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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
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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`
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Table 1_
Table 2.
Table 3.
Table 4.
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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.
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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
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crops in rotation with strawberry has the potential to provide a sustainable alternatl've
to chemical herbicides and maintain soil quality.
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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
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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
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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.
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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).
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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
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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
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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.
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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
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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
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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
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(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).
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