Retrospective Theses and Dissertations Iowa State University Capstones, Theses andDissertations

1-1-1999

Cover crops reduce weeds and maintain soil qualityfor sustainable strawberryJillene Rae SummersIowa State University

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Recommended CitationSummers, Jillene Rae, "Cover crops reduce weeds and maintain soil quality for sustainable strawberry" (1999). Retrospective Theses andDissertations. 17482.https://lib.dr.iastate.edu/rtd/17482

Cover crops reduce weeds and maintain soil quality for sustainable strawberry

production

by

Jillene Rae Summers

A thesis submitted to the graduate faculty

in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

Major: Horticulture

Major Professor: Gall R. Nonnecke

Iowa State University

Ames, Iowa

1999

ll

Graduate College

Iowa State University

This is to certify that the Master's thesis of

Jillene Rae Summers

has met the thesis requirements of Iowa State University.

Signatures have been redacted for privacy

iii

TABLE OF CONTENTS

LIST OF TABLES

ABSTRACT

CHAPTER ONE. GENERAL INTRODUCTIONIntroductionThesis OrganizationLiterature ReviewLiterature Cited

CHAPTERll^/O. COVER CROPS REDUCE VVEEDS ANDMAINTAIN SOIL QUALITY FOR SUSTAINABLESTRAWBERRY PRODUCTION

AbstractIntroductionMaterials and MethodsResults and DiscussionLiterature Cited

CHAPTER THREE. GENERAL CONCLUSIONSRecommendations for Future Research

APPENDIX: ADDITIONAL TABLE

ACKNOWLEDGEMENTS

.2>TTC\lCNr`

TrC\ICr)r`CJCO(aTrrTrC\JC`JC\J

COGt)CVCI?

Table 1_

Table 2.

Table 3.

Table 4.

lV

LIST OF TABLES

\^/eed cover percentages of research plots plantedto one of six cover crops, strawberry, or cultivated-baresoil at the Iowa State University Horticulture ResearchStation, Ames, Iowa.

Soil physical properties of soil samples taken fromresearch plots receiving one of six cover crops,strawberry, or cultivated-bare soil at the Iowa StateUniversity Horticulture Research Station, Ames, Iowa.

Soil chemical properties of soil samples taken fromresearch plots receiving one of six cover crops,strawberry, or cultivated-bare soil at the Iowa StateUniversity Horticulture Research StatI-On, Ames, Iowa.

Percentage organic carbon and macroaggregate massfor soil samples from research plots receiving one of sixcover crops, strawberry, or cultivated-bare soil at theIowa State University Horticulture Research Station,Ames, Iowa. Data were grouped and averaged fornative perennial vegetation and annual vegetation for1997 and 1998.

Appendix Table 1. Analysis of variance for the dependentvariables weed cover percentage,macroaggregate mass, percentage organiccarbon, total nitrogen, and pH for the years1997 and 1998.

22

23

24

25

29

V

ABSTRACT

The traditional cover crops Lo/,'t,m pereme L. and Songht,m sudanense

(Piper) Stapf. are used in rotation with strawberry (Fraga#'a xananassa Duch.) in the

Midwest to improve soil quality and suppress weeds. The objective of this study was

to investigate how cover crops affect weed populations and soil physical and

chemical properties when used in rotation with strawberry. The experiment was

established in 1996 at the Iowa State University Horticulture Station, Ames, Iowa.

Treatmer\ts were cover crops of Rudbeckia hirta L., Panicum virgatum L.,

Sorghastrum avenaceum (NIlchx) Nash, Andropogon gerardii V'T+m., Tagetes erecta

L. 'Crackerjack', i. perenne, S. sudanense, i. xar,anassa Duch. lHoneoyeI, and

cultivated-bare soil. Weed cover percentage was determined visually. Soil quality

was determined by measuring macroaggregate mass (aggregate stability), bulk

density, water infiltration, percentage organic carbon (C), total nitrogen (N), and pH.

Plots planted to i, pereme had 100% weed cover in 1997 and 1998, with Sefar,-a

faber,a,I Herrm. being the dominant weed species in those plots. Use of cover crops

reduced weed populations in 1998 by as much as 870/o, depending on treatment.

Plots of F, xananassa had greater macroaggregate mass than the cultivated-bare

so-ll plots in 1997. The fastest water infiltration rates occurred in plots of S.

st,cyar,ense (2.1 um/s), and I. erecfa (2.4 um/s), and in cultivated-bare soil (2.0

um/s). Percent organic C was higher in plots of F. xananassa, i. pereme, and R,

h,-rfa than in S. suc/anense and cultivated-bare soil. Use of native perennial cover

vi

crops in rotation with strawberry has the potential to provide a sustainable alternatl've

to chemical herbicides and maintain soil quality.

1

CHAPTER ONE. GENERAL INTRODUCTION

Introduction

A cover crop is a plant cover that is grown to reduce weed populations,

increase porosity and water infiltration, add organic matter to the soil, and reduce

soil erosion. Cover crops may be incorporated into the soil to add organic matter or

left on the soil surface as living mulch. Cover crops are typically grown in rotation

with a cash crop or during times of the year when the cash crop cannot be grown

(fallow season) (Gliessman,1998).

Traditional cover crops, such as Lo/,'L,m Pereme (Perennial ryegrass) and

So,glum sudanense (sorghum sudangrass) are used in crop rotation by commercial

strawberry growers in the midwestern United States to control weeds and improve

soil quality. There are only three herbicides labeled nationally for strawberry use

during production years. Two of these cannot be used for at least three months after

planting (Pritts and Kelly, 1993), which provides an open field for weed populations

to establish. Currently, methyl bromide is used as a fumigant to kill weed seeds and

soil-borne pathogenic fungi that damage strawberry. Since methyl bromide may

deplete the atmospheric ozone layer, its use in the United States will be eliminated

by the year 2005 (U.S. EPA, personal communication).

As current chemicals become limited, nonchemical, alternative weed-

management strategies need to be identified for strawberry production to be

sustainable. Cover crops provide many benefits including increased water

infiltration, increased aggregate stability, reduced pathogen severity, and reduced

weed populations (Bordelon and Wellerl 1997). Cover crops, especially native

2

prairie grasses, have extensive root systems that improve soil structure, increase

soil stability, reduce runoff of water, and increase soil organic matter (Schultz et al.,

1996; Cohenl 1993).

The objective of this study was to determine which cover crop(s) would be

best suited in rotation in commercial strawberry production, as indicated by reduced

weed populations and improved soil physical and chemical properties.

Thesis organization

This thesis is written to include a General Introduction, a manuscript to be

submitted to HorfTechno/ogy, and General Conclusions. The manuscript is written

following the format outlined by the journal.

Literature review

Cover crops in strawberry production and weed management

ln a study by Smeda and Putnam (1988), cover crops of rye (Seca/e ceres/e

L. l\^theeler'), wheat (Triticum aestivum L. {Yorkstar'), barley (Hordeum vulgare L.

'Barsoy'), and oats (Avena sat,-va L. 'Gary') were sown l'nto a two-year old strawberry

field. The cover crops were killed in the spring with fluazifop-butyl and cover crop

residues remained on the soil surface. Weed biomass was lower in plots with cover

crops than in unseeded control plots. The cover crop treatments had no effect on

strawberry yield.

Pritts and Kelly (1993) seeded cover crops of marigold (Tagefes erecfa L.),

sudangrass (Sorghum sudanense (P-lper) Stapf.), buckwheat (Polygonum fagopyrum

3

L.), and ryegrass (Lo/,'um pereme L.) the spring before planting strawberry.

Strawberry was also planted as a cover crop, to simulate a continuous planting of

strawberry. They found that cover crops reduced weed populations by as much as

80% and did not affect strawberry growth or yield. Sudangrass and marigold

suppressed weed growth. Strawberries were planted into killed sods of the cover

crops. Results showed that ryegrass, buckwheat, and stale seedbed offered the

same protection against weeds as did cultivation plus herbicide. ln a second

experiment, cover crops were interplanted in an established strawberry planting.

Cover crops of sudangrass, tall fescue, and marigold provided better weed control

than diphenamid. However, strawberry yI'eldS On early-Planted Plots Of tall fescue,

marigold and sudangrass were significantly lower than late-planted sudangrass,

untreated plots, and herbicI'de treatments. A second experiment was conducted

using sudangrass as an interplanted cover crop. Sudangrass was interseeded after

strawberry renovation and mowed twice a year. This management system

controlled weeds, especially when used with straw mulch during the winter. This

alternative potentially eliminates the need for herbicides.

Cloutier and Lamarre (1997) used interseeded cover crops including spring

wheat (Triticum aestivum L.)I cr`lmson clover (Trifolium incarnatum L), sudan grass

(So,ghum st,c/anense (Piper) Stapf.), annual alfalfa (MecJ,'cago sat,tva L. tNitro'),

spring oat (Avena sat,tva L. lCapital'), barley (HordeL,m W/gate L. lLe;g3rj), fall rye

(Seca/e ceres/e L. tPrima'). They found that the interseeded cover crops provided

effective control of weeds. Strawberry yields were higher in plots treated with

herbicide and straw mulch than in any of the interseeded cover crop treatments.

4

Newenhouse and Dana (1989) used perennial ryegrass (Lo/,-um perenne L.),

Kentucky bluegrass (Poa prafens,'s L.), and winter wheat (Tw'f,-cwm aesf,'vum L.) as

living mulch. The grasses were planted between strawberry rows. Their results

showed that plots with living mulch had higher yields than did control plots during

years that brought windstorms, likely because strawberry plants were protected by

cover crops. Yield was not affected during years without windstorms. competition

from the mulches reduced strawberry growth, but did not hinder high yields. weed

growth was reduced between rows, enough to omit cultivation.

VVhitworth (1995) used crimson clover (Tn'fo/,'um incamafum L.), wheat

(Tr7'f,'Cum aeSfeVum L.), and rye (Seca/e ceres/e L.) as cover crops. This study

investigated the effects of cover-crop residues and cover-crop incorporation on weed

control and strawberry growth. Surface residues from cover crops caused the death

of, or reduced growth of, weeds and smaller strawberry plants. Plots with residues

incorporated into the soil produced larger strawberry root systems and decreased

weed growth.

Cover crops and soil quality

Physical and chemical properties of soil affect the capacity of a soil to

support plant life and biological activity. Many physical and chemical properties are

directly correlated to soil organic matter content. Physical properties include

aggregate stability, bulk density, and water infiltration. Chemical properties include

organic carbon (C), total nitrogen (N), and pH (Karlen and Stott, 1994; Singer and

Munnsl 1987).

5

Singer and Munns (1987) define bulk density as the dry mass (weight) of soil

Per unit Of bulk volume. Bulk density measurements are necessary for "proper

interpretation of the importance of change in magnitude in other chemical and

biochemical soil components" (Doran and Parkin,1994). Bulk density is an

important soil property relating to plant growth (Karlen and Stott,1994), because

bulk densi-ty, in combination with soil texture and aggregate size and type, influence

pore Size distribution and COnSequently, Water infiltration (Singer and Munns, 1987).

Aggregate stability is the ability of soil to withstand pressure from outside

forces. Soil aggregates are separated into two categories based on size.

Macroaggregates are 2 250 um in diameter, and microaggregates are < 250 Llm in

diameter. Macroaggregates have a higher concentration of organic matter than

microaggregates, and the organic matter associated with the macroaggregates is

more labile than that associated with microaggregates (Cambardella and Elliot,

1993). Cover crops increase aggregate stability (Sainju and Singh,1997; Ram,

1960; Rogers and Giddens, 1957), and aggregate synthesis and stability are highly

influenced by organic matter concentration (Karlen and Stott, 1994).

Water infiltration is the entry of water into the soil. lt is a key variable in soil-

water relationships. Water infiltration rate can determine the amount of water that

gets into the soil. Also determined is the amount of water remaining on the soil

surface, that water responsible for flooding and erosion (Singer and Munns, 1987).

Extended periods of saturated soils can kill strawberry rootlets and provide the

proper environment for fungi that destroy roots (Galletta and Bringhurst, 1990).

Schultz et aI. (1996) report that the extensive root systems of prairie grasses

6

contribute to better soil porosity (versus the root systems of cultivated crops) and to

faster water infiltratI'On rates. Cover crops increase water infiltration by adding

organic matter to the soil (Sainju and Singh,1997; Karlen et al.,1992).

Soil organic matter (SOM) is regarded as "the single most important indicator

of soil quality" (Larson and Pierce,1991). Nelson and Sommers (1982) list the

following effects that SOM has on soil properties: 1 ) the capacity of a soil to supply

N, phosphorus (P), sulfur (S), and trace metals to plants, 2) infiltration and retention

of water, and 3) degree of aggregation and overall structure that affect air and water

relationships. Karlen and Stott (1994) agreed that SOM is important in the formation

of soil aggregates.

The main building block of SOM is C, but N, P, and S are also key

components. Nitrogen, P, and S are essential elements for plant growth (Singer and

Munns,1987). Carbon is the prominent element in SOM, comprising from 48-58%

of the total weight (Nelson and Sommers,1982).

Prairie grass root systems are more extensive and deeper than those of

cultivated crops and festucoid grasses. Their deep roots contribute to better soil

porosity than in soil planted to cultivated crops and yield faster water infiltration

rates. Faster infiltration rates reduce erosion, and less erosion improves soil quality.

These extensive root systems also contribute organic matter to the soil (schultz et

al.,1996).

The various components of SOM (living organisms, plant and animal organic

residues) hold as much as 99% of the total N found in the soil (Sikora et al., 1996;

Magdoff, 1992). Nitrogen cycling in the soil is higher in soils planted to cover crops

7

than in soils only fertilized with N, due to more biological activity in cover-cropped

soils (Sainju and Singh,1997; Radke et al.,1988). Extensive root systems of non-

leguminous cover crops are capable of taking up NO3-, effectively reducing leaching

(Kuo et al., 1995). Nitrogen is immobilized if the c:N ratio of cover-crop residue is

too high (Sainju and Singh,1997; Aulakh et al.,1991). Therefore, the C:N ratio

should be < 25 (Wagger, 1989), or the N concentration of cover crop residues

should be between 16.6 -18.9 g.kg-1 (sainJ'u and Singh,1997; lritani and Arnold,

1960). Soils left unplanted lose organic matter faster than soils planted to cover

crops (Elliott,1986); hence, soil organl-c C and N are sustained when land

management includes incorporation of cover crops (Sainju and Singh, 1997).

The measure of hydrogen ion concentration in soil solution, or pH, is a

measure of acid intensity (Singer and Munns,1987). Strawberry plants require a pH

range of 5.5 -7.5 (Galletta and Bringhurst,1990) for optimal growth and

development.

Cover crops can be valuable in rotation with strawberry. Weed populations

are reduced', soil quality is improved through the addition of organic matter; and soil

erosion is reduced due to faster water infiltration rates. The use of cover crops

promotes sustainable strawberry production.

Literature Cited

Aulakh, M.S., J.W. Doran, D.T. Walters, A.R. Mosier, and D.D. Francis.1991.Crop residue type and placement effects on denitrification andmineralization. Soil Sci. Soc. Amer. J. 55:1020-1025.

8

Bordelon B,P. and S.C. Weller. 1997. Preplant cover crops affect weed and vinegrowth in first-year vineya+ds. HortScience 32.I 1040-1043.

Cambardella, C.A. and E.T. EIliot.1993. Carbon and nitrogen distrI-butiOn inaggregates from cultivated and native grassland soI'lS. Soil Sci. Soc. Amer.J. 57:1071-1076.

Cloutier, D.C. and M. Lamarre.1997. Weed control and winter protection ofstrawberries in Quebec using jnterseeded crops. proc. Acta Hort. 439(2):893-897.

Cohen, D. (ed.).1993. Biological communities: Iowa prairies. Iowa Assn. ofNaturalists BuI. no. lAN-203

Doran, J.W. and T.B. Parkin.1994. Defining and assessing soil quality, p. 3-21.ln: Soil Science Society of America. Definl'ng soil quality for a sustainableenvironment. Soil Sci. Soc. Amer. Spec. Publ. no. 35.

Elliott, E.T.1986. Aggregate structure and carbon, nitrogen and phosphorus innative and cultivated soils. Soil Sci. Soc. Amer. J. 50:627-633.

Galletta, G.J. and R.S. Bringhurst.1990. Strawberry management, p. 83-153. ln:G.J. Galletta and D.E. Himelrick (eds.). Small fruit crop management.Prentice Hall, Englewood Cliffs, N.J.

Gliessman, S.R.1998. Agroecology: ecological processes in sustainableagriculture. Ann Arbor Press, Chelsea, Mich.

lritani, W.M. and C.Y. Arnold.1960. Nitrogen release of vegetable crop residuesduring incubation as related to their chemical composition. soil sci. 89:74-82.

Karlen, D.L. and D.E. Stott.1994. A framework for evaluating physical andchemical indicators of soil qualityl p. 53-72. ln: Soil Science Society ofAmerica. Defining soil quality for a sustainable environment. soil sci. soc.Amer. Spec. Publ. no. 35.

Karlen, D.L., N.S.lash, and P.W. Unger.1992. Soil and crop management effectson soil quality indicators. Amer. J. Alternative Agric. 7:48-55.

Kuo, S., E.J. Jellum, and U.M. Sainju.1995. The effect of winter cover cropping onsoil and water quality, p.56-64. Proc. Western Nutrient Mgt. Conf., SaltLake Cl'ty, Utah.

Larson, W.E. and F.J. Pierce.1991. The dynamic of soil quality as a measure ofsustainable management, p. 37-51. ln: Soil Science Society of

9

America. Defl-nl'ng sol'I quality for a sustainable environment. soil Sci. Soc.Amer. Spec. Publ. no. 35:

Magdoff, F.1992. Bul'lding soils for better crops: organic matter management.Univ. of Nebraska Press, Lincoln.

Nelson, D.Wl and L.E. Sommers.1982. Total carbon, organic carbon, and organicmatter. Methods of soil analysis, part 2. Chemical and microbiologl'calproperties--Agronomy monograph no. 9 (2nd ed.). Amer. Soc. Agron., Soil Sci.Soc. Amer., Madison, VVIs. 539-577.

Newenhouse, A.C. and M.N. Dana.1989. Grass living mulch for strawberries. J.Amer. Soc. Hort. Sci.114.'859-62.

Pritts, M.P. and M.J. Kelly.1993. Alternative weed management strategies forstrawberries. Acta Hort. 348:321 -327.

Radke, J.K., R.W. Andrews, R.R. Janke, and S.E. Peters.1988. Low inputcroppI'ng Systems and effl'CienCy Of Water and nitrogen use, p. 193-218. ln:W.L. Hargrove (ed.). Cropping strategies for efficient use of water andnitrogen. Amer. Soc. Agron. Spec. Publ. 51. Amer. Soc. Agron., Crop Sci.Soc. Amer., and Soil Sci. Soc. Amer., MadI'SOn, Wis.

Ram, D.N.I M.T. Vittum, and P.J. Zwerman.1960. An evaluatl-on of certain wintercover crops for the control of splash erosion. Agron. J. 52.'479-482.

Rogers, T.H. and J.E. Giddens.1957. Green manure and cover crops. p. 252-257.ln Soil, the 1957 Yearbook of Agriculture. U.S. Dept. of Agric. U.S. Govt.Printing Office, Washington, D.C.

Sainju, U.M. and B.P. Singh.1997. VVInter cover crops for sustainableagricultural systems: influence on soil properties, water quality, and cropyields. HorfScience 32:21 -28.

Schultz, R.C., A. KuehI, J.P. Colletti, P. Wray, T. lsenhart.1996. Stewards of ourStreams: riParian buffer Systems. Iowa State Univ. Ext. BuI. Pm-1626a.

Sikora, L.J., V. Yakovchenko, C.A. Cambardella, and J.W. Doran.1996.Assessing soil quality by testing organic matter, p. 41-50. ln: Soil ScienceSociety of America. Soil organic matter.I analysis and interpretation. soilSci. Soc. Amer. J. Spec. PubI. no. 46.

Singer, M.J. and D.N. Munns.1987. Soils: an introduction. Macmillan, New York.

10

Smeda, R.J. and A.R. Putnam.1988. Cover crop suppression of weeds andinfluence on strawberry yields. HorfScience 23: 132-134.

Wagger, M.G. 1989. Time of desiccatI-On effects On Plant composition andsubsequent nitrogen release from several winter annual cover crops.Agron. J. 81 :236-241.

Whitworth, J.L.1995. The ability of some cover crops to suppress commonweeds of strawberry fields. J. Sustainable Agr. 7.

iE.

CHAPTER ll^/O. COVER CROPS REDUCE WEED POPULATIONS AND

MAINTAIN SOIL QUALITY FOR SUSTAINABLE STRAWBERRY PRODUCTION

A paper to be submitted to HorfTechno/ogy

Jillene R. Summers, Gall R. Nonnecke, Cynthia A. Cambardella, and

Richard C. Schultz

ALDDITIONALINDEXWOF`DS. Sorghastrum avenaceum, Panicum virgatum, Rudbeckia

hirta, Sorghum sudanense, Fragaria xananassa, macroaggregate mass, water

infiltration, organic matter

ABSTRACT. Lo/,-i,m perenne L. and Sorght,m st,danense (Piper) Stapf. are the

traditional cover crops used in rotation with strawberry (Fragaw'a xananassa Duch.)

in the Midwest to suppress weeds and improve soil quality. The objective of this

study was to investigate how cover crops affect weed populations and soil physical

and chemical properties when used in rotation with strawberry. The experiment was

established in 1996 at the Iowa State University Horticulture Station, Ames, Iowa.

Treatmehis were cover crops of Rudbeckia hiria L., Panicum virgatum L.,

Sorghastrum avenaceum (M'lchx.) Mash, Andropogon gerardii V`itm., Tagetes erecta

L. 'Crackerjackl, i. perenne, S. sudanensel F. xananassa Duch. tHoneoyeJ, and

cultivated-bare soil. Weed cover percentage was determined visually. Soil quality

was determined by measuring macroaggregate mass (aggregate stabilI-ty), bulk

12

density, water infiltration, percentage organic carbon, total nitrogen, and pH. Bulk

density and water infiltratl-on were additional variables determined in 1998. Use of

cover crops reduced weed populations in plots of cultivated-bare soil (4%), S,

sudaner,se (13%), and P, v,+gait,m (150/o). Plots planted to i. pereme had 100O/o

weed cover in 1997 and 1998, with SefarJ-a rabeW',' Herrm. being the dominant weed

species in those plots. Plots of i. xananassa had greater macroaggregate mass

than the cultivated-bare soil plots in 1997, but there were no differences among

treatments in 1998. The fastest water infiltration rates occurred in plots of annual

vegetation of S. st,danense (2.1 um/s) and I. ereofa (2.4 um/s) and cultivated-bare

soil (2.0 um/s). Use of native perennial cover crops in rotation with strawberry has

the potential to provide a sustainable alternative to chemical herbicides and maintain

soil quality.

Introduction

Lolium perenne (perennial ryegrass) and Sorghum sudanense (sorghum

sudangrass), traditional cover crops, are used in crop rotation by commercial

strawberry growers in the midwestern United States to control weeds and improve

soil quality. Few herbicides are labeled nationally for matted-row strawberry

production. Two of these cannot be used for three to four months after planting

(Pritts and Kelly, 1993), providing an open field for weed population establishment.

As current chemical herbicides become limited, nonchemical, alternative

weed management strategies need to be identified for sustainable strawberry

production. Cover crops provide many benefits, including reduced weed

13

POPulatiOnS, increased aggregate stability, increased water infiltration, and reduced

pathogen severity, (Bordelon and Weller,1997). Cover crops, especially native

Prairie grasses, are known for their extensive root Systems that improve SOil

structure and increase soil organic matter (Schultz et al.,1996; Cohen,1993).

The objective of this study was to determine which cover crops would be best

suited in rotation with commercial strawberry production as determined by reduced

weed populations and improved soil physical and chemical properties.

Materials and methods

FIELD METHODS 1996. Research plots were established on 22 May, at the Iowa State

University Horticulture Research Station, Ames, Iowa. Plots were in a continuous

planting of strawberry for the previous ten years. The research area was tilled

before establishment of treatments. The soil was a Clarion loam, a mixed mesic

Typic Hapludoll (DeWtt,1984). Research plots were arranged in a randomized

complete-block design with three replications of nine treatments. Each treatment

plot was 6.1 m by 6.1 m. A 1.8-m buffer separated treatment plots, and a o.3-m

buffer separated replications. Dormant crowns of F. xananassa 'Honeoye' (Indiana

Berry and Plant Co., Huntingbu'rg, lnd.) were planted in a matted row design with

three rows per plot for the i. xananassa treatment plots. The rows were 1.2 m

apart, and the plants were spaced 0.9 m apart within the rows. A 1.8-m strip of bare

soil was left between the outside of the treatment plot and the outer rows of the

strawberry plarfus. Sorghastrum avenaceum, Andropogon gerardii, Rudbeckia hirfa

14

(Nature's Wayl Woodburn, Iowa), pan,'cum v,-ngafum (Ion Exchange, Harper's Ferry,

Iowa), and S. sL,danenSe (Sexauer, Des Moines, Iowa) were sown into their

treatment plots at 1.2 g.m-2. Lo/,-um mu/f,-fi/orum Lam. (Sexauer, Des Moines, Iowa)

was sown at 3.4 g.m-2. Tagefes ereofa 'Crackerjack' (Stokes Seeds, lnc., Fredonia,

N.Y.) was sown at 0.6 g.m-2. All seeds were mixed with = 200 g of moist sand for

even distribution, then broadcasted and raked into the soil by hand. cover crops

were not interplanted with strawberry. PIots were irrigated as needed to obtain a

total of 2.5 cm of water per week from June through September.

FIELD METHODS 1997. The research plot was burned on 14 April with a

kerosene/gasoline mix in a drip torch. On 6 June, seeds of all cover crops were

distributed by rates and methods described previously for 1996. ln 1997, annual

cover crops (i. pe,enne, S. sL,daner,Se, and I. erecfa) were seeded into soil tilled to

a depth of 15 cm on 6 June. We used i. pereme (Seed Research of Oregon, Inc.,

Corvallis, Ore.) rather than i. mt,/i,-#ort,m, under the assumption that the perennial

ryegrass would establish more quickly and easily than the annual rye. perennial

natilve cover crops (S. avenaceum, P. virgatuml A. gerardii, and R. hirfa) were over-

seeded into non-tilled plots. PIots of F. xar,anassa were renovated ll July by

narrowing the matted row to 20.3 cm through tillage. Urea was applied at the rate of

5.6 g m-2. cultivated-bare soil plots were tilled on 6 June, 23 July, and 18 Sept.

Irrigation of the plots was similar to 1996. Weed-cover percentage was determined

visually on 9 Sept. by viewing plots from a 3.1 -m ladder, after cover crop treatments

reached maturity. Weed-cover percentages were estimated by two people, using a

scale of 0-100% weed coverage, and averaged. VVhere the estimates varied by 2

15

10%, the treatment plot was re-evaluated. soil samples were collected from all

replications on 3 Oct. with a 3.8-cm-diameter soil probe. Samples were taken to a

depth of 15 cm. Beginning at 1.5 m from the northwest corner of each treatment

plot, one sample was taken every 1.5 m along a diagonal transect, which ran from

the northwest corner to the southeast corner of each treatment plot, totaling four

samples, that were combined into one sample for analysis for each replication.

FIELD METHODS 1998. The research plot was burned on 24 April in the same manner

as in the previous year. No overseeding of prairie species was done because

greenhouse-grown plugs of S. avenaceum, P. virgatum, A. gerardiil and R. hirfa

were planted in their respective treatment plots; they were planted from 18 June to 2

July. We thought that planting plugs would lead to better establishment of these

crops. On 27 July, seeds of i. pereme, S. sudanense and I. ereofa were mixed

with sand, and they were broadcasted and raked into plots by hand after the soil was

tilled to a depth of 15 cm. Plots were irrigated with water at a rate of 2.5 cm.hr-1 for 1

hr on 27 July and throughout the season as described in 1996. Control plots were

tilled 2 July, 6 Aug., and 3 Sept. Fragaw'a xananassa plots were renovated on 10

July as described in 1997 but were not amended with urea. weed cover percentage

was determined on 25 Sept., as described for the previous year. After three years,

plots planted to A. gerard,-,I had few plants; the treatment was eliminated due to poor

establishment and data analysis did not include this treatment. From 10 Oct.

through 23 Oct., soil samples were taken from all replications following the same

procedures used in 1997. Water infiltration rates were measured from 9 Oct.

through 14 Oct., using the same transect and spacing used previously -ln soil

16

Sampling in 1997; rates were measured at four locations within each treatment plot.

VVhere obvious perfurbations occurred in the soil (tractor wheel tracks, sampling

holes from previous year), infiltrometers were placed on the opposite side of the

transect. Metal cylinders that were 15.6 cm in diameter served as infiltrometers.

The infiltrometers were placed over existing plant cover, and no attempt was made

to remove above-ground thatch prior to recording water infiltration rates. The

cylinders were located at least 0.3 m away from the transect and were placed into

the ground to a depth of 7.6 cm. The cylinders were lined w-lth plastic and filled with

500 ml H20. Head space was measured on opposite sides of the cylinder. Then the

liner was removed from the cylinder, and water infiltration rate was recorded. soil

was at =5% moisture.

LABORATORY METHODS. Soil core samples were analyzed for aggregate stability by

determining the mass of the macroaggregate fraction of the soil. A 100 g dry soil

sample was wet sieved (sieve sizes were 4.0 mm, 2.0 mm,1.0 mm, 500 Hm, and

250 um). Aggregates remaining on each sieve were backwashed into loaf pans,

oven dried at 71 oC, and weighed. Organic carbon and total nitrogen were

determined using dry combustion in a Carlo/Erba NA 1500 NCS analyzer (Haake

Buchler Instruments, Paterson, NJ). Values of pH were measured by mixing 5 g of

soil with 5 ml water`and then stirring the slurry. The slurry was allowed to sit for 30

minutes and was stirred again. The agitated slurry was sampled with a Corning pH

meter 240 for pH determination. ln 1998, the four soil samples from each treatment

plot were combined for pH determinations. Bulk density was determined using the

following equation: Bulk density = Weight of dry soilIVolume of soil (Brady,1974).

17

The soil was weighed at field moist conditl'ons. A 5 g sample of each soil sample

was oven dried and reweighed to determine percent moisture of the soil. Total

weight of dry soil was then calculated.

STATISTICAL ANALYSIS. PROC ANOVA was used to generate analyses of variance

with the Statistical AnalysI'S System (SAS Institute, Cary, N.C.). VVhen F values

were significant, least significant differences (LSD) at P < 0.05 were calculated.

Results and discussion

There were significant main effects of treatments for weed cover percentage,

percent organic carbon, and total nitrogen in 1997 and for weed cover percentage

and water infiltration rate in 1998 (Appendix Table 1 ).

Plots with the lowest weed cover in 1997 were cultivated-bare soil (54o/o), F,

xananassa (54oyo;), R. hirfa (570y(o), and T. erecta (68Oy{o) (Table 1). Panicum virgatum

did not differ from R. h,'rfa and I. erecfa. Because all treatments were difficult to

establish in 1997, weed species, primarily Sefan,'a fabefi7-,', dominated the research

plots (2 54%). ln 1998, weed cover percentages were by as much as 87% ih the

cover Crop treatments. The observed reduction in weed cover percentages is

probably due to better establishment of plots in 1998, from self-reseeding of cover-

crop plants, and the presence of the plugged plants. Smeda and Putnam (1988),

Pritts and Kelly (1993), and VVhitworth (1995) also found that cover crops

Suppressed Weed growth. ln Our experiment, data from 1998 showed that using a P.

virgaft,m or S. st,danense cover crop was as effective at reducing weed cover as

18

cultivating bare soil. As expected, cultivated-bare soil offered one of the best

defenses against.weed pressures in both years. Lo/,'t,m perenne was out-competed

by weeds (primarily SefaH'a fabe#',I), having 100% weed cover in both years.

Macroaggregate mass of soil from cover crop treatment plots did not show

significant differences except that F. xananassa had greater macroaggregate mass

than cultivated-bare soil in 1997 (Table 2). CJltivation causes destruction of plant

material and reduces macroaggregates to microaggregates (Cambardella and Elliot,

1993). Macroaggregates generally contain more soil organic matter than

microaggregates, and an increase in macroaggregate mass is an indication of

improved soil quality. We expect that the presence of cover crops will add organic

matter to the soil in the long run. However, we did not expect to see a difference in

a short period of time (one year).

Bulk density measurements in all treatments were similar to each other (Table

2). Strawberry plants grow best in light, porous soils (lower bulk density) (Galletta

and Bringhurst,1990). Bulk density is a function of soil texture (Brady,1974).

Water infiltration rates were highest in plots of I ereofa (2.4 um/s), S.

sudar,erse (2.1 um/s), and cultivated-bare soil (2.0 um/s) (Table 2). Fasterwater

infiltration rates were expected in undisturbed soils that contain large root systems,

such as those of native prairie plants. However, water infiltration rates in our

experiment were fastest in soils of three-year-old plots grown with the annual cover

crops of I. ereofa and S. st,danense in addition to the cultivated-bare soil treatment.

These results disagree with Shennan (1992), who reports slow infiltration rates in

bare-fallow plots. These results also contradict what Schultz et al. (1996) describe

19

regarding the influence of prairie grasses on infiltration. They report that perennial

root systems increase sol'l porosity and lead to faster water infiltration rates. The

root systems of our prairie species plots may not have been fully developed by the

third year of the experiment. An alternative explanation for our observations is that

the thick thatch that develops under native perennial vegetation physically prevents

water entry into the soil. The dominance of s. faber,-,I in the i. perenne plots created

a thatch that likely controlled water movement in a manner similar to native perennial

vegetation. In the cultivated-bare soil treatment and the annual vegetation

treatments, water infiltration probably occurred largely along preferential flow paths

in the dry soil. Future research could investigate water infiltration rates of soils

under longer-established native prairie plants used as cover crops.

PIots with S. st,danense and cultivated-bare soil had lower organic C

percentages than plots with F. xananassa, i. perenne, and R. A,'rfa in 1997 (Table

3). Groody (1990) observed that bare-fallow plots had significantly lower organic c

than plots with cover-crop treatments, when analyzing soil at o-5 cm. VVhen

analyzing soil at 0-20 cm over four years, no significant changes occurred in organic

C (Shennan,1992). The addition of urea to the F. xar,anassa plots likely caused an

increase in soil microbial accumulation of C, which led to a concomitant increase in

soil organic C.

Total N data mirrored those data for organic C in both years (Table 3).

Shennan (1992) found no significant changes in total N over four years in soil at o-

20 cm.

20

Values of pH were not different between treatments and were in the range

acceptable for strawberry production (5.5-7.5) (Table 3).

Two very important indicators of soil quality are percentage organic c (Larson

and Pierce,1991) and macroaggregate mass. Grouping the data by type of

vegetation provided a supplementary method to evaluate percent organI'C Carbon

and macroaggregate mass. vegetatl'on grouping included native perennial

vegetation (R. A,'rfa, S. avenaceum, and P. v,+gafum), annual vegetation (i. perenne,

S. suc/anense, and I. erecfa), perennial strawberry (F. xananassa), and cultivated-

bare soil.

We assume that perennial strawberry provides a good environment for the

maintenance of soil quality. The matted-row plant cover throughout the season and

use of straw mulch over winter and through harvest contribute to reduced

disturbance of the soil. Trends show that plots with perennial strawberry had the

highest percentage organic carbon,and macroaggregate mass (Table 4). The trends

show that native perennial vegetation improved soil quality over annual vegetation

and cultivated-bare soil in both years of the experiment. The cultivated-bare soil

provided the poorest soil quality because there is no plant cover and soil is

constantly disturbed, thus percentage organic carbon and macroaggregate mass are

the lowest.

Literature cited

Bordelon B.P. and S.C. Weller.1997. Preplant cover crops affect weed and vinegrowth in first-year vineyards. HortScience 32: 1040-1043.

21

Brady, N.C.1974. The nature and properties of soils. 8th ed. Macmillan. N.Y.

Cambardella, C.A. and E.T. Elliot.1993. Carbon and nitrogen distribution inaggregates from cultivated and native grassland soils. Soil Sci. Soc. Amer.J. 57:1071-1076.

Cohen, D. (ed.).1993. Biological communities: Iowa prairies. Iowa Assn. ofNaturalists Bul. no. lAN-203.

DeVVItt, T.A.1984. Soil survey of Story County, Iowa. U.S. Dept. ofAgr. SoilConservation Serv. Bul.0-403-900 QL 3.

Galletta, G,J. and R.S. Bringhurst.1990. Strawberry management, p. 83-153. ln.IG.J. Galletta and D.E. Himelrick. Small fruit crop management. prenticeHall, Englewood Cliffs, N.J.

Groody, K. 1990. Implications for cover crop residue incorporation and mineralfertilizer applications upon crust strength and seedling emergence. MSThesis, Univ. of Californial Davis.

Larson, W.E. and F.J. Pierce.1991. The dynamic of soil quality as a measure ofsustainable management, p. 37-51. ln: Soil Science Society ofAmerica. Defining soil quality for a sustainable environment. Soil Sci. Soc.Amer. Spec. Publ. no. 35.

Pritts, M.P. and M.J. Kelly.1993. Alternative weed management strategies forstrawberries. Acta Hort. 348:321 -327.

Schultz, R.C., A. KuehI, J.P. Colletti, P. Wray, T. lsenhart.1996. Stewards of ourstreams: riparian buffer systems. Iowa State Univ. Ext. BuI. Pm-1626a.

Shennan, C.1992. Cover crops, nitrogen cycling, and soil properties in semi-irrigated vegetable production systems. HortScience 27:7.

Smeda, R.J. and A.R. Putnam.1988. Cover crop suppression of weeds andinfluence on strawberry yields. HortScience 23: 132-134.

Whitworfh, J.L.1995. The ability of some cover crops to suppress commonweeds of strawberry fields. J. Sustainable Agr. 7.

22

Table 1. Weed cover percentages of research plots planted to one of six cover

crops, strawberry, or cultivated-bare soil at the Iowa State University

Horticulture Research Station, Ames, Iowa.

Weed cover (%)z

Treatment 1997y 1998y

Lol-Ium Perenne

Sorghum sudanense

Tagetes erecta

Sorghastrum avenaceum

Panicum virgatum

Rudbeckia hirfa

Fragaria xananassa

Cultivated bare soil

LSDo.o5

100

94

68

95

79

57

54

54

16

100

13

37

83

15

78

91

4

15

z Percentages were determined visually and are based on a possible maximum of

100% weed coverage. Data were collected when the cover crop treatments reached

maturity.

y Mean separation within columns, P < 0.05.

23

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25

Table 4. Percentage organic carbon and macroaggregate mass for soil samples

from research plots receiving one of six cover crops, strawberry, or cultivated-

bare soil at the Iowa State University Horticulture Research Station, Ames,

Iowa. Data were grouped and averaged for native perennial vegetation and

annual vegetation for 1997 and 1998.

Organic carbon (%) Macroaggregate mass (g)

Vegetatl'on type 1997 1998 1997 1998

Perennial strawberryZ

Native perennial vegetationy

Annual vegetationx

Cultivated-bare soilw

2.01 2.00

1.80 1.98

1.70 1.88

1.53 1.84

44.3 37.3

41.5 35.7

40.5 34.7

37.6 31.8

z F. xananassa

y S. avenaceum, P. virgatum, , R. hirta

x L. perenne, S. sudanense, T. erecta

w Minimum plant cover

26

CHAPTER THREE. GENERAL CONCLUSIONS

Weed populations were reduced in treated plots of cultivated-bare soil, and in

plots of the cover crops Sorghum sudanensel and Panicum virgatum between t:wo

years. Better establishment of these cover crop treatments contributed to lower

weed populations in the second year. Macroaggregate mass results were lower in

cultivated-bare soil plots,I likely due to destruction of the aggregates by continuous

cultivation. Water infiltration was faster in plots that were cultivated; preferential flow

paths created by tillage may have allowed for faster infiltration. All treatments

maintained pH values within an acceptable range for strawberry. Trends in

percentage organic C and macroaggregate mass show that undisturbed soils and

the presence of plant cover allow for improved soil quality. Perennial strawberry in

matted-row production provides ideal conditions for maintaining soil quality. Cover

crops of native perennial vegetation in their undisturbed soils performed well and

have the potential for use in rotation with strawberry. Management of native

perennial vegetation must include keeping weed populations to a minimum to allow

for better establishment of the native species. Additional field research should

measure soil quality variables in plantings of cover crops that have been established

for a longer time period than three years.

Recommendations for Future Research

Subsequent research, based upon results of this study, might include planting

strawberries into the treatment plots after removing the cover crops. The cover

27

crops could be killed, incorporated, or act as a living mulch. soil quality variables

should continue to be measured to evaluate soil quality over a longer time period.

This could be done after better establishment of the cover crops (i.e. reducing the

effect of weeds on soil quality as much as possible).

28

APPENDIX= ADDITIONAL TABLE

29

Appendix Table 1. Analysis of variance for the dependent variables, weed cover

percentage, macroaggregate mass, percentage organic carbon, total

nitrogen, and pH for the years 1997 and 1998, and bulk density and water

infiltration rate for the year 1998.

Weed cover percentage (1997 and 1998)

Source

Year

Replication

Treatment

Year * Treatment

Error

Corrected total

DF

1

2

7

7

30

47

Mean square P > F

6154.00852 0.0001

306.3333

3816.9993

2569.6957

71.7611

0.0234

0.0001

0.0001

Macroaggregate mass (1997)

Source

Replication

Treatment

Error

Corrected total

Mean square P > F

135.287387 0.0026

14.739398 0.4549

14.352392

30

Appendix Table 1. (Continued)

Macroaggregate mass (1998)

Source

Replication

Treatment

Error

Corrected total

DF

2

7

14

23

Mean square

85.881050

10.891714

10.159812

P>F

0.0039

0.4295

Percent organic carbon (1997)

Source

Replication

Treatment

Error

Corrected total

DF

2

7

14

23

Mean square

0.01791667

0.11864226

0.03684048

P>F

0.6249

0.0298

Percent organic carbon (1998)

Source

Replication

Treatment

Error

Corrected total

Mean square

0.22807917

0.03153333

0.05313155

P>F

0.0352

0.7513

31

Appendix Table 1. (Continued)

Total nitrogen (1997)

Source

Replication

Treatment

Error

Corrected total

Mean square

0.00026250

0.00078036

0.00024821

P>F

0.3735

0.0324

Total nitrogen (1998)

Source

Replication

Treatment

Error

Corrected total

Mean square

0.00042917

0.00024226

0.00031012

P>F

0.2829

0.6137

pH (1997)

Source

Replication

Treatment

Error

Corrected total

Mean square

0.05345000

0.04255655

0.12624048

P>F

0.6629

0.9235

32

Appendix Table 1. (Continued)

Source

Replication

Treatment

Error

Corrected total

Mean square

0.02311667

0.05037083

0.12129762

P>F

0.8286

0.8771

Bulk density (1998)

Source

Replication

Treatment

Error

Corrected total

DF

2

7

14

23

Mean square

0.01006667

0.00445179

0.00384286

P>F

0.1080

0.3842

Water infiltration rate (1998)

Source

Replication

Treatment

Error

Corrected total

Mean square

0.69875000

3.1552381

0.2425595

P>F

0.0896

0.0001

33

ACKNOWLEDGEMENTS

I thank all of the people that helped me to attain my goal of becoming a

woman with a Master's degree. Initially, I need to thank smokey McKinney for

believing in me and recruiting me to Iowa State. Thanks also to his family for taking

such good care of me during my career at Iowa State.

I thank the following faculty and staff of Iowa State for their aid in their areas

of expertise: Dr. Tom lsenharf for knowledge of prairie specl'es, field help, and as a

prairie plant seed source; and Jody Ohmacht and her lab crew-Briana, Connie,

LeeAnn, and Cindy--who made lab work fun and helped me out when I needed it

most.

Thanks to the faculty and staff of the Horticulture Department who continually

teased-good-heartedly, of course-and made my stay in the department one l'll

remember for a good long time. Thanks to the Hort Farm staff, who so willingly gave

me their time.

Thanks to the past and present graduate students who have become more

than co-workers. You are all dear friends. Thanks for your help in the field. Tom,

Melita, Michael, Craigl Rhonda, Melissa, and Anj, you mean the world to me, and I

pray that we're in touch always.

l{A roomful of thank you's" to Dr. Gall Nonnecke, for not giving up on me and

for keeping me at it for as long as it's taken. You are a true mentor.

Many thanks, too, to my graduate committee-Dr. Richard Schultz for your

humor and patience, Dr. Cynthia Cambardella for being another true mentor and for

34

so graciously allowing me the use of your lab, and Dr. Nick Christians for your quiet,

yet professional, presence and for Italy.

Mom, Dad, Sara, and Gus...just thanks for being there-the WHOLE way! I

love you.

I thank God for watching over me and guiding me.