk densities above 1.1 Mg/m3.These trends were reflected in a significantinteraction (P < 0.05) between soil bulk densityand ryegrass line, with tillers/pot for Yatsyn 1being greater than for NZA1 but not NZA3 (Fig.3). This trend continued at harvest 5, but tiller

numbers for Yatsyn 1 exceeded both NZA1 andNZA3 (Fig. 3) and the interaction between soilbulk density and ryegrass line was not significant.

Root length and diameter

Ryegrass root length was strongly correlated withroot dry weight and so only root length data havebeen presented. Root length in Stratum A increasedas the bulk density of Stratum B increased,suggesting that poor root penetration into StratumB had an effect on root growth in the layer above.Root length in Strata B and C decreased (P < 0.05)as bulk density increased from 0.9 to 1.2 Mg/m3

(Table 4). Stratum B showed the greatest decrease(40%) in root length as bulk density increasedfrom 0.9 to 1.0 Mg/m3 after which only a slightdecrease occurred. A visual inspection of the soil

Table 2 The effect of soil bulk density on herbage yield/pot (g) at 5 harvestsand total cumulative herbage yield over the trial. Total yield includes plantstubble weight at harvest 5.

Bulk density(Mg/m3)

0.91.01.11.2

LSD5%

l(50)a

0.020.080.020.010.02

2(83)

0.180.190.110.080.07

Harvest

3(106)

0.480.290.200.110.07

4(142)

1.771.721.370.780.18

5(171)

1.431.431.180.800.11

Totalyield

4.724.573.632.380.95

aDays from planting.

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Houlbrooke et al.—Effects of soil bulk density on ryegrass

100% T

433

50 83 106 142Days from planting

Fig. 3 Tiller production of each line at successiveharvests. Bars represent

1.0 1.1 1.2Bulk density (Mg/m3) of Stratum B

Fig. 4 Distribution of root length in pot sections asaffected by different bulk densities in Stratum B.

at the completion of the experiment showed that atthe higher two bulk densities used, roots hadaccumulated at the interface between A and BStrata.

The bulk density of Stratum B had a largeinfluence on the distribution of root length over thethree pot strata. For example, in the low bulk density

Table 3 The effect of soil bulk density on tillernumbers/pot at each harvest (over all ryegrass lines).

Bulk density(Mg/m3)

0.91.01.11.2LSD5%

aDays from

l(50)a

2.62.42.22.0-

planting.

2(83)

6.24.33.63.01.7

Harvest

3(106)

15.110.38.45.84.2

4(142)

29.132.825.417.76.3

5(171)

36.340.238.224.7

7.4

treatment (0.9 Mg/m3), 30 % of roots were foundin Stratum A, 60% in B and 10% in C. In contrast,at the high bulk density of 1.2 Mg/m3 the proportionof roots found in Stratum A was greater than 60%,with approximately 35% in B and less than 5% inStratum C (Fig. 4). The ratio of root length inStratum B to A gives an indication of rootpenetration into the different soil densities inStratum B. This ratio decreased with increasingbulk density in Stratum B.

Averaged over all treatments, Yatsyn 1 hadapproximately 25% more roots (P < 0.05) than didNZA1 or NZA3 in Stratum A. In Strata B and C,no significant differences between ryegrass linesexisted (Table 5). However, Table 6 shows thatroot length of NZA1 decreased (P < 0.05) by 90%with increasing bulk density, whereas effects onNZA3 and Yatsyn 1 were inconsistent. Yatsyn 1had the least root length in the 1.0 Mg/m3 treatment,

Table 4 Total root length (m) per pot stratum as affected by soil bulk densityin Stratum B. Total root length per pot computed by summing Strata means.

Bulk density(Mg/m3)

0.91.01.11.2LSD5o / o

Stratum A(0-4 cm)

9.612.215.012.84.7

Stratum B(4-14 cm)

20.411.210.38.47.4

Stratum C(14-24 cm)

3.52.21.60.71.3

Total

33.525.626.921.9-

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434 New Zealand Journal of Agricultural Research, 1997, Vol. 40

and with further increases of bulk density, rootlength was similar.

Estimates of mean root diameter in Stratum Bshowed no differences between ryegrasses (range0.51-0.56 mm).

DISCUSSION

The results presented in this paper indicate thatmechanical impedance may adversely affectryegrass growth at bulk densities lower than areusually reported. Many studies have suggestedmechanical impedance may affect plant growth ata range of bulk densities greater than 1.3 Mg/m3,and penetration resistances greater than 2.0 MPa(e.g., Vepraskas 1988; Asady & Smucker 1989;Gregory 1989; Cook et al. 1996). Root elongationrates have been shown to be affected at penetrationresistances lower than used in this study for someplants (Gregory 1989), although corresponding bulkdensities were usually much higher and other plantparameters may not be affected. It is suggested,therefore, that bulk density is not a good indicatorof mechanical impedance in some soils. It is likelythat mechanical impedance was the main limitingfactor in the columns in this study as moisturecontents were maintained regularly throughout theexperiment to prevent water stress occurring, airfilled porosity was similar or greater than in otherstudies investigating mechanical impedance (Grable1971 ; Cannell 1977; Cook et al. 1996), and nutrientswere thoroughly mixed into all columns prior toplanting. Furthermore, the visual observation ofplant roots accumulating at the interface of StrataA and B at the two highest bulk densities, combinedwith the increase in root material in Stratum A,supports the suggestion that the plants had difficultypenetrating the subsoil as mechanical impedanceincreased.

The main effects of subsoil mechanicalimpedance were only observed after harvest 2,indicating that the plants had to penetrate beyond

the favourable soil conditions of Stratum A beforeroot growth was affected. The growth of all threeperennial ryegrasses used was affected byincreasing soil bulk density from 0.9 to only 1.0Mg/m3, which caused penetration resistance toincrease by approximately 30%. The plant responseto changing soil conditions was clearly indicatedby both root and shoot growth. Overall, root lengthwas decreased by up to 50% for the range of soildensities created in the subsoil (Table 4).

In response to the bulk density treatmentsimposed, Yatsyn 1 produced the greatest total rootlength and weight and had the greatest herbageyield and tiller production (Table 5; Fig. 1, 3).Yatsyn 1 also showed the least response toincreasing soil bulk density as expressed by rootlength (Table 6), herbage yield (Fig. 1 ), and tillering(Fig. 2). Thus, it may be the least sensitive of thethree ryegrasses to the limitations imposed by soilsof high bulk density. Although NZA1 and NZA3showed similar reductions in total root growth,NZA1 was the most affected by increasing soilbulk density as it had the least roots growing intoStrata B and C (Table 5), produced considerablyless herbage yield (Fig. 1), and showed reducedtillering in response to increasing soil compactionat a lower bulk density than did NZA3 (Fig. 2).

The different shoot responses of the ryegrassesto increased bulk density appear to be directreflections of the degree of root development asthere was no change in their root/shoot ratios withincreased bulk density, which was similar to thefinding of Cook et al. (1996). Bennie (1991)suggested that differences in the ability to overcomemechanical impedance observed between differentplants may reflect their ability to grow roots inuncompacted soils. Our results support thissuggestion as Yatsyn 1 produced significantly moreroots in the low density surface stratum of the pots(Stratum A) than did NZA1 or NZA3 and alsoslightly more in Stratum B (Table 5). The superiorroot production of Yatsyn 1 compared with NZA1

Table 5 Root length (m) of each ryegrass in each potstratum. Total root length was computed by summingStrata means.

Table 6 The effect of soil bulk density in Stratum Bon ryegrass root length (m). LSD 5%= 12.8 forcomparison of interaction means.

Ryegrass

NZA1NZA3Yatsyn 1LSD5%

Stratum A(0-4 cm)

11.49.8

16.04.1

Stratum B(4-14 cm)

11.212.114.26.4

Stratum C(14-24 cm)

1.82.12.11.1

Total

24.423.932.3

Bulk density(Mg/m3)

0.91.01.11.2

NZA1

20.614.76.92.6

NZA3

18.010.411.68.2

Yatsyn 1

22.18.3

12.313.9

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Houlbrooke et al.—Effects of soil bulk density on ryegrass 435

and NZA3, when subjected to increasing soilimpedance, was reflected in better overall shootgrowth from Yatsyn 1.

ACKNOWLEDGMENTS

We wish to thank Margaret Auger for providing benchspace in the University of Waikato, Biology glasshousefor this trial. We thank Harold Henderson, CatherineCameron (AgResearch, Ruakura), and Rhonda Hooper(Dairying Research Corporation) for statistical adviceand analysis of the data, and the Ellett Trust forcontributing to the funding of this research. New ZealandAgriseeds Ltd provided the seed of the ryegrasses used.

REFERENCES

Anderson, G.; Pidgeon, J. D.; Spencer, H. B.; Parks, R.1980: A new hand-held recording penetrometerfor soil studies. Journal of soil science 31:279-297.

Asady, G. H.; Smucker, J. M. 1989: Compaction androot modifications of soil aeration. Soil ScienceSociety of America journal 53: 251-254.

Bengough, A. G.; Mullins, E. C. 1990: Mechanicalimpedance to root growth: a review ofexperimental techniques and root growthresponses. Journal of soil science 40: 341-358.

Bennie, A. T. P. 1991: Growth and mechanicalimpedance. In: Plant roots-the hidden half. YoarWaisel, Amram Eshel. Pp. 393-413.

Campbell, D. J.; Henshall, J. K. 1991: Bulk density. In:Smith, K. A.; Mullins E. C. ed. Soil analysis-physical methods. New York, Marcel Dekker,Inc. Pp. 329-366.

Cannell, R. Q. 1977: Soil aeration and compaction inrelation to root growth and soil management.Pp. 1-68. In: Coaker, T. H. ed. Applied biologyVol. II. London, Academic Press. Pp. 1-68.

Chapman, R.; Allbrook, R. F. 1987: The effects ofsubsoiling compacted soils under grass-aprogress report. Proceedings of the AgronomySociety of New Zealand 17: 55-58.

Cook, A.; Marriott, C. A.; Seel, W.; Mullins, C. E.1996: Effects of soil mechanical impedance onroot and shoot growth of Lolium perenne L.,Agrostis capillaris and Trifolium repens L.Journal of experimental botany 47: 1075-1084.

Cumberland, G. L.; Honore, E. N. 1970: An evaluationof Mangere ryegrass. New Zealand agriculturalscience, January 1970: 21-24.

Dexter, A. R. 1986: Model experiments on the behaviourof roots at the interface between a tilled seed bedand a compacted subsoil. Plant and soil 95:123-133.

Grable, A. R. 1971: Effects of compaction on contentand transmission of air in soils. In: Barnes K. K.ed. Compaction of agricultural soils. Monograph1. Michigan, American Society of AgriculturalEngineers. Pp. 154-164.

Gregory, P. J. 1993: Growth and functioning of plantroots. In: Wild A. ed. Russell's soil conditionsand plant growth. Longman, Avon. Pp. 113-167.

Harris, W. L. 1971: Methods of measuring soilcompaction. In: Barnes, K. K. ed. Compactionof agricultural soils. Monograph 1. Michigan,American Society of Agricultural Engineers.Pp. 9-14.

Hewitt, A. E. 1992: New Zealand Soil Classification.DSIR Land Resources scientific report no. 19.133p.

Hillel, D. 1980: Fundamentals of soil physics. NewYork, Academic Press.

Houlbrooke, D. J. 1996: Subsoiling and soil compac-tion effects on soil physical properties andpasture response. Unpublished MSc thesis, TheUniversity of Waikato, Hamilton, NewZealand.

Kuznetsova, I. V. 1990: Optimum bulk density.Pochvovedeniye no. 5: 43-54.

Lewis, D. W.; McConchie, D. M. 1993: Analyticalsedimentology. New York, Chapman & Hall.

Marshall, T. J.; Holmes, J. W. 1988: Soil physics.Second edition. Cambridge, CambridgeUniversity Press.

Matthew, C. 1992: A study of seasonal root and tillerdynamics in swards of perennial ryegrass.Unpublished PhD thesis, Massey University,Palmerston North, New Zealand.

McLaren, R. G.; Cameron, K. C. 1996: Soil science.Auckland, Oxford University Press. 304p.

Roberts, A. H. C.; Edmeades, D. C. 1993: Fertiliser useon dairy farms. Dairying Research CorporationLtd & New Zealand Pastoral AgricultureResearch Institute Ltd.

Seager, R. H. 1984: Standard techniques used inphosphate trials carried out in a glasshouse. NewZealand Fertiliser Manufacturers ResearchAssociation research reports. Pp. 272-278.

Thorn, E. R.; van Vught, V. T.; McCabe, R. J. 1996:Growth and persistence of perennial ryegrasscultivars with different tolerances to "pulling"during grazing. Proceedings of the New ZealandGrassland Association 58: 67-72.

Vepraskas, M. J. 1988: Bulk density values diagnosticof restricted root growth in coarse textured soils.Soil Science Society of America journal 52:1117-1121.

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