Soil & Tillage Research, 8 (1986) 277--287 277 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

C O N S E R V A T I O N T I L L A G E E F F E C T S O N S O I L P R O P E R T I E S A N D Y I E L D O F C O R N A N D S O Y A B E A N S I N I N D I A N A *

EILEEN J. KLADIVKO, DONALD R. GRIFFITH and JERRY V. MANNERING

Department of Agronomy, Purdue University, West Lafayette, IN 47907 (U.S.A.)

(Accepted for publication 11 April 1986)

ABSTRACT

Kladivko, E.J., Griffith, D.R. and Mannering, J.V., 1986. Conservation tillage effects on soil properties and yield of corn and soya beans in Indiana. Soil Tillage Res., 8: 277--287.

Long-term studies have been conducted on 7 soils throughout the state of Indiana, to determine the effectiveness of a range of tillage systems in producing high crop yields while reducing erosion and improving soil physical properties. The soils studied included one sandy loam, one loam, three silt loams and two silty clay loams with a range of organic matter contents and slope and drainage classes. Tillage systems studied included conventional moldboard plow, chisel plow, disking, ridge till-plant and no-till. Con- servation tillage systems resulted in higher soil water contents, lower soil temperatures, more organic matter and more water-stable aggregates near the surface, and higher bulk densities than conventional tillage systems. On sloping, well-drained, low organic matter soils, conservation tillage produced corn (Zea mays L.) yields that were equal or better than yields from conventional tillage. On poorly<lrained, low organic matter , poorly structured soils, the soil structure under conservation tillage tended to improve with time as soil organic matter and aggregation increased. Corn and soya bean (Glycine max) yields have also improved with time and often exceed those from conventionally tilled soils. On poorly drained soils high in organic matter that were cropped to continuous corn, conservation tillage generally yielded less than conventional systems due to low soil temperatures and excess wetness in spring. Significant interactions of tillage system and crop rotation existed on the two poorly-drained soils on which rotation was an experimental variable.

INTRODUCTION

C o n s e r v a t i o n t i l l age s y s t e m s w e r e d e v e l o p e d i n i t i a l l y as a m e t h o d f o r c o n t r o l l i n g soi l e r o s i o n . T h e y e n c o m p a s s a w i d e v a r i e t y o f s p e c i f i c s y s t e m s r a n g i n g f r o m n o - t i l l , w h i c h l e aves t h e so i l s u r f a c e a l m o s t c o m p l e t e l y c o v e r e d w i t h r e s i d u e s , t o t w i s t e d ch i s e l s y s t e m s w h i c h m a y l eave o n l y 20% s u r f a c e c o v e r . I n c r e a s e d a c c e p t a n c e o f c o n s e r v a t i o n t i l l a ge t e c h n i q u e s b y f a r m e r s

*Journal Paper no. 10,296 of the Purdue University Agricultural Experiment Station.

0167-1987/86/$03.50 © 1986 Elsevier Science Publishers B.V.

278

has been due in part to the potential for reducing labor and machinery costs compared with conventional moldboard plowing. Advances in chemical herbicide technology have made it possible to control weeds wi thout me- chanical cultivation, and improvements in machinery design permit accurate seed placement even in untilled soft. This eliminates many of the former reasons for full-width tillage, and reduced tillage or no-tillage systems are now technically feasible. The decision of which tillage system to use on a particular field can now be more appropriately answered on the basis of soil properties and crop performance rather than on pest problems or ma- chinery limitations.

Long-term tillage studies were conducted o n 5 soil series throughout the state of Indiana from 1967 to 1973, and two newer studies were begun in 1975 and 1980, respectively. The objectives of the studies were to meas- ure crop growth and yield under a range of different tillage systems, soil types and locations (latitudes) within the state. Additional objectives were to determine the effect of the tillage system on soil physical properties and erosion control. Studies by other researchers at Purdue University concentrated on fertilization practices, machinery adaptations and the control of weeds, insects and diseases. Results of the studies led to the development of a practical guide for selection of appropriate tillage systems for different soils in the state and region (Galloway et al., 1977). Long- term studies on two softs are continuing, to provide more detailed infor- mation on soil physical properties as well as crop growth and yield.

The objectives of this paper are to summarize and discuss data from 3 long-term conservation tillage studies conducted in Indiana. Emphasis will be placed on the relationship between soil physical properties and crop growth and yield.

MATERIALS AND METHODS

From 1967 to 1973, studies were conducted on 5 soil series at Purdue University Regional Farms in northern, eastern and southern Indiana. A detailed description of tillage systems, cultural practices and sampling procedures is provided by Griffith et al. (1973). In this study, the distance between the northern and southern locations was about 290 km (Table I).

Tracy sandy loam (Typic Hapludalf) and Sebewa loam (formerly Runny- mede loam) (Typic Argiaquoll) are located in the northern Indiana outwash plain. Blount silt loam (Aeric Ochraqualf} and Pewamo silty clay loam (Typic ArgiaquoU) are on a till plain in eastern Indiana. The Bedford silt loam soil (Typic Fragiudult) in southern Indiana has a rather constant fragipan layer at a depth of 50--60 cm and is wet in the spring and somewhat droughty in summer.

These 5 soils were all cropped to continuous corn (Zea mays L) during the 7 years of the study. Treatments were replicated 4 times in each experi-

TABLE

I

Characteristics of

7 Ind

iana

soils use

d in

lon

g-te

rm t

illage st

udie

s

Soi

l se

ries

Co

nten

t (% w

/w) i

n Dr

aina

ge

0--2

0 cm d

epth

of

Natu

ral

Sand

Si

lt Cl

ay

Ol~I

T

ile

spac

ing

(m)

Slo

pe

(~)

Sur

face

co

lor

Pare

nt mat

eria

l an

d soil characteristics

Lat

itu

de

(no

rth

)

Tra

cy s

andy

lo

am

59

39

3 1,

4 W

ell

Seb

ewa

loam

48

45

8

3.0

Po

orl

y

Bed

ford

sil

t lo

am

10

84

6 1.

7 M

od

erat

ely

wel

l

Blo

un

t si

lt l

oam

21

59

21

1.

7 S

om

ewh

at

po

orl

y

Pew

amo

sil

ty c

lay

loam

14

47

39

3.

3 P

oo

rly

C

halm

ers

silt

y cl

ay l

oam

9

59

32

4.0

Po

orl

y

Cle

rmo

nt

silt

lo

am

17

73

10

1.3

Po

orl

y

20

O--

2

0--

2

1--

3

0--

2

0--

2

0--

2

0--

2

Bro

wn

Bla

ck

Bro

wn

Gra

y

Bla

ck

Bla

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Gra

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Lo

amy

mat

eria

l ov

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stra

tifi

ed,

shal

ey,

acid

san

d an

d gr

avel

L

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ov

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ton

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sid

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m

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ss o

ver

silt

y cl

ay l

oam

til

l

Loe

ss o

ver

lo

am

till

L

oess

ove

r si

lty

sed

imen

ts

41 °

26'

41 °

26'

38 °

53'

40 °

15'

40 ° 1

5'

40 °

28'

39 °

02'

t~

¢~

280

ment in a randomized block design. Row spacing was 76 cm and all plots were 8 rows wide and at least 100 m long.

In 1975, a study was established on a Chalmers silty clay loam (Typic Haplaquoll) in north-central Indiana. Crop rotations studied on this soil were continuous corn, corn/soya bean rotation and continuous soya beans (Glycine max). Treatments were replicated 4 times in a randomized split- plot design, with crop rotation as the main plots and tillage treatment as the split-plot. Tillage plots were 12 rows wide and 46 m long with 76 cm row spacing.

In 1980, a study was established in southeastern Indiana on a Clermont silt loam (Typic Ochraqualf). A fragipan-like horizon below a depth of about 90 cm may restrict drainage in the spring. Crop rotations studied on this soil were continuous corn and a corn/soya bean rotation. Experi- mental design was similar to that on the Chalmers soil, except that plots were either 46 or 61 m long.

Tillage systems studied include: (1) conventional moldboard plow (au- tumn or spring) followed by disking and/or field cultivation in the spring; (2) chisel plow (autumn or spring) followed by discing and/or field culti- vation; (3) disking as both the primary and secondary tillage treatment; (4) ridge till plant, and (5) no-tillage. Moldboard plow has been the "con- ventional" form of tillage for corn and soya beans in this region, and all the other systems studied were to some extent "conservation" tillage due to surface residue cover and/or soil roughness.

The ridge till--plant system consists of planting in pre-formed ridges by scraping the top 2--5 cm of soil from the ridge and reforming the ridge with a disk-hiller or winged-sweep during a cultivation operation when the corn is about 45--60 cm tall. Ridges can be reformed after harvest if soil wetness prevents timely cultivation and ridging in early summer. For soya beans in these studies, ridges were formed after harvest of the beans rather than at cultivation. For no-till planting, the planter is equipped with 3--5- cm-wide non-powered fluted coulters to open a slot for the seed. The same rates of fertilizer, herbicide and insecticide were used in all tillage systems at a given site. Rates were adjusted to try to remove any limitations to crop growth and yields caused by pest problems or nutrient deficiencies.

RESULTS AND DISCUSSION

Residue cover

The location of crop residues on or in the soil is altered by tillage and affects many soil physical properties and processes. The percentage of the soil surface covered by residue increases as tillage intensity decreases (Table II). In the 1967--1973 study, annual corn residue production on these plots ranged from 4.9--8.6 Mg ha -~. The current studies have shown some greater residue cover levels, partially due to higher corn yields and there-

TABLE II

Proportion of soil surface covered by residues (%), immediately after planting

281

Tillage Previous crop system Corn a Corn b Soya beans b

Moldboard plow 1 2 1 Chisel 19 35 8 Dis k 13 37 17 Ridge 8 56 36 No-till 76 97 88

aAverage of 4 soils in 1967--1973 study. bAverage of Chalmers and Clermont soils, 1983.

fore greater residue production (up to 13.6 Mg ha-l). In addition, the ridge till--plant system as it is currently used in these plots, moves less soil from the ridge than previously, thus less residue is covered by soil. There is less residue cover following soya beans than following corn in all the conser- vation tillage systems. Cover values with any given tillage system may vary widely due to amount of residue produced, distribution by the harvester, condition of residue at tillage or planting and depth of tillage.

Soil organic matter

After 7 years of continuous corn on 2 soils of widely different initial organic matter contents, soil organic matter content was greater under conservation tillage than conventional tillage, due to residues located at or near the surface with reduced tillage. The Tracy sandy loam had organic matter contents (0--10 cm depth) of 1.5 and 1.9% (w/w) for plow and no-till systems, respectively (Fernandes, 1976), while the Chalmers silty clay loam (0--7.5 cm depth) had levels of 4.1 and 4.8% (w/w), respectively (Cruz, 1982). Increases in soil organic matter contents with conservation tillage systems have been measured by many researchers (Blevins et al., 1977; Dick, 1983).

Soil physical properties

Aggregate stability Water-stable aggregates are important for minimizing crusting and erosion,

maximizing water and air entry into the soil and allowing good seedling emergence. The water stability of soil aggregates was increased by reduced tillage, with the largest increases noted under no-till systems. In the 1967-- 1973 study, the water-stable aggregation index in the surface 0--5-cm layer was doubled in the no-till system compared with the plowed system, after 5 years of continuous corn (Mannering et al., 1975). Periodic plowing of

282

no-til l p lo t s d e s t r o y e d the pos i t ive inf luence o f no-ti l l on surface aggregat ion, as was d o c u m e n t e d on a p lo t which had been no-t i l led fo r 3 years , p lowed the fou r th yea r and no-ti l led the f i f th year . Aggregat ion (0--5 cm dep th ) in the p lo t p low ed on ly once in 5 years was equal to t ha t o f the p lo t p lowed all 5 years (Manner ing et al., 1975) .

Cur ren t s tudies have shown tha t s ignificant i m p r o v e m e n t in aggregat ion of the poo r ly - s t r uc t u r ed C l e r m o n t soil occur red by the th i rd yea r of con- t inuous no-till , b o t h wi th c o n t i n u o u s corn and the c o r n / s o y a bean r o t a t i o n (Table I I I ) . Chisel and disk sys tems p r o d u c e d in t e rmed ia t e levels o f aggre- gat ion. The high organic m a t t e r Chalmers soil also showed increases in aggre- gate s tabi l i ty wi th r educed tillage, as well as the o f t en observed p h e n o m e - non of less aggregat ion with increasing n u m b e r s of years of soya beans in the r o t a t i o n {Table IV).

S o i l w a t e r c o n t e n t

Conserva t ion tillage sys tems general ly had higher soil wa t e r con t en t s dur ing a large pa r t o f the growing season, due to increased inf i l t ra t ion and

TABLE III

Mean weight diameter (mm) of water,table aggregates a from the 0--7.5-cm depth be- tween rows (no wheel traffic) on Clermont silt loam, July 19825

Tillage Crop system 1981 Corn Soya beans Corn

1982 Corn Corn Soya beans

Spring plow 1.5 0.9 0.9 Autumn chisel 1.8 1.8 1.4 Spring disk 2.1 2.0 1.9 No-tillage 2.3 2.7 2.3

aAccording to Kemper and Chepil (1965). bSource: Kladivko et al., 1983.

TABLE IV

Mean weight diameter (mm) of water,stable aggregates a from the 0--7.5-cm depth be- tween rows on Chalmers silty clay loam, before spring tillage, April 1983

Tillage Crop system 1982 Corn Soya beans Corn Soya beans

1983 Corn Corn Soya beans Soya beans

Autumn plow 0.9 0.9 0.9 0.6 Autumn chisel 1.7 1.2 1.1 0.8 Ridge 2.2 1.4 1.5 0.8 No-tillage 2.7 1.7 1.7 1.5

aAccording to Kemper and Chepil (1965).

283

decreased evapora t ion . Water con ten t s were increased more by reduced tillage on the sandy and sloping soils (Tracy and Bedford , respect ively) than on the Sebewa and Chalmers soils (Mannering et al., 1975; Cruz, 1982) . Higher wate r con ten t s can be a benef i t during mid-summer , bu t can be a disadvantage in the spring on na tura l ly poor ly dra ined soils tha t are slow to dry and to warm, causing de layed planting and reduced early c rop growth.

Soil temperature Maximum and daffy average soil t empera tu re s were lower under conser-

vat ion tillage than unde r conven t iona l tillage, especially during the first 4--6 weeks af ter plant ing (Gri f f i th et al., 1977) . Lower soil t empera tu re s were caused by higher residue cover amo u n t s and soil water con ten ts . During the first 4 weeks af ter plant ing in 1983, the poor ly -dra ined Chalmers silty clay loam had up to 8°C lower m a x i m u m daily t empera tu re s in the surface 4 cm in no-till corn residue than in p lowed plots , wi th an average

TABLE V

Average maximum soil temperature (°C) (0--4 cm) during the first 4 weeks after planting a on Chalmers silty clay loam, 1983

Tillage Previous crop system Corn Soya beans

Plow 22.6 22.4 Chisel 21.4 22.5 Ridge 21.7 22.5 No-tillage 17.7 20.5

aAverage of 10 readings from 13 May to 7 June.

T A B L E VI

Average daily m a x i m u m soil t e m p e r a t u r e (T) in the 0 - - 1 0 - c m d ep th and he igh t (H) of corn 8 weeks a f te r p lant ing a 'b and corn yields a ' c , at 2 l a t i tudes in Ind iana

Tillage s y s t e m N o r t h e r n Ind iana S o u t h e r n Ind iana

• T ra c y sandy l o a m Sebewa l o a m Bedford silt l o a m T H Yield T H Yield T H Yield (°C) ( cm) (Mg ha-1) (°C) ( cm) (Mg ha -l) (°C) ( cm) (Mg ha - l )

Spr ing p low, disk 22.4 112 7.6 21.7 109 8.4 26.1 208 5.8 A u t u m n chisel, field 20.1 99 7.8 19 .6 107 $.1 24.2 193 6.4 cul t ivate Ridge 21.1 107 8.6 d 20.8 102 8.3 25.1 206 6.7 d No-tillage 18.8 97 7.8 18.2 86 7.2 23.4 221 6.3

a s o u r c e : Gr i f f i th et al., 1977 . bAve rage of 3rd and 4 th years in s ame tillage s y s t e m, con t inuous corn. CAverage f r o m 1 9 6 7 - - 1 9 7 3 , c o n t i n u o u s corn. dCul t iva t ion e f fec t f r o m ridge f o r m i n g m a y have i m p r o v e d yield on these low organic m a t t e r soils. O the r sys t ems were n o t cu l t iva ted .

284

difference in maximum temperatures of 5°C {Table V). Temperature dif- ferences among tillage systems were much less following soya beans than following corn. During the first 8 weeks after planting on 3 soils in 1969-- 1970 {Table VI), maximum daily soil temperatures in the 0--10-cm depth averaged 3--4°C lower under no-till continuous corn than under plowing.

Crop growth and yield

The effect of tillage systems on crop growth and yield varies because crop growth is seldom directly related to a particular soil proper ty but is a complex integration of numerous genetic and environmental factors. The major factors that determine the relative success of reduced tillage for corn and soya bean production in the Cornbelt are discussed below, with examples from the long-term studies in Indiana.

Latitude or length o f growing season Conservation tillage is generally more successful in the southern portions

of the region due to the longer growing season. Early season corn growth rates in below-normal rainfall years were greater under no-till than under plowed plots on the Bedford silt loam in southern Indiana, where the water saved with no-till apparently had a greater effect on growth than the lower soil temperatures {Table VI). In northern Indiana, the lower soil temper- atures under no-till corresponded with reduced corn growth rates compared with plowing, for both the well drained Tracy sandy loam and the poorly drained Sebewa loam. Because of the relatively short growing season in the northern Cornbelt, delays in germination and early growth may sig- nificantly reduce crop yields.

Drainage class and slope Reduced tillage is usually more successful on well-drained or sloping softs

than on poorly drained soils. Higher soil water contents that accompany reduced tillage can be a major benefit on well-drained soils, but the excess water delays planting and crop growth and decreases soil warming on poorly drained soils. The Tracy and Sebewa softs are both in the same location in northern Indiana and had similar soil temperatures, but the well-drained Tracy sandy loam had equal yields with no-till and plowing {Table VI) in spite of slower initial growth with no-till, whereas the poorly drained Sebewa had 14% lower yields with no-till than with plowing. Slower growth and lower yields with no-till on poorly drained softs are not due to temperature alone, as the Tracy and Sebewa data indicate, but the exact causes of the yield depression are not known. Allelopathy, excess wetness and aeration problems may all contribute to the reduced growth under no-till on these soils.

285

Crop rotation

A corn/soya bean rotat ion is better adapted to no-till than is continuous corn. Corn yields on the poorly drained Chalmers soil were only slightly lower for no-till than plow in rotat ion (Table VII), but were much lower for no-till than plow in continuous corn. This significant tillage X rotation interaction has also been found on poorly drained softs in Ohio (Dick and Van Doren, 1985). The data in Table VII also show the increase in corn yields across all tillage systems, when corn is grown in rotation compared with monoculture corn. The tillage × rotation interaction for soya beans is not as pronounced as for corn, with no-till beans consistently yielding less than plowed beans on this soil, regardless of rotation.

TABLE VII

Average corn and soya bean yields (Mg ha -1) in response to tillage system and crop rotation, Chalmers silty clay loam during 1980--1984 a

Tillage Previous crop Corn Soya beans Soya beans Corn system Crop Corn Corn Soya beans Soya beans

Autumn plow 10.7 11.6 3.6 3.8 Autumn chisel 10.3 11.4 3.3 3.6 Ridge 10.4 11.6 3.4 3.6 No-tillage 9.1 11.2 3.2 3.3

aSixth to tenth year of the study.

Surface soil structure and aggregation

Conservation tillage systems have a greater positive effect on crop growth and yield on low-organic matter, poorly-structured soils than on high- organic matter, well-structured soils. Recent studies on the poorly drained Chalmers and Clermont soils have illustrated this concept. During the 10 years of the experiment on the Chalmers, no-till continuous corn has yielded less than the plowed plots in 7 years and yielded more than the plowed plots only once. On the poorly-structured Clermont soil, continuous corn yields were lower with no-till than with plowed systems during the first three years, but higher during the last 2 years of the 5-year experiment (Table VIII). No-till corn yields in the corn/soya bean rotat ion were equal to plowed yields the first three years, but greater than plowed yields the last two years. In contrast with the Chalmers soil (Fig. 1), it appears that the per- formance of the no-till system may be improving with time on the Clermont. In 1983 there was a severe drought during June and July and all yields were greatly reduced. No-till yields may have exceeded plowed yields due to extra

286

TABLE VIII

Corn yield (Mg ha ~) with 5 tillage systems on Clermont silt loam in southern Indiana during 1980--1984

Previous Tillage 1980 1981 1982 1983 1984 Average crop system

Corn Spring plow 7.1 7.7 11.7 3.3 9.1 7.8 Autumn chisel 7.6 8.2 12.1 3.6 9.7 8.3 Spring disk 7.3 7.8 11.3 3.2 9.1 7.8 Ridge -- -- -- 3.3 9.4 -- No-tillage 6.6 6.6 10.0 4.1 9.8 7.4

Soya bean Spring plow 7.3 7.6 12.3 3.1 9.3 7.9 Autumn chisel 7.0 7.4 11.7 4.1 8.9 7.8 Spring disk 7.5 7.5 12.3 4.4 9.4 8.2 Ridge -- -- -- 4.0 9.8 -- No-tillage 7.5 7.3 12.3 4.8 10.3 8.4

soil wa te r storage. In b o t h 1983 and 1984 heavy ra ins to rms occu r r ed shor t ly a f te r p lant ing, which f o r m e d a crus t on exp osed soil. A l though the re was no reduc t ion in s tand on any of the tilled plots c o m p a r e d wi th the no-till p lots , the greater aggregate s tabi l i ty and surface p r o t e c t i o n on the no-till p lo t s m a y have a l lowed for fas ter ge rmina t ion and be t t e r aera t ion . Plant heights at 4 and 8 weeks were greater for no-till t han for any of the o the r tillage systems.

Even t hough the C le rmon t soil is ra ted as general ly no t well a d a p t e d to no-tiU (Gal loway et al., 1977) , the i m p r o v e m e n t in soil surface s t ruc tu re wi th t ime m a y ou tweigh the po ten t i a l de t r imen ta l e f fec t s o f p o o r drainage. Stengel et al. (1984) n o t e d tha t phys ica l p rope r t i e s of some soils cons idered " p r o b l e m a t i c " for d i rec t drilling were i m p r o v e d by d i rec t drilling, thus raising t h e m a class in the sui tabi l i ty rankings for d i rec t drilling. The po- tent ia l i m p r o v e m e n t s in soil s t ruc tu re wi th t ime in no-ti l l f ields needs fu r t he r invest igat ion, especial ly fo r p o o r l y - s t r u c t u r e d soils.

CONCLUSIONS

(1) In general , no-til l p roduces higher yields than p lowed sys t ems on sloping, wel l -drained soils and in the m o r e sou the r ly sec t ions of the Corn- belt .

(2) On high organic m a t t e r , p o o r l y dra ined soils, no-til l is n o t well adap t ed in c o n t i n u o u s corn , b u t in a c o r n / s o y a bean r o t a t i o n it p roduces on ly slightly less t han p l owed sys tems.

(3) On poo r ly - s t ruc tu red , low organic m a t t e r , p o o r l y dra ined soils, no-til l c o n t i n u o u s corn general ly yields less than p lowing dur ing the f irst several years , bu t m a y improve wi th t ime as soil s t ruc tu re improves . With a co rn /

287

soya bean rotation, no-till yields may exceed plow yields after the first several years of soil structural improvement.

REFERENCES

Blevins, R.L., Thomas, G.W. and Cornelius, P.L., 1977. Influence of no-tillage and nitro- gen fertilization on certain soil properties after 5 years of continuous corn. Agron. J., 69: 383--386.

Cruz, J.C., 1982. Effect of crop rotat ion and tillage systems on some soil physical proper- ties, root distribution and crop production. Ph.D. Thesis, Purdue University, W. Lafayette, IN, 220 pp.

Dick, W.A., 1983. Organic carbon, nitrogen and phosphorus concentrations and pH in soil profiles as affected by tillage intensity. Soil Sci. Soc. Am. J., 47: 102--107.

Dick, W.A. and Van Doren, Jr., D.M., 1985. Continuous tillage and rotat ions combi- nations effects on corn, soya bean and oat yields. Agron. J., 77: 459--465.

Fernandes, B., 1976. The effect of tillage systems on soil physical properties. Ph.D. Thesis, Purdue University, W. Lafayette, IN, 123 pp.

Galloway, HAl. , Griffith, D.R. and Mannering, J.V., 1977. Adaptabi l i ty of various tillage-planting systems to Indiana soils. Coop. Ext. Serv. Pub. AY-210, Purdue Uni- versity, W. Lafayette, IN, 15 pp.

Griffith, D.R., Mannering, J.V., Galloway, H.M., Parsons, S.D. and Richey, C.B., 1973. Effect of eight tillage-planting systems on soil temperature, percent stand, plant growth and yield of corn on five Indiana soils. Agron. J., 65 : 321--326.

Griffith, D.R., Mannering, J.V. and Moldenhauer, W.C., 1977. Conservation tillage in the Eastern Corn Belt. J. Soil Water Conserv., 32: 20--28.

Kemper, W.D. and Chepil, W.S., 1965. Size distribution of aggregates. In: C.A. Black (Editor), Methods of Soil Analysis, Part 1. Am. Soc. Agron., Monograph No. 9, pp. 499--510.

Kladivko, E.J., Griffith, D.R. and Mannering, J.V., 1983. Conservation tillage studies on a Clermont silt loam soil. Proc. 1982 Indiana Acad. Sci., 92: 441--445.

Mannering, J.V., Griffith, D.R. and Richey, C.B., 1975. Tillage for moisture conservation. Am. Soc. Agr. Eng. Paper No. 75-2523.

Stengel, P., Douglas, J.T., Guerif, J., Go~ , M.J., Monnier, G. and Cannell, R.Q., 1984. Factors influencing the v~ia t ion of some properties of soils in relation to their suita- bil i ty for direct drilling. Soil Tillage Res., 4: 35--53.