RELATIVE MEASURES OF SOIL BULK DENSITY TO CHARACTERIZE COMPACTION IN TILLAGE STUDIES ON FINE SANDY LOAMS

RELATIVE MEASURES OF SOIL BULK DENSITY TO CHARACTERIZE COMPACTION IN TILLAGE STUDIES ON FINE SANDY LOAMS M. R. CARTER Research Statin, Agriculture Canada, Charlttetwn, Prince Edward Island, Canadn ClA7M8. Cntibutin n.713, received23 Oct. 1989, accepted 12 Apr. I9%). Cenren, M. R. 1990. Relative measures f sil bulk density t characterize cmpactin in tillage studies n fine sandy lams. Can. J. Sil Sci: 70:.425-433. Field studies cncerned with sil physical prperties require methds t quantify and characterize sil cmpactin, especially fr tillage experiments. The bulk densities f a Charlttetwn and Gwanbrae hne sandy lam were related t a maximum r standard cmpactin state fr each sil t btain a measure f relative cmpactin. The tw sils were bth classified as Orthic Hum-Ferric P^dzls. Relative cmpactin was clsely related t the vlume f macrpres (r':0.85). Over a 3-yr perid, muldbard plughing lsened the sil t give an average relative cmpactin f'17 %. Subsequent sil cnslidatin and settling increased relative cmpactin t 84% ver the grwing seasn. Direct-drilling maintained relative cmpactin at a limited range f 88-917. Relative grain yield f cereals was related (r':0.69) using a plynmial curve t relative cmpactin. A range f 77-84% relative cmpactinwasassciatedwitharelativegrainyield >g5%.arelativecmpactinf34-89% was cnsidered the equilibrium sil density level fr the tw sils under study. This range was related t a macrpre vlume f 13.5-l7 which is adequate fr permeability but pssibly inadequate fr ptimum sil aeratin under a humid sil misture regime. Overall, relative cmpactin prvided a useful index r standard t assess changes in sil bulk density and prved t be a bilgically meaningful sil physical parameter. Key wrds: Sil cmpactin indices, relative cmpactin, Pdzlic sil, fine sandy lam, cereal yield [Mesures relatives de la densit6 apparente de sls pur caractdriser la cmpactin dans des 6tudes sur la pr6paratin de lams sableux fins.l Titre abr6g6: Mesures relatives de la densitd apparente de sls. Les 6tudes en plein champ prtant sur les prpri6t6s physiques du sl exigent des m6thdes pur quantifier et caract6riser la cmpactin du sl, en particulier dans les exp6riences de pr6paratin du sl. Les densit6s apparentes des lams sableux Charlttetwn et Gwanbrae nt 6t6 li6es ir un 6tat de cmpactin maximal u nrmal pur chaque sl afin d'btenir une mesure de la cmpactin relative. Les deux sls nt 6t6 class6s cmme des pdzls hum-ferriques rthiques. La cmpactin relative est 6tritement crr6l6e au vlume des macrpres (R':0,85). Au curs d'une p6ride de tris ans, le labur ir la charrue classique a ameubli le sl pur dnner une cmpactin relative myenne de'17 %. La cnslidatin et le rappuyage subs6quents des sls nt augmentd la cmpactin relative d84% au curs de la saisn de crissance. Le semis direct a maintenu la cmpactin relative dans un 6cart restreint de 98 i 9l%. On a li6 le rendernent grainier relatif de c6r6ales (R'=0,69) )r la cmpactin relative au myen d'une curbe plynmiale. Un 6cart de7'7 it84% de cmpactin relative est assci6 a un taux de rendement grainier > 95%. Un taux de cmpactin de 84 ir 89 % est cnsid6re6 crnme la densit6 apparente du sl h l'6quilibre, et ce, pur les deux types de sls b l'6tude. Cet 6cart est li6 a un vlume des macrpres de 13,5 it IO%, ce qui Can. J. Sil Sci. 70: A5-433 (Aug. f990) 425

426 canadtan JuRNAL F sri- scrence est suffisant pur assurer I'impermdabilit6, mais peut-ctre insuffisant pur assurer une a6ratin ptimale en rdgime de sls humides. Dans I'ensemble, la cmpactin relative ffre un indice u une nnne utile pur dvaluer les changements dans la densit6 apparente des sls et s'avere un paramdtre physique des sls bilgiquement significatif. Mts cl6s: Indices de cmpactin du sl, cmpactin relative, sl pdzlique, lam sableux fin, rendement de c6r6ales Excessive sil cmpactin is a majr frm f sil degradatin and can adversely influence sil chemical, bilgical, and physical prcesses. Gupta and Allmaras (1987) have emphasized the need t characterize and quantiff the effect f cmpactin n sil structure. In this regard, measurements f sil structure such as bulk density and pre size distributin characterize sil cmpactin at the macrscpic scale (Gupta et al. 1989). Klen (1987) and Gupta et al. (1989) als emphasized that bulk density is an imprtant factr influencing sil micrscpic prperties (e. g., large pres, hydraulic cnductivity, penetratin resistance) which are significant in land utilizatin. Fr tillage studies, macrprsity prvides a useful index f sil cmpactin effects n pre gemetry, especially fr medium-textured sils (Duglas 1986; Carter 1988). Sil pre gemetry in the field is subject t tempral variatin. Therefre, sil prsity usually reflects the sil cnditin at time f sampling rather than sme final r permanent cnditin. Furthermre, the ptimum range f sil prsity r related prperties (e.g., bulk density) fr plant prductiviry is unknwn fr many sils (Cassel 1982). In additin t tempral variatin, changes in particle size distributin r rganic carbn dwn the sil prfile can influence the spatial variability f prsity and density measurements. The prblem f data interpretatin in regard t abslute measurements f sil cmpactin indices, fr field studies, can be apprached in several ways. Bulk density and pre vlume can be expressed in relative terms (Sane et al. l98l). The expressin factual bulk density as a percentage f sme standard r maximum cmpactin state (Erikssn et al. 1974; Pidgen and Sane 1977; Carter and Kunelius 1986; Carter 1987) has prven useful in characterizing sil cmpactin and crp respnse n different sil types, especially fr mineral sils (Hdkanssn 1988). A further apprach is based n the cncept f nrmal r equilibrium bulk density (Heinnen 1977). ln field situatins, sil bulk density depends n land use r vehicular traffic and natural prcesses (e.g., drying/wetting, freezing/thawing cycles) invlved with sil cnslidatin r strucfure regeneratin. Under cnditins f minimal vehicular traffic, the resulting bulk density wuld be a functin f the natural prcesses which are strngly dependent n sil envirnment, particle size distributin, and rganic carbn. Determinatin f the equilibrium bulk density may prve useful fr in situ assessment f sil suitability fr minimum tillage, and prvide a reference value in sil structure studies (Heinnen 1986). In this study the bulk density f tw fine sandy lams was assessed in field experiments using the cncept f relative and equilibrium cmpactin. The specific bjectives were (1) t determine the relatinship between relative cmpactin and macrprsity, (2) t assess tempral changes in relative cmpactin and assess values fr equilibrium bulk density, and (3) t determine the relatinship between relative cmpactin and the relative yield f cereal crps. MATERIALS AND METHODS The study was cnducted ver 4 yr at tw sites near Charlttetwn, Prince Edward Island. The sils at sites I and 2 were a Charlttetwn fine sandy lam and a Gwanbrae light fine sandy lam, respectively. Bth sils are classified as well-drained, Orthic Hum-Ferric Pdzls (MacDugall et al. 1988). These sils have a relatively high silt and fine sand cntent (Table l). At bth sites, tillage experiments cmpsed f muldbard plughing and direct drilling cmparisns prvided a range in sil structure t assess relative measures f sil

CARTER - RELATIVE MEASURES OF SOIL BULK DENSITY! 4 g @ E E N E d q 6 v 3 O q a,i + I >E!b0 tr> tr>\ > t es N EE b =!< dxi E dai N >:! ii 6 55 -ii +t +l n\ na r: ;A +l +l r\.i -: -i^ +t +l O,m q\ NN # m 6- dcj +t n \ -i ci @$ hq +s ON lr -., 9X 9: ='e ;F E= PA,F* Ha 'F! :tr :q F6 A; ++ cmpactin. Muldbard plughing was cnducted t the 16- t 20-cm sil depth and fllwed by tw passes f a spring-tined cultivatr t the l0-cm sil depth. At site 1, plughing was cnducted in the spring. Spring barley (Hrdeum vulgare L.) was established in 1983 and 1985 and spring wheat (Triticum aestivum L. ) in 1984. At site 2, plughing was cnducted in the fall, and winter wheat was established in 1985 and 1986. Seeding at bth sites was cnducted using a triple-disc Tye drill. Tillage experiments and sil prperties fr site I are described mre fully by Carter (1988) and Carter et al. (1988). The design fthe experiments was a randmized cmplete blck with three replicates at site I and fur replicates at^site 2. The size f each plt was 360 and 180 m' at sites I and 2, respectively. Relative Cmpactin Sil cres (80 x 80 mm) were used t determine bulk density and prsity. Cres were btained five t seven times ver the grwing seasn each year, at the 0- [ 8-cm sil depth, at site l. At site 2, cres were cllected frm the same depth at the end f the secnd year. At each sampling time, 5- l0 cres were btained frm each plt frm nntrafficked areas. After determinatin f misture release characteristics (see later), the cres were ven-dried at 105'C and weighed t btain dry bulk density. The standard r maximum cmpactin state used t calculate relative cmpactin was determined using the Prctr test. Bulk sil samples were btained frm the treatment replicates at each site in the last year f the study. After air-drying, the sils were passed thrugh a 19-mm sieve, adjusted t between 5 and l0% misture entent using distilled water, and then cmpacted in a standard (1000 cm3) mld using a2.5-kg rammer falling freely frm a height f 300 mm (Vickers 1983). The sil was addsd t the mld in three layers and 25 blws f the rammer were applied t each layer. The ttal energy density applied t each mld was 597 kj m-' (Burke et al. 1986). The cmpactin test was repeated at increasing sil misture levels t btain a relatinship between bulk density and sil misture. The maximum sil bulk density (dry mass basis) wa$ determined graphically frm the abve relatinship. Relative cmpactin was btained by dividing the actual sil bulk density derived frm the sil cres by the maximum sil bulk density determined frm the Prctr test, and expressing the result n a percentage basis. Previus studies (Pidgen and Sane 1977; Carterand Kunelius 1986; Caner

428 CANADIAN JOURNAL OF SOIL SCIENCE 1987) have generally used the Prctr test t achieve a maximum cmpactin state; sme ther studies, hwever, use a static lad f 200 kpa t achieve a^cmpacted reference state (Erikssn et al. 1974; Hakanssn 1988). Macrprsity Misture release characteristics were determined n each sil cre prir t calculatin f bulk density. Sil cres were saturated at rm temperature in metal trays cntaining distilled water. After saturatin the cres were weighed and placed n a tensin table which cntained a tensin medium f glass beads (30-pm diameter) supprted and cvered by layers f nyln mesh (Tpp and Zebchuk 1979). A tensin f 6 kpa was applied t prvide an estimate f macrpre vlume (equivalent pre diameter >50 prm) (Carter 1988). Equilibrium Bulk Density Dry bulk densities determined frm the sil cres were used t calculate equilibrium bulk density. Changes in bulk density at site I were assessed ver time t determine the rate f recmpactin after muldbard plughing, secndary tillage, and seeding peratins had been cnducted. The directdrilled plts at site I were als mnitred t determine equilibrium bulk density. The maximum dry bulk density achieved after ne grwing seasn. when the sil had been expsed t numerus wetting/drying cycles and cnslidatin prcesses, was cnsidered t apprximate the specific equilibrium bulk density fr that sil type. Yield f Cereals Grain yields were btained using 0.25-m2 quadrats (5-10 per replicate). In sme years, yields were als btained using a small cmbine harvester. The samples were air-dried, threshed, and weighed. Overall, the small area sampled with the quadrats was cnsidered adequate t determine yield differences since the plant stands were relatively even. Grain yields were cmpared n a relative yield basis in each specific experiment by expressing each treatment as a percentage f the treatment with the highest yield. Relative grain yield was cmpared t relative cmpactin based n bulk densities derived frm sil cres btained between seeding and heading. RESULTS AND DISCUSSION Relative Cmpactin and Macrprsity A clse relatinship was evident between relative cmpactin and macrprsity in the Ap hrizn f the Charlttetwn and Gwanbrae sils (Fig. l). A macrprsity range f 12-14% (vl/vl), identified as adequate t maintain an ptimum aerbic envirnment under a humid sil misture regime in sandy lams (Carter 1988), crrespnded t a relative cmpactin range f 83-86%. Erikssn et al. (1974) indicated an ptimum range f 84-89% relative cmpactin fr yield f spring-seeded cereals ver a 4-yr perid in dry and wet years fr sils with less than20% clay cntent. Generally, macrpres are very sensitive t stress and are susceptible t cllapse during the first stages f sil cmpactin (Gupta et al. 1989). A reductin in macrpre vlume will cause a cncmitant change in ther sil micrscpic physical prperties such as a reductin in hydraulic cnductivity (Duglas 1986; Carter 1988) and air permeability (Grenevelt et al. 1984; Duglas 1986; Ball et al. 198& 1989), and increases in penetratin resistance r sil strength (Carter 1990). Bulk density (a macrscpic sil physical measurement) is als related t macrpre vlume (Carter 1988). On this basis, relative cmpactin may serve as a useful and easily measurable parameter t indicate changes in the abve micrscpic sil physical prperties. Tempral Changes in Relative Cmpactin and Equilibrium Bulk Density Changes in relative cmpactin ver the grwing seasn n the Charlttetwn sil fr 3 yr are given in Fig.2. As expected, muldbard-plughing fllwed by secndary tillage decreased relative cmpactin, althugh the actual decrease varied each year. Over the grwing seasn, the density f the muldbard-plughed sil increased in a linear fashin. Fr the 3-yr perid, the mean and standard deviatin f the crrelatin ceffi cient, Y- intercept and regres s in ceffi - cient were 0.9'7 5 +O.Ol7, 7 6.6 + 5.6, and 5. 2 * 3. I (10-2), respectively. Using this relatinship, the relative cmpactin f a Charlttetwn sandy lam wuld be 77 7 after muldbard plughing and gradually increase t 84% ver a 150-d grwing seasn. A relative cmpactin f 84% wuld represent a relative

CARTER RELATIVE MEASURES OF SOIL BULK DENSITY 429-25 MACROPRES (%) = 70.9-0.684 (REL. COMPACTION) OO 12 = 0.849 UJ =f 20 6U' s ts at, E t() = 15 10 70 CHARLOTTETOWN A GOWANBRAE A 80 RELATIVE COMPACTION (%I Fig. l. Relatinship between relative cmpactin and macrprsity in the Charlftetwn and Gwanbrae Ap hrizns. macrpre vlume f 13.5% (Fig. 1). Since the sil cres were btained frm nntrafficked areas, the increase in relative density in the plughed treatment wuld be related t natural changes in pre gemetry and sil matrix ver time (Gupta et al. 1989) due t cnslidatin, slaking, settling, and wetting and drying (Heinnen 1977, 1986). In cntrast t the plughed treatment, the relative cmpactin under direct drilling had a limited range (88-91%) ver the grwing seasn (Fig. 2). The verall average and standard deviatin were 89.3 and 2.6, respectively. Evidently, a relative cmpactin f 89% represents the nrmal r equilibrium density level fr the Charlttetwn sil when tillage is mitted. This density level crrespnds t a macrpre vlume f l0% (Fig. l), which is adequate fr functinal prsity r permeability (Carter 1988) but inadequate fr ptimum sil aeratin under 90 100 humid sil cnditins (Carter et al. 1988; Carter and Jhnstn 1989). In general, these results agree with previus sil suitability studies which indicated that Pdzlic sils in Prince Edward Island had lw resistance t cmpactive frces and wuld require sme frm f sil lsening t maintain ptimum sil structure (Carter 1987). On sandy and sandy clay lams in Sctland an equilibrium relative cmpactin f 9O% was btained under direct drilling (Pidgen and Sane 1977). This level f equilibrium bulk density was assciated with the specific management and machinery usage emplyed in the study. In general, a relative cmpactin f 9O% redtced yields f spring barley unless relatively high rates fn (i.e., 150 kg ha-') were applied. Subsequent studies at the same site with winter barley, hwever, have shwn that cntinued, lng-term direct drilling, due t significant increases in rganic

430 CANADIAN JOURNAL OF SOIL SCIENCE Ee z C) - s = c) ul xtl Itt E70 0 50 100 150 200 TIME (DAYS AFTER PLOUGHING} Fig. 2. Change in relative cmpactin ver time in a muldbard-plughed and directdrilled Charlttetwn Ap hrizn. carbn and develpment f a stable structure at the sil surface, reduced the equilibrium sil density level and increased crp prductivity in cmparisn t shrt-term direct drilling (Ball et al. 1989). The cncept f nrmal r equilibrium bulk density as a reference value fr sil cmpactin was develped mainly fr lwer sil depths r subsils (Heinnen 1977). In this regard, the nrmal bulk density was a functin f a sil's silt and clay cntent, since rganic carbn levels were usually negligible. Adaptatin f the cncept t Ap hrizns, hwever, must take int accunt rganic carbn especially where tillage systems cause a redistributin f crp residues resulting in accumulatins f rganic matter at the sil surface. Such accumulatins wuld reinfrce the resistance t sil cmpactin (Ball et al. 1989) especially fr sils wittr high silt r hne sand cntents (Carter 1987). O DIRECT DRILLED Cmparisn f Relative Cmpactin t Relative Yield f Cereals A significant (P:0.01) relatinship was bserved between relative cmpactin and the relative prductivity f cereals fr the Charlttetwn and Gwanbrae sils (Fig. 3). A range f 77.5-84% relative cmpactin was assciated with a relative grain yield >95%. The higher end f this range crrespnds t a macrpre vlume f 13.5% (Fig. 1) and t the equilibrium relative cmpactin r equilibrium bulk density (Fig. 2). This agrees with previus studies which shwed that fr the humid sil-misture regime f Prince Edward Island, the vlume fmacrpres shuld exceed l2-147 t prvide an ptimum level f air-filled pre space (Carter 1988). Marked decreases in relative grain yield as relative cmpactin exceeded 84% emphasize that the equilibrium density under direct drilling (i.e.,89%, Fig. 2) is t

CARTER RELATIVE MEASURES OF SOIL BULK DENSITY - 431 Ref.viefd = 47.41 (Rel.cmpactin) - O.2926 (Rel.cmpactin12-1822 R2 = 0.686 J uj t J'l Ee <L J IJJ c 701-----_r 70 75 I 80 I 85 RELATIVE COMPACTION Fig. 3. Relatinship between relative cmpactin and relative yield f cereals high fr ptimum crp prductivity, unless relatively dry sil cnditins prevail. Furthermre, the gradual increase in relative cmpactin ver the grwing seasn in the plughed treatment underlines the need fr regular sil lsening in the Charlttetwn and Gwanbrae sils. In general, the relatinship between sil cmpactin and plant grwth r yield has been established fr nly a few sils (Cassel 1982). This is because a bulk density which limits plant grwth is relative nly t a specific sil type. Furthermre, bulk density is a macrscpic measurement (Gupta et al. 1989) which affects micrscpic sil physical prperties such as sil strength (Carter 1990), hydraulic cnductivity (Grenevelt et al. 1984; Duglas 1986; Carter 1988; Ball etal. 1988), andpre size distributin and water retentin (Duglas 1986; Carter 1988). These micrscpic r behaviral prperties, either individually r 90 cllectively, are the main influences (direct r indirect) n plant grwth rather than bulk density per se. Fr example, at lw bulk density a large vlume f macrpres may adversely influence the sil water strage capacity, while at high bulk density, limiting values fr macrpre vlume, sil strength, and air and water permeability culd reduce rt grwth and plant prductivity. In additin t the relatinship between bulk density and subsequent changes in micrscpic sil structure, the influence f sil envirnment must als be cnsidered (Vrhees 1987). A certain bulk density may be ptimum under a semi-arid misture regime, but deleterius under a hur,nid misture regime due t changes in relative saturatin and subsequent pr sil aeratin (Carter 1988; Carter and Jhnstn 1989). In view f the abve discussin, the cncept f relative cmpactin prvides a degree 95 100

432 CANADIAN JOURNAL OF SOIL SCIENCE f nrmality t bulk density measurements (Erikssn et al. 1974; Pidgen and Sane 1977; Hikanssn 1988). Relative cmpactin is generally independent f sil particle size distributin fr mineral sils, althugh sil misture regime will influence the piimum relative cmpactin fr cereal grain yield (Hdkanssn 1988). Thus, the bulk density f different mineral sils under similar sil envirnments can be evaluated using relative cmpactin. In this regard, the range fr ptimum relative cmpactin under wet cnditins in Sweden (Erikssn et al. 1974\ crrespnded t the ptimum range fund in this study. CONCLUSIONS Relative and equilibrium cmpactin prved t be useful cncepts t characterize sil cmpactin in field studies. Relative cmpactin was clsely related t macrprsity and relative crp yield. The latter bservatin supprts the view that relative cmpactin is a bilgically ^ meaningful sil physical parameter (Hdkanssn 1988). Reducing r mitting sil tillage n bth the Charlttetwn and Gwanbrae fine sandv lam caused the relative cmpactin t exce& 89%. ln cmparisn, relative cmpactin in a muldbard-plughed sil gradually increased frm77 t 84% ver a 150-d grwing seasn. A relative cmpactin range f 84-89% was cnsidered t represent the equilibrium sil density level fr these sils. The relative cmpactin range f 84-89% crrespnded t a macrpre vlume f 13.5-10.0%, indicating the need fr regular sil lsening t maintain ptimum sil aeratin in these sils under a humid misture regime. ACKNOWLEDGMENTS I wish t acknwledge the technical assistance f Mr. D. C. Mutch f the Charlttetwn Research Statin. Ball, B. C., Lang, R. W., O'Sullivan, M. F. and Hunter, R. 1988. Gas diffusin. fluid flw and derived pre cntinuity indices in relatin t vehicle traffic and tillage. J. Sil Sci. 39t 327-339. Ball, B. C., Lang, R. W., O'Sullivan, M. F. and Franklin, M. F. 1989. Cultivatin and nitrgen requirements fr cntinuus winter barley n a Gleysl and a Cambisl. Sil Tillage Res. 13: 333-352. Burke, W., Gabriels, D. and Buma, J. (eds.) 1986. Sil structure assessment. A. A. Balkema, Rtterdam, The Netherlands, 92 pp. Carter, M. R. 1987. Physical prperties f sme Prince Edward Island sils in relatin t their tillage requirement and suitability fr direct drilling. Can. J. Sil Sci. 672 473-487. Carter, M. R. f988. Tempral variability f sil macrprsity in a fine sandy lam under muldbard plughing and direct drilling. Sil Tillage Res. 12:37-51. Carter, M. R. f990. Relatinship f strength prperties t bulk density and macrprsity in cultivated lamy sand t lam sils. Sil Tillage Res. 15: 257-268. Carter. M. R. and Kunelius. H. T. 1986. Cmparisn f tillage and direct drilling fr Italian ryegrass n the prperties fa frne sandy lam sil. Can. J. Sil Sci. 66t 197-207. Carter, M. R. and Jhnstn, H. W. 1989. Assciatin f sil macrprsity and relative saturatin with rt rt severity f spring cereals. Plant Sil 120: 149-152. Carter, M. R., Jhnstn, H. W. and Kimpinski, J. 198& Direct drilling and sil lsening fr spring cereals n a fine sandy lam in Atlantic Canada. Sil Tillage Res. 12: 365-384. Cassel, D. K. 1982. Tillage effects n sil bulk density and mechanical impedance. Pages 45-61 1n P. W. Unger and D. M. Van Dran, Jr., eds. Predicting tillage effects n sil physical prperties and prcesses. ASA Publ. N. 44. Am. Sc. Agrn. and Sil Sci. Am., Madisn, Wis. Duglas, J. T. f986. Macrprsity and permeability f sme sil cres frm England and France. Gederma 37:221-231. Erilrssn, J., Hikanssn, I. and Danfrs,8.ln4. The effect f sil cmpactin n sil structure and crp yields. Bull. 354, Swedish Inst. Agric. Eng., English translatin by J. Kristian Aase, USDA, Agricultural Res. Service, Sidney, Mas. l0l pp. Grenevelt, P. H., Kay, B. D. and Grant, C. D. 1984. Physical assessment f sil with respect t rting ptential. Gederma 34: 101-114. Gupta, S. C. and Allmaras, R. R. 1987. Mdels t assess the susceptibility f sils t excessive cmpactin. Adv. Sil Sci. 6: 65-100. Gupta, S. C., Sharma, P. P. and DeFranchi, S. A. 1989. Cmpactin effects n sil structure. Adv. Agrn. 422 3ll-338.

CARTER RELATIVE MEASURES OF SOIL BULK DENSITY 433 - Hikanssn, I. 1988. A methd frcharacterizing the state f cmpactness f an arable sil. Pages l0l-105 1n J. Drescher, R. Hm, and M. de Bdt, eds. Impact f water and external frces n sil structure. Catena: Supplement II, Catena Verlag, Cremlingen, Federal Republic f Germany. Heinnen, R. 1977. Tward "nrmal" sil bulk density. Sil Sci. Sc. Am. J.4l:. 1214-1215. Heinnen, R. 1986. Alleviatin f sil cmpactin by natural frces and cultural practices. Pages 285-29'l In R. Lal, P. A. Sanchez, and R. W. Cummings, Jr. eds. Land clearing and develpment in the trpics. A. A. Balkema, Rtterdam, The Netherlands. Klen, A. J. 1987. Defrmatin and cmpactin f elemental sil vlumes and effects n mechanical sil prperties. Sil Tillage Res. l0: 5-19. MacDugall, J. I., Veer, C. and Wilsn, F., 198& Sils f Prince Edward Island. Land Resurce Centre, Cnt. N. 83-54. Research Branch, Agriculture Canada, Charlttetwn, P.E.I. 210 pp. Pidgen, J. D. and Sane, B. D. 1977. Effects f tillage and direct drilling n sil prperties during the grwing seasn in a lng-term barley mn-culture system. I. Agric. Sci. (Camb.) 88: 43r-442. Sane, B. D., Blackwell, P. S., Dicksn' J. W. and Painter, D. J. 1981. Cmpactin by agricultural vehicles: A review. 1 Sil and wheel characteristics. Sil Tillage Res. 1: 20'l-237. Tpp, G. C. and Zebchuk, M. 1979. The determinatin f sil-water desrptin curves fr sil cres. Can. J. Sil Sci. 592 19-26. Vickers, B. 1983. Labratry wrk in sil mechanics, 2nd ed. Granada, I-ndn, U.K. 170 pp. Vrhees. W. B. 1987. Assessment f sil susceptibility t cmpactin using sil and climatic data bases. Sil Tillage Res. l0: 29-38.